Fluorinated bis(hydroxymethyl) cyclobutyl purines and pyrimidines

Abstract: Antiviral activity is exhibited by compounds having the formula and its pharmaceutically acceptable salts wherein R₁ is wherein R₂ is hydrogen, methyl, fluoro, chloro, bromo, iodo, hydroxy or amino; R₃ is fluoro, chloro, bromo, iodo, hydrogen, methyl, trifluoromethyl, ethyl, n-propyl, 2-fluoroethyl, 2-chloroethyl, or wherein R₄ is chloro, bromo, iodo, hydrogen, methyl or trifluoromethyl; R₅ is alkyl; R₆ is hydrogen, alkyl, substituted alkyl, or aryl; and R₇ and R₈ are independently hydrogen,

Description: In EP-A-0 335 355 bis(hydroxymethyl)cyclobutyl purines and pyrimidines are disclosed which possess antiviral activity. The cyclobutyl ring of these compounds is not fluorinated. CA 75 (1971) Abstract No. 2055h discloses the compound CHF:C(OEt)OB z . Antiviral activity is exhibited by compounds having the formula 1 and its pharmaceutically acceptable salts wherein R₁ is R₃ is hydrogen, methyl, fluoro, chloro, bromo, iodo, or and R₄ is chloro, bromo, or iodo. Preferred compounds of formula 1 are when R₁ is The most preferred compound is (1α,2β,3α,4β)-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. This compound exists as the following two enantiomers: [1S-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one, which is preferred, and [1R-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. The compounds of formula 1 , and the pharmaceutically acceptable salts thereof, are antiviral agents that can be used to treat viral infection in mammalian species such as domesticated animals (e.g., dogs, cats, horses and the like) and humans, and avian species (e.g., chickens and turkeys). The compounds of formula 1 wherein R₁ is are effective against one or more of the following viruses: herpes simplex virus 1 and 2, varicellazoster virus, cytomegalovirus and murine leukemia virus. They are also believed to be active against a variety of other DNA and retroviruses. Exemplary DNA viruses in addition to those named above include other herpes viruses (e.g. , Epstein-Barr virus, pseudorabies virus, human herpes virus 6, and the like), poxviruses (e.g. , vaccinia virus, monkey pox and myoma), papovaviruses (e.g. , the papilloma viruses), hepatitis B virus, and adenoviruses. Exemplary retroviruses in addition to that named above include human T-cell lymphotrophic viruses -I and -II, feline leukemia virus, and equine infectious anemia virus. The compounds of formula 1 , wherein R₁ is and the enantiomer [1R-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one are belived to be active against the various retroviruses and DNA viruses named above with the exception of herpes simplex 1 and 2, varicella-zoster virus, and cytomeglovirus. All of the other compounds of formula 1 are believed to be active against one or more of the following viruses: herpes simplex virus 1 and 2, varicella-zoster virus, cytomegalovirus, human immuno deficiency virus (HIV), and the other retroviruses and DNA viruses described above. The compounds of this invention may be administered parenterally (for example, by intravenous, intraperitoneal or intramuscular injection), orally or topically. The compounds may be administered orally or parenterally in an amount effective to treat the infection. The dosage will, of course, depend on the severity of the infection, but will likely be in the range of about 1.0 to 50 mg/kg of body weight. The desired dose may be administered several times daily at appropriate intervals. For infections of the eye, or other external tissues (e.g. , mouth and skin) the compositions may be applied to the infected part of the body of the patient topically as an ointment, cream, aerosol, gel, powder, lotion, suspension or solution (e.g. , as in eye drops). The concentration of the compound in the vehicle will, of course, depend on the severity of the infection, but will likely be in the range of about 0.1 to 7% by weight. A compound of formula 1 wherein R₁ is can be prepared from an intermediate of formula wherein P is a protecting group such as benzyl or silyl, and X is a leaving group such as a straight or branched chain lower alkyl of 1 to 5 carbons, phenyl, substituted lower alkyl or substituted phenyl sulfonate well known in the art ( e.g ., benzenesulfonyloxy, p-toluenesulfonyloxy, methanesulfonyloxy, p-nitrophenylsulfonyloxy, or trifluoromethylsulfonyloxy). The term "silyl" refers to silyl protecting groups well known in the art [ e.g ., t-butyldimethylsilyl, t-butyldiphenylsilyl, (triphenylmethyl)dimethylsilyl, methyldiisopropylsilyl, or triisopropylsilyl]. Reaction of a compound of formula 2 with a protected form of guanine such as a compound of formula in the presence of a base such as potassium carbonate, sodium hydride, or potassium hydride in an aprotic polar solvent such as dimethylformamide, dimethylsulfoxide, or sulfolane (tetramethylene sulfone) yields the corresponding compound of formula Optionally, the reaction can be run in the presence of a metal chelating catalyst such as 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) or 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane). Alternatively, treatment of a compound of formula 2 with a tetraalkylammonium salt of a compound of formula 3 (in an aprotic polar solvent such as dimethylformamide, dimethylsulfoxide or sulfolane) yields a compound of formula 4 . A tetraalkylammonium salt of a compound of formula 3 can be prepared by treatment of a compound of formula 3 (in an aprotic polar solvent such as dimethylformamide) with a tetraalkylammonium hydroxide such as aqueous tetra-n-butylammonium hydroxide and subsequent azeotropic removal of water. Alternatively, a tetraalkylammonium salt of a compound of formula 3 can be prepared by treatment of a compound of formula 3 (in a chlorinated hydrocarbon solvent such as methylene chloride) with a tetraalkylammonium hydroxide (such as aqueous tetra-n-butylammonium hydroxide). Removal of the water layer, drying of the organic layer (with a drying agent such as sodium or magnesium sulfate), filtration, and subsequent removal of the volatiles under vacuum provides a tetraalkylammonium salt of a compound of formula 3 . Removal of the protecting groups from a compound of formula 4 yields a compound of formula 1 wherein R₁ is When the group P in compound 4 is a silyl protecting group, removal of the P group can be accomplished using fluoride ion (e.g. , tetrabutylammonium fluoride in tetrahydrofuran). The purine O-benzyl protecting group can then be removed with aqueous alcoholic mineral acid or by hydrogenolysis (e.g. , palladium hydroxide on carbon in cyclohexene and ethanol). When the protecting group P in 4 is benzyl, removal of all the benzyl protecting groups can be effected using sodium in liquid ammonia, hydrogenolysis (e.g. , palladium hydroxide on carbon in cyclohexene and ethanol), or boron trichloride in dichloromethane. Reaction of a compound of formula 2 with compound under conditions analogous to those used in the preparation of compound 4 provides a compound of formula Removal of the protecting group P provides a compound of formula 1 wherein R₁ is For example, when the protecting group P in 6 is silyl, the protecting group can be removed by treatment with fluoride ion (e.g. , tetrabutylammonium fluoride). When the protecting group P in 6 is benzyl, removal of the P group can be performed by treatment with boron trichloride. Acid hydrolysis (e.g. , using hot aqueous hydrochloric acid) of the chloro group of a compound of formula 1 wherein R₁ is provides a compound of formula 1 wherein R₁ is A compound of formula 2 can be prepared as follows: The hydroxyl groups of the known compound of formula 7 [see W.A. Slusarchyk, et al. , Tetrahedron Lett ., 6453 (1989) and European Patent 335,355-A] can be protected with a protecting group "P" such as benzyl or silyl by methods known in the art, yielding a compound of formula 8 . Hydrolysis of the ketal in compound 8 using, for example, p-toluenesulfonic acid in acetone or aqueous sulfuric acid in acetonitrile, provides a compound of formula 9 . Treatment of a compound of formula 9 (wherein P, for example, is a benzyl protecting group) with a sterically hindered amide base such as lithium 2,2,6,6-tetramethylpiperidide, lithium diisopropylamide, or lithium bis(trimethylsilyl)amide (preferentially, lithium 2,2,6,6-tetramethylpiperidide), followed by the addition of a fluorinating agent such as perchloryl fluoride [see for example, C.L.J. Wang, P.A. Grieco, F.J. Okuniewicz, Chem.Commun. , 48 (1976)] provides compounds of formula 10 as a mixture of (2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and (2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone. Optionally, perchloryl fluoride can be substituted by other fluorinating agents such as acetyl hypofluorite or N-fluoroalkylsulfonamides [see for example, S. Rozen and M. Brand, Synthesis , 665 (1985); W.E. Barnette, J. Amer. Chem. Soc. , 106 , 452 (1984)]. Alternatively, a compound of formula 9 can be treated with a sterically hindered amide base, followed by the addition of an enolate trapping agent such as chlorotrimethylsilane to provide a compound of formula 11 . Treatment of a compound of formula 11 with a fluorinating agent such as acetyl hypofluorite, trifluoromethyl hypofluorite, 2-fluoropyridinium trifluoromethanesulfonate, or a N-fluoroalkylsulfonamide, provides compounds of formula 10 [see for example, S. Rozen, et al. , J. Org. Chem. , 50 , 4753 (1985); W. J. Middleton and E.M. Bingham, J. Amer. Chem. Soc. , 102 , 4845 (1980); T. Umenoto, K. Kawada and K. Tomita, Tetrahedron Lett. , 4465 (1986); W. E. Barnette, J. Amer. Chem. Soc. , 106 , 452 (1984)]. Reduction of compounds of formula 10 with a reducing agent such as lithium trisiamylborohydride provides a compound of formula 12 and an equal amount of (1α,2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol which can be separated by chromatography. Alternatively, reducing agents such as lithium aluminum hydride, sodium borohydride, lithium borohydride, lithium tri-sec-butylborohydride or diisobutylaluminum hydride can be used. The compound of formula 12 can be converted to a compound of formula 2 by methods known in the art. For example, treatment of 12 with p-toluenesulfonyl chloride in pyridine, or methanesulfonyl chloride in pyridine, or methylene chloride/triethylamine, yields a compound of formula 2 wherein X is p-toluenesulfonyloxy or methanesulfonyloxy, respectively. The above description is shown in the following schematic: Reaction of the compound of formula 2 with a compound of formula by methodology analogous to that used to prepare a compound of formula 4 , and subsequent removal of the protecting groups P, yields the corresponding compound of formula 1 wherein R₁ is Alternatively, this compound of formula 1 can be prepared by reaction of a compound of formula with a compound of formula 2 by methods analogous to those used in the preparation of a compound of formula 4 . This affords the corresponding compound of formula Treatment of a compound of formula 21 with hot ammonia in an alcohol (such as, methanol or ethanol) and subsequent deprotection of the protecting groups P yields the corresponding compound of formula 1 wherein R₁ is The compound of formula wherein R₃ is hydrogen, fluoro, methyl, ethyl, n-propyl, 2-chloroethyl, or 2-fluoroethyl can be prepared by reaction of the corresponding compound of formula with a compound of formula 2 in the presence of a base such as potassium carbonate, sodium hydride, or potassium hydride, in an aprotic polar solvent (e.g. , dimethylformamide, dimethylsulfoxide, or sulfolane), in the optional presence of 18-crown-6 or 15-crown-5, to yield an intermediate of formula Alternatively, treatment of a compound of formula 2 with a mono-tetraalkylammonium salt of a compound of formula 24 in an aprotic polar solvent such as dimethylformamide, dimethylsulfoxide or sulfolane affords a compound of formula 25 . A mono-tetraalkylammonium salt of a compound of formula 24 can be prepared by treatment of a compound of formula 24 (in an aprotic polar solvent such as dimethylformamide) with a tetraalkylammonium hydroxide, such as aqueous tetra-n-butylammonium hydroxide, and subsequent azeotropic removal of water. Removal of the protecting groups P from a compound of formula 25 provides the corresponding compound of formula 23 . For example, when P is a silyl group, deprotection can be accomplished with fluoride ion. When P is a benzyl group, deprotection can be accomplished by hydrogenolysis (e.g. , palladium hydroxide on carbon in cyclohexene and ethanol) or by treatment with boron trichloride. The compound of formula 24 wherein R₃ is 2-chloroethyl or 2-fluoroethyl can be prepared by methods known in the art [H. Griengl, et al. , J. Med. Chem. , 30 , 1199 (1987); J. Med. Chem. , 28 , 1679 (1985)]. The compound of formula 23 wherein R₃ is fluoro can also be prepared from the corresponding compound 23 wherein R₃ is hydrogen and the hydroxy groups are optionally protected with a group such as acetyl by fluorination with trifluoromethyl hypofluorite using methodology known in the art. For example, see M.J. Robins, et al. , J. Amer Chem. Soc. , 93 , 5277 (1971) and Chem. Commun ., 18 (1972); T.S. Lin, et al. , J. Med. Chem. , 26 , 1691 (1983). The compounds of formula 23 wherein R₃ is 2-chloroethyl and 2-fluoroethyl can also be prepared from a compound of formula wherein P₂ and P are different protecting groups wherein P₂ can be selectively removed in the presence of P. For example, when P₂ is a silyl, trityl or substituted trityl group, P can be a benzyl group. Similarly, when P₂ is an acyl or benzyl group, P can be a silyl protecting group. Selective removal of the protecting group P₂ yields a compound of formula 25 wherein R₃ is 2-hydroxyethyl. Treatment of this compound with triphenylphosphine-carbon tetrachloride and subsequent removal of protecting groups P affords the compound of formula 23 wherein R₃ is 2-chloroethyl. Similar treatment using triphenylphosphine-N-bromosuccinimide or triphenylphosphine N-bromosuccinimide-tetrabutylammonium iodide in place of triphenylphosphine-carbon tetrachloride (e.g. , see H. Griengl, et al. , J. Med. Chem. , 28 , 1679 (1985)) affords compounds of formula 25 wherein R₃ is 2-bromoethyl or 2-iodoethyl, respectively. Subsequent treatment with fluoride ion, followed by removal of protecting groups P, provides the compound of formula 23 wherein R₃ is 2-fluoroethyl. When P is a silyl group, deprotection will occur upon treatment with fluoride ion. Alternatively, treatment of a compound of formula 25 , wherein R₃ is 2-hydroxyethyl, with diethylaminosulfur trifluoride provides, upon removal of the protecting groups p, a compound of formula 23 wherein R₃ is 2-fluoroethyl. The compound of formula 26 can be prepared by reaction of a compound of formula with a compound of formula 2 by methods analogous to those used for the preparation of 25 wherein, for example, R₃ is hydrogen, methyl or ethyl. The compound of formula 27 can be prepared from the corresponding free alcohol by methods known in the art. The compound of formula wherein R₃ is hydrogen, fluoro, methyl, ethyl, n-propyl, 2-chloroethyl, or 2-fluoroethyl can be prepared from the corresponding compound of formula 25 by methods known in the art. See, for example, I. Wempner, et al. , in "Synthetic Procedures in Nucleic Acid Chemistry", Vol. 1, W.W. Zorbach and R.S. Tipson, Eds., Interscience Publishers, N.Y., p. 299, 1968; T.S. Lin, et al. , J. Med. Chem. , 26 , 1691 (1983); P. Herdewijn, et al. , J. Med. Chem. , 28 , 550 (1985). Deprotection yields the corresponding compound of formula 28 . Alternatively, the compound of formula 28 , wherein R₃ is fluoro, hydrogen, methyl, ethyl, n-propyl, 2-chloroethyl, or 2-fluoroethyl, can be prepared by reaction of the corresponding compound of formula with a compound of formula 2 in the presence of a base such as potassium carbonate, sodium hydride, or potassium hydride in an aprotic solvent (e.g. , dimethylformamide, dimethylsulfoxide, or sulfolane), in the optional presence of 18-crown-6 or 15-crown-5, and subsequent removal of the protecting groups. Optionally, the amino (-NH₂) group in 29 can be protected, e.g. , with an acyl group. Removal of this protecting group can be accomplished using sodium methoxide in methanol or methanolic ammonia. Alternatively, treatment of a compound of formula 2 with a tetraalkylammonium salt of a compound of formula 29 in an aprotic polar solvent such as dimethylformamide, dimethylsulfoxide or sulfolane provides a compound of formula 28 . A tetraalkylammonium salt of a compound of formula 29 can be prepared by treatment of a compound of formula 29 (in an aprotic polar solvent such as dimethylformamide) with a tetraalkylammonium hydroxide such as aqueous tetra-n-butylammonium hydroxide and subsequent azeotropic removal of water. Alternatively, the compound of formula 28 wherein R₃ is fluoro can be prepared from the corresponding compound wherein R₃ is hydrogen by fluorination with trifluoromethyl hypofluorite using methodology known in the art. Fluorination can also be performed on the compounds of formula 28 wherein R₃ is hydrogen and the hydroxyl and/or amino groups are protected, for example, by an acyl group. After fluorination, deprotection using methanolic ammonia or aqueous hydroxide affords the compound of formula 28 wherein R₃ is fluoro. See, for example, M.J. Robins, et al. , J. Amer. Chem. Soc. , 93 , 5277 (1971) and Chem. Commun. , 18 (1972); T.S. Lin, et al. , J. Med. Chem. , 26 , 1691 (1983). The compounds of formula 23 and 28 wherein R₃ is chloro, bromo, or iodo can be prepared from the corresponding compounds of formula 23 and 28 wherein R₃ is hydrogen by methods known in the art. See, for example, "Basic Principals in Nucleic Acid Chemistry", Vol. 1, P.O.P. Ts'O, Ed., Academic Press, N.Y., p. 146, 1974; P.K. Chang in "Nucleic Acid Chemistry" Part 3, L.B. Townsend and R.S. Tipson, Eds., John Wiley and Sons, N.Y., p.46, 1986. The compounds of formula 23 and 28 wherein R₃ is trifluoromethyl can be prepared from the corresponding compounds of formula 23 and 28 wherein R₃ is iodo and the hydroxy and amino (-NH₂) groups are protected, for example, by an acyl, by treatment with trifluoromethyl iodide and copper according to procedures known in the art. Subsequent deprotection using methanolic ammonia or sodium methoxide in methanol yields the corresponding compound of formulas 23 and 28 wherein R₃ is trifluoromethyl. See, for example, Y. Kobayashi, et al. , J. Chem. Soc. Perkin 1 , 2755 (1980); S. Lin, et al. , J. Med. Chem. , 26 , 1691 (1983). The compounds of formula 23 and 28 wherein R₃ is and R₄ is chloro, bromo, iodo, hydrogen, methyl or trifluoromethyl can be prepared from the corresponding compounds of formula 23 and 28 wherein R₃ is iodo or -HgCl via organopalladium intermediates. The compounds of formula 23 and 28 wherein R₃ is -HgCl can be prepared from the corresponding compounds of formula 23 and 28 wherein R₃ is hydrogen by methods known in the art. See, for example, M. Ashwell, et al ., Tetrahedron , 43 , 4601 (1987); M.E. Perlman, et al. , J. Med. Chem. , 28 , 741 (1985); P. Herdewijn, et al. , J. Med. Chem. , 28 , 550 (1985); D.E. Bergstrom, et al. , J. Med. Chem. , 27 , 279 (1984); and references in E. DeClercq, et al. , Pharmac. Ther. , 26 , 1 (1984). Unless otherwise stated, the stereochemistry shown for the compounds of this invention and intermediates leading to compounds of this invention is relative, not absolute. It is drawn to show that in the compounds of this invention, the base represented by R₁ is trans to both the vicinal -CH₂OH substituent and the fluorine atom, and the -CH₂OH substituents are trans to each other. The compounds of formula 1 can exist as levorotatory or dextrarotatory enantiomers or as mixtures thereof. All of these forms are within the scope of this invention. For example, the 1S-enantiomer of the compound of formula 1 wherein R₁ is and R₇ and R₈ are hydrogen, [1S-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)-cyclobutyl]-1,9-dihydro-6H-purin-6-one, can be prepared from the 1R-enantiomer of the compound of formula 2 wherein X is p-toluenesulfonoxy and P is benzyl, [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis-[(phenylmethoxy)methyl]cyclobutanol,4-methylbenzenesulfonate ester, using methods analogous to those used in the preparation of the corresponding racemic compound of formula 1 . Reaction of [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)-methyl]cyclobutanol, 4-methylbenzenesulfonate ester, with a compound of formula 3 in the presence of a base, such as potassium carbonate, in an aprotic polar solvent, such as dimethylformamide, and subsequent removal of the benzyl protecting groups yields [1S-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. [1R-(1α,2β,3α,4β)]-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one can be prepared in an analogous fashion from [1S-(1α,2α,3β,4α)]-2-fluoro-3,4-bis((phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester. [1R-(1α,2α,3β,4α)]-2-Fluoro-3,4-bis-[(phenylmethoxy)methyl]cyclobutanol,4-methylbenzeflesulfonate ester, can be prepared from the corresponding alcohol, [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis-[(phenylmethoxy)methyl]cyclobutanol, by methods known in the art. [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol can be prepared from (2S- trans )-2,3-bis[(phenylmethoxy)-methyl]cyclobutanone by treatment with a sterically hindered base (such as lithium 2,2,6,6-tetramethylpiperidide), followed by addition of a fluorinating agent (such as perchlorylfluoride), and subsequent reduction of the resulting α-fluoro ketone with, for example, lithium trisiamylborohydride. Alternatively, (2S- trans )-2,3-bis[(phenylmethoxy)methyl]cyclobutanone can be treated with a sterically hindered base (such as 2,2,6,6-tetramethylpiperidide), followed by the addition of chlorotrimethylsilane, treatment of the resulting enol ether with a fluorinating agent (such as 2-fluoropyridinium trifluoromethanesulfonate), and subsequent reduction of the resulting α-fluoro ketone with, for example, lithium trisiamylborohydride. [1S-(1α,2α,3β,4α)]-2-Fluoro-3,4-bis[phenylmethoxy)methyl]cyclobutanol,4-methylbenzenesulfonate ester can be prepared in an analogous fashion from (2R- tran s)-2,3-bis[(phenylmethoxy)methyl]cyclobutanone. (2S- trans )-2,3-Bis[(phenylmethoxy)]methyl]cyclobutanone can be prepared as described below: Complexation of (-)-dimenthylfumarate with diisobutylaluminum chloride (or diethylaluminum chloride; -78°C in toluene or toluene/hexane), followed by addition of 1,1-dimethoxyethylene, provides, after recrystallization of the crude product, (1S- trans )-3,3-dimethoxy-1,2-cyclobutane-dicarboxylic acid, di-(-)-menthyl ester. Reduction of (1S- trans )-3,3-dimethoxy-1,2-cyclobutanedicarboxylic acid, di-(-)-menthyl ester with lithium aluminum hydride provides (1S- trans )-3,3-dimethoxy-1,2-cyclobutanedimethanol. (1S- trans )-3,3-dimethoxy-1,2-cyclobutanedimethanol is then converted to (2S- trans )-1,1-dimethoxy-2,3-bis[(phenylmethoxy)-methyl]cyclobutane by methods known in the art. Hydrolysis of (2S- trans )-1,1-dimethoxy-2,3-bis-[(phenylmethoxy)methyl]cyclobutane with, for example, aqueous sulfuric acid in acetonitrile provides (2S- trans )-2,3-bis[(phenylmethoxy)methyl]cyclobutanone. (2R- trans )-2,3-bis[(phenylmethoxy)methyl]-cyclobutanone can be prepared in an analogous fashion from (+)-dimenthylfumarate. The intermediate [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol can alternatively be prepared by the procedure shown in the reaction scheme below: R₁₀ is lower alkyl of 1 to 5 carbons or both R₁₀ groups are joined together by an alkylene group of 2 or 3 carbons; and P is a protecting group; and R₉ is a group selected from: These R₉ groups are obtained from the optically active alcohols (+)- or (-)-menthol, (+)- or (-)-methyl- or ethyl lactate, (+)- or (-)-pantolactone, (+)- or (-)-α-phenethyl alcohol, (+)- or (-)-borneol, (+)- or (-)-α,β-isopropylideneglycerol, (+)- or (-)-3-methyl-2-butanol and the like (i.e., wherein R₉OH is (+)- or (-)-menthol, (+)- or (-)-methyl or ethyl lactate, (+)- or (-)-pantolactone, (+)- or (-)-α-phenethyl alcohol, (+)- or (-)-borneol, (+)- or (-)-α,β-isopropylideneglycerol, (+)- or (-)-3-methyl-2-butanol) by removal of the hydroxy group. Preferred R₁₀ groups are methyl and ethyl, especially ethyl. Compounds 32a , 32b , 33a , 33b , 34a , 34b , 10a and 10b are optically active and their absolute stereochemistry is as follows: for 32a , 32b , 33a , and 33b , the absolute configuration at the cyclobutyl 1-position is S; for 34a and 10a , the absolute configuration at the cyclobutyl 2-position is S; for 34b and 10b , the absolute configuration at the cyclobutyl 2-position is R. The relative steroechemistry of these compounds is as depicted in the scheme. The compounds of formula 32a and 32b can be prepared by reaction of a compound of formula 31 with a compound of formula 30 in the presence of a Lewis acid. Examples of suitable Lewis acids include aluminum compounds such as diethylaluminum chloride, diisobutylaluminum chloride, ethylaluminum dichloride, isopropoxyaluminum dichloride, aluminum trichloride and the like, boron compounds such as boron trifluoride, boron trichloride and the like, titanium compounds such as titanium tetrachloride, dichlorodiisopropoxytitanium and the like, and tin compounds such as tin tetrachloride, tin trichloride and the like. Preferably, the Lewis acid is a di(lower alkyl)aluminum chloride wherein each lower alkyl is of 2 to 4 carbons and most preferably is diethylaluminum chloride. Preferably, the R₉ group for the compound of formula 30 is derived from (-)-menthol, i.e., R₉ is The ratio of the resulting epimeric fluoro compounds 32a and 32b may vary according to the conditions of the reaction and the particular R₉ and R₁₀ groups chosen in the reactants 30 and 31, respectively. For example, in the case wherein R₉ of compound 30 is derived from (-)-menthol, R₁₀ of compound 31 is ethyl, and the Lewis acid is diethylaluminum chloride, the ratio of resulting 32a and 32b is approximately 1:5 after recrystallization of the crude product mixture. The compounds of formulas 33a and 33b can be prepared by reaction of the compounds of formulae 32a and 32b with a reducing agent such as lithium aluminum hydride, lithium borohydride, and the like, in a solvent such as tetrahydrofuran, diethyl ether, and the like, preferably with lithium aluminum hydride in tetrahydrofuran. The compounds of formulas 34a and 34b can be prepared by protection of the hydroxy groups of compounds of formulas 33a and 33b with benzyl or silyl protecting groups as previously defined. Preferably the protecting group P is a benzyl group. The compounds of formula 10a and 10b can be prepared by hydrolysis of the compounds of formulas 34a and 34b with an acid catalyst such as sulfuric acid, hydrochloric acid, p-toluenesulfonic acid, and the like, preferably sulfuric acid, in a solvent or solvent mixture such as water, water-acetonitrile, water-dioxane, acetone, and the like, preferably water-acetonitrile. If in the mixture of isomers 10a and 10b , the isomer 10b is the predominant isomer, then the mixture of 10a and 10b can be treated with a basic catalyst such as N,N-diisopropylethylamine, 1,8-diazabicyclo[5.4.0]undec-7-ene and the like, preferably 1,8-diazabicyclo[5.4.0]undec-7-ene, in a solvent such as tetrahydrofuran, acetonitrile, and the like, preferably acetonitrile, to effect epimerization of the fluorine so that the isomer 10a is the predominant isomer or exclusive isomer of the mixture. The desired intermediate [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol can then be prepared by reduction of 10a or the mixture of 10a and 10b with a reducing agent, preferably lithium trisiamylborohydride, followed by chromatography on silica gel. The starting material of formula 31 in the above procedure can be prepared by the method shown in the reaction scheme below: wherein R₁₄ is a lower alkyl of 1 to 5 carbons and R₁₃ is chloro, bromo, or iodo. A compound of formula 36 can be prepared by reducing a compound of formula 35 with diisobutylaluminum hydride, lithium aluminum hydride, and the like, preferably with diisobutylaluminum hydride, followed by treatment of the crude product with an acid catalyst such as sulfuric acid, hydrochloric acid, and the like, preferably hydrochloric acid, in the presence of a lower alkyl alcohol or ethylene glycol or propylene glycol, preferably ethyl alcohol. In the compound of formula 35 , R₁₃ is preferably chloro and R₅ is preferably ethyl. The compounds of formula 35 are either commercially available or can be prepared by methods known in the art. The compound of formula 31 can be prepared by treatment of the compound of formula 36 with a hindered base such as potassium t-butoxide, lithium diisopropylamide, and the like, preferably potassium t-butoxide. The starting materials of formula 30 wherein R₉ is as defined above are commercially available (e.g., the compound of formula 30 wherein R₉ is the group obtained by removal of the hydroxy group from (-)-menthol is available from Aldrich Chemical Company) or can be readily prepared by methods known in the art (see e.g., Heathcock, C.H., et al., J. Med. Chem., 32 , 197(1989); Scharf, H.D., Chem. Ber., 119 , 3492, (1986); Helmchem, G., et al., DE 3702084). Analogously, by selection of an appropriate R₉ the above procedure can be utilized to prepare the intermediate [1S-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol. In particular, when R₉ is the group obtained by removal of the hydroxy from (+)-menthol, i.e., R₉ is the [1S-(1α,2α,3β,4α)] enantiomer is obtained. The separate [1S-(1α,2β,3α,4β)]- and [1R-(1α,2β,3α,4β)]-enantiomeric forms of the remaining compounds of formula 1 can be prepared from [1R-(1α,2α,3β,4α)]- and [1S-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, respectively, using methods analogous to those used in the preparation of the corresponding racemic compounds of formula 1 . The compounds of formula 1 wherein R₁ is wherein R₂ = H and can form acid addition salts with inorganic or organic acids. Illustrative are the halide (e.g. , chloride and bromide), alkylsulfonate, sulfate, phosphate and carboxylate salts. The compounds of formula 1 wherein R₁ is wherein R₂ is hydrogen can form basic salts with inorganic and organic bases. Illustrative are alkali metal salts (e.g. , sodium and potassium), alkaline earth metal salts (e.g. , calcium and magnesium), ammonium and substituted ammonium salts. The following examples are specific embodiments of this invention. Unless otherwise noted, the compounds exemplified have been prepared as racemic mixtures. Example 1 (1α,2β,3α,4β)-2-Amino-9-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one A. trans -3,3-Diethoxy-1,2-cyclobutanedimethanol To a suspension of 1.97 g (52.0 mmole) of lithium aluminum hydride in 150 ml of diethyl ether at 0°C was added over 15 minutes a solution of 10.0 g (34.7 mmole) of trans -3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, diethyl ester [W.A. Slusarchyk, et al ., Tetrahedron Lett. , 6453 (1989) and European Patent 335,355-A] in 100 ml of anhydrous diethyl ether. The mixture was warmed to room temperature and stirred for 16 hours. The reaction was quenched by the addition of sodium sulfate decahydrate followed by the addition of 10 ml of 3M NaOH. The solids were removed by suction filtration and washed with ethyl acetate. Rotary evaporation of the solvent provided 7.21 g of the title compound as a viscous, colorless oil. B. trans -1,1-Diethoxy-2,3-bis[(phenylmethoxy)methyl]cyclobutane Sodium hydride (3.33 g, 83.3 mmole, 60% dispersion in mineral oil) was washed with pentane and then suspended in 50 ml of dry dimethylformamide. The suspension was cooled to 0°C and 10.0 ml of benzyl bromide was added to the reaction flask. To the resulting mixture was added a solution of 7.36 g (35 mmole) of trans -3,3-diethoxy-1,2-cyclobutanedimethanol in 20 ml of dry dimethylformamide. The mixture was warmed to room temperature and stirred for 8 hours. The mixture was diluted with diethyl ether (300 ml) and washed with saturated aqueous NaHCO₃ (2 x 150 ml). The aqueous layer was washed with diethyl ether (2 x 50 ml) and the combined organic layers were dried over Na₂SO₄. The solvent was removed by rotary evaporation and the residue was taken up in diethyl ether and dried over Na₂SO₄. Rotary evaporation of the solvent gave 14.1 g of residue which was purified by flash chromatography on silica gel (0% to 5% ethyl acetate - petroleum ether) affording 10.4 g of title compound that was homogenous by TLC. C. trans -2,3-bis[(phenylmethoxy)methyl]cyclobutanone To a solution of 10.4 g (27.0 mmol) of trans -1,1-diethoxy-2,3-bis[(phenylmethoxy)methyl]cyclobutane in 300 ml of acetonitrile was added 100 ml of 0.5M H₂SO₄. The solution was stirred overnight at room temperature. Next, the solution was diluted with ethyl acetate (1200 ml) and washed in succession with water (400 ml), saturated aqueous NaHCO₃ (400 ml), and brine (400 ml). The organic layer was dried over Na₂SO₄, and the solvent was removed by rotary evaporation. Toluene was added to the residue, followed by rotary evaporation in order to effect a toluene-water azeotrope. Flash chromatography on silica gel (10% to 20% ethyl acetate - petroleum ether) provided 7.85 g of pure cyclobutanone. D. (2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone To a solution of 0.41 ml (2.4 mmol) of dry 2,2,6,6-tetramethylpiperidine in 4 ml of dry tetrahydrofuran at -78°C was added 0.84 ml of n-butyllithium (2.0 mmol; 2.4M in hexane). The resulting solution was stirred at -78°C for 20 minutes. To this was added, via cannula over 2 minutes, a cold (-78°C) solution of 0.500 g (1.51 mmol) of trans -2,3-[bis(phenylmethoxy)methyl]cyclobutanone in 2 ml of dry tetrahydrofuran. The resulting solution was stirred for 30 minutes at -78°C and then warmed to -35°C. Perchloryl fluoride (∼5 g) was bubbled through the reaction mixture at a rapid rate for 30 seconds. The mixture was purged with Argon for 5 minutes at -35°C and for 10 minutes at 0°C. The mixture was quenched by dropwise addition of 8 ml of 1M KI, and the mixture was stirred for 15 minutes at room temperature. The reaction contents were extracted with diethyl ether and the combined organic layers were washed with 5% aqueous sodium bisulfite. The organic layer was dried over Na₂SO₄, and the solvent was removed by rotary evaporation. The residue was dried by azeotropic rotary evaporation with toluene to provide 0.57 g of crude product containing an ∼1:1 ratio of (2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and its C2-epimer, (2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone. E. (1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol To a solution of 19.3 ml (19.3 mmol) of lithium trisiamylborohydride (1.0 M in tetrahydrofuran) in 12 ml of dry tetrahydrofuran at -78°C was added, via cannula over 3 minutes, a cold (-78°C) solution of 3.55 g (∼9.7 mmol) of a crude ∼1:1 mixture of (2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and (2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone in 12 ml of dry tetrahydrofuran. The mixture was stirred at -78°C for 30 minutes, and then warmed to 0°C and stirred for 30 minutes. The reaction contents were quenched by the slow addition of saturated aqueous NaHCO₃ (60 ml) and 30% H₂O₂(12 ml). The mixture was warmed to room temperature and stirred for 15 minutes. Next, the mixture was diluted with brine (75 ml) and extracted with ethyl acetate (300 ml, then 2 x 100 ml). The combined organic layers were washed with brine (100 ml) and dried over Na₂SO₄. Rotary evaporation of the solvent provided 3.37 g of crude material. Flash chromatography on silica gel (20% to 30% ethyl acetate-hexane) gave in order of elution 644 mg of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 556 mg of an unidentified material, and 694 mg of (1α,2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol. F. (1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester To a solution of 340 mg (1.03 mmol) of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol in 1 ml of dry pyridine was added 972 mg (5.10 mmol) of p-toluenesulfonyl chloride. The mixture was heated at 60°C for 7 hours. The mixture was cooled to room temperature, diluted with diethyl ether (80 ml), and washed with water (4 x 25 ml), saturated aqueous NaHCO₃ (25 ml), and brine (25 ml). The organic layer was dried over Na₂SO₄, and rotary evaporation of the solvent gave 1.49 g of crude material. Flash chromatography on silica gel (10% to 20% ethyl acetate-hexane) gave 465 mg of the title compound as a white solid. G. (1α,2β,3α,4β)-9-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine To a solution of 450 mg (0.929 mmol) of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester in 5 ml of dry dimethylformamide were added 1.12 g (4.64 mmol) of 2-amino-6-(phenylmethoxy)-9H-purine, 963 mg (6.97 mmol) of K₂CO₃ (powdered and dried under high vacuum at 80°C for 18 hours) and 1.23 g (4.64 mmol) of 18-crown-6. The mixture was heated at 110°C for 18 hours. The mixture was cooled to room temperature and filtered. The solvent was removed by rotary evaporation. The residue was dissolved in ethyl acetate and adsorbed onto silica gel. The ethyl acetate was removed by rotary evaporation and flash chromatography on silica gel (20% to 50% ethyl acetate-hexane) afforded 202 mg of the title compound. H. (1α,2β,3α,4β)-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one To a solution of 158 mg (0.285 mmol) of (1α,2β,3α,4β)-9-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine in 14 ml of 95% ethyl alcohol were added 7 ml of cyclohexene and 80 mg of Pd(OH)₂/C (20%). The mixture was heated at reflux for 15 hours. The hot mixture was filtered through celite which was then rinsed with 50% aqueous methyl alcohol (150 ml). Rotary evaporation of the filtrate gave 85 mg of a white solid. The solid was suspended in water and chromatographed (CHP-20P resin, Mitsubishi Chemical Co., 35 - 150µ; 0% to 20% acetonitrile-water) to afford 62.9 mg of material which was suspended in water and freeze-dried to provide 52.6 mg of (1α,2β,3α,4β)-2-amino-9-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one as a white solid, mp 220°C (dec). ¹H NMR (270 MHz; DMSO-d₆) δ: 2.16 - 2.33 (m, 2H), 3.45 - 3.55 (m, 2H), 3.55 - 3.70 (m, 2H), 4.53 - 4.68 (m, 1H), 4.74 (t, 1H, J = 5.3 Hz), 4.81 (t, 1H, J = 5.3 Hz), 5.08 (ddd, 1H, J = 55.7 Hz, J = 7.0 Hz, J = 7.0 Hz), 6.43 (bs, 2H), 7.80 (s, 1H), 10.59 (bs, 1H). Example 2 (1α,2β,3α,4β)-3-(6-Amino-9H-purin-9-yl)-4-fluoro-1,2-cyclobutanedimethanol A. (1α,2β,3α,4β)-9-[2-Fluoro-3,4-bis[(phenyl methoxy)methyl]cyclobutyl]-9H-purin-6-amine To a solution of 579 mg (1.19 mmol) of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester in 12 ml of dry dimethylformamide were added 484 mg (3.58 mmol) of adenine (dried under vacuum at 80°C for 24 hours), 658 mg (4.76 mmol) of K₂CO₃ (powdered and dried under vacuum at 80°C for 24 hours), and 946 mg (3.58 mmol) of 18-crown-6. The mixture was heated at 135°C for 5 hours. The mixture was cooled to room temperature and filtered. The solids were washed with ethyl acetate and the solvent removed by rotary evaporation. The residue was flash chromatographed on silica gel (1% to 5% CH₃OH-CHCl₃) to afford 297 mg of the title compound. B. (1α,2β,3α,4β)-3-(6-Amino-9H-purin-9-yl)-4-fluoro-1,2-cyclobutanedimethanol To a solution of 262 mg (0.586 mmol) of (1α,2β,3α,4β)-9-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-9H-purin-6-amine in 20 ml of 95% ethyl alcohol were added 10 ml of cyclohexene and 130 mg of Pd(OH)₂/C (20%). The mixture was heated at reflux for 16 hours, and another 70 mg of catalyst was added. Reflux was continued for an additional 24 hours at which time TLC (3:1 CHCl₃-CH₃OH) indicated that the reaction was complete. The hot mixture was filtered through celite which was rinsed with hot methyl alcohol (100 ml) and hot 50% aqueous methyl alcohol (100 ml). Rotary evaporation of the filtrate gave 160 mg of crude product. Chromatography (CHP-20P resin, 0% to 20% acetonitrile-water) afforded 137 mg of a white solid which was suspended in water and freeze-dried to provide 137 mg of (1α,2β,3α,4β)-3-(6-amino-9H-purin-9-yl)-4-fluoro-1,2-cyclobutanedimethanol as a white solid, mp 198 - 201°C. ¹H NMR (270 MHz; DMSO-d₆) δ: 2.15 - 2.35 (m, 1H), 2.35 - 2.48 (m, 1H), 3.50 - 3.60 (m, 2H), 3.60 - 3.70 (m,2H), 4.70 - 4.90 (m, 3H), 5.28 (ddd, 1H, J = 55.7 Hz, J = 7.0 Hz, J = 7.0 Hz), 7.21 (bs, 2H), 8.14 (s, 1H), 8.24 (s, 1H). Example 3 (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl) cyclobutyl]-5-methyl-2,4(1H,3H)-pyrimidinedione A. (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-5-methyl-2,4(1H,3H)-pyrimidinedione To a suspension of 1.39 g (11.0 mmol) of thymine in 25 ml of dimethylformamide at room temperature was added 6.5 ml (40% wt; 10.0 mmol) of aqueous tetrabutylammonium hydroxide. The mixture was stirred at room temperature for 1 hour. The solvents were removed in vacuo at 55°C. The resulting residue was dissolved in approximately 10 ml of dry dimethylformamide and the solvent was removed in vacuo (this procedure was repeated three times to remove water). The resulting tetrabutylammonium salt, which appeared as a colorless solid, was dried overnight under high vacuum (0.1 mm Hg). The salt was dissolved in 7 ml of dry dimethylformamide to give approximately 10 ml of solution. Then, 7.5 ml (∼7.5 mmol) of the resulting solution was added to 623 mg (1.29 mmol) of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester. The mixture was stirred at room temperature for 5 minutes to dissolve the tosylate and then heated at 100°C for 6 hours. The reaction mixture was cooled to room temperature and allowed to stir for 30 minutes. Acetic acid (0.86 ml, 15 mmol) was added and the volatiles were removed by rotary evaporation to give a gummy residue. The residue was suspended in 50 ml each of water and ethyl acetate and then stirred in the presence of 2.5 g of slightly damp AG-MP-50 (Na⁺) resin for 30 minutes at room temperature. The solids were removed by filtration, and the filter cake was thoroughly washed with additional quantities of both water and ethyl acetate. The ethyl acetate layer was separated and dried over Na₂SO₄. The solvent was removed by rotary evaporation to give 639 mg of crude product. Flash chromatography on silica gel (1:1 ethyl acetate-hexane) afforded 284 mg of the title compound. B. (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-methyl-2,4(1H,3H)-pyrimidinedione To a solution of 270 mg (0.616 mmol) of (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-5-methyl-2,4(1H,3H)-pyrimidinedione in 20 ml of 95% ethyl alcohol were added 10 ml of cyclohexene and 200 mg of Pd(OH)₂/C (20%). The mixture was heated at reflux under argon for 24 hours. The hot mixture was filtered through celite which was rinsed with hot methyl alcohol (100 ml) and hot 50% aqueous methyl alcohol (100 ml). Rotary evaporation of the filtrate gave 175 mg of crude product. Flash chromatography on silica gel (0% to 20% isopropyl alcohol-chloroform) afforded 102 mg of a white solid which was suspended in water and freeze-dried to provide 92 mg of (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-methyl-2,4(1H,3H)-pyrimidinedione as a white solid, mp 50 - 55°C. ¹H NMR (270 MHz, DMSO-d₆) δ: 1.72 (s, 3H), 1.95 - 2.15 (m, 2H), 3.35 - 3.45 (m, 2H), 3.45 - 3.55 (m, 2H), 4.48 - 4.68 (m, 3H), 4.90 (ddd, 1H, J = 55.7 Hz, J = 7.0 Hz, J = 7.0 Hz), 7.48 (d, 1H, J = 1.2 Hz), 11.18 (bs, 1H). Example 4 (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)pyrimidinedione A. (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2,4(1H,3H)pyrimidinedione To a suspension of 1.14 g (10.2 mmol) of uracil in 25 ml of dimethylformamide at room temperature was added 6.1 ml of aqueous tetrabutylammonium hydroxide (40% wt; 9.3 mmol). The mixture was stirred at room temperature for 1 hour. The solvents were removed in vacuo at 55°C. The resulting residue was dissolved in approximately 5 ml of dry dimethylformamide and the solvent was removed in vacuo (this procedure was repeated four times to remove water). The resulting tetrabutylammonium salt, which appeared as a colorless solid, was dried overnight under high vacuum (0.1 mm Hg). The salt was suspended in 9 ml of dry dimethylformamide and 623 mg (1.29 mmol) of (1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester was added. The resulting mixture was heated at 100°C for 6 hours (when the temperature reached 55°C, the tetrabutylammonium salt dissolved yielding a homogeneous mixture). The reaction mixture was cooled to room temperature and allowed to stir for 30 minutes. Acetic acid (1.11 g, 18.5 mmol) was added and the volatiles were removed by rotary evaporation to give a gummy residue. The residue was suspended in 62 ml each of water and ethyl acetate, and stirred for 45 minutes at room temperature. Next, 13.0 g of slightly damp AG-MP-50 (Na⁺) resin was added and the mixture stirred for 30 minutes at room temperature. The resin and precipitated uracil were removed by filtration and the filter cake was thoroughly washed with additional quantities of both water and ethyl acetate. The ethyl acetate layer was separated and dried over Na₂SO₄. The solvent was removed by rotary evaporation, and purification of the residue by flash chromatography on silica gel (1:2 ethyl acetate-hexane) afforded 253 mg of the title compound as a viscous oil. B. (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2,4(1H,3H)-pyrimidinedione To a solution of 253 mg (0.60 mmol) of (1α,2β,3α,4β)-1-[2-fluoro-3,4-[bis(phenylmethoxy)methyl]cyclobutyl]-2,4(1H,3H)-pyrimidinedione in 20 ml of 95% ethyl alcohol were added 202 mg of Pd(OH)₂/C and 9.5 ml of cyclohexene. The reaction mixture was heated at reflux for 4.5 hours. TLC (20% CH₃OH-CH₂Cl₂) showed no starting material remaining. The mixture was cooled slightly, filtered through celite, and washed with 50% aqueous methyl alcohol (100 ml). The solvent was removed by rotary evaporation, and purification of the residue by chromatography (CHP-20P resin; 0% to 10% CH₃CN-H₂O) afforded 111 mg of the title compound as a clear oil upon evacuation of the volatiles. C. (1α,2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)pyrimidinedione To a solution of 83 mg (0.34 mmol) of (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2,4(1H,3H)-pyrimidinedione in 5 ml of dioxane and HNO₃ (0.40 ml, 0.8N) was added 129 mg (0.51 mmol) of iodine. The mixture was heated at reflux for 3 hours. TLC (6:3:1 CHCl₃-CH₃OH-NH₄OH, silica) showed no starting material remaining. The mixture was cooled to room temperature and concentrated in vacuo to yield a dark red residue that was stored at -20°C for 48 hours. After warming to room temperature, the crude material was suspended in H₂O and chromatographed (CHP-20P resin; 10% to 20% CH₃CN-H₂O). The purified material was suspended in water and freeze-dried to afford 110 mg of (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)-pyrimidinedione as an opaque solid, mp 211 - 213°C. ¹H NMR (400 MHz; DMSO-d₆) δ: 2.00 - 2.20 (m, 2H), 3.35 - 3.65 (m, 4H), 4.53 - 4.66 (m, 1H), 4.70 (t, 1H, J = 5.3 Hz), 4.75 (t, 1H, J = 5.1 Hz), 5.01 (ddd, 1H, J = 55.7 Hz, J = 7.0 Hz, J = 7.0 Hz), 8.14 (s, 1H), 11.64 (s, 1H). Example 5 [1S-(1α,2β,3α,4β)]-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one A. (1S-trans)-3,3-Dimethoxy-1,2-cyclobutanedicarboxylic acid, di-(-)-menthyl ester Di-(-)-menthylfumarate (100 g) was dissolved in 400 ml dry toluene and cooled to -75°C under argon. To this solution was added with stirring diisobutylaluminum chloride (99.5 ml) over 15 minutes. The resulting orange mixture was stirred at -75°C for 15 minutes and 1,1-dimethoxyethylene (24.7 g; Wiley Organics) was added over 15 minutes. After stirring the reaction mixture for 10 minutes at -78°C, methanol (30 ml) was added over 15 minutes and the mixture was stirred for 30 minutes. Hexane (250 ml) was added over 5 minutes followed by the addition of 20% aqueous sodium hydroxide (40 ml) over 15 minutes at -60°C to -40°C. The reaction mixture was slowly (over 45 minutes) allowed to warm to 10°C and anhydrous magnesium sulfate (40 g) was added. The mixture was allowed to come to room temperature, was filtered, and the filtrate was concentrated in vacuo to afford an oil (119.5 g) which solidified under vacuum. The crude product was recrystallized from methanol-water (95:5) to afford 102 g of the title compound (isomerically pure as judged by HPLC) as a white solid, m.p. 89°C, [α] D = -28.5° (c = 1, CHCl₃). Alternatively, the title compound can be prepared by using diethylaluminum chloride instead of diisobutylaluminum chloride, as described below. A solution of diethylaluminum chloride (1M in hexane, 5.1 ml) was added dropwise to a stirred solution of di(-)-menthyl fumarate (1.0 g) in 5 ml toluene under nitrogen at -78°C. The reaction mixture was stirred at that temperature for 15 minutes, followed by the addition of 0.247 g 1,1-dimethoxyethylene (Wiley Organics). The reaction mixture was stirred at -78°C for 30 minutes, then was carefully added to a mixture of 50 ml hexane and 20 ml saturated aqueous sodium bicarbonate solution. The organic phase was washed with additional saturated sodium bicarbonate solution (2 x 20 ml), water (3 x 20 ml), and was dried (magnesium sulfate) and concentrated in vacuo giving 1.23 g of a thick oil. The crude mixture was recrystallized from methanol-water (95:5) affording 0.98 g of the title compound pure by HPLC and NMR. B. (1S-trans)-3,3-Dimethoxy-1,2-cyclobutanedimethanol To a suspension of 2.09 g (55 mmol) of lithium aluminum hydride in 365 ml of tetrahydrofuran was added over 10 minutes a solution of 17.5 g (36.4 mmol) of (1S-trans)-3,3-dimethoxy-1,2-cyclobutanedicarboxylic acid, di-(-)-menthyl ester in 75 ml of anhydrous tetrahydrofuran. The mixture was heated at 55°C for 1 hour and then cooled to 0°C. The reaction was quenched by the addition of 2.1 ml of water over 5 minutes followed by the addition of 2.1 ml of 15% aqueous sodium hydroxide. The reaction mixture was diluted with 6.3 ml of water and allowed to stir for 30 minutes. Magnesium sulfate (25 g) was added to the reaction mixture and stirring was continued for an additional 30 minutes. The solids were removed by filtration through a pad of celite and the filter cake was washed several times with additional ethyl acetate (2 x 150 ml). The solvents were removed in vacuo , and the residue was placed under vacuum to remove any traces of solvent. The residue was dissolved in 100 ml of heptane and washed with water (4 x 25 ml). The combined aqueous layers were extracted with 25 ml of heptane. The aqueous portion was saturated with ammonium sulfate (45 g) and extracted with ethyl acetate (4 x 50 ml). The combined ethyl acetate extracts were dried over anhydrous magnesium sulfate and the solvent was removed in vacuo to provide 6.28 g of the title compound as a colorless oil. C. (2S-trans)-1,1-Dimethoxy-2,3-bis[(phenylmethoxy)methyl]cyclobutane Sodium hydride (3.5 g, 88 mmol, 60% dispersion in mineral oil) was suspended in 50 ml of dry dimethylformamide. The suspension was cooled to 0°C, and 10.5 ml (88.3 mmol) of benzyl bromide was added to the reaction flask. To the resulting mixture was added a solution of 7.73 g (43.7 mmol) of (1S-trans)-3,3-dimethoxy-1,2-cyclobutanedimethanol in 20 ml of dry dimethylformamide. The mixture was warmed to room temperature and stirred overnight. The reaction was incomplete by TLC analysis. Additional quantities of sodium hydride (350 mg) and benzyl bromide (1.05 ml) were added. The reaction mixture was stirred for 2 hours longer, at which time TLC indicated that the reaction was complete. The mixture was diluted with diethyl ether (300 ml) and washed with saturated aqueous sodium bicarbonate (2 x 150 ml). The aqueous layer was washed with diethyl ether (100 ml) and the combined organic layers were dried over Na₂SO₄. The solvent was removed by rotary evaporation and the residue was purified by flash chromatography (0% to 5% ethyl acetate-hexane) to afford 12.2 g of title compound that was homogenous by TLC. D. (2S-trans)-2,3-Bis[(phenylmethoxy)methyl]cyclobutanone To a solution of 12.0 g (33.7 mmol) of (2S-trans)-1,1-dimethoxy-2,3-bis[(phenylmethoxy)methyl]cyclobutane in 340 ml of acetonitrile was added 112 ml of 0.5 M H₂SO₄. The solution was stirred at room temperature overnight. Then the solution was diluted with ethyl acetate (1200 ml) and washed in succession with water (400 ml), saturated aqueous sodium bicarbonate, and brine (400 ml). The organic layer was dried over sodium sulfate, and the solvent was removed by rotary evaporation. Flash chromatography (10% to 20% ethyl acetate-hexane) of the crude material provided 7.65 g of the title compound. E. [2S-(2α,3β,4α)]-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone To a solution of 0.71 ml (4.2 mmol) of dry 2,2,6,6-tetramethylpiperidine in 8 ml of dry tetrahydrofuran at -78°C was added 1.75 ml of n-butyllithium (3.86 mmol; 2.22 M in hexane). The resulting solution was stirred at -78°C for 30 minutes. To this was added, via cannula over 1-2 minutes, a cold (-78°C) solution of 1.00 g (3.22 mmol) of (2S-trans)-2,3-bis[(phenylmethoxy)methyl]cyclobutanone in 2 ml of dry tetrahydrofuran. The resulting solution was stirred for 30 minutes at -78°C and then warmed to -35°C. Perchloryl fluoride (∼2.5 g) was bubbled through the reaction mixture at a steady rate for 15 seconds. The mixture was purged with Argon for 5 minutes at -35°C and for 5 minutes at 0°C. The mixture was quenched by the dropwise addition of 15 ml of 1M aqueous KI and the mixture was stirred for 15 minutes at room temperature. The reaction contents were extracted with diethyl ether and the combined organic layers were washed with 5% aqueous sodium thiosulfate. The organic layer was dried over Na₂SO₄ and the solvent was removed by rotary evaporation. The residue was dried by azeotropic rotary evaporation with toluene to provide 1.13 g of crude fluoroketone. F. [1R-(1α,2α,3β,4α)]-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol To a solution of 26 ml (26 mmol) of lithium trisiamylborohydride (1.0 M in tetrahydrofuran) in 20 ml of dry tetrahydrofuran at -78°C was added, via cannula over a few minutes, a cold (-78°C) solution of 4.30 g (∼12.9 mmol) of crude [2S-(2α, 3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone in 15 ml of dry tetrahydrofuran. The mixture was stirred at -78°C for 30 minutes and then warmed to 0°C and stirred for 30 minutes. The mixture was warmed to room temperature briefly and then cooled to 0°C. The mixture was quenched by the slow addition of saturated aqueous sodium bicarbonate (80 ml) and 30% hydrogenperoxide (16 ml). The mixture was warmed to room temperature and stirred for 20 minutes. This quenched reaction mixture was combined with that from a ∼10.7 mmol reaction. The resulting mixture was diluted with brine (200 ml) and extracted with ethyl acetate (600 ml, then 2 x 100 ml). The combined organic layers were washed with brine (100 ml) and dried over Na₂SO₄. Rotary evaporation of the solvent provided 7.68 g of crude material. Flash chromatography (20% to 50% ethyl acetate-hexane) gave 2.38 g of crude desired alcohol, which was rechromatographed (20% ethyl acetate-hexane) affording 1.29 g of the title compound as a viscous oil. G. [1R-(1α,2α,3β,4α)]-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester To a solution of 1.22 g (3.69 mmol) of [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol in 3.6 ml of dry pyridine was added 3.52 g (18.4 mmol) of p-toluenesulfonyl chloride. The mixture was heated at 60°C for 7 hours. The mixture was cooled to room temperature, diluted with diethyl ether (250 ml) and washed with water (4 x 100 ml), saturated aqueous sodium bicarbonate (100 ml) and brine (100 ml). The organic layer was dried over Na₂SO₄, and rotary evaporation of the solvent gave 5.4 g of crude material. Flash chromatography (10 to 20% ethyl acetate-hexane) afforded 0.842 g of the title compound as a white solid. H. [1S-(1α,2β,3α,4β)]-9-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine To a solution of 832 mg (1.72 mmol) of [1R-(1α,2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester in 9 ml of dry dimethylformamide were added 2.1 g (8.6 mmol) of 2-amino-6-(phenylmethoxy)-9H-purine, 1.78 g (12.9 mmol) of K₂CO₃ (powdered and dried under vacuum at 75°C for 18 hours), and 2.3 g (8.6 mmol) of 18-crown-6. The mixture was heated at 110°C for 18 hours. The mixture was cooled to room temperature and filtered. The solvent was removed by rotary evaporation, and the residue was purified by flash chromatography (20% to 50% ethyl acetate-hexane) to afford 268 mg of the title compound. I. [1S-(1α,2β,3α,4β)]-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one To a solution of 265 mg (0.479 mmol) of [1S-(1α,2β,3α,4β)]-9-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine in 31 ml of 95% ethyl alcohol were added 15 ml of cyclohexene and 133 mg of Pd(OH)₂/C (20%). The mixture was heated at reflux for 22 hours. The hot mixture was filtered through celite, which was rinsed with 50 ml of hot ethyl alcohol. Rotary evaporation of the solvent gave 118 mg of a white solid. The solid was dissolved in 8 ml of 10% acetonitrile-water and chromatographed (CHP-20P resin; 0% to 40% acetonitrile-water). The solvent was removed by rotary evaporation and the product was freeze-dried to provide 86 mg of [1S-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one as a white solid, mp 234°C (dec). [α]²² D -8.0 (c = 1, H₂O); ¹H NMR (270 MHz; DMSO-d₆) δ: 2.15 - 2.35 (m, 2H), 3.45 - 3.55 (m, 2H), 3.55 - 3.65 (m, 2H), 4.53 - 4.67 (m, 1H), 4.78 (t, 1H, J = 5.3 Hz), 4.84 (t, 1H, J = 5.3 Hz), 5.08 (ddd, 1H, J = 55.7 Hz, J = 7.0 Hz, J = 7.0 Hz), 6.44 (bs, 1H), 7.81 (s, 1H), 10.57 (bs, 1H). Example 6 [1S-(1α,2β,3α,4β)]-4-Amino-1-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-2(1H)-pyrimidinone A. Chlorofluoroacetaldehyde diethyl acetal To a stirred solution of 100 g. (709 mmol) of ethyl chlorofluoroacetate in 550 mL dry toluene at -75° to -78°C under argon was added dropwise 780 mL of a solution of diisobutylaluminum hydride (1M in toluene, 780 mmol). The reaction mixture was stirred at that temperature for 30 minutes, then transferred via cannula to a stirred solution of 230 g. (6.3 moles) anhydrous HCl in absolute ethanol (1000 mL) at -78°C. The mixture was allowed to come to room temperature and was fractionally distilled using a 2 foot long Vigreux column until ca. 1800 mL distillate had been collected. The remaining material was flash distilled (slowly increasing the vacuum from 150 mm to 0.1 mm until no more liquid remained in the pot) and collected separately. The flash distillate was combined with 25 mL ethanol and subjected to a second fractional distillation under atmospheric pressure, affording 55 g. of the title compound as a colorless liquid, boiling point 152 - 154°C. Partial ¹H NMR (270 MHz, CDCl₃)δ5.94 (dd, J = 5.3, 50.4 Hz, H-CF). B. 2-Fluoro-1,1-diethoxyethene To a stirred suspension of 50.0 g.(445.6 mmol) potassium t-butoxide in 250 mL tetrahydrofuran at 0°C under argon was added dropwise a solution of 55.0g. (321.6 mmol) chlorofluoroacetaldehyde diethyl acetal in 50 mL dry tetrahydrofuran. The reaction mixture was stirred at 35°C for 12 hours, refluxed for one hour, and then fractionally distilled using a 1 foot long Vigreux column until 250 mL of solvent had been collected. The remaining material was flash distilled (slowly increasing the vacuum from 150 mm to 0.1 mm until no more liquid remained in the pot) and collected separately. The flash distillate was subjected to a second fractional distillation under atmospheric pressure, affording 29.5 g. of the title compound as a colorless liquid, boiling point 123 - 126°C. Partial ¹H NMR (270 MHz, CDCl₃)δ6.35 (d, J=77.4 Hz, 1H, H-CF). C. [1S-(1α,2β,4α)]-4-Fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester To a stirred solution of 29.03 g. (73.95 mmol) of (-)-dimenthyl fumarate (Aldrich Chemical Co.) in 400 mL dry toluene at -78°C under argon was added dropwise 175 mL (175 mmol) of a solution of diethylaluminum chloride (1M in hexane). The resulting dark orange reaction mixture was stirred at that temperature for 15 minutes followed by the addition of the freshly prepared 2-fluoro-1,1-diethoxyethene dropwise over 5 minutes at -78°C. The reaction mixture was stirred at that temperature for 30 minutes, quenched by sequential dropwise addition of methanol (10 mL), 20% NaOH (10 mL), hexane (100 mL) and magnesium sulfate (20 g). The mixture was slowly allowed to come to room temperature, filtered and the filtrate concentrated in vacuo, affording a thick colorless oil. The crude mixture was recrystallized twice, first from pentane, followed by hexane, affording 13.9 g. of title compound along with its epimer [1S-(1α,2β,4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester (ratio ca. 5:1 by NMR). Partial ¹H NMR of the title compound (270 MHz, CDCl₃)δ5.06 (dd, J = 8.0, 54.2 Hz; 1H, H-CF), 4,72 (dt, J = 4.7, 11.1 Hz, 2H), 3.78 - 3.37 (m, 4H). D. [1S-(1α,2β,4α)]-4-Fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol A solution of 13.8 g. (26.2 mmol) of the mixture of [1S-(1α,2β,4α)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester and its epimer [1S-(1α,2β,4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester (ratio about 5:1) in 50 mL dry tetrahydrofuran was added dropwise under argon to a stirred suspension of 1.49 g (39.3 mmol) lithium aluminum hydride in 100 mL tetrahydrofuran at 0°C. The mixture was stirred at room temperature for 2 hours, quenched at 0°C by sequentially adding 1.5 mL water, 4.5 mL 1N NaOH and 1.5 mL water. The solids were removed by filtration and the filtrate was concentrated in vacuo . The crude product was subjected to flash chromatography on silica gel (hexane-ethyl acetate) giving 5.1 g. of a colorless oil containing the title compound and its epimer [1S-(1α,2β,4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol (ratio about 5:1 by NMR). Partial ¹H NMR of the title compound (270 MHz, CDCl₃)δ5.02 (dd, J = 6.6, 54.0 Hz, 1H, H-CF). E. [2R-(2α,3α,4β)-2-Fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane To a stirred suspension of 2.252 g. (56.3 mmol, 60% in oil) of ether-washed NaH in 10 mL dimethylformamide under argon at 10-15°C was added sequentially 5.62 mL (47.3 mmol) benzyl bromide and a dimethylformamide solution (20 mL) containing 5.0 g (22.5 mmol) of the mixture of [1S-(1α,2β,4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol and [1S-(1α,2β,4α)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol (ratio about 1:5). The reaction mixture was stirred at room temperature for 12 hours, diluted with ether and washed with water. The organic phase was dried over magnesium sulfate and concentrated in vacuo . The crude oil was subjected to flash chromatography on silica gel (hexane to hexane-ether 5:1) affording 8.5 g of the title compound and its epimer [2S-(2α,3β,4α)-2-fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane (ratio about 5:1). Partial ¹H NMR of the title compound (270 MHz, CDCl₃)δ4.95 (dd, J = 5.9, 53.9 Hz, 1H, H-CF). F. [2R-(2α,3α,4β)]-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone A solution of 8.4 g. (20.9 mmol) of the mixture of [2R-(2α,3α,4β)-2-fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane and its epimer [2S-(2α,3β, 4α)-2-fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane (ratio about 5:1) in 150 mL acetonitrile and 75 mL 0.5 M sulfuric acid was refluxed for 3 hours. The solution was allowed to come to room temperature, diluted with ethyl acetate and washed with saturated sodium bicarbonate. The organic layer was dried (MgSO₄) and concentrated in vacuo , giving 6.8 g. of the title compound and its epimer, [2S-(2α,3β,4α)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone (ratio by NMR = about 5:1). Partial ¹H NMR of the title compound (270 MHz, CDCl₃)δ 5.58 (ddd, J = 3.51, 9.38, 53.35 Hz, 1H, H-CF). G. [2S-(2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone To a stirred solution of 6.8 g. (20.73 mmol) of the mixture of [2R-(2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and its epimer [2S-(2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone (ratio about 5:1) in 300 mL dry acetonitrile was added 0.31 mL (2.07 mmol) 1,8-diazabicyclo[5.4.0]undec-7-ene over 10 minutes at 0°C. The mixture was allowed to come to room temperature and was stirred for 3 hours (monitored by ¹H NMR). The mixture was diluted with 500 mL ethyl acetate, washed with a 1:1 mixture of 20% sulfuric acid and saturated sodium sulfate solutions (3 x 500 mL). The organic phase was concentrated approximately to 100 mL, diluted with 250 mL methylene chloride, dried (MgSO₄) and concentrated in vacuo to afford 6.8 g. of the title compound and its epimer [2R-(2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone (ratio about ≧ 5:1). Partial ¹H NMR of the title compound (270 MHz, CDCl₃) δ 5.43 (ddd, J = 3.5, 9.4, 53.6 Hz, 1H, H-CF). H. [1R-(1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol A solution of 22.8 mℓ(22.8 mmol) of lithium trisiamylborohydride (1M in tetrahydrofuran) was added dropwise under argon at -78°C. to a stirred solution of 6.8 g. (20.7 mmol, crude from the previous step) of the mixture of [2S-(2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and [2R-(2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone (ratio about 5:1) in 200 mL dry tetrahydrofuran. The reaction mixture was stirred at -78°C for 15 minutes, 0°C for 15 minutes and quenched with 14 mL saturated sodium bicarbonate solution and 7 mL 30% hydrogen peroxide. The mixture was stirred at 0°C for 30 minutes, diluted with ethyl acetate and washed sequentially with saturated sodium bicarbonate and brine. The organic phase was dried (MgSO₄) and concentrated in vacuo , affording a gummy residue which was subjected to flash chromatography on silica gel (hexane to 10 - 20% ethyl acetate in hexane) to give 4.81 g. of the title compound as a colorless oil. Partial ¹H NMR (270 MHz, CDCl₃)δ 4.78 (td, J = 6.5, 54.0 Hz). I. [1R-(1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester A mixture of 13.87 g. (72.73 mmol) tosyl chloride and 4.8 g. (14.55 mmol) of [1R-(1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol in 10 mL dry pyridine was heated at 70°C for 20 hours. TLC (silica gel; hexane-ethyl acetate 3:1) indicated incomplete reaction. Additional tosyl chloride (6.94 g., 36.4 mmol) was added to the reaction mixture which was heated at 100°C for 5 hours. The mixture was cooled to room temperature, diluted with ethyl acetate, washed with water and then brine. The organic phase was dried (MgSO₄), concentrated in vacuo and the residue flash chromatographed on silica gel (hexane to 5 - 20% ethyl acetate in hexane) affording 2.3 g. of the title compound and 2.4 g. of unreacted [1R-(1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol (eluted after the tosylate). Partial ¹H NMR (270 MHz, CDCl₃) δ 7.85 (d, J = 8.2 Hz, 2H) 7.38 - 7.19 (s, 12 H), 5.08 (m, 1H, HC-OTs), 4.80 (td, J = 6.5, 52.2 Hz), 4.48 (s, 2H, benzyl), 4.42 (AB quartet, J = 11.7 Hz, 2H, benzyl). J. [1S-(1α,2β,3α,4β)]-1-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2,4(1H,3H)-pyrimidinedione To a suspension of 4.63 g. (41.3 mmol) of uracil in 10 mL dimethylformamide was added 21.4 mL (36.2 mmol) of a 1.5 M aqueous solution of tetrabutylammonium hydroxide. The mixture was stirred at room temperature for 10 minutes, concentrated in vacuo , and the residue redissolved and concentrated 4 times from 35 mL dry dimethylformamide. The resulting gummy uracil tetrabutylammonium salt was dried overnight at 65°C. under vacuum (0.1 mm.). To a solution of this salt in 30 mL dry dimethylformamide was added 2.3 g. (5.2 mmol) [1R-(1α,2α, 3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester and the mixture was stirred at 100°C for 12 hours. The reaction mixture was cooled to room temperature followed by the addition of acetic acid (2.48 g., 41.3 mmol) and concentration in vacuo . The residue was dissolved in 150 mL water and 150 mL ethyl acetate and the resulting solution was stirred for 2 hours with 150 g. AGMP-50 resin (Na⁺ form). The mixture was filtered and the aqueous layer of the filtrate extracted with 3 x 100 mL ethyl acetate. The combined organic phase was dried (MgSO₄), concentrated in vacuo and the residue subjected to flash chromatography on silica gel (hexane to 1:1 hexane - ethyl acetate) affording 1.35 g. of the title compound as a pale gummy solid. Partial ¹H NMR (270 MHz, CDCl₃) δ 7.19 (d, J = 8.2, 1H), 5.13 (td, J = 6.5, 54.5 Hz, 1H, H-CF). K. [1S-(1α,2β,3α,4β)]-4-Amino-1-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2(1H)pyrimidinone To a stirred solution of 230 mg. (0.54 mmol) of [1S-(1α,2β,3α,4β)]-1-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2,4(1H,3H)-pyrimidinedione in dry pyridine (1.5 mL) under argon was added 0.239 mL (1.47 mmol) p-chlorophenyl dichlorophosphate. The reaction mixture was stirred at room temperature for 10 minutes and 207 mg. (2.99 mmol) 1,2,4-triazole was added, stirred at room temperature for 4 days and concentrated in vacuo . The residue was dissolved in methylene chloride, washed with water and saturated sodium bicarbonate. The organic phase was dried over magnesium sulfate and concentrated in vacuo . To the resulting triazole derivative dissolved in 5 mL dioxane was added 5 mL 29% ammonium hydroxide and the mixture was stirred at room temperature for 24 hours. The mixture was concentrated in vacuo , redissolved in methylene chloride and washed with 5% NaOH followed by water. The organic layer was dried over magnesium sulfate, concentrated in vacuo and the residue subjected to flash chromatography on silica gel (2-10% ethanol in ethyl acetate) affording 0.14 g. of the title compound as a gummy solid. Partial ¹H NMR (270 MHz, CDCl₃) δ 7.30 (m, 12 H), 7.22 (d, J = 7.62 Hz, 1H), 5.65 (d, J = 7.62 Hz, 1H), 5.18 (td, J = 6.55, 55.7 Hz, 1H, H-CF). L. [1S-(1α,2β,3α,4β)]-4-Amino-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2(1H)-pyrinidinone A mixture of 0.14 g. (0.332 mmol) [1S-(1α,2β, 3α,4β)]-4-amino-1-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2(1H)-pyrimidinone, 20% palladium hydroxide on carbon (200 mg), and cyclohexene (7.5 mL) in 15 mL 95% ethanol was heated under reflux for 4 hours. The mixture was allowed to come to room temperature and filtered through celite. The filtrate was concentrated in vacuo and the residue was subjected to CHP-20 chromatography (water to 1% acetonitrile in water) affording 60 mg. of the title compound as a white solid, m.p. 145 - 150°C, [α] D = -6.5° (water, C = 2). ¹H NMR (270 MHz, DMSO-d₆) δ 8.70 (br s, 1H), 8.01 (br s, 1H), 7.89 (d, J = 7.0 Hz, 1H), 5.97 (d, J = 7.0 Hz, 1H), 4.98 (td, J = 7.0, 55.1 Hz, 1H, H-CF), 4.68 - 4.53 (m, 1H in a deuterium exchanged spectrum in presence of D₂O, 3H in the original unchanged spectrum), 3.66 - 3.48 (m, 4H), 2.25 - 2.07 (m., 2H) Example 7 [1S-(1α(E),2β,3α,4β)-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-bromoethenyl)-2,4(1H,3H)pyrimidinedione A. [1S-(1α,2β,3α,4β)]-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2,4(1H,3H)-pyrimidinedione To a stirred solution of 1.35 g. (3.2 mmol) of [1S-(1α,2β,3α,4β)]-1-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-2,4(1H,3H)-pyrimidinedione in 95% ethanol (100 mL) was added sequentially cyclohexene (50 mL) and 20% palladium hydroxide on carbon (1 g). The mixture was heated under reflux for 4 hours, allowed to come to room temperature, filtered through celite and concentrated in vacuo . The residue was subjected to CHP-20 chromatography (water to 1% acetonitrile-water), affording 0.62 g. of the title compound as a colorless gummy solid. Partial ¹H NMR (270 MHz, DMSO-d₆) δ 11.31 (br s, 1H, NH), 7.68 (d, J = 8.2 Hz, 1H), 5.62 (d, J = 8.2 Hz, 1H), 4.95 (td, J = 6.4, 55.7 Hz, 1H, H-CF). B. [1S-(1α,2β,3α,4β)]-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)pyrimidinedione A mixture of 0.62 g. (2.54 mmol) [1S-(1α,2β, 3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2,4(1H,3H)pyrimidinedione, 0.968 g. (3.81 mmol) iodine, and 3 mL 0.8 N nitric acid in 30 mL dioxane was heated under reflux for 4 hours. The mixture was allowed to come to room temperature and was adjusted to pH 7 using saturated sodium bicarbonate solution. The mixture was concentrated and the residue subjected to CHP-20 chromatography (water to 0 - 20% acetonitrile-water) affording 0.53 g. of the title compound as a white solid. Partial ¹H NMR (270 MHz, DMSO-d₆) δ 11.68 (br s, 1H, NH), 8.15 (s, 1H, C6-H), 5.11 (td, J = 6.5, 55.1 Hz, 1H, H-CF). C. [1S-[1α(E),2β,3α,4β]]-3-[1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,2,3,4-tetrahydro-2,4-dioxo-5-pyrimidinyl]-2-propenoic acid, methyl ester To 2.5 mL of deoxygenated, anhydrous dioxane under argon was added sequentially 33 mg (0.135 mmol) palladium acetate, 71 mg (0.27 mmol) triphenylphosphine, and 0.565 mL (4.05 mmol) triethylamine. The mixture was stirred at 70° C for 15 minutes. A solution of 0.5 g. (1.35 mmol) [1S-(1α,2β,3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)pyrimidinedione in 2.5 mL of deoxygenated dioxane was added to the dark brown reaction mixture, followed by the addition of 0.487 mL (5.4 mmol) methyl acrylate. The reaction mixture was stirred at 70°C. for 4 hours, allowed to come to room temperature, filtered through celite and concentrated in vacuo . The residue was triturated from methylene chloride affording 400 mg. of the title compound as a light brownish solid. Partial ¹H NMR (270 MHz, DMSO-d₆) δ 11.65 (br s, 1H,NH), 8.34 (s, 1H, C6-H), 7.43 (d, J = 16 Hz, 1H), 6.90 (d, J = 16 Hz,1H). D. [1S-[1α(E),2β,3α,4β]]-3-[1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,2,3,4-tetrahydro-2,4-dioxo-5-pyrimidinyl]-2-propenoic acid A solution of 390 mg. (1.19 mmol) [1S-[1α(E), 2β,3α,4β]]-3-[1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,2,3,4-tetrahydro-2,4-dioxo-5-pyrimidinyl]-2- propenoic acid methyl ester in 8 mL 2N KOH was stirred at room temperature for 1 hour. The reaction mixture was concentrated in vacuo and the residue dissolved in ice cold 10% HCl (2 mL). The pH was adjusted to 1.7 with additional 10% HCl. The mixture was kept in the refrigerator for 3 hours after which the solids were filtered, washed with cold water and dried to afford 232 mg. of crude product. The combined filtrate was concentrated to one half volume and kept overnight in a refrigerator. Filtration afforded an additional 35 mg. product. This was combined with the previous batch and recrystallized from water, affording 233 mg. of the title compound. Partial ¹H NMR (270 MHz, DMSO-d₆) δ 12.13 (br s, 1H, carboxylic acid), 11.63 (br s, 1H,NH), 8.27 (s, 1H, H-C6), 7.36 (d, J = 15.8 Hz, 1H), 6.82 (d, J = 15.8 Hz, 1H), 4.93 (td, J = 5.9, 55.7 Hz, 1H, H-CF). E. [1S-(1α(E),2β,3α,4β)]-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-bromoethenyl)-2,4(1H,3H)-pyrimidinedione To a stirred suspension of 90 mg. (0.287 mmol) of [1S-[1α(E),2β,3α,4β]]-3-[1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,2,3,4-tetrahydro-2,4-dioxo-5-pyrimidinyl]-2-propenoic acid and 86.1 mg. (0.86 mmol) potassium bicarbonate in 1 mL dry dimethylformamide was added a solution of 51 mg. (0.287 mmol) N-bromosuccinimide in dry dimethylformamide (0.51 mL) over 15 minutes. The reaction mixture was stirred under argon for 1 hour at room temperature, filtered and concentrated in vacuo . The residue was subjected to CHP-20 chromatography (25 mL slurry) using a 0 to 33% acetonitrile-water continuous gradient (600 mL) to afford a white residue. Recrystallization of this material from water gave 60 mg of the title compound, m.p. 185 - 187°C (dec.), [α] D = -11.6° (methanol, C = 0.32). ¹H NMR (270 MHz, DMSO-d₆) δ 11.57 (br s, 1H, NH), 7.94 (s, 1H, H-C₆), 7.28 (d, J = 13.2 Hz, 1H, bromovinyl), 7.16 (d, J = 13.2 Hz, 1H, bromovinyl), 4.92 (td, J = 6.6, 55.4 Hz, 1H, H-CF), 4.80 - 4.62 (m, 3H), 3.55 (m, 2H), 3.48 (m,2H), 2.22 - 2.02 (m, 2H). Example 8 [1S-(1α(E),2β,3α,4β)]-1-[2-Fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-iodoethenyl)-2,4(1H,3H)pyrimidinedione A mixture of 70 mg. (0.223 mmol) [1S-[1α(E), 2β,3α,4β]]-3-[1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,2,3,4-tetrahydro-2,4-dioxo-5-pyrimidinyl]-2-propenoic acid and 48.3 mg. (0.446 mmol) potassium acetate in 0.5 mL dimethylformamide was stirred under argon at room temperature for 15 minutes followed by the addition of a solution of 50.2 mg. (0.223 mmol) N-iodosuccinimide in 0.5 mL dimethylformamide over 15 minutes. The reaction mixture was heated at 50°C for 2.5 hours, filtered and the filtrate concentrated in vacuo . The crude product was subjected to CHP-20 chromatography (using a 0 to 30% acetonitrile in water gradient) affording 60 mg. of the title compound as a white solid m.p. 93 - 96°C, [α] D = -6.5° (methanol, C = 0.333). ¹H NMR (270 MHz, DMSO-d₆) δ 11.55 (br s, 1H, NH), 7.93 (s, 1H, H-C6), 7.24 (d, J = 14.5 Hz, 1H, iodovinyl), 7.16 (d, J = 14.5 Hz, 1H, iodovinyl), 4.92 (td, J = 6.6, 55.4 Hz, 1H, H-CF), 4.80 - 4.60 (m, 3H), 3.55 (m, 2H), 3.50 (m, 2H), 2.18 -2.02 (m, 2H). Example 9 [1R-(1α,2β,3α,4β)]-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one A. [1R-(1α,2β,4α)]-4-Fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(+)-menthyl ester To a solution of 22.7 g. (58 mmol) (+)-dimenthylfumerate in 115 ml. of dry degassed toluene at -78°C under an argon atmosphere was added 114 ml (1.0 M, 1.14 mmol) of a solution of diethylaluminum chloride in hexanes dropwise over 30 minutes keeping the reaction temperature between -74° and -78°C. After the addition was complete the reaction was stirred at -78°C. for an additional 30 minutes. 2-Fluoro-1,1-diethoxyethene (8.6 g., 64 mmol) was added over a 5 minute period and then the reaction was stirred for 1.5 hours at -78°C. A saturated solution of sodium bicarbonate (150 ml) was added and the resulting mixture was allowed to warm to room temperature. The aqueous mixture was extracted with 250 ml. of hexanes, washed with water and dried over sodium sulfate. The solvent was evaporated in vacuo yielding 34.6 g. of crude material. The crude material was crystallized from pentane yielding 10.2 g. of the title compound as a colorless solid. B. [1R-(1α,2β,4α)]-4-Fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol A solution of 8.6 g. (16.3 mmol) [1R-(1α,2β, 4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(+)-menthyl ester in 40 ml. of dry tetrahydrofuran was added dropwise to a suspension of 0.928 g. (24.6 mmol) lithium aluminum hydride in 150 ml. of dry tetrahydrofuran under an argon atmosphere at room temperature. After the addition was complete the reaction was heated at 50°C. for 1.5 hours. Water (1 ml.) was carefully added and the reaction mixture stirred for 15 minutes. Additions of 1.0 ml. of a 15% sodium hydroxide solution and 3.4 ml. of water were made and the reaction stirred for 30 minutes at room temperature. Magnesium sulfate (24.5 g.) was added and the mixture stirred for 15 minutes. The solids were removed by filtration and the filter cake was washed with ethyl acetate. The organic solution was concentrated in vacuo to afford the crude product which was column chromatographed on silica gel (250 ml.) eluting with 25% ethyl acetate/hexanes (1000 ml.) followed by 60% ethyl acetate/hexanes (2000 ml.). The appropriate fractions were combined and the solvents removed in vacuo yielding 3.25 g. of the title compound as a colorless oil. C. [2S-(2α,3α,4β)]-2-Fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane A solution of 3.22 g. (14.5 mmol) of [1R-(1α,2β,4α)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedimethanol in 3.5 ml. of dry dimethylformamide was added over a 5 minute period to a stirred mixture of sodium hydride (60% in oil, 1.16 g., 29 mmol) and 3.8 ml. (31.9 mmol) benzyl bromide in 5 ml. of dry dimethylformamide at 0°C under an argon atmosphere. The reaction mixture was allowed to slowly warm to ambient temperature and was stirred overnight. Additional amounts of benzyl bromide (0.380 ml., 55 mg.) and 60% sodium hydride (120 mg.) were added. The reaction mixture was then allowed to stir for 5 hours. The mixture was diluted with ether and washed with saturated sodium bicarbonate (3 x 40 ml.). The aqueous portions were back extracted with ether (2 x 50 ml.) and the combined ether extracts were dried over anhydrous magnesium sulfate. The solvent was removed in vacuo yielding the crude product as an oily residue which was column chromatographed on silica gel (800 ml.) eluting with hexanes (1500 ml.) followed by 5% ethyl acetate/hexanes (3000 ml.). The appropriate fractions were combined and the solvents removed in vacuo yielding 4.3 g. of the title compound. D. [2S-(2α,3α,4β)]-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone A solution of 4.1 g. (10.2 mmol) of [2S-(2α,3α,4β)]-2-fluoro-1,1-diethoxy-3,4-bis[(phenylmethoxy)methyl]cyclobutane in a mixture of 60 ml. of acetonitrile and 30 ml. of 0.5 M aqueous sulfuric acid was heated at 90°C. for 3 hours. The mixture was cooled to room temperature and diluted with 800 ml. of ethyl acetate. The mixture was washed with 600 ml. of a saturated solution of sodium bicarbonate and 600 ml. of brine. The aqueous mixture was back extracted with ethyl acetate and the combined ethyl acetate extracts were dried over anhydrous magnesium sulfate. The ethyl acetate was removed in vacuo yielding 3.2 g. of the title compound as a colorless oily residue. E. [2R-(2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone A solution of 3.2 g. (9.7 mmol) of [2S-(2α,3α,4β)]-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone in 775 ml. of acetonitrile was added over a 10 minute period to a solution of 0.221 ml. (9.7 mmol) 1,8-diazabicyclo[5.4.0]undec-7-ene in acetonitrile at 0°C. under an argon atmosphere. The reaction mixture was stirred at 0° C. under an argon atmosphere for 15 minutes and was then allowed to warm to ambient temperature and stirred for 4 hours. The reaction mixture was diluted with 3500 ml. of ethyl acetate and washed with a (1:1) mixture of 20% sulfuric acid/saturated sodium sulfate solution (3 x 3500 ml). The ethyl acetate mixture was dried over anhydrous magnesium sulfate and the solvent removed in vacuo yielding 3.05 g. of the title compound (ratio about ≧ 5:1) as a colorless oily residue. F. [1S-(1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol To a solution of 3.0 g. (9.1 mmol) of the mixture of [2R-(2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone and its epimer [2S-(2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone (ratio about ≧ 5:1) in 275 ml. of dry tetrahydrofuran at -78°C. under an argon atmosphere was added a solution of lithium trisiamylborohydride in hexanes (1.0 M, 14 ml.). The reaction mixture was stirred at -78°C for 15 minutes and then allowed to warm to room temperature. A saturated aqueous sodium bicarbonate solution (275 ml.) was added followed by 3 ml. of a 30% hydrogen peroxide solution. The reaction mixture was stirred for 15 minutes and then extracted with 400 ml. of ethyl acetate. The ethyl acetate extract was washed with a solution of brine and dried over anhydrous magnesium sulfate. The solvent was removed in vacuo yielding 2.5 g. of crude product as an oily residue which was column chromatographed on silica gel (250 ml.) eluting with 10% ethyl acetate/hexanes (500 ml.), 15% ethyl acetate/hexanes and 20% ethyl acetate/hexanes (1000 ml.). The appropriate fractions were combined and the solvents removed in vacuo to afford 840 mg. of the title compound as a colorless oil. G. [1S-(1α,2α,3β,4α)-2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester A solution of 800 mg. (2.4 mmol) of [1S-(1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol and 2.29 g. (12 mmol.) p-toluenesulfonyl chloride in 2.3 ml. of dry pyridine was heated at 60°C under an argon atmosphere and then stirred overnight at ambient temperature. Additional p-toluenesulfonyl chloride (458 mg., 2.4 mmol.) was added and the reaction was reheated at 65°C. for an additional 1 hour. The reaction was cooled to room temperature and diluted with 80 ml. of ether. The mixture was washed with water (3 x 50 ml.), a saturated solution of sodium bicarbonate (75 ml.) and brine (75 ml.). The mixture was dried over anhydrous sodium sulfate and the ether removed in vacuo yielding the crude product as a colorless oily residue. The crude product was column chromatographed on silica gel (500 ml.) eluting with 10% ethyl acetate/hexanes (3000 ml.) followed by eluting with 20% ethyl acetate/hexanes (4000 ml.). The appropriate fractions were combined and the solvents removed in vacuo yielding 1.1 g. of the title compound as a colorless solid. H. [1R-(1α,2β,3α,4β)]-9-[2-Fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine A mixture of 1.1 g. (2.77 mmol) of [1S-(1α,2α,3β,4α)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanol, 4-methylbenzenesulfonate ester, 2.74 g. (11.35 mmol.) O-benzylguanine and 3.0 g. (11.35 mmol) 18-crown-6 in 9 ml. of dry dimethylformamide was heated at 110°C. under an argon atmosphere for 20 hours. The reaction mixture was cooled to room temperature and was filtered and the solids washed with methanol. The solvents were evaporated in vacuo yielding a brown oily residue which was column chromatographed on silica gel (400 ml.) eluting with hexanes (1200 ml.), 20% ethyl acetate/hexanes (1200 ml.) and 40% ethyl acetate/hexanes (2000 ml.). The appropriate fractions were combined and the solvents removed in vacuo yielding 248 mg. of the title compound as a colorless solid. J. [1R-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one A mixture of 245 mg. (0.443 mmol) of [1R-(1α, 2β,3α,4β)]-9-[2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutyl]-6-(phenylmethoxy)-9H-purin-2-amine, 135 mg. of 20% palladium hydroxide, and 15 ml. cyclohexene were dissolved in 30 ml. of 95% ethanol and the mixture heated at reflux for 18 hours. The reaction mixture was cooled to room temperature, filtered through celite and the filter cake washed with additional ethanol. The solvents were removed in vacuo yielding the crude product as a colorless oil. The crude material was column chromatographed on CHP-20 resin eluting with water (100 ml.) followed by eluting with a 40% acetonitrile-water/water gradient (600 ml.). The appropriate fractions were combined and the solution concentrated in vacuo to remove acetonitrile. The resulting aqueous solution was lyophilized yielding 94 mg. (75%) of the title compound as a colorless solid, m.p. 235°C. (dec.), [α] 22 / D = +8.4 (c = 1, water). Example 10 Treatment of Viral Infection in Cell Culture in Vitro Assays were performed in cell culture systems to determine the concentrations of compounds that are effective in preventing several kinds of viral infections. The assays are described below, and-the results are presented in Table 1 . Abbreviations: HSV-1 (herpes simplex virus type 1, strain Schooler), HSV-2 (herpes simplex virus type 2, strain 186), VZV (varicella zoster virus, strain ELLEN), HCMV (human cytomegalovirus, strain AD 169), MuLV (murine leukemia virus, strain CAS). Cell Culture Assays: HSV-1, HSV-2, HCMV, and VZV antiviral assays: Virus was adsorbed to WI-38 cell culture monolayers in 6 well culture plates (Costar, Cambridge, MA) for 1 hour prior to addition of maintenance medium containing duplicate dilutions of the test compound. Inhibition of plaque development was evaluated on fixed and stained monolayers after 4 days incubation at 37°C for HSV-1 and HSV-2 and after 6-7 days incubation at 37°C for HCMV and VZV. ID₅₀ values were determined from the drug concentration which conferred at least a 50% plaque reduction compared to virus controls. MuLV antiviral assay: Antiviral assays using MuLV were performed with some modification, as described by Rowe et al. and Shannon et al. . SC-1 cells were planted at approximately 2 x 10⁵ cell per well in 6 well plates. After overnight incubation at 37°C, the cell cultures were sensitised with DEAS-Dextran for one hour at 37°C, rinsed and inoculated with MuLV. Cultures were re-fed with growth medium containing different concentrations of the test compound. After three more days at 37°C, cultures were re-fed with fresh medium plus test compounds and incubated at 37°C for an additional 3 days. Cultures were then washed to remove medium, ultraviolet light irradiated, and planted with approximately 5 x 10⁵ XC cells per well in cell growth medium containing the appropriate concentration of the test compound. The cultures were then incubated for an additional 4 days, with a re-feed using growth medium containing test compound at the second day following XC cell overlay. Finally the cultures were rinsed, stained and syncytial plaques were counted. References: Rowe, W.P., Pugh, W.E., and Hartley, J.W., (1970), Plaque Assay Techniques for Murine Leukemia Viruses, Virology , 42 : 1136-1139. Shannon, W.M., Brockman, R.W., Westbrook L., Shaddix, S., and Shabel, F.M., (1974), Inhibition of Gross Leukemia Virus-Induced Plaque Formation in XC Cells by 3-Deazauridine, J. Natl. Cancer Inst. , 52 :199-205. Compound ID₅₀(µM) for the following viruses HSV-1 HSV-2 VZV HCMV MuLV I 0.7 - 1.8 0.7 1.8 - 3.5 3.5 - 35 3.5 - 7 II 0.7 - 1.8 0.4 - 0.7 ≦0.7 18 - 35 ND* III 37 7.5 - 19 37 37 ND* IV >286 >286 28.6-286 >286 ND* V >252 >252 2.5-25.2 >252 ND* *ND = not determined Compound I is (1α,2β,3α,4β)-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. Compound II is [1S-(1α,2β,3α,4β)]-2-Amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. Compound III is (1α,2β,3α,4β)-3-(6-Amino-9H-purin-9-yl)-4-fluoro-1,2-cyclobutanedimethanol Compound IV is [1S-(1α(E),2β,3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-bromoethenyl)-2,4-(1H,3H)-pyrimidinedione. Compound V is [1S-(1α(E),2β,3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-iodoethenyl)-2,4-(1H,3H)-pyrimidinedione.

Claims: A compound having the formula and its pharmaceutically acceptable salts wherein R₁ is R₃ is hydrogen, methyl, fluoro, chloro, bromo, iodo, trifluoromethyl, ethyl, n-propyl, 2-fluoroethyl, 2-chloroethyl, or and R₄ is chloro, bromo, iodo, hydrogen, methyl, or trifluoromethyl. A compound in accordance with claim 1 wherein R₁ is A compound in accordance with claim 2 wherein R₁ is A compound in accordance with claim 2 wherein R₁ is A compound in accordance with Claim 1, (1α,2β,3α,4β)-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. A compound in accordance with claim 5, [1S-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. A compound in accordance with Claim 5, [1R-(1α,2β,3α,4β)]-2-amino-9-[2-fluoro-3,4-bis-(hydroxymethyl)cyclobutyl]-1,9-dihydro-6H-purin-6-one. A compound in accordance with Claim 1, (1α,2β,3α,4β)-3-(6-amino-9H-purin-9-yl)-4-fluoro-1,2-cyclobutanedimethanol. A compound in accordance with Claim 1, (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-methyl-2,4-(1H,3H)-pyrimidinedione. A compound in accordance with Claim 1, (1α,2β,3α,4β)-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-iodo-2,4(1H,3H)-pyrimidinedione. A compound in accordance with Claim 1 wherein R₁ is R₃ is and R₄ is Br or I. A compound in accordance with Claim 1, [1S-(1α(E), 2β,3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-iodoethenyl)-2,4(1H,3H)pyrimidinedione. A compound in accordance with Claim 1, [1S-(1α(E),2β,3α,4β)]-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-5-(2-bromoethenyl)-2,4(1H,3H)pyrimidinedione. A compound in accordance with Claim 1, [1S-(1α,2β,3α,4β)]-4-amino-1-[2-fluoro-3,4-bis(hydroxymethyl)cyclobutyl]-2(1H)pyrimidinone. A compound of the formula wherein P is a hydroxy protecting group selected from benzyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, (triphenylmethyl)dimethylsilyl, methyldiisopropylsilyl and triisopropylsilyl and the absolute stereochemistry at the position bearing the secondary hydroxy group is R. A compound of the formula wherein X is a leaving group selected from benzenesulfonyloxy, p-toluenesulfonyloxy, methanesulfonyloxy, p-nitrophenylsulfonyloxy, and trifluoromethylsulfonyloxy; and P is a hydroxy protecting group selected from benzyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, (triphenylmethyl)dimethylsilyl, methyldiisopropylsilyl, and triisopropylsilyl; and the absolute stereochemistry at the position bearing the group X is R. A process for the preparation of the optically active compound 10a wherein P is a hydroxy protecting group selected from benzyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, (triphenylmethyl)dimethylsilyl, methyldiisopropylsilyl, and triisopropylsilyl, which comprises (a) reacting a compound of the formula 30 with a compound of the formula 31 in the presence of a Lewis acid to yield a mixture of isomers of the formulas 32a and 32b wherein R₉ is and R₁₀ is lower alkyl of 1 to 5 carbons or both R₁₀ groups are joined together by an alkylene group of 2 or 3 carbons; (b) reacting the mixture of 32a and 32b with a reducing agent to form the corresponding mixture of compounds of the formula 33a and 33b (c) protecting the hydroxy groups of the mixture of compounds 33a and 33b with a group (P) to form the corresponding mixture of compounds of the formula 34a and 34b (d) hydrolyzing the mixture of compounds of formula 34a and 34b using an acid catalyst to form the mixture of compounds of formula 10a and 10b (e) treating the mixture of isomers 10a and 10b with a basic catalyst to effect epimerization of the fluorine so that 10a becomes the predominant or exclusive isomer of the mixture. The process according to Claim 17 wherein the Lewis acid is selected from the group consisting of diethylaluminum chloride, diisobutylaluminum chloride, ethylaluminum dichloride, isopropoxyaluminum dichloride, aluminum trichloride, boron trifluoride, boron trichloride, titanium tetrachloride, dichlorodiisopropoxytitanium, tin tetrachloride, and tin trichloride. The process according to Claim 17 wherein the Lewis acid is a di(lower alkyl)aluminum chloride wherein each lower alkyl is of 2 to 4 carbons, and R₁₀ is methyl or ethyl. The process according to Claim 17 wherein R₉ is R₁₀ is ethyl; and the Lewis acid is diethylaluminum chloride. A compound having the formula 32a or 32b wherein R₉ and R₁₀ are as defined in claim 17. A compound according to Claim 21 wherein R₉ is and R₁₀ is ethyl. A compound according to Claim 22 [1S-(1α,2β,4α)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester or [1S-(1α,2β,4β)]-4-fluoro-3,3-diethoxy-1,2-cyclobutanedicarboxylic acid, bis-(-)-menthyl ester. A compound having the formula 10a or 10b wherein P is as defined in claim 17 and wherein the absolute stereochemistry at the 2-position of 10a is S and the absolute stereochemistry at the 2-position is 10b is R. A compound according to Claim 24, [2R-(2α,3α,4β)-2-fluoro-3,4-bis[(phenylmethoxy)methyl]cyclobutanone [2S-(2α,3β,4α)-2-fluoro-3, 4-bis[(phenylmethoxy)methyl]cyclobutanone. A pharmaceutical composition comprising a compound of any of Claims 1 to 14. An antiviral agent comprising a compound of any of Claims 1 to 14. Use of a compound of any of Claims 1 to 14 for the preparation of an antiviral agent. A process for preparing compounds of formula 1 according to claim 1 and pharmaceutically acceptable salts comprising a) when R₁ is reacting an intermediate of formula 2 wherein X is a leaving group selected from benzenesulfonyloxy, p-toluenesulfonyloxy, methanesulfonyloxy, p-nitrophenylsulfonyloxy, and trifluoromethylsulfonyloxy; and P is a hydroxy protecting group selected from benzyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, (triphenylmethyl) dimethylsilyl, methyldiisopropylsilyl and triisopropylsilyl; with a benzyl protected guanine of formula 3 followed by removal of the P hydroxy protecting groups and the guanine benzyl protecting group; b) when R₁ and R₃ is hydrogen, methyl, fluoroethyl, n-propyl, 2-chloroethyl, or 2-fluoroethyl, reacting an intermediate of formula 2 with a compound of the formula R₁-H followed by removal of the P hydroxy protecting groups; c) when R₁ is and R₃ is chloro, bromo or iodo, reacting a compound of formula 1 wherein R₁ is with a source of chlorine, bromine, or iodine; and d) when R₁ is and R₄ is chloro, bromo or iodo, hydrogen, methyl or trifluoromethyl, reacting a compound of formula 1 wherein R₁ is with an organopalladium compound by methods known in the art.

Inventors: AHMAD SALEEM, GOODFELLOW VAL S, VITE GREGORY D, ZAHLER ROBERT, AHMAD, SALEEM, GOODFELLOW, VAL S., VITE, GREGORY D., ZAHLER, ROBERT
Applicants: SQUIBB & SONS INC, E.R. SQUIBB & SONS, INC.
Application year: 1991
Main classification: C07D473
IPC codes: A61K31, C07D473, C07D239, C07C43, C07D317, C07D307, C07C49, C07C45, C07C69, A61P31