METHODS AND COMPOSITIONS FOR TREATING FLAVIVIRUSES AND PESTIVIRUSES

Abstract: A method and composition for treating a host infected with flavivirus or pestivirus comprising administering an effective flavivirus or pestivirus treatment amount of a described 1', 2' or 3'-modified nucleoside or a pharmaceutically acceptable salt or prodrug thereof, is provided.

Description: METHODS AND COMPOSITIONS FOR TREATING FLAVIVIRUSES AND PESTIVIRUSES FIELD OF THE INVENTION This invention is in the area of pharmaceutical chemistry, and in particular, is a compound, method and composition for the treatment of flaviviruses and pestiviruses. This application claims priority to U.S. provisional application no. 60/207,674, filed on May 26, 2000 and U.S. provisional application no. 60/283,276, filed on April 11, 2001. BACKGROUND OF THE INVENTION Pestiviruses and flaviviruses belong to the Flaviviridae family of viruses along with hepatitis C virus. The pestivirus genus includes bovine viral diarrhea virus (BNDV), classical swine fever virus (CSFN, also called hog cholera virus) and border disease virus (BDN) of sheep (Moennig, N. et al. Adv. Vir. Res. 1992, 41, 53-98). Pestivirus infections of domesticated livestock (cattle, pigs and sheep) cause significant economic losses worldwide. BNDV causes mucosal disease in cattle and is of significant economic importance to the livestock industry (Meyers, G. and Thiel, H.-J., Advances in Virus Research, 1996, 47, 53- 118; Moennig V., et al, Adv. Vir. Res. 1992, 41, 53-98). Human pestiviruses have not been as extensively characterized as the animal pestiviruses. However, serological surveys indicate considerable pestivirus exposure in humans. Pestivirus infections in man have been implicated in several diseases including congenital brain injury, infantile gastroenteritis and chronic diarrhea in human immunodeficiency virus (HIN) positive patients. M. Giangaspero et al., Arch. Virol. Suppl., 1993, 7, 53-62; M. Giangaspero et al., Int. J. Std. Aids, 1993, 4 (5): 300-302. The flavivirus genus includes more than 68 members separated into groups on the basis of serological relatedness (Calisher et al., J Gen. Virol, 1993, 70, 37-43). Clinical symptoms vary and include fever, encephalitis and hemorrhagic fever. Fields Virology, Editors: Fields, B. Ν., Knipe, D. M., and Howley, P. M., Lippincott-Raven Publishers, Philadelphia, PA, 1996, Chapter 31, 931-959. Flaviviruses of global concern that are associated with human disease include the dengue hemorrhagic fever viruses (DHF), yellow fever virus, shock syndrome and Japanese encephalitis virus. Halstead, S. B., Rev. Infect. Dis., 1984, 6, 251-264; Halstead, S. B., Science, 239:476-481, 1988; Monath, T. P., New Eng. J. Med., 1988, 319, 641-643. Examples of antiviral agents that have been identified as active against the flavivirus or pestiviruses include: (1) interferon and ribavirin (Battaglia, A.M. et al., Ann. Pharmacother, 2000,. 34, 487- 494); Berenguer, M. et al. Antivir. Ther., 1998, 3 (Suppl. 3), 125-136); (2) Substrate-based NS3 protease inhibitors (Attwood et al, Antiviral peptide derivatives, PCT WO 98/22496, 1998; Attwood et al, Antiviral Chemistry and Chemotherapy 1999, 10, 259-273; Attwood et al, Preparation and use of amino acid derivatives as anti-viral agents, German Patent Pub. DE 19914474; Tung et al. Inhibitors of serine proteases, particularly hepatitis C virus NS3 protease, PCT WO 98/17679), including alphaketoamides and hydrazinoureas, and inhibitors that terminate in an electrophile such as a boronic acid or phosphonate (Llinas-Brunet et al, Hepatitis C inhibitor peptide analogues, PCT WO 99/07734). (3) Non-substrate-based inhibitors such as 2,4,6-trihydroxy-3-nitro-benzamide derivatives (Sudo K. et al, Biochemical and Biophysical Research Communications, 1997, 238, 643-647; Sudo K. et al. Antiviral Chemistry and Chemotherapy, 1998, 9, 186), including RD3-4082 and RD3-4078, the former substituted on the amide with a 14 carbon chain and the latter processing apara- phenoxyphenyl group; (4) Thiazolidine derivatives which show relevant inhibition in a reverse-phase HPLC assay with an NS3/4A fusion protein and NS5A/5B substrate (Sudo K. et al, Antiviral Research, 1996, 32, 9-18), especially compound RD-1-6250, possessing a fused cinnamoyl moiety substituted with a long alkyl chain, RD4 6205 and RD4 6193; (5) Thiazolidines and benzanilides identified in Kakiuchi N. et al. J. EBS Letters 421, 217-220; Takeshita N. et al. Analytical Biochemistry, 1997, 247, 242-246; (6) A phenan-threnequinone possessing activity against protease in a SDS-PAGE and autoradiography assay isolated from the fermentation culture broth of Streptomyces sp., Sch 68631 (Chu M. et al, Tetrahedron Letters, 1996, 37, 7229-7232), and Sch 351633, isolated from the fungus Penicillium griscofuluum, which demonstrates activity in a scintillation proximity assay (Chu M. et al, Bioorganic and Medicinal Chemistry Letters 9, 1949-1952); (7) Selective NS3 inhibitors based on the macromolecule elgin c, isolated from leech (Qasim M.A. et al, Biochemistry, 1997, 36, 1598-1607); (8) Helicase inhibitors (Diana G.D. et al, Compounds, compositions and methods for treatment of hepatitis C, U.S. Pat. No. 5,633,358; Diana G.D. et al, Piperidine derivatives, pharmaceutical compositions thereof and their use in the treatment of hepatitis C, PCT WO 97/36554); (9) Polymerase inhibitors such as nucleotide analogues, gliotoxin (Ferrari R. et al. Journal of Virology, 1999, 73, 1649-1654), and the natural product cerulenin (Lohmann V. et al, Virology, 1998, 249, 108-118); (10) Antisense phosphorothioate oligodeoxynucleotides (S-ODN) complementary to sequence stretches in the 5' non-coding region (NCR) of the virus (Alt M. et al, Hepatology, 1995, 22, 707-717), or nucleotides 326-348 comprising the 3' end of the NCR and nucleotides 371-388 located in the core coding region of the JJCN RΝA (Alt M. et al, Archives of Virology, 1997, 142, 589-599; Galderisi U. et al, Journal of Cellular Physiology, 1999, 181, 251-257); (11) Inhibitors of JRES-dependent translation (Ikeda Ν et al, Agent for the prevention and treatment of hepatitis C, Japanese Patent Pub. JP-08268890; Kai Y. et al. Prevention and treatment of viral diseases, Japanese Patent Pub. JP-10101591); (12) Nuclease-resistant ribozymes (Maccjak, D. J. et al, Hepatology 1999, 30, abstract 995); and (13) Other miscellaneous compounds including 1-amino-alkylcyclohexanes (U.S. Patent No. 6,034,134 to Gold et al), alkyl lipids (U.S. Pat. No. 5,922,757 to Chojkier et al), vitamin E and other antioxidants (U.S. Pat. No. 5,922,757 to Chojkier et al), squalene, amantadine, bile acids (U.S. Pat. No. 5,846,964 to Ozeki et al), N-(phosphonoacetyl)-L-aspartic acid, (U.S. Pat. No. 5,830,905 to Diana et al), benzenedicarboxamides (U.S. Pat. No. 5,633,388 to Diana et al), polyadenylic acid derivatives (U.S. Pat. No. 5,496,546 to Wang et al), 2',3'- dideoxyinosine (U.S. Pat. No. 5,026,687 to Yarchoan et α/.), and benzimidazoles (U.S. Pat. No. 5,891,874 to Colacino et al). hi view of the severity of diseases associated with pestiviruses and flaviviruses, and their pervasiveness in animal and man, it is an object of the present invention to provide a compound, method and composition for the treatment of a host infected with flavivirus or pestivirus. SUMMARY OF THE INVENTION Compounds, methods and compositions for the treatment of a host infected with a flavivirus or pestivirus infection are described that includes an effective treatment amount of a 3-D- or -L-nucleoside of the Formulas (I) - (XVIII), or a pharmaceutically acceptable salt or prodrug thereof. In a first principal embodiment, a compound of Formula I, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (I) wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a second principal embodiment, a compound of Formula II, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (II) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabihzed phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R or R 3 is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). hi a third principal embodiment, a compound of Formula III, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (III) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including onophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a fourth principal embodiment, a compound of Formula IV, or a pharmaceutically acceptable salt or prodrug thereof, is provided: wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a fifth principal embodiment, a compound of Formula V, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (V) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , τ R.2 or R is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a sixth principal embodiment, a compound of Formula VI, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (VI) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). hi a seventh principal embodiment, a compound selected from Formulas Nil, VIII and IX, or a pharmaceutically acceptable salt or prodrug thereof, is provided: wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3) chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . In a eighth principal embodiment, a compound of Formulas X, XI and XII, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (X) (XI) (XII) wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . In a ninth principal embodiment a compound selected from Formulas XIII, XIV and XV, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (XIII) (XIV) (XV) wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R 3 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . In a tenth principal embodiment the invention provides a compound of Formula XVI, or a pharmaceutically acceptable salt or prodrug thereof: (XVI) wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving • . 1 group which when admimstered in vivo is capable of providing a compound wherein R or R is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower allcyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a pi bond; and X is O, S, SO 2 or CH 2 . In a eleventh principal embodiment the invention provides a compound of Formula XVII, or a pharmaceutically acceptable salt or prodrug thereof: (XVII) wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 or R 2 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; 7 0 9 R and R are independently hydrogen, OR , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a pi bond; and X is O, S, SO 2 or CH 2 . In an twelfth principal embodiment, the invention provides a compound of Formula XVIII, or a pharmaceutically acceptable salt or prodrug thereof: (xviπ) wherein: Base is a purine or pyrimidine base as defined herein; 1 9 R and R independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 or R 2 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(lower- alkyl)amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a pi bond; X is O, S, SO 2 or CH 2 . The /3-D- and /3-L-nucleosides of this invention may inhibit flavivirus or pestivirus polymerase activity. These nucleosides can be assessed for their ability to inhibit flavivirus or pestivirus polymerase activity in vitro according to standard screening methods. i one embodiment the efficacy of the anti-flavivirus or pestivirus compound is measured according to the concentration of compound necessary to reduce the plaque number ofthe virus in vitro, according to methods set forth more particularly herein, by 50% (i.e. the compound's EC 50 ). hi preferred embodiments the compound exhibits an EC 50 of less than 15 or preferably, less than 10 micromolar in vitro. In another embodiment, the active compound can be administered in combination or alternation with another anti-flavivirus or pestivirus agent. In combination therapy, effective dosages of two or more agents are administered together, whereas during alternation therapy an effective dosage of each agent is administered serially. The dosages will depend on absorption, inactivation and excretion rates of the drug as well as other factors known to those of skill in the art. It is to be noted that dosage values will also vary with the severity of the condition to be alleviated. It is to be further understood that for any particular subject, specific dosage regimens and schedules should be adjusted over time according to the individual need and the professional judgment of the person administering or supervising the administration ofthe compositions. HCV is a member ofthe Flaviviridae family; however, now, HCV has been placed in a new monotypic genus, hepacivirus. Therefore, in one embodiment, the flavivirus or pestivirus is not HCV. Nonlimiting examples of antiviral agents that can be used in combination with the compounds disclosed herein include: (1) an interferon and/or ribavirin (Battaglia, A.M. et al, Ann. Pharmacother. 34:487- 494, 2000); Berenguer, M. et al. Antivir. Ther. 3(Supρl. 3):125-136, 1998); (2) Substrate-based NS3 protease inhibitors (Attwood et al, Antiviral peptide derivatives, PCT WO 98/22496, 1998; Attwood et al, Antiviral Chemistry and Chemotherapy 10.259-273, 1999; Attwood et al, Preparation and use of amino acid derivatives as anti-viral agents, German Patent Publication DE 19914474; Tung et al. Inhibitors of serine proteases, particularly hepatitis C virus NS3 protease, PCT WO 98/17679), including alphaketoamides and hydrazinoureas, and inhibitors that terminate in an electrophile such as a boronic acid or phosphonate. Llinas-Brunet et al, Hepatitis C inhibitor peptide analogues, PCT WO 99/07734. (3) Non-substrate-based inhibitors such as 2,4,6-trihydroxy-3-nitro-benzamide derivatives(Sudo K. et al, Biochemical and Biophysical Research Communications, 238:643- 647, 1997; Sudo K. et al. Antiviral Chemistry and Chemotherapy 9:186, 1998), including RD3-4082 and RD3-4078, the former substituted on the amide with a 14 carbon chain and the latter processing a αr -phenoxyphenyl group; (4) Thiazolidine derivatives which show relevant inhibition in a reverse-phase HPLC assay with an NS3/4A fusion protein and NS5A/5B substrate (Sudo K. et al, Antiviral Research 32:9-18, 1996), especially compound RD-1-6250, possessing a fused cinnamoyl moiety substituted with a long alkyl chain, RD4 6205 and RD4 6193; (5) Thiazolidines and benzanilides identified in Kakiuchi N. et al. J. EBS Letters 421:217-220; Takeshita N. et al Analytical Biochemistry 247:242-246, 1997; (6) A phenan-threnequinone possessing activity against protease in a SDS-PAGE and autoradiography assay isolated from the fermentation culture broth of Streptomyces sp., Sch 68631 (Chu M. et al, Tetrahedron Letters 37:7229-7232, 1996), and Sch 351633, isolated from the fungus Penicillium griscofuluum, which demonstrates activity in a scintillation proximity assay (Chu M. et al, Bioorganic and Medicinal Chemistry Letters 9:1949-1952); (7) Selective NS3 inhibitors based on the macromolecule elgin c, isolated from leech (Qasim M.A. et al, Biochemistry 36:1598-1607 ', 1997); (8) Helicase inhibitors (Diana G.D. et al, Compounds, compositions and methods for treatment of hepatitis C, U.S. Patent No. 5,633,358; Diana G.D. et al, Piperidine derivatives, pharmaceutical compositions thereof and their use in the treatment of hepatitis C, PCT WO 97/36554); (9) Polymerase inhibitors such as nucleotide analogues, gliotoxin (Ferrari R. et al. Journal of Virology 73:1649-1654, 1999), and the natural product cerulenin (Lohmann V. et al, Virology 249:108-118, 1998); (10) Antisense phosphorothioate oligodeoxynucleotides (S-ODN) complementary to sequence stretches in the 5' non-coding region (NCR) of the virus (Alt M. et al, Hepatology 22:707-717, 1995), or nucleotides 326-348 comprising the 3' end ofthe NCR and nucleotides 371-388 located in the core coding region of the IICV RNA (Alt M. et al, Archives of Virology 142:589-599, 1997; Galderisi U et al, Journal of Cellular Physiology 181:251-257, 1999); (11) Inhibitors of IRES-dependent translation (Ikeda N et al, Agent for the prevention and treatment of hepatitis C, Japanese Patent Publication JP-08268890; Kai Y. et al. Prevention and treatment of viral diseases, Japanese Patent Publication JP-10101591); (12) Nuclease-resistant ribozymes. (Maccjak D.J. et al, Hepatology 30 abstract 995, 1999); and (13) Other miscellaneous compounds including 1-amino-alkylcyclohexanes (U.S. Patent No. 6,034,134 to Gold et al), alkyl lipids (U.S. Patent No. 5,922,757 to Chojkier et al), vitamin E and other antioxidants (U.S. Patent No. 5,922,757 to Chojkier et al), squalene, amantadine, bile acids (U.S. Patent No. 5,846,964 to Ozelci et al), N- (phosphonoacetyl)-L-aspartic acid, (U.S. Patent No. 5,830,905 to Diana et al), benzenedicarboxamides (U.S. Patent No. 5,633,388 to Diana et al), polyadenylic acid derivatives (U.S. Patent No. 5,496,546 to Wang et al), 2',3'-dideoxyinosine (U.S. Patent No. 5,026,687 to Yarchoan et al.), and benzimidazoles (U.S. Patent No. 5,891,874 to Colacino et al). BRIEF DESCRIPTION OF THE FIGURES Figure 1 provides the structure ofvarious non-limiting examples of nucleosides ofthe present invention, as well as other known nucleosides, FIAU and Ribavirin, which are used as comparative examples in the text. Figure 2 is a line graph of the pharmacokinetics (plasma concentrations) of β-D-2'- CH 3 -riboG administered to Cynomolgus Monkeys over time after administration. Figure 3a and 3b are line graphs of the pharmacokinetics (plasma concentrations) of β-D-2'-CH 3 -riboG administered to Cynomolgus Monkeys either intravenously (3a) or orally (3b) over time after administration. Figure 4 depicts line graphs of the results of the cell protection assay of β-D-2'-CH 3 - riboG against BVDV. Figure 5 depicts line graphs of the results of the cell protection assay of ribavirin against BVDV. Figure 6 are line graphs of the cell protection assay of β-D-2'-CH 3 -riboG, β-D-2'- CH 3 -riboC, β-D-2'-CH 3 -riboU, β-D-2'-CH 3 -riboA and ribavirin. Figure 7 are line graphs of the results of the plaque reduction assay for β-D-2'-CH 3 - riboU, β-D-2'-CH 3 -riboC and β-D-2'-CH 3 -riboG. Figure 8 is an illustration of plaque reduction based on increasing concentrations of β-D-2'-CH 3 -riboU. Figure 9 is a line graph of the results of the yield reduction assay for β-D-2'-CH 3 - riboG, depicting a 4 log reduction at 9 μM. Figure 10 is an illustration of the yield reduction based on increasing concentrations of β-D-2'-CH 3 -riboC. DETAILED DESCRIPTION OF THE INVENTION The invention as disclosed herein is a compound, method and composition for the treatment of pestiviruses and flaviviruses in humans and other host animals, that includes the administration of an effective flavivirus or pestivirus treatment amount of an /3-D- or β-L- nucleoside as described herein or a pharmaceutically acceptable salt or prodrug thereof, optionally in a pharmaceutically acceptable carrier. The compounds of this invention either possess antiviral (i.e., anti-flavivirus or pestivirus) activity, or are metabolized to a compound that exhibits such activity. hi summary, the present invention includes the following features: (a) /3-D- and /3-L-nucleosides, as described herein, and pharmaceutically acceptable salts and prodrugs thereof; (b) /3-D- and /3-L-nucleosides as described herein, and pharmaceutically acceptable salts and prodrugs thereof for use in the treatment or prophylaxis of a flavivirus or pestivirus infection, especially in individuals diagnosed as having a flavivirus or pestivirus infection or being at risk for becoming infected by flavivirus or pestivirus; (c) use of these /3-D- and /3-L-nucleosides, and pharmaceutically acceptable salts and prodrugs thereof in the manufacture of a medicament for treatment of a flavivirus or pestivirus infection; (d) pharmaceutical formulations comprising the /3-D- and /3-L-nucleosides or pharmaceutically acceptable salts or prodrugs thereof together with a pharmaceutically acceptable carrier or diluent; (e) /3-D- and /3-L-nucleosides as described herein substantially in the absence of enantiomers of the described nucleoside, or substantially isolated from other chemical entities; (f) processes for the preparation of /3-D- and /3-L-nucleosides, as described in more detail below; and (g) processes for the preparation of /3-D- and /3-L-nucleosides substantially in the absence of enantiomers of the described nucleoside, or substantially isolated from other chemical entities. Flaviviruses included within the scope of this invention are discussed generally in Fields Virology, Editors: Fields, B. N., Knipe, D. M., and Howley, P. M., Lippincott-Raven Publishers, Philadelphia, PA, Chapter 31, 1996. Specific flaviviruses include, without limitation: Absettarov, Alfuy, Apoi, Aroa, Bagaza, Banzi, Bouboui, Bussuquara, Cacipacore, Carey Island, Dakar bat, Dengue 1, Dengue 2, Dengue 3, Dengue 4, Edge Hill, Entebbe bat, Gadgets Gully, Hanzalova, Hypr, Ilheus, Israel turkey meningoencephalitis, Japanese encephalitis, Jugra, Jutiapa, Kadam, Karshi, Kedougou, Kokobera, Koutango, Kumlinge, Kunjin, Kyasanur Forest disease, Langat, Louping ill, Meaban, Modoc, Montana myotis leukoencephalitis, Murray valley encephalitis, Naranjal, Negishi, Ntaya, Omsk hemorrhagic fever, Phnom-Penh bat, Powassan, Rio Bravo, Rocio, Royal Farm, Russian spring-summer encephalitis, Saboya, St. Louis encephalitis, Sal Vieja, San Perlita, Sau arez Reef, Sepik, Sokuluk, Spondweni, Stratford, Tembusu, Tyuleniy, Uganda S, Usutu, Wesselsbron, West Nile, Yaounde, Yellow fever, and Zika. Pestiviruses included within the scope of this invention are discussed generally in Fields Virology, Editors: Fields, B. N., Knipe, D. M., and Howley, P. M., Lippincott-Raven Publishers, Philadelphia, PA, Chapter 33, 1996. Specific pestiviruses include, without limitation: bovine viral diarrhea virus ("BVDV"), classical swine fever virus ("CSFV," also called hog cholera virus), and border disease virus ("BDV"). I. Active Compound, and Physiologically Acceptable Salts and Prodrugs Thereof In a first principal embodiment, a compound of Formula I, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (I) wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula I, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: R 1 , R 2 and R 3 are independently H or phosphate (preferably H); X 1 is H; X 2 is H orNH 2 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH 2 or OH. In a second principal embodiment, a compound of Formula II, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (II) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R 3 is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula II, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: R 1 , R 2 and R 3 are independently H or phosphate (preferably H); X 1 is H; X 2 is H or NH 2 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH 2 or OH. In a third principal embodiment, a compound of Formula III, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (III) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving • • 1 9 group which when admimstered in vivo is capable of providing a compound wherein R , R or R 3 is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are mdependently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula III, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: R 1 , R 2 and R 3 are independently H or phosphate (preferably H); X 1 is H; X 2 is H or NH 2 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH 2 or OH. In a fourth principal embodiment, a compound of Formula IV, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (TV) wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group 1 " which when administered in vivo is capable of providing a compound wherein R , R or R is independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula IV, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: 1 9 " R , R and R are independently H or phosphate (preferably H); X 1 is H or CH 3 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH 2 or OH. hi a fifth principal embodiment, a compound of Formula V, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (V) wherein: 1 9 " R , R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R 3 is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula V, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: R 1 , R 2 and R 3 are independently H or phosphate (preferably H); X 1 is H or CH 3 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH or OH. In a sixth principal embodiment, a compound of Formula VI, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (VI) wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO- alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 5 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). In a preferred subembodiment, a compound of Formula VI, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: R 1 , R 2 and R 3 are independently H or phosphate (preferably H); X 1 is H or CH 3 ; and Y is hydrogen, bromo, chloro, fluoro, iodo, NH or OH. In a seventh principal embodiment, a compound selected from Formulas VII, VIII and IX, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (VII) (VIII) (IX) wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving 1 9 group which when administered in vivo is capable of providing a compound wherein R , R or R is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), CF 3) chloro, bromo, fluoro, iodo, NO 2 , NH , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . In a first preferred subembodiment, a compound of Formula VII, VIII or IX, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently hydrogen or phosphate; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . hi a second preferred subembodiment, a compound of Formula VII, VIII or IX, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are hydrogens; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . In a third preferred subembodiment, a compound of Formula VII, NIII or IX, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently hydrogen or phosphate; R is alkyl; and X is O. In a eighth principal embodiment, a compound of Formula X, XI or XII, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (X) (XI) (XII) wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R 3 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH , -NH(lower alkyl), -NH(acyl), - N(loweralkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . In a first preferred subembodiment, a compound of Formula X, XI or XII, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently hydrogen or phosphate; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . In a second preferred subembodiment, a compound of Formula X, XI or XII, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are hydrogens; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . hi a third preferred subembodiment, a compound of Formula X, XI or XII, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H or phosphate; R 6 is alkyl; and X is O. In even more preferred subembodfments, a compound of Formula XI, or its pharmaceutically acceptable salt or prodrug, is provided: (XI) wherein: Base is a purine or pyrimidine base as defined herein; optionally substituted with an amine or cyclopropyl (e.g., 2-amino, 2,6-diamino or cyclopropyl guanosine); and R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo i ■s capable of provi •ding a compound wherein R 1 or R9 is independently H or phosphate. In a ninth principal embodiment a compound selected from Formula XIII, XIV or XV, or a pharmaceutically acceptable salt or prodrug thereof, is provided: (XIII) (XIV) (XV) wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 or R is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) ; and X is O, S, SO 2 or CH 2 . In a first preferred subembodiment, a compound of Formula XIII, XIN or XV, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently hydrogen or phosphate; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . In a second preferred subembodiment, a compound of Formula XIII, XIV or XV, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are hydrogens; R 6 is alkyl; and X is O, S, SO 2 or CH 2 . In a third preferred subembodiment, a compound of Formula XIII, XIV or XN, or a pharmaceutically acceptable salt or prodrug thereof, is provided wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently hydrogen or phosphate; R 6 is alkyl; and X is O. In a tenth principal embodiment the invention provides a compound of Formula XVI, or a pharmaceutically acceptable salt or prodrug thereof: (XVI) wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a pi bond; and X is O, S, SO 2 or CH 2 . In a first preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 , alkyl, alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O, S, SO 2 or CH 2 . hi a second preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl, alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O, S, SO 2 or CH 2 . In a third preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl, alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 , alkyl, alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (5) R 8 and R 10 are H; and (6) X is O, S, SO 2 or CH 2 . In a fourth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl, alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 , alkyl, alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (5) R and R are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O. In a fifth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R is alkyl; (4) R 7 and R 9 are independently OR 1 ; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O, S, SO 2 or CH . In a sixth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R is alkyl; (4) R 7 and R 9 are independently OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 8 and R 10 are H; and (6) X is O, S, SO 2 , or CH 2 . In a seventh preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O. hi a eighth prefened subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when admimstered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are hydrogen; and (6) X is O, S, SO 2 or CH 2 . In a ninth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O. In a tenth prefened subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R are independently OR , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O- alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 8 and R 10 are hydrogen; and (6) X is O. hi an eleventh preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are hydrogen; and (6) X is O, S, SO 2 or CH 2 . In a twelfth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are hydrogen; and (6) X is O, S, SO 2 , or CH 2 . hi a thirteenth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or 1 f pyrimidine base as defined herein; (2) R is independently H or phosphate; (3) R is alkyl; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O. In a fourteenth preferred subembodiment, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (5) R 8 and R 10 are hydrogen; and (6) X is O. In even more prefened subembodiments, a compound of Formula XVI, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is guanine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is cytosine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is thymine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is uracil; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is phosphate; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is ethyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R δ is propyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is butyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 is hydrogen and R 9 is hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is S; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is SO 2 ; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 and R 10 are hydrogen; and (6) X is CH 2 ; In a eleventh principal embodiment the invention provides a compound of Formula XVII, or a pharmaceutically acceptable salt or prodrug thereof: (XVII) wherein: Base is a purine or pyrimidine base as defined herein; R 1 is H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a pi bond; and X is O, S, SO 2 or CH 2 . In a first preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)- amino; (5) R 10 is H; and (6) X is O, S, SO 2 , or CH 2 . In a second preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O, S, SO 2 or CH 2 . hi a third preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)- amino; (5) R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O. In a fourth preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H; and (6) X is O, S, SO 2 or CH 2 . In a fifth preferred subembodiment, a compound of Formula XVII, or its pharaiaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O. In a sixth preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is mdependently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 0 9 and R are independently hydrogen, OR , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 10 is H; and (6) X is O. In a seventh preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H; and (6) X is O. In an eighth preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)-amino; (5) R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O, S, SO 2 , or CH 2 . In a ninth preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H; and (6) X is O, S, SO 2 , or CH 2 . In a tenth preferred subembodiment, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 ; (5) R 10 is H; and (6) X is O, S, SO 2 , or CH 2 . hi even more preferred subembodiments, a compound of Formula XVII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is guanine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is cytosine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is thymine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is uracil; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is phosphate; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is ethyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is propyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is butyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is S; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is SO 2 ; or (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 10 is hydrogen; and (6) X is CH 2 . In an twelfth principal embodiment the invention provides a compound of Formula XVIII, or a pharmaceutically acceptable salt or prodrug thereof: (XVIII) wherein: Base is a purine or pyrimidine base as defined herein; R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br- vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), - N(lower alkyl) 2 , -N(acyl) 2 ; R and R are independently hydrogen, OR , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, lower alkylamino, or di(loweralkyl)amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a pi bond; X is O, S, SO 2 or CH 2 . In a first preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (5) R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; and (6) X is O, S, SO 2 or CH 2 . hi a second preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di-(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O, S, SO 2 or CH 2 . hi a third preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(lower-alkyl)amino; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 8 is H; and (6) X is O, S, SO 2 or CH 2 . In a fourth preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 8 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O. In a fifth preferred subembodiment, a compound of Formula XNTfl, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, ΝO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H; and (6) X is O, S, SO 2 , or CH 2 . In a sixth prefened subembodiment, a compound of Formula XVIII, or its pharaiaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; and (6) X is O. In a seventh preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (4) R 7 0 9 and R are independently hydrogen, OR , alkyl (including lower alkyl), alkenyl, alkynyl, Br- vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; (5) R 8 is H; and (6) X is O. In an eighth prefened subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, hydroxy, O-alkyl, O-alkenyl, chloro, bromo, fluoro, iodo, NO 2 , amino, loweralkylamino or di(loweralkyl)amino; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H; and (6) X is O, S, SO 2 or CH 2 . In a ninth preferred subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R 1 is independently H or phosphate; (3) R 6 is alkyl; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H; and (6) X is O, S, SO 2 , or CH 2 . In a tenth prefened subembodiment, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is a purine or pyrimidine base as defined herein; (2) R is independently H or phosphate; (3) R is alkyl; (4) R 7 and R 9 are independently OR 2 ; (5) R 8 is H; and (6) X is O. i even more prefened subembodiments, a compound of Formula XVIII, or its pharmaceutically acceptable salt or prodrug, is provided in which: (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is guanine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is cytosine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is thymine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is uracil; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is phosphate; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is ethyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is propyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is butyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is O; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is S; (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is SO 2 ; or (1) Base is adenine; (2) R 1 is hydrogen; (3) R 6 is methyl; (4) R 7 and R 9 are hydroxyl; (5) R 8 is hydrogen; and (6) X is CH 2 . The /3-D- and /3-L-nucleosides of this invention belong to a class, of anti-flavivirus or pestivirus agents that may inhibit flavivirus or pestivirus polymerase activity. Nucleosides can be screened for their ability to inhibit flavivirus or pestivirus polymerase activity in vitro according to screening methods set forth more particularly herein. One can readily determine the spectrum of activity by evaluating the compound in the assays described herein or with another confirmatory assay. In one embodiment the efficacy of the anti-flavivirus or pestivirus compound is measured according to the concentration of compound necessary to reduce the plaque number ofthe virus in vitro, according to methods set forth more particularly herein, by 50% (i.e. the compound's EC50). hi preferred embodiments the compound exhibits an EC 50 of less than 15 or 10 micromolar. HCV is a member ofthe Flaviviridae family; however, now, HCV has been placed in a new monotypic genus, hepacivirus. Therefore, in one embodiment, the flavivirus or pestivirus is not HCV. The active compound can be administered as any salt or prodrug that upon administration to the recipient is capable of providing directly or indirectly the parent compound, or that exhibits activity itself. Nonlimiting examples are the pharmaceutically acceptable salts (alternatively refened to as "physiologically acceptable salts"), and a compound, which has been alkylated or acylated at the 5 '-position, or on the purine or pyrimidine base (a type of "pharmaceutically acceptable prodrug"). Further, the modifications can affect the biological activity ofthe compound, in some cases increasing the activity over the parent compound. This can easily be assessed by preparing the salt or prodrug and testing its antiviral activity according to the methods described herein, or other methods known to those skilled in the art. II. Definitions The term alkyl, as used herein, unless otherwise specified, refers to a saturated straight, branched, or cyclic, primary, secondary, or tertiary hydrocarbon of typically to C 10 , and specifically includes methyl, trifluoromethyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl, isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl, 3-methylpentyl, 2,2-dimethylbutyl, and 2,3-dimethylbutyl. The term includes both substituted and unsubstituted alkyl groups. Moieties with which the alkyl group can be substituted are selected from the group consisting of hydroxyl, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, or phosphonate, either unprotected, or protected as necessary, as known to those skilled in the art, for example, as taught in Greene, et al, Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, hereby incorporated by reference. The term lower alkyl, as used herein, and unless otherwise specified, refers to a to C 4 saturated straight, branched, or if appropriate, a cyclic (for example, cyclopropyl) alkyl group, including both substituted and unsubstituted forms. Unless otherwise specifically stated in this application, when alkyl is a suitable moiety, lower alkyl is preferred. Similarly, when alkyl or lower alkyl is a suitable moiety, unsubstituted alkyl or lower alkyl is prefened. The term alkylamino or arylamino refers to an amino group that has one or two alkyl or aryl substituents, respectively. The term "protected" as used herein and unless otherwise defined refers to a group that is added to an oxygen, nitrogen, or phosphorus atom to prevent its further reaction or for other purposes. A wide variety of oxygen and nitrogen protecting groups are known to those skilled in the art of organic synthesis. The term aryl, as used herein, and unless otherwise specified, refers to phenyl, biphenyl, or naphthyl, and preferably phenyl. The term includes both substituted and unsubstituted moieties. The aryl group can be substituted with one or more moieties selected from the group consisting of hydroxyl, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, or phosphonate, either unprotected, or protected as necessary, as known to those skilled in the art, for example, as taught in Greene, et al, Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The temi alkaryl or alkylaryl refers to an alkyl group with an aryl substituent. The term aralkyl or arylalkyl refers to an aryl group with an alkyl substituent. The term halo, as used herein, includes chloro, bromo, iodo, and fluoro. The term purine or pyrimidine base includes, but is not limited to, adenine, N - alkylpurines, N 6 -acylpurines (wherein acyl is C(O)(alkyl, aryl, alkylaryl, or arylalkyl), N 6 - benzylpurine, N 6 -halopurine, N 6 -vinylpurine, N 6 -acetylenic purine, N 6 -acyl purine, N -hydroxyalkyl purine, N -thioalkyl purine, N -alkylpurines, N -alkyl-6-thiopurines, thymine, cytosine, 5-fluorocytosine, 5-methylcytosine, 6-azapyrimidine, including 6- azacytosine, 2- and/or 4-mercaptopyrmidine, uracil, 5-halouracil, including 5-fluorouracil, C 5 -alkylpyrimidines, C 5 -benzylpyrimidines, C 5 -halopyrimidines, C 5 -vinylpyrimidine, C 5 - acetylenic pyrimidine, C 5 -acyl pyrimidine, C 5 -hydroxyalkyl purine, C 5 -amidoρyrimidine, C 5 - cyanopyrimidine, C 5 -nitropyrimidine, C 5 -aminopyrimidine, N 2 -alkylpurines, N 2 -alkyl-6- thiopurines, 5-azacytidinyl, 5-azauracilyl, triazolopyridinyl, imidazolopyridinyl, pynolopyrimidinyl, and pyrazolo-pyrimidinyl. Purine bases include, but are not limited to, guanine, adenine, hypoxanthine, 2,6-diaminopurine, and 6-chloropurine. Functional oxygen and nitrogen groups on the base can be protected as necessary or desired. Suitable protecting groups are well known to those skilled in the art, and include tnmethylsilyl, dimethylhexylsilyl, t-butyldimethylsilyl and t-butyldiphenylsilyl, trityl, alkyl groups, and acyl groups such as acetyl and propionyl, methanesulfonyl, and p-toluenesulfonyl. Alternatively, the purine or pyrimidine base can optionally substituted such that it forms a viable prodrug, which can be cleaved in vivo. Examples of appropriate substituents include acyl moiety, an amine or cyclopropyl (e.g., 2-amino, 2,6-diamino or cyclopropyl guanosine). The term acyl refers to a carboxylic acid ester in which the non-carbonyl moiety of the ester group is selected from straight, branched, or cyclic alkyl or lower alkyl, alkoxyalkyl including methoxymethyl, aralkyl including benzyl, aryloxyalkyl such as phenoxymethyl, aryl including phenyl optionally substituted with halogen, to C 4 alkyl or C \ to C alkoxy, sulfonate esters such as alkyl or aralkyl sulphonyl including methanesulfonyl, the mono, di or triphosphate ester, trityl or monomethoxytrityl, substituted benzyl, trialkylsilyl (e.g. dimethyl- t-butylsilyl) or diphenylmethylsilyl. Aryl groups in the esters optimally comprise a phenyl group. The term "lower acyl" refers to an acyl group in which the non-carbonyl moiety is lower alkyl. As used herein, the term "substantially free of or "substantially in the absence of refers to a nucleoside composition that includes at least 85 or 90% by weight, preferably 95% to 98 % by weight, and even more preferably 99% to 100% by weight, of the designated enantiomer of that nucleoside. In a preferred embodiment, in the methods and compounds of this invention, the compounds are substantially free of enantiomers. Similarly, the term "isolated" refers to a nucleoside composition that includes at least 85 or 90% by weight, preferably 95% to 98 % by weight, and even more preferably 99% to 100% by weight, of the nucleoside, the remainder comprising other chemical species or enantiomers. The term "independently" is used herein to indicate that the variable, which is independently applied, varies independently from application to application. Thus, in a compound such as R"XYR", wherein R" is "independently carbon or nitrogen," both R" can be carbon, both R" can be nitrogen, or one R" can be carbon and the other R" nitrogen. The terai host, as used herein, refers to an unicellular or multicellular organism in which the virus can replicate, including cell lines and animals, and preferably a human. Alternatively, the host can be carrying a part of the flavivirus or pestivirus genome, whose replication or function can be altered by the compounds of the present invention. The term host specifically refers to infected cells, cells transfected with all or part of the flavivirus or pestivirus genome and animals, in particular, primates (including chimpanzees) and humans. In most animal applications ofthe present invention, the host is a human patient. Veterinary applications, in certain indications, however, are clearly anticipated by the present invention (such as chimpanzees). The term "pharmaceutically acceptable salt or prodrug" is used throughout the specification to describe any pharmaceutically acceptable form (such as an ester, phosphate ester, salt of an ester or a related group) of a nucleoside compound which, upon administration to a patient, provides the nucleoside compound. Pharmaceutically acceptable salts include those derived from pharmaceutically acceptable inorganic or organic bases and acids. Suitable salts include those derived from alkali metals such as potassium and sodium, alkaline earth metals such as calcium and magnesium, among numerous other acids well known in the pharmaceutical art. Pharmaceutically acceptable prodrugs refer to a compound that is metabolized, for example hydrolyzed or oxidized, in the host to form the compound of the present invention. Typical examples of prodrugs include compounds that have biologically labile protecting groups on a functional moiety of the active compound. Prodrugs include compounds that can be oxidized, reduced, aminated, deaminated, hydroxylated, dehydroxylated, hydrolyzed, dehydrolyzed, alkylated, dealkylated, acylated, deacylated, phosphorylated, dephosphorylated to produce the active compound. The compounds of this invention possess antiviral activity against flavivirus or pestivirus, or are metabolized to a compound that exhibits such activity. III. Nucleotide Salt or Prodrug Formulations In cases where compounds are sufficiently basic or acidic to form stable nontoxic acid or base salts, administration of the compound as a pharmaceutically acceptable salt may be appropriate. Examples of pharmaceutically acceptable salts are organic acid addition salts formed with acids, which form a physiological acceptable anion, for example, tosylate, methanesulfonate, acetate, citrate, malonate, tartarate, succinate, benzoate, ascorbate, - ketoglutarate, and α-glycerophosphate. Suitable inorganic salts may also be formed, including, sulfate, nitrate, bicarbonate, and carbonate salts. Pharmaceutically acceptable salts may be obtained using standard procedures well known in the art, for example by reacting a sufficiently basic compound such as an amine with a suitable acid affording a physiologically acceptable anion. Alkali metal (for example, sodium, potassium or lithium) or alkaline earth metal (for example calcium) salts of carboxylic acids can also be made. Any of the nucleosides described herein can be administered as a nucleotide prodrug to increase the activity, bioavailabihty, stability or otherwise alter the properties of the nucleoside. A number of nucleotide prodrug ligands are known. In general, alkylation, acylation or other lipophilic modification of the mono, di or triphosphate of the nucleoside will increase the stability of the nucleotide. Examples of substituent groups that can replace one or more hydrogens on the phosphate moiety are alkyl, aryl, steroids, carbohydrates, including sugars, 1,2-diacylglycerol and alcohols. Many are described in R. Jones and N. Bischofberger, Antiviral Research, 27 (1995) 1-17. Any of these can be used in combination with the disclosed nucleosides to achieve a desired effect. The active nucleoside can also be provided as a 5'-phosphoether lipid or a 5 '-ether lipid, as disclosed in the following references, which are incorporated by reference herein: Kucera, L.S., N. Iyer, E. Leake, A. Raben, Modest E.K., D.L.W., and C. Piantadosi, "Novel membrane-interactive ether lipid analogs that inhibit infectious H1N-1 production and induce defective virus formation," AIDS Res. Hum. Retro Viruses, 1990, 6, 491-501; Piantadosi, C, J. Marasco C.J., S.L. Morris-Νatschke, K.L. Meyer, F. Gumus, J.R. Surles, K.S. Ishaq, L.S. Kucera, Ν. Iyer, CA. Wallen, S. Piantadosi, and E.J. Modest, "Synthesis and evaluation of novel ether lipid nucleoside conjugates for anti-HIV activity," J. Med. Chem., 1991, 34, 1408-1414; Hosteller, K.Y., D.D. Richman, D.A. Carson, L.M. Stuhmiller, G.M. T. van Wijk, and H. van den Bosch, "Greatly enhanced inhibition of human immunodeficiency virus type 1 replication in CEM and HT4-6C cells by 3'-deoxythymidine diphosphate dimyristoylglycerol, a lipid prodrug of 3,-deoxythymidine," Antimicrob. Agents Chemother., 1992, 36, 2025-2029; Hosetler, K.Y., L.M. Stuhmiller, H.B. Lenting, H. van den Bosch, and D.D. Richman, "Synthesis and antiretroviral activity of phospholipid analogs of azidothymidine and other antiviral nucleosides." J. Biol. Chem., 1990, 265, 61127. Nonlimiting examples of U.S. patents that disclose suitable lipophilic substituents that can be covalently incorporated into the nucleoside, preferably at the 5' -OH position of the nucleoside or lipophilic preparations, include U.S. Patent Nos. 5,149,794 (Sep. 22, 1992, Yatvin et al); 5,194,654 (Mar. 16, 1993, Hostetler et al, 5,223,263 (June 29, 1993, Hostetler et al.); 5,256,641 (Oct. 26, 1993, Yatvin et al.); 5,411,947 (May 2, 1995, Hostetler et al.); 5,463,092 (Oct. 31, 1995, Hostetler et al); 5,543,389 (Aug. 6, 1996, Yatvin et al.); 5,543,390 (Aug. 6, 1996, Yatvin et al); 5,543,391 (Aug. 6, 1996, Yatvin et al.); and 5,554,728 (Sep. 10, 1996; Basava et al.), all of which are incorporated herein by reference. Foreign patent applications that disclose lipophilic substituents that can be attached to the nucleosides ofthe present invention, or lipophilic preparations, include WO 89/02733, WO 90/00555, W0 91/16920, W0 91/18914, W0 93/00910, W0 94/26273, WO 96/15132, EP 0 350 287, EP 93917054.4, and WO 91/19721. IV. Combination and Alternation Therapy It has been recognized that drug-resistant variants of viruses can emerge after prolonged treatment with an antiviral agent. Drug resistance most typically occurs by mutation of a gene that encodes for an enzyme used in viral replication. The efficacy of a drug against flavivirus or pestivirus infection can be prolonged, augmented, or restored by administering the compound in combination or alternation with a second, and perhaps third, antiviral compound that induces a different mutation from that caused by the principle drug. Alternatively, the pharmacokinetics, biodistribution or other parameter of the drug can be altered by such combination or alternation therapy. In general, combination therapy is typically preferred over alternation therapy because it induces multiple simultaneous stresses on the virus. Nonlimiting examples of antiviral agents that can be used in combination or alternation with the compounds disclosed herein include: (1) an interferon and/or ribavirin (Battaglia, A.M. et al, Ann. Pharmacother. 34:487- 494, 2000); Berenguer, M. et al. Antivir. Ther. 3(Suppl. 3):125-136, 1998); (2) Substrate-based NS3 protease inhibitors (Attwood et al, Antiviral peptide derivatives, PCT WO 98/22496, 1998; Attwood et al, Antiviral Chemistry and Chemotherapy 10.259-273, 1999; Attwood et al, Preparation and use of amino acid derivatives as anti-viral agents, German Patent Publication DE 19914474; Tung et al. Inhibitors of serine proteases, particularly hepatitis C virus NS3 protease, PCT WO 98/17679), including alphaketoamides and hydrazinoureas, and inhibitors that terminate in an electrophile such as a boronic acid or phosphonate. Llinas-Brunet et al, Hepatitis C inhibitor peptide analogues, PCT WO 99/07734. (3) Non-substrate-based inhibitors such as 2,4,6-trihydroxy-3-nitro-benzamide derivatives(Sudo K. et al, Biochemical and Biophysical Research Communications, 238:643- 647, 1997; Sudo K. et al. Antiviral Chemistry and Chemotherapy 9:186, 1998), including RD3-4082 and RD3-4078, the former substituted on the amide with a 14 carbon chain and the latter processing a αr -phenoxyphenyl group; (4) Thiazolidine derivatives which show relevant inhibition in a reverse-phase HPLC assay with an NS3/4A fusion protein and NS5A/5B substrate (Sudo K. et al, Antiviral Research 32:9-18, 1996), especially compound RD-1-6250, possessing a fused cinnamoyl moiety substituted with a long alkyl chain, RD4 6205 and RD4 6193; (5) Thiazolidines and benzanilides identified in Kakiuchi N. et al. J. EBS Letters 421:217-220; TakeshitaN. et al Analytical Biochemistry 24 -.242-246, 1997; (6) A phenan-threnequinone possessing activity against protease in a SDS-PAGE and autoradiography assay isolated from the fermentation culture broth of Streptomyces sp., Sch 68631 (Chu M. et al, Tetrahedron Letters 37:7229-7232, 1996), and Sch 351633, isolated from the fungus Penicillium griscofuluum, which demonstrates activity in a scintillation proximity assay (Chu M. et al, Bioorganic and Medicinal Chemistry Letters 9:1949-1952); (7) Selective NS3 inhibitors based on the macromolecule elgin c, isolated from leech (Qasim M.A. et al, Biochemistry 36:1598-1607, 1997); (8) Helicase inhibitors (Diana G.D. et al, Compounds, compositions and methods for treatment of hepatitis C, U.S. Patent No. 5,633,358; Diana G.D. et al, Piperidine derivatives, pharmaceutical compositions thereof and their use in the treatment of hepatitis C, PCT WO 97/36554); (9) Polymerase inhibitors such as nucleotide analogues, gliotoxin (Fenari R. et al. Journal of Virology 73:1649-1654, 1999), and the natural product cerulenin (Lohmann V. et al, Virology 249:108-118, 1998); (10) Antisense phosphorothioate oligodeoxynucleotides (S-ODN) complementary to sequence stretches in the 5' non-coding region (NCR) of the virus (Alt M. et al, Hepatology 22:707-717, 1995), or nucleotides 326-348 comprising the 3' end ofthe NCR and nucleotides 371-388 located in the core coding region of the IICV RNA (Alt M. et al, Archives of Virology 142:589-599, 1997; Galderisi U et al, Journal of Cellular Physiology 181:251-257, 1999); (11) Inhibitors of IRES-dependent translation (Ikeda N et al, Agent for the prevention and treatment of hepatitis C, Japanese Patent Publication JP-08268890; Kai Y. et al. Prevention and treatment of viral diseases, Japanese Patent Publication JP-10101591); (12) Nuclease-resistant ribozymes. (Maccjak D.J. et al, Hepatology 30 abstract 995, 1999); and (13) Other miscellaneous compounds including 1-amino-alkylcyclohexanes (U.S. Patent No. 6,034,134 to Gold et al), alkyl lipids (U.S. Patent No. 5,922,757 to Chojkier et al), vitamin E and other antioxidants (U.S. Patent No. 5,922,757 to Chojkier et al), squalene, amantadine, bile acids (U.S. Patent No. 5,846,964 to Ozeki et al), N- (phosphonoacetyl)-L-aspartic acid, (U.S. Patent No. 5,830,905 to Diana et al), benzenedicarboxamides (U.S. Patent No. 5,633,388 to Diana et al), polyadenylic acid derivatives (U.S. Patent No. 5,496,546 to Wang et al), 2',3'-dideoxyinosine (U.S. Patent No. 5,026,687 to Yarchoan et al), and benzimidazoles (U.S. Patent No. 5,891,874 to Colacino et al). V. Pharmaceutical Compositions Host, including humans, infected with flavivirus or pestivirus, or a gene fragment thereof can be treated by administering to the patient an effective amount of the active compound or a pharmaceutically acceptable prodrug or salt thereof in the presence of a pharmaceutically acceptable carrier or diluent. The active materials can be administered by any appropriate route, for example, orally, parenterally, intravenously, intradermally, subcutaneously, or topically, in liquid or solid form. A prefened dose of the compound for flavivirus or pestivirus infection will be in the range from about 1 to 50 mg/kg, preferably 1 to 20 mg/kg, of body weight per day, more generally 0.1 to about 100 mg per kilogram body weight of the recipient per day. The effective dosage range of the pharmaceutically acceptable salts and prodrugs can be calculated based on the weight ofthe parent nucleoside to be delivered. If the salt or prodrug exhibits activity in itself, the effective dosage can be estimated as above using the weight of the salt or prodrug, or by other means known to those skilled in the art. The compound is conveniently administered in unit any suitable dosage form, including but not limited to one containing 7 to 3000 mg, preferably 70 to 1400 mg of active ingredient per unit dosage form. A oral dosage of 50-1000 mg is usually convenient. Ideally the active ingredient should be administered to achieve peak plasma concentrations ofthe active compound of from about 0.2 to 70 μM, preferably about 1.0 to 10 μM. This may be achieved, for example, by the intravenous injection of a 0.1 to 5% solution of the active ingredient, optionally in saline, or administered as a bolus of the active ingredient. The concentration of active compound in the drug composition will depend on absorption, inactivation, and excretion rates of the drug as well as other factors known to those of skill in the art. It is to be noted that dosage values will also vary with the severity of the condition to be alleviated. It is to be further understood that for any particular subject, specific dosage regimens should be adjusted over time according to the individual need and the professional judgment ofthe person administering or supervising the administration ofthe compositions, and that the concentration ranges set forth herein are exemplary only and are not intended to limit the scope or practice of the claimed composition. The active ingredient may be administered at once, or may be divided into a number of smaller doses to be administered at varying intervals of time. A prefened mode of administration of the active compound is oral. Oral compositions will generally include an inert diluent or an edible carrier. They may be enclosed in gelatin capsules or compressed into tablets. For the purpose of oral therapeutic administration, the active compound can be incorporated with excipients and used in the form of tablets, troches or capsules. Pharmaceutically compatible binding agents, and/or adjuvant materials can be included as part ofthe composition. The tablets, pills, capsules, troches and the like can contain any of the following ingredients, or compounds of a similar nature: a binder such as microcrystalline cellulose, gum tragacanth or gelatin; an excipient such as starch or lactose, a disintegrating agent such as alginic acid, Primogel, or corn starch; a lubricant such as magnesium stearate or Sterotes; a glidant such as colloidal silicon dioxide; a sweetening agent such as sucrose or saccharin; or a flavoring agent such as peppermint, methyl salicylate, or orange flavoring. When the dosage unit form is a capsule, it can contain, in addition to material ofthe above type, a liquid carrier such as a fatty oil. hi addition, dosage unit forms can contain various other materials which modify the physical form ofthe dosage unit, for example, coatings of sugar, shellac, or other enteric agents. The compound can be administered as a component of an elixir, suspension, syrup, wafer, chewing gum or the like. A syrup may contain, in addition to the active compounds, sucrose as a sweetening agent and certain preservatives, dyes and colorings and flavors. The compound or a pharmaceutically acceptable prodrug or salts thereof can also be mixed with other active materials that do not impair the desired action, or with materials that supplement the desired action, such as antibiotics, antifungals, anti-inflammatories, or other antivirals, including other nucleoside compounds. Solutions or suspensions used for parenteral, intradermal, subcutaneous, or topical application can include the following components: a sterile diluent such as water for injection, saline solution, fixed oils, polyethylene glycols, glycerine, propylene glycol or other synthetic solvents; antibacterial agents such as benzyl alcohol or methyl parabens; antioxidants such as ascorbic acid or sodium bisulfite; chelating agents such as ethylenediaminetetraacetic acid; buffers such as acetates, citrates or phosphates and agents for the adjustment of tonicity such as sodium chloride or dextrose. The parental preparation can be enclosed in ampoules, disposable syringes or multiple dose vials made of glass or plastic. If administered intravenously, prefened carriers are physiological saline or phosphate buffered saline (PBS). In a prefened embodiment, the active compounds are prepared with carriers that will protect the compound against rapid elimination from the body, such as a controlled release formulation, including implants and microencapsulated delivery systems. Biodegradable, biocompatible polymers can be used, such as ethylene vinyl acetate, polyanhydrides, polyglycolic acid, collagen, polyorthoesters and polylactic acid. Methods for preparation of such formulations will be apparent to those skilled in the art. The materials can also be obtained commercially from Alza Corporation. Liposomal suspensions (including liposomes targeted to infected cells with monoclonal antibodies to viral antigens) are also prefened as pharmaceutically acceptable carriers. These may be prepared according to methods known to those skilled in the art, for example, as described in U.S. Patent No. 4,522,811 (which is incorporated herein by reference in its entirety). For example, liposome formulations may be prepared by dissolving appropriate lipid(s) (such as stearoyl phosphatidyl ethanolamine, stearoyl phosphatidyl choline, arachadoyl phosphatidyl choline, and cholesterol) in an inorganic solvent that is then evaporated, leaving behind a thin film of dried lipid on the surface of the container. An aqueous solution of the active compound or its monophosphate, diphosphate, and/or triphosphate derivatives is then introduced into the container. The container is then swirled by hand to free lipid material from the sides ofthe container and to disperse lipid aggregates, thereby forming the liposomal suspension. VI. Processes for the Preparation of Active Compounds The nucleosides of the present invention can be synthesized by any means known in the art. In particular, the synthesis of the present nucleosides can be achieved by either alkylating the appropriately modified sugar, followed by glycosylation or glycosylation followed by alkylation of the nucleoside. The following non-limiting embodiments illustrate some general methodology to obtain the nucleosides ofthe present invention. A. General Synthesis of 1 '-C-Branched Nucleosides l'-C-Branched ribonucleosides ofthe following structure: wherein BASE is a purine or pyrimidine base as defined herein; 7 0 9 R and R are independently hydrogen, OR , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a pi bond; 1 9 R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; R 6 is an alkyl, halogeno-alkyl (i.e. CF 3 ), alkenyl, or alkynyl (i.e. allyl); and X is O, S, SO 2 or CH 2 can be prepared by one ofthe following general methods. 1) Modification from the lactone The key starting material for this process is an appropriately substituted lactone. The lactone can be purchased or can be prepared by any known means including standard epimerization, substitution and cyclization techniques. The lactone can be optionally protected with a suitable protecting group, preferably with an acyl or silyl group, by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The protected lactone can then be coupled with a suitable coupling agent, such as an organometallic carbon nucleophile, such as a Grignard reagent, an organolithium, lithium dialkylcopper or R 6 -SiMe 3 in TBAF with the appropriate non-protic solvent at a suitable temperature, to give the l'-alkylated sugar. The optionally activated sugar can then be coupled to the BASE by methods well known to those skilled in the art, as taught by Townsend Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. For example, an acylated sugar can be coupled to a silylated base with a lewis acid, such as tin tetrachloride, titanium tetrachloride or trimethylsilyltriflate in the appropriate solvent at a suitable temperature. Subsequently, the nucleoside can be deprotected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. In a particular embodiment, the l'-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 1. Alternatively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2'-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 1 1) Coupling 2) Optional Deprotection Optional Deprotection 2. Alternative method for the preparation of 1 '-C-branched nucleosides The key starting material for this process is an appropriately substituted hexose. The hexose can be purchased or can be prepared by any known means including standard epimerization (e.g. via alkaline treatment), substitution and coupling techniques. The hexose can be selectively protected to give the appropriate hexa-furanose, as taught by Townsend Chemistry of Nucleosides and Nucleotides. Plenum Press, 1994. The 1 '-hydroxyl can be optionally activated to a suitable leaving group such as an acyl group or a halogen via acylation or halogenation, respectively. The optionally activated sugar can then be coupled to the BASE by methods well known to those skilled in the art, as taught by Townsend Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. For example, an acylated sugar can be coupled to a silylated base with a lewis acid, such as tin tetrachloride, titanium tetrachloride or trimethylsilyltriflate in the appropriate solvent at a suitable temperature. Alternatively, a halo-sugar can be coupled to a silylated base with the presence of trimethylsilyltriflate. The l'-CH 2 -OH, if protected, can be selectively deprotected by methods well known in the art. The resultant primary hydroxyl can be functionalized to yield various C-branched nucleosides. For example, the primary hydroxyl can be reduced to give the methyl, using a suitable reducing agent. Alternatively, the hydroxyl can be activated prior to reduction to facilitate the reaction; i.e. via the Barton reduction. In an alternate embodiment, the primary hydroxyl can be oxidized to the aldehyde, then coupled with a carbon nucleophile, such as a Grignard reagent, an organolithium, lithium dialkylcopper or R 6 -SiMe 3 in TBAF with the appropriate non-protic solvent at a suitable temperature. In a particular embodiment, the 1 '-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 2. Alternatively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2 '-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 2 hi addition, the L-enantiomers conesponding to the compounds of the invention can be prepared following the same general methods (1 or 2), beginning with the conesponding L-sugar or nucleoside L-enantiomer as starting material. B . General Synthesis of 2 ' -C-Branched Nucleosides 2'-C-Branched ribonucleosides ofthe following structure: wherein BASE is a purine or pyrimidine base as defined herein; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a pi bond; 1 9 R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; R 6 is an alkyl, halogeno-alkyl (i.e. CF 3 ), alkenyl, or alkynyl (i.e. allyl); and X is O, S, SO 2 or CH 2 can be prepared by one ofthe following general methods. 1. Glycosylation ofthe nucleobase with an appropriately modified sugar The key starting material for this process is an appropriately substituted sugar with a 2' -OH and 2'-H, with the appropriate leaving group (LG), for example an acyl group or a halogen. The sugar can be purchased or can be prepared by any known means including standard epimerization, substitution, oxidation and reduction techniques. The substituted sugar can then be oxidized with the appropriate oxidizing agent in a compatible solvent at a suitable temperature to yield the 2'-modified sugar. Possible oxidizing agents are Jones reagent (a mixture of chromic acid and sulfuric acid), Collins 's reagent (dipyridine Cr(VI) oxide, Corey's reagent (pyridinium chlorochromate), pyridinium dichromate, acid dichromate, potassium permanganate, MnO 2 , ruthenium tetroxide, phase transfer catalysts such as chromic acid or permanganate supported on a polymer, Cl 2 -pyridine, H 2 O 2 - ammonium molybdate, NaBrO 2 -CAN, NaOCl in HOAc, copper chromite, copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone) and N-bromosuccinimide. Then coupling of an organometallic carbon nucleophile, such as a Grignard reagent, an organolithium, lithium dialkylcopper or R 6 -SiMe 3 in TBAF with the ketone with the appropriate non-protic solvent at a suitable temperature, yields the 2'-alkylated sugar. The alkylated sugar can be optionally protected with a suitable protecting group, preferably with an acyl or silyl group, by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The optionally protected sugar can then be coupled to the BASE by methods well known to those skilled in the art, as taught by Townsend Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. For example, an acylated sugar can be coupled to a silylated base with a lewis acid, such as tin tetrachloride, titanium tetrachloride or trimethylsilyltriflate in the appropriate solvent at a suitable temperature. Alternatively, a halo-sugar can be coupled to a silylated base with the presence of trimethylsilyltriflate. Subsequently, the nucleoside can be deprotected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. hi a particular embodiment, the 2'-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 3. Altematively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis. John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2'-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 3 1) Coupling 2) Optional Deprotection 2. Modification of a pre-formed nucleoside The key starting material for this process is an appropriately substituted nucleoside with a 2'-OH and 2'-H. The nucleoside can be purchased or can be prepared by any known means including standard coupling techniques. The nucleoside can be optionally protected with suitable protecting groups, preferably with acyl or silyl groups, by methods well known to those skilled in the art, as taught by Greene et al Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The appropriately protected nucleoside can then be oxidized with the appropriate oxidizing agent in a compatible solvent at a suitable temperature to yield the 2'-modified sugar. Possible oxidizing agents are Jones reagent (a mixture of chromic acid and sulfuric acid), Collins 's reagent (dipyridine Cr(NI) oxide, Corey's reagent (pyridinium chlorochromate), pyridinium dichromate, acid dichromate, potassium permanganate, MnO 2 , ruthenium tetroxide, phase transfer catalysts such as chromic acid or permanganate supported on a polymer, Cl 2 -pyridine, H O 2 -ammonium molybdate, ΝaBrO -CAΝ, NaOCl in HOAc, copper chromite, copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone) and iV-bromosuccinimide. Subsequently, the nucleoside can be deprotected by methods well known to those skilled in the art, as taught by GreeneGreene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. In a particular embodiment, the 2 '-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 4. Alternatively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2'-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 4 Optional Deprotection Optional Deprotection In another embodiment of the invention, the L-enantiomers are desired. Therefore, the L-enantiomers can be conesponding to the compounds of the invention can be prepared following the same foregoing general methods, beginning with the conesponding L-sugar or nucleoside L-enantiomer as starting material. C. General Synthesis of 3 '-C-Branched Nucleosides 3'-C-Branched ribonucleosides ofthe following structure: wherein BASE is a purine or pyrimidine base as defined herein; "7 Q 9 R and R are independently hydrogen, OR , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R is H, alkyl (including lower alkyl), chlonne, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a pi bond; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 is independently H or phosphate; R 6 is an alkyl, halogeno-alkyl (i.e. CF 3 ), alkenyl, or alkynyl (i.e. allyl); and X is O, S, SO 2 or CH 2 can be prepared by one ofthe following general methods. 1 Glycosylation ofthe nucleobase with an appropriately modified sugar The key starting material for this process is an appropriately substituted sugar with a 3' -OH and 3'-H, with the appropriate leaving group (LG), for example an acyl group or a halogen. The sugar can be purchased or can be prepared by any known means including standard epimerization, substitution, oxidation and reduction techniques. The substituted sugar can then be oxidized with the appropriate oxidizing agent in a compatible solvent at a suitable temperature to yield the 3 '-modified sugar. Possible oxidizing agents are Jones reagent (a mixture of chromic acid and sulfuric acid), Collins's reagent (dipyridine Cr(VI) oxide, Corey's reagent (pyridinium chlorochromate), pyridinium dichromate, acid dichromate, potassium permanganate, MnO 2 , ruthenium tetroxide, phase transfer catalysts such as chromic acid or permanganate supported on a polymer, Cl 2 -pyridine, H 2 O 2 - ammonium molybdate, NaBrO 2 -CAN, NaOCl in HOAc, copper chromite, copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone) and N-bromosuccinimide. Then coupling of an organometallic carbon nucleophile, such as a Grignard reagent, an organolithium, lithium dialkylcopper or R -SiMe 3 in TBAF with the ketone with the appropriate non-protic solvent at a suitable temperature, yields the 3 '-C-branched sugar. The 3 '-C-branched sugar can be optionally protected with a suitable protecting group, preferably with an acyl or silyl group, by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The optionally protected sugar can then be coupled to the BASE by methods well known to those skilled in the art, as taught by Townsend Chemistry of Nucleosides and Nucleotides, Plenum Press, 1994. For example, an acylated sugar can be coupled to a silylated base with a lewis acid, such as tin tetrachloride, titanium tetrachloride or trimethylsilyltriflate in the appropriate solvent at a suitable temperature. Alternatively, a halo-sugar can be coupled to a silylated base with the presence of trimethylsilyltriflate. Subsequently, the nucleoside can be deprotected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. In a particular embodiment, the 3 '-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 5. Alternatively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2'-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 5 1) Coupling 2) Optional Deprotection 1) Optional Protection 2) Optional Reduction tion 2. Modification of a pre-formed nucleoside The key starting material for this process is an appropriately substituted nucleoside with a 3'-OH and 3'-H. The nucleoside can be purchased or can be prepared by any known means including standard coupling techniques. The nucleoside can be optionally protected with suitable protecting groups, preferably with acyl or silyl groups, by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. The appropriately protected nucleoside can then be oxidized with the appropriate oxidizing agent in a compatible solvent at a suitable temperature to yield the 2 '-modified sugar. Possible oxidizing agents are Jones reagent (a mixture of chromic acid and sulfuric acid), Collins 's reagent (dipyridine Cr(VI) oxide, Corey's reagent (pyridinium chlorochromate), pyridinium dichromate, acid dichromate, potassium permanganate, MnO 2 , ruthenium tetroxide, phase transfer catalysts such as chromic acid or permanganate supported on a polymer, Cl 2 -pyridine, H 2 O 2 -ammonium molybdate, NaBrO 2 -CAN, NaOCl in HOAc, copper chromite, copper oxide, Raney nickel, palladium acetate, Meerwin-Pondorf-Verley reagent (aluminum t-butoxide with another ketone) and 7V-bromosuccinimide. Subsequently, the nucleoside can be deprotected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991. In a particular embodiment, the 3 '-C-branched ribonucleoside is desired. The synthesis of a ribonucleoside is shown in Scheme 6. Alternatively, deoxyribo-nucleoside is desired. To obtain these nucleosides, the formed ribonucleoside can optionally be protected by methods well known to those skilled in the art, as taught by Greene et al. Protective Groups in Organic Synthesis, John Wiley and Sons, Second Edition, 1991, and then the 2'- OH can be reduced with a suitable reducing agent. Optionally, the 2 '-hydroxyl can be activated to facilitate reduction; i.e. via the Barton reduction. Scheme 6 Optional Deprotection Optional Deprotection hi another embodiment of the invention, the L-enantiomers are desired. Therefore, the L-enantiomers can be conesponding to the compounds of the invention can be prepared following the same foregoing general methods, beginning with the conesponding L-sugar or nucleoside L-enantiomer as starting material. EXAMPLES Example 1: Preparation of l'-C-methylriboadenine via 6-amino-9-(l-deoxy-β-D- psicofuranosvDpurine The title compound could also be prepared according to a published procedure (J. Farkas, and F. Sonn, "Nucleic acid components and their analogues. XCIV. Synthesis of 6- amino-9-(l-deoxy-β-D-psicofuranosyl)purine" Collect. Czech. Chem. Commun. 1967, 32, 2663-2667; J. Farkas", Collect. Czech. Chem. Commun. 1966, 31, 1535) (Scheme 7). Scheme 7 6-Benzamidopurine chloromercuri salt In a similar manner, but using the appropriate sugar and pyrimidine or purine bases, the following nucleosides of Formula I are prepared. (I) wherein: Alternatively, the following nucleosides of Formula IN are prepared, using the appropriate sugar and pyrimidine or purine bases. (TV) wherein: Alternatively, the following nucleosides of Formula VII are prepared, using the appropriate sugar and pyrimidine or purine bases. (Nil) wherein: Alternatively, the following nucleosides of Fonnula NIII are prepared, using the appropriate sugar and pyrimidine or purine bases. (NIII) wherein Alternatively, the following nucleosides of Formula IX are prepared, using the appropriate sugar and pyrimidine or purine bases. (IX) wherein: Alternatively, the following nucleosides of Formula XVI are prepared, using the appropriate sugar and pyrimidine or purine bases. (XVI) wherein: Example 2: Preparation of 2'-C-methylriboadenine The title compound was prepared according to a published procedure (R.E. Harry- O'kuru, J.M. Smith, and M.S. Wolfe, "A short, flexible route toward 2 '-C-branched ribonucleosides", J.Org. Chem. 1991, 62, 1754-1759) (Scheme 8). Scheme 8 (a) Dess-Martin periodinane; (b) MeMgBr / TiCl 4 ; (c) BzCl, DMAP, Et 3 N; (d) bis(trimethylsilyl)acetamide, N 6 -benzoyl adenine, TMSOTf; (e) NH 3 / MeOH In a similar manner, but using the appropriate sugar and pyrimidine or purine bases, the following nucleosides of Formula II are prepared. (II) wherein: ill Alternatively, the following nucleosides of Formula V are prepared, using the appropriate sugar and pyrimidine or purine bases. (V) wherein: Alternatively, the following nucleosides of Formula X are prepared, using the appropriate sugar and pyrimidine or purine bases. (X) wherein: Alternatively, the following nucleosides of Formula XI are prepared, using the appropriate sugar and pyrimidine or purine bases. (XI) wherein: Alternatively, the following nucleosides of Formula XII are prepared, using the appropriate sugar and pyrimidine or purine bases. (XII) wherein: Alternatively, the following nucleosides of Formula XVII are prepared, using the appropriate sugar and pyrimidine or purine bases. (XVII) wherein: Example 3: Preparation of 3'-C-methylriboadenine The title compound can be prepared according to a published procedure (R.F. Nutt, M. J. Dickinson, F.W. Holly, and E. Walton, "Branched-chain sugar nucleosides. in. 3 '-C- methyladenine ", J.Org. Chem. 1968, 33, 1789-1795) (Scheme 9). Scheme 9 (a) RuO 2 / NaIO 4 ; (b) MeMgl / TiCl 4 ; (c) HCl / MeOH / H 2 O; (d) BzCl / pyridine; (e) AcBr, HBr / AcOH; (f) chloromercuri-6-benzamidopurine; (g) NH 3 / MeOH. In a similar manner, but using the appropriate sugar and pyrimidine or purine bases, the following nucleosides of Formula III are prepared. (III) wherein: Alternatively, the following nucleosides of Formula VI are prepared, using the appropriate sugar and pyrimidine or purine bases. (Ni) wherein: Alternatively, the following nucleosides of Formula XIII are prepared, using the appropriate sugar and pyrimidine or purine bases. (XIII) wherein: Alternatively, the following nucleosides of Formula XIN are prepared, using the appropriate sugar and pyrimidine or purine bases. (XIN) wherein: Alternatively, the following nucleosides of Formula XV are prepared, using the appropriate sugar and pyrimidine or purine bases. R° (XN) wherein: Alternatively, the following nucleosides of Formula XVIII are prepared, using the appropriate sugar and pyrimidine or purine bases. (XVIII) wherein: VII. Anti-Flavivirus or Pestivirus Activity Compounds can exhibit anti-flavivirus or pestivirus activity by inhibiting flavivirus or pestivirus polymerase, by inhibiting other enzymes needed in the replication cycle, or by other pathways. EXAMPLES The test compounds were dissolved in DMSO at an initial concentration of 200 μM and then were serially diluted in culture medium. Unless otherwise stated, baby hamster kidney (BHK-21) (ATCC CCL- 10) and Bos Taurus (BT) (ATCC CRL 1390) cells were grown at 37°C in a humidified CO 2 (5%) atmosphere. BHK-21 cells were passaged in Eagle MEM additioned of 2 mM L-glutamine, 10% fetal bovine serum (FBS, Gibco) and Earle's BSS adjusted to contain 1.5 g/L sodium bicarbonate and 0.1 mM non-essential amino acids. BT cells were passaged in Dulbecco's modified Eagle's medium with 4 mM L-glutamine and 10% horse serum (HS, Gibco), adjusted to contain 1.5 g/L sodium bicarbonate, 4.5 g/L glucose and 1.0 mM sodium pyruvate. The vaccine strain 17D (YFV-17D) (Stamaril®, Pasteur Merieux) and Bovine Viral Dianhea virus (BVDN) (ATCC NR-534) were used to infect BHK and BT cells, respectively, in 75 cm 2 bottles. After a 3 day incubation period at 37°C, extensive cytopathic effect was observed. Cultures were freeze-thawed three times, cell debris were removed by centrifugation and the supernatant was aliquoted and stored at -70°C. YFN-17D and BVDV were titrated in BHK-21 and BT cells, respectively, that were grown to confluency in 24-well plates. Example 4; Phosphorylation Assay of Nucleoside to Active Triphosphate To determine the cellular metabolism of the compounds, HepG2 cells were obtained from the American Type Culture Collection (Rockville, MD), and were grown in 225 cm 2 tissue culture flasks in minimal essential medium supplemented with non-essential amino acids, 1% penicillin-streptomycin. The medium was renewed every three days, and the cells were subcultured once a week. After detachment ofthe adherent monolayer with a 10 minute exposure to 30 mL of trypsin-EDTA and three consecutive washes with medium, confluent HepG2 cells were seeded at a density of 2.5 x 10 6 cells per well in a 6-well plate and exposed to 10 μM of [ 3 H] labeled active compound (500 dpm/pmol) for the specified time periods. The cells were maintained at 37°C under a 5% CO 2 atmosphere. At the selected time points, the cells were washed three times with ice-cold phosphate-buffered saline (PBS). Intracellular active compound and its respective metabolites were extracted by incubating the cell pellet overnight at -20°C with 60% methanol followed by extraction with an additional 20 μL of cold methanol for one hour in an ice bath. The extracts were then combined, dried under gentle filtered air flow and stored at -20°C until HPLC analysis. The preliminary results ofthe HPLC analysis are tabulated in Table 1. Table 1 Example 5: Bioavailability Assay in Cynomolgus Monkeys Within 1 week prior to the study initiation, the cynomolgus monkey was surgically implanted with a chronic venous catheter and subcutaneous venous access port (VAP) to facilitate blood collection and underwent a physical examination including hematology and serum chemistry evaluations and the body weight was recorded. Each monkey (six total), received approximately 250 uCi of 3 H activity with each dose of active compound, namely β- D-2'-CH -riboG at a dose level of 10 mg/kg at a dose concentration of 5 mg/mL, either via an intravenous bolus (3 monkeys, IN), or via oral gavage (3 monkeys, PO). Each dosing syringe was weighed before dosing to gravimetrically detennine the quantity of formulation administered. Urine samples were collected via pan catch at the designated intervals (approximately 18-0 hours pre-dose, 0-4, 4-8 and 8-12 hours post-dosage) and processed. Blood samples were collected as well (pre-dose, 0.25, 0.5, 1, 2, 3, 6, 8, 12 and 24 hours post- dosage) via the chronic venous catheter and NAP or from a peripheral vessel if the chronic venous catheter procedure should not be possible. The blood and urine samples were analyzed for the maximum concentration (C max ), time when the maximum concentration was achieved (T max ), area under the curve (AUC), half life of the dosage concentration (T> /2 ), clearance (CL), steady state volume and distribution (N ss ) and bioavailability (F), which are tabulated in Tables 2 and 3, and graphically illustrated in Figures 2 and 3, respectively. Table 2: Oral Bioavailability in Monkeys Table 3: Experimental Pharmacokinetics of β-D-2'-CH 3 -riboG in Cynomolgus Monkeys Example 6: Bone Marrow Toxicity Assay Human bone marrow cells were collected from normal healthy volunteers and the mononuclear population was separated by Ficoll-Hypaque gradient centrifugation as described previously by Sommadossi J-P, Carlisle R. "Toxicity of 3'-azido-3'- deoxythymidine and 9-(l,3-dihydroxy-2-propoxymethyl)guanine for nonnal human hematopoietic progenitor cells in vitro" Antimicrobial Agents and Chemotherapy 1987; 31:452-454; and Sommadossi J-P, Schinazi RF, Chu CK, Xie M-Y. "Comparison of cytotoxicity of the (-)- and (+)-enantiomer of 2',3'-dideoxy-3'-thiacytidine in normal human bone manow progenitor cells" Biochemical Pharmacology 1992; 44:1921-1925. The culture assays for CFU-GM and BFU-E were performed using a bilayer soft agar or methylcellulose method. Drugs were diluted in tissue culture medium and filtered. After 14 to 18 days at 37°C in a humidified atmosphere of 5% CO 2 in air, colonies of greater than 50 cells were counted using an inverted microscope. The results in Table 4 are presented as the percent inhibition of colony formation in the presence of drug compared to solvent control cultures. Table 4: Human Bone Marrow Toxicity CFU-GM and BFU-E Clonogenic Assays IC 50 in μM Treatment CFU-GM BFU-E ribavirin ~ 5 ~ 1 β-D-2'-CH 3 -riboA > 100 > 100 β-D-2'-CH 3 -riboU > 100 > 100 β-D-2'-CH 3 -riboC > 10 > 10 β-D-2'-CH 3 -riboG > 10 > 100 Example 7; Mitochondria Toxicity Assay HepG2 cells were cultured in 12-well plates as described above and exposed to various concentrations of drugs as taught by Pan-Zhou X-R, Cui L, Zhou X-J, Sommadossi J- P, Darley-Usmer NM. "Differential effects of antiretroviral nucleoside analogs on mitochondrial function in HepG2 cells" Antimicrob Agents Chemother 2000; 44:496-503. Lactic acid levels in the culture medium after 4 day drug exposure was measured using a Boehringer lactic acid assay kit. Lactic acid levels were nonnalized by cell number as measured by hemocytometer count. The prehrmnary results from this assay are tabulated in Table 5. Table 5: Mitochondrial Toxicity Study (L-lactic acid assay) Example 8; Cytotoxicity Assay Cells were seeded at a rate of between 5 x 10 3 and 5 x 10 4 /well into 96-well plates in growth medium overnight at 37°C in a humidified CO 2 (5%) atmosphere. New growth medium containing serial dilutions ofthe drugs was then added. After incubation for 4 days, cultures were fixed in 50% TCA and stained with sulforhodamineB. The optical density was read at 550 nm. The cytotoxic concentration was expressed as the concentration required to reduce the cell number by 50% (CC 50 ). The data is tabulated in Table 6. Table 6: MDBK versus Human Hepatoma Example 9; Cell Protection Assay (CPA) The assay was performed essentially as described by Baginski, S. G.; Pevear, D. C; Seipel, M.; Sun, S. C. C; Benetatos, C. A.; Chunduru, S. K.; Rice, C. M. and M. S. Collett "Mechanism of action of a pestivirus antiviral compound" PNAS USA 2000, 97(14), 7981- 7986. MDBK cells (ATCC) were seeded onto 96-well culture plates (4,000 cells per well) 24 hours before use. After infection with BVDV (strain NADL, ATCC) at a multiplicity of infection (MOI) of 0.02 plaque forming units (PFU) per cell, serial dilutions of test compounds were added to both infected and uninfected cells in a final concentration of 0.5% DMSO in growth medium. Each dilution was tested in quadruplicate. Cell densities and virus inocula were adjusted to ensure continuous cell growth throughout the experiment and to achieve more than 90% virus-induced cell destruction in the untreated controls after four days post-infection. After four days, plates were fixed with 50% TCA and stained with sulforhodamine B. The optical density ofthe wells was read in a microplate reader at 550 nm. The 50% effective concentration (EC 50 ) values were defined as the compound concentration that achieved 50% reduction of cytopathic effect of the virus. The results are tabulated in Table 7. Figures 4 and 5 provide a graphical illustration of the methodology used to arrive at the 50% effective concentration (EC 50 ) values for β-D-2'-CH 3 -riboG and ribavirin. Figure 6 compares the results of the CPA for β-D-2'-CH 3 -riboG, β-D-2'-CH 3 -riboC, β-D-2'-CH 3 - riboU, β-D-2'-CH -riboA and ribavirin Table 7: Cell Protection Assay Example 10: Plaque Reduction Assay For each compound the effective concentration was determined in duplicate 24-well plates by plaque reduction assays. Cell monolayers were infected with 100 PFU/well of virus. Then, serial dilutions of test compounds in MEM supplemented with 2% inactivated serum and 0.75% of methyl cellulose were added to the monolayers. Cultures were further incubated at 37°C for 3 days, then fixed with 50% ethanol and 0.8% Crystal Violet, washed and air-dried. Then plaques were counted to determine the concentration to obtain 90% virus suppression and tabulated in Table 8. Figure 7 is a graphical illustration ofthe results from the Plaque Reduction Assay. Figure 8 is an image of BVDV plaque formation in the presence of increasing concentrations of β-D-2'-CH 3 -riboU. Table 8: Viral Suppression via Plaque Reduction Assay Example 11: Yield Reduction Assay For each compound the concentration to obtain a 6-log reduction in viral load was determined in duplicate 24-well plates by yield reduction assays. The assay was performed as described by Baginski, S. G.; Pevear, D. C; Seipel, M.; Sun, S. C. C; Benetatos, C. A.; Chunduru, S. K.; Rice, C. M. and M. S. Collett "Mechanism of action of a pestivirus antiviral compound" PNAS USA 2000, 97(14), 7981-7986, with minor modifications. Briefly, MDBK cells were seeded onto 24-well plates (2 x 105 cells per well) 24 hours before infection with BVDV (NADL strain) at a multiplicity of infection (MOI) of 0.1 PFU per cell. Serial dilutions of test compounds were added to cells in a final concentration of 0.5% DMSO in growth medium. Each dilution was tested in triplicate. After three days, cell cultures (cell monolayers and supernatants) were lysed by three freeze-thaw cycles, and virus yield was quantified by plaque assay. Briefly, MDBK cells were seeded onto 6-well plates (5 x 105 cells per well) 24 h before use. Cells were inoculated with 0.2 mL of test lysates for 1 hour, washed and overlaid with 0.5% agarose in growth medium. After 3 days, cell monolayers were fixed with 3.5% formaldehyde and stained with 1% crystal violet (w/v in 50% ethanol) to visualize plaques. The plaques were counted to determine the concentration to obtain a 6- log reduction in viral load as tabulated in Table 9. Figure 9 is a graphical illustration ofthe results from the Yield Reduction Assay. Figure 8 is an image of BVDV yield reduction in the presence of increasing concentrations of β-D-2'-CH 3 -riboC. Table 9: Concentration to Obtain 6-log Reduction Cone, for 6-log Reduction (μM) β-D-2'-CH 3 -riboU 120 β-D-2'-CH 3 -riboG 20 β-D-2'-CH 3 -riboC 20 β-D-2'-CH 3 -riboA 9 Example 12: Comparative Cytotoxicity Table 10 summarizes the cytoxicity of two compounds of this invention, β-D-l'-CH 3 - riboA and β-D-2'-CH -riboA, in comparison to RBV ("ribavirin"), in various cell systems. Table 10: Comparative Cytotoxicity* (CC 5 0) * Compound concentration (μM) required to reduce the viability of cells by 50%. The chemical structures for β-D-F-CH 3 -riboA and β-D-2'-CH 3 -riboA are as follows: β-D-l'-CH 3 -riboA β-D-2'-CH 3 -riboA Table 11 summarizes the antiviral activity of β-D-l'-CH 3 -riboA and β-D-2'-CH 3 - riboA against several viruses within the flavivirus and pestivirus genuses. Table 11: Comparative Antiviral Activity* (EC50) * Compound concentration (μM) required to reduce the plaque number by 50%. The following virus-cell system were used: BNDC-BT, YFN-BHK, PICO (Cosxackie Bl and Polio SabinVVSV - Vero. Table 12 summarizes the antiviral activity and toxicity of β-D-2'-methyl-riboG, β-D- 2'-methyl-riboC and β-D-2'-methyl-riboU, against a couple of viruses within the flavivirus and pestivirus genuses. Table 12: Comparative Antiviral Activity* (EC50) BVDV YFV EC 50 * CC 50 ** EC50* CC 50 ** β-D-2'-CH 3 -riboG 2 >100 1.2 20 β-D-2'-CH 3 -riboC 3.7 >100 70 >100 β-D-2'-CH 3 -riboU 20 >100 33 >100 * Compound concentration (μM) required to reduce the plaque number by 50%. The following virus-cell system were used: BVDC-BT and YFV-BHK. * Compound concentration (μM) required to reduce the viability of cells by 50%. The chemical structures for β-D-2'-CH 3 -riboG, β-D-2'-CH 3 -riboC and β-D-2'-CH 3 - riboU are as follows: β-D-2'-CH 3 -riboG β-D-2'-CH 3 -riboC β-D-2'-CH 3 -riboU Table 13 summarizes the anti-viral activity of several compounds of this invention against BVDV in three different assays. Table 13: for BVDV This invention has been described with reference to its prefened embodiments. Variations and modifications ofthe invention, will be obvious to those skilled in the art from the foregoing detailed description ofthe invention.

Claims: We Claim: 1. A compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 2. A compound of Formula II: or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 3. A compound of Formula III: or a pharmaceutically acceptable salt thereof, wherein: 1 " R , R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 4. A compound of Formula IV: (IN) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable 1 T of providing a compound wherein R , R and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 5. A compound of Formula N: (V) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 6. A compound of Formula NI: or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharaiaceutically acceptable leaving group which when administered in vivo is 1 9 • capable of providing a compound wherein R , R and R are mdependently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 7. A compound selected from Formulas Nil, NIII and IX: (VII) (NIII) (IX) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3> chloro, bromo, fluoro, iodo, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . 8. A compound of Formulas X, XI and XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, S0 2 or CH 2 . 9. A compound selected from Formulas XIII, XIV and XV: (XIII) (XIN) (XN) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is • 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 10. A compound of Formula XVI: (XVI) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are mdependently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 11. A compound of Formula XVII: (xvπ) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 12. A compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; R is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 13. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 14. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 15. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 16. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 17. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 18. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 19. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 20. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 21. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 22. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 23. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 24. A compound ofthe structure: or a pharmaceutically acceptable salt thereof. 25. The compound as described in any of the preceding claims 1-24, wherein the said compound is in the form of a dosage unit. 26. The compound as described in claim 187, wherein the dosage unit contains 10 to 1500 mg of said compound. 27. The compound as described in claim 187 or 188, wherein said dosage unit is a tablet or capsule. 28. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are mdependently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 29. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula II: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 30. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula III: (III) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; 1 9 X and X are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 31. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula IN: (IN) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 32. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula V: (N) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 33. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula NI: (NI) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 34. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formulas VII, VIII or IX: (VH) (VIII) (IX) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; 1 9 ^ R , R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3; chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 35. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula X, XI or XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 36. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XIII, XIV or XV: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; 1 9 ^ R , R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 "3 capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 37. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVI: (XVI) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 or -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 38. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(0)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 39. A phannaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent, wherein: Base is a purine or pyrimidine base as defined herein; 1 9 R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when admimstered in vivo is ι 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl) amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 40. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 41. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 42. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 43. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 44. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 45. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 46. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 47. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of stmcture: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 48. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 49. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 50. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 51. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier or diluent. 52. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 53. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula II: (II) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 4. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula III: (ni) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; 1 9 X and X are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 5. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula IN: (IN) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 6. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula N: (V) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 57. A phannaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula VI: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 58. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula VII, VIII or IX: (VTJ) (VIII) (IX) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -0(lower alkyl), -O(alkenyl), CF 3) chloro, bromo, fluoro, iodo, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . 59. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula X, XI or XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . 60. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Fonnula XIII, XIN or XV: (XIII) (XIV) (XV) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 61. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVI: (XVI) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; *7 0 7 IΠ R Q R I D alternatively, R and R , R and R , R and R , or R and R can come together to form a bond; and X is O, S, SO 2 , or CH 2 . 62. A phannaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; 1 9 R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 63. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl) amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 64. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 65. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 66. A phannaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 67. A pharmaceutical composition for the freahnent or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 68. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 69. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 70. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 71. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 72. A phannaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 73. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 74. A pharmaceutical composition for the treatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 75. A pharmaceutical composition for the freatment or prophylaxis of a flavivirus or pestivirus in a host, comprising an effective amount of a compound of structure: or a pharmaceutically acceptable salt thereof, in combination with one or more other antivirally effective agents. 76. The pharmaceutical composition as described in any of the preceding claims 28-75, wherein the said compound is in the form of a dosage unit. 77. The pharmaceutical composition as described in claim 76, wherein the dosage unit contains 10 to 1500 mg of said compound. 78. The pharmaceutical composition as described in claim 75 or 76, wherein said dosage unit is a tablet or capsule. 79. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable 1 9 ^ ■ of providing a compound wherein R , R and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X and X are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 80. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula II: or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 81. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula III: (III) or a pharmaceutically acceptable salt thereof, wherein: R , R and R are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 82. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula IN: (IN) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 83. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula N: (V) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is • 1 9 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR R 5 or SR ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , ΝR 4 ΝR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 84. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula VI: (Ni) or a pharmaceutically acceptable salt thereof, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 85. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula NIL NIII or IX: (Nil) (NIII) (IX) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3j chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . 86. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula X, XI or XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 87. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Fonnula XIII, XIN or XV: (XIII) (XIN) (XN) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R 2 and R 3 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, S0 2 or CH 2 . 88. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XNI: (xvi) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 89. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; i o i in alternatively, R and R , or R and R can come together to form a bond; and X is O, S, S0 2 or CH 2 . 90. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; 1 9 R and R are independently hydrogen, OR , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 91. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising admimstering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 92. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 93. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 94. A method for the freahnent or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 95. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 96. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 97. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 98. A method for the freahnent or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 99. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 100. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 101. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. 102. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof. . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; 1 9 X and X are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula II: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is • 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula III: (III) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula IN: (TV) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable 1 9 " of providing a compound wherein R , R and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula V: (V) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 108. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula VI: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 ^ ■ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 109. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula VII, VIII or IX: (VII) (VIII) (IX) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharaiaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3; chloro, bromo, fluoro, iodo, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising admimstering an anti-virally effective amount of a compound of Formula X, XI or XII: (X) (XI) (xπ) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 "\ capable of providing a compound wherein R , R and R are independently H or phosphate; R is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 111. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XIII, XIV or XV: (XIII) (XIV) (XV) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; 1 9 ^ • R , R and R are mdependently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 112. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XVI: (XVI) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, S0 2 or CH 2 . . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . . A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an anti-virally effective amount of a compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more other antivirally effective agents, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, S0 2 or CH 2 . 115. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 116. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 117. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 118. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 119. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 120. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 121. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 122. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 123. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 124. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 125. A method for the treatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 126. A method for the freatment or prophylaxis of a flavivirus or pestivirus infection in a host, comprising administering an antivirally effective amount of a compound of the structure: or a pharmaceutically acceptable salt thereof, in combination or alternation with one or more antivirally effective agents. 127. Method of treatment as described in any ofthe preceding claims 79-126, wherein the said compound is in the form of a dosage unit. 128. Method of treatment as described in claim 127, wherein the dosage unit contains 10 to 1500 mg of said compound. 129. Method of treatment as described in claim 127 or 128, wherein said dosage unit is a tablet or capsule. 130. A use of a compound of Fonnula I: (I) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound of Formula II: (II) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR , NR 4 R 5 or SR ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound of Formula III: (III) or a pharmaceutically acceptable salt thereof, for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 133. A use of a compound of Formula IN: (IN) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower allcyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R , R and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 134. A use of a compound of Formula V: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 135. A use of a compound of Formula VI: (NI) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound selected from Fonnulas VII, VIII and IX: (VII) (VIII) (DC) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3> chloro, bromo, fluoro, iodo, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . . A use of a compound of Formulas X, XI and XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R is hydrogen, OR , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 138. A use of a compound selected from Formulas XIII, XIV and XV: (XIII) (XIN) (XN) or a pharaiaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, S0 2 or CH 2 . 139. A use of a compound of Formula XNI: (XNI) or a pharmaceutically acceptable salt thereof, for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . . A use of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 141. A use of a compound of Formula XNIII: (XNIII) or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R and R are independently H or phosphate; R δ is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(0)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl)amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 142. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 143. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 144. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 145. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 146. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 147. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 148. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 149. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus. 150. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus. 151. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 152. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 153. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 154. A use of a compound of Formula I: (I) or a pharmaceutically acceptable salt thereof, for the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are mdependently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when admimstered in vivo is capable 1 9 ' • of providing a compound wherein R , R and R are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X and X are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound of Formula II: (II) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are is independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound of Formula III: (III) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 9 " capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 and X 2 are independently selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 157. A use of a compound of Formula IN: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H, phosphate (including mono-, di- or triphosphate and a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , ΝR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 158. A use of a compound of Formula V: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 * \ capable of providing a compound wherein R , R and R are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). 159. A use of a compound of Formula VI: (VI) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 , R 2 and R 3 are independently H or phosphate; and Y is hydrogen, bromo, chloro, fluoro, iodo, OR 4 , NR 4 R 5 or SR 4 ; X 1 is selected from the group consisting of H, straight chained, branched or cyclic alkyl, CO-alkyl, CO-aryl, CO-alkoxyalkyl, chloro, bromo, fluoro, iodo, OR 4 , NR 4 NR 5 or SR 4 ; and R 4 and R 5 are independently hydrogen, acyl (including lower acyl), or alkyl (including but not limited to methyl, ethyl, propyl and cyclopropyl). . A use of a compound selected from Formulas VII, VIII and IX: (VII) (VIII) (IX) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituex ts as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherei •n R 1 , R 9 and R ^ are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, 2-Br-ethyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), CF 3; chloro, bromo, fluoro, iodo, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 , or CH 2 . . A use of a compound of Formulas X, XI and XII: (X) (XI) (XII) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other phannaceutically acceptable leaving group which when administered in vivo is 1 9 ' capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 is hydrogen, OR 3 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , - NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 162. A use of a compound selected from Formulas XIII, XIV and XV (XIII) (XIN) (XN) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 , R 2 and R 3 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is 1 ^ capable of providing a compound wherein R , R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, ΝO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; and X is O, S, SO 2 or CH 2 . 163. A use of a compound of Formula XNI: (XNI) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the freahnent or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(0)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH 2 , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 8 and R 10 are independently H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , R 7 and R 10 , R 8 and R 9 , or R 8 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 164. A use of a compound of Formula XVII: (XVII) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharmaceutically acceptable leaving group which when administered in vivo is capable of providing a compound wherein R and R are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), -O(lower alkyl), -O(alkenyl), chlorine, bromine, iodine, NO 2 , NH , -NH(lower alkyl), -NH(acyl), -N(lower alkyl) , -N(acyl) 2 ; R 10 is H, alkyl (including lower alkyl), chlorine, bromine, or iodine; alternatively, R 7 and R 9 , or R 7 and R 10 can come together to form a bond; and X is O, S, SO 2 or CH 2 . . A use of a compound of Formula XVIII: (XVIII) or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus, wherein: Base is a purine or pyrimidine base as defined herein; R 1 and R 2 are independently H; phosphate (including monophosphate, diphosphate, triphosphate, or a stabilized phosphate prodrug); acyl (including lower acyl); alkyl (including lower alkyl); sulfonate ester including alkyl or arylalkyl sulfonyl including methanesulfonyl and benzyl, wherein the phenyl group is optionally substituted with one or more substituents as described in the definition of aryl given herein; a lipid, including a phospholipid; an amino acid; a carbohydrate; a peptide; a cholesterol; or other pharaiaceutically acceptable leaving group which when admimstered in vivo is capable of providing a compound wherein R 1 and R 2 are independently H or phosphate; R 6 is hydrogen, hydroxy, alkyl (including lower alkyl), azido, cyano, alkenyl, alkynyl, Br-vinyl, -C(O)O(alkyl), -C(O)O(lower alkyl), -O(acyl), -O(lower acyl), -O(alkyl), - O(lower alkyl), -O(alkenyl), chloro, bromo, fluoro, iodo, NO 2 , NH 2 , -NH(lower alkyl), - NH(acyl), -N(lower alkyl) 2 , -N(acyl) 2 ; R 7 and R 9 are independently hydrogen, OR 2 , alkyl (including lower alkyl), alkenyl, alkynyl, Br-vinyl, O-alkenyl, chlorine, bromine, iodine, NO 2 , amino, loweralkylamino, or di(loweralkyl) amino; R 8 is H, alkyl (including lower alkyl), chlorine, bromine or iodine; alternatively, R 7 and R 9 , or R 8 and R 9 can come together to form a bond; and X is O, S, SO 2 or CH 2 . 166. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 167. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 168. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 169. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 170. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus. 171. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 172. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 173. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 174. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 175. A use of a compound of the structure: or a phannaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 176. A use of a compound of the structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment or prophylaxis of a host infected with the flavivirus or pestivirus. 177. A use of a compound ofthe structure: or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the freatment or prophylaxis of a host infected with the flavivirus or pestivirus. 178. Use of the compound as described in any of the preceding claims 130-177, wherein the said compound is in the form of a dosage unit. 179. Use of the compound of claim 101, wherein the dosage unit contains 178 to 1500 mg of said compound. 180. Use of the compound of claim 178 or 179, wherein said dosage unit is a tablet or capsule.

Inventors: LACOLLA PAOLO, SOMMADOSSI JEAN-PIERRE, LACOLLA, PAOLO, SOMMADOSSI, JEAN-PIERRE
Applicants: LACOLLA PAOLO, NOVIRIO PHARMACEUTICALS LTD, SOMMADOSSI JEAN PIERRE, UNIV CAGLIARI, LACOLLA, PAOLO, NOVIRIO PHARMACEUTICALS LIMITED, SOMMADOSSI, JEAN-PIERRE, UNIVERSITA DEGLI STUDI DI CAGLIARI
Application year: 2001
Main classification: C07H19
IPC codes: A61K31, A61P1, A61P25, A61P31, A61P43, C07H19