NUCLEOSIDE DERIVATIVES AND PROCESS FOR THEIR PREPARATION

Abstract: Nucleoside derivs. of formula (I) and (II) are new, where B is a residue of a purine, pyrimidine or imidazole base (pref. adenine, guanine, uridine or cytosine); R1 and R3 are pivaloyloxy, tosyloxy, dimethoxytrityloxy or benzoyl; R2 is mesyloxy, tosyloxy or trifluoromethanesulphonyloxy; R4 is H or a protecting gp.; R31 is MsO, TsO, PhO-CS-O-, OH, H or (1-imidazolyl)thiocarbonyloxy; R32 is TsO, MsO, trifluoromethanesulphonyloxy, pivaloyloxy, benzoyl, OH, SH, alkylthio, arylthio, azido, isothiocyanato, halogen or H; at least one of R31 and R32 is H; R4O and R31 are not both OH. Pref. in (I), R2 = MsO and R1 = R3 = pivaloyloxy; or R1 is dimethoxytrityloxy, R2 is MsO and R3 is TsO. In (II), R31 = OH, R32 = H; or R31 = H, R32 = azido; or R31 = H, R32 = halogen. Cpds. (I) are made from the underivatised nucleosides by reaction with acid chlorides such as MsCl or DMTrCl; they are then converted to (II) by redn. under basic conditions, e.g. with borohydride in the presence of an alkali or alkaline earth alkoxide (pref. magnesium methoxide).

Description: FIELD OF THE INVENTION The present invention relates to novel intermediates useful for synthesizing various dideoxynucleoside derivatives, synthetic processes thereof, and novel dideoxynucleoside derivatives which may be synthesized from these intermediates and may be used as effective antiviral agent. PRIOR ART Dideoxynucleosides have now been paid attention as an effective antiviral agent for AIDS virus (HTLV III) which is a kind of retrovirus. Effectiveness of azidothymidine, dideoxycytidine, dideoxyadenosine, etc. has hitherto been confirmed, and researches on other dideoxynucleoside derivatives are also proceeded with (Inhibition of the in vitro infectivity and cytopathic effect of human T-lymphotrophic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) by 2',3'-dideoxynucleosides; by Hiroaki Mitsuya and Samuel Broder, Proc. Natl. Acad. Sci. USA, vol. 83, pp. 1911 - 1915, March 1986, Medical Sciences). 2',3'-Dideoxyadenosine, 2',3'-dideoxyinosine, 2',3'-dideoxyguanosine and 2',3'-dideoxycytidine are sold by BIOCHEMICAL CO. in USA. These compounds are prepared using a conventional method for preparing 2',3'-dideoxycytidine (Horwitz et al., J. Org. Chem., 32 (3), 817 - 818 (1967)) or a conventional method for preparing 2',3'-dideoxyguanosine (Prisbe et al., Synth. Commun. 1985, 15 (5), 401 - 409). European Patent Publication No. 206497A discloses the use of 2',3'-dideoxyinosine as an antiviral agent. Further, the aforementioned article of Hiroaki Mitsuya et al., and Japanese Patent Unexamined Published Application No. 61-280500 disclose the use of various 2',3'-dideoxynucleosides and their derivatives as an antiviral agent, particularly against AIDS virus. As for a synthetic method of nucleoside intermediates useful for synthesizing various dideoxynucleoside derivatives, there are prior researches by M. J. Robins, Nyilas, Chattopadhyaya, Reese, Sasaki and others. However, these synthetic methods are not fully satisfactory for industrial synthesis of dideoxynucleoside derivatives from the reason that steps required for the synthesis are long, complicated steps including an oxidation reaction follow, reagents not suitable for a large scale preparation are used, etc. Literatures with respect to these synthetic methods are enumerated below. 1) M. J. Robins et al., J. Org. Chem. 39 , 2564 (1974). 2) F. Hansske and M. J. Robins, Tetrahedron Lett., 26 , 4295 (1985). 3) F. Hansske, D. Madej, and M. J. Robins, Tetrahedron, 40 , 125 (1984). 4) F. Hansske and M. J. Robins, J. A. Chem. Soc., 105 , 6736 (1983). 5) A. Nyilas and J. Chattopadhyaya, Synthesis, 1986 , 196. 6) H. Bazin and J.Chattopadhyaya, Synthesis, 1985 , 1108. 7) D. G. Norman and C. B. Reese, Synthesis, 1983 , 304. 8) T. Sasaki, K. Minamoto and S. Tanizawa, J. Org. Chem. 38 , 2896 (1973). Among dideoxynucleoside derivatives, one of which is now practically used as a AIDS therapeutic agent is azidothymidine. However, azidothymidine brings about strong side effects and has only an inadequate AIDS virus growth-inhibiting effect. Therefore, researches on various dideoxynucleoside derivatives must successively be proceeded with. OBJECTS OF THE INVENTION An object of the present invention is to provide novel dideoxynucleoside derivatives which are effective as antiviral agents. Another object of the present invention is to provide novel nucleoside derivatives which are usable as intermediates useful for synthesis of various dideoxynucleoside derivatives. Further object of the present invention is to provide simple synthetic methods suitable for a large scale synthesis of the above intermediates. DESCRIPTION OF THE INVENTION The present invention relates to nucleoside derivatives represented by the following general formula I : wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protectivie group or an imidazolyl group optionally having a protective group; R¹ and R³ are a PivO- group, a TsO- group, a DMTrO- group or a benzoyl group; R² is a MsO- group, a TflO-group or a TsO- group. Further, the present invention relates to nucleoside derivatives represented by the following general formula II : wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or an imidazolyl group optionally having a protective group; R¹ is OR⁴ wherein R⁴ is a protective group or a hydrogen atom; R³ is an azido group, a halogen atom or a hydrogen atom; R 3' is a MsO- group, a TsO- group, a a or a hydroxyl group; provided that when B is a cytosine residue optionally having a protective group, at least one of R¹ and R 3' is other than a hydroxyl group and R³ is not an azido group; further, when B is an adenine residue optionally having a protective group or an uracil residue optionally having a protective group, at least one of R¹ and R 3' is other than a hydroxyl group. Further the present invention relates to a process for synthesizing a nucleoside derivative represented by the formula [II']: (wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or an imidazolyl group optionally having a protective group; R¹ is OR⁴ wherein R⁴ is a protective group or a hydroxyl group; R³ is a hydrogen atom; and R 3' is a hydroxyl group), which comprises treating a compound represented by the formula [I']: (wherein B is a purine base residue optinally having a protective group, a pyrimidine base residue optionally having a protective group or an imidazolyl group optionally having a protective group; R¹ and R³ are a PivO- group, a TsO- group, a DMTrO- group, a benzoyl group or a hydroxyl group; and R² is a MsO- group, a TflO- group or a TsO- group) with a base and a reducing agent. Further the present invention relates to the process for selectively eliminating the Piv- group of B alone in the nucleoside derivatives represented by the formula [I'']: (wherein B is a purine base residue protected by a Piv-group, a pyrimidine base residue protected by a Piv-group or an imidazolyl group protected by a Piv-group; R¹ and R³ are a PivO- group; R² is a MsO- group, TflO- group, TsO- group, hydroxyl group or a hydrogen atom), which comprises treating nucleoside derivatives [I''] with an alkaline compound. The present invention is explained in more detail below. Base B in the nucleoside derivatives [I] and [II] of the present invention usually binds to the 1'-position of the ribose through a nitrogen atom in the base B, but may bind to the 1'-position of the ribose through a carbon atom therein. In the present specification, Ms- means a mesyl group (CH₃SO₂-), Ts- means a tosyl group Tfl-means a trifluoromethanesulfonyl group (CF₃SO₂-), Piv- means a pivaloyl group [(CH₃)₃CO-] and DMTr- means a 4,4'-dimethoxytrityl group. Further, the purine base means purine derivatives having close relation to a living body. These purine derivatives are included in nucleic acids: adenines and guanines in coenzymes; uric acid, hypoxanthine, xanthine, and the like which have close relation to purine metabolism of a living body; caffeine, theophylline, theobromine and the like which are called purine alkaloids; isoguanine in a nucleoside called crotonoside; methylaminopurine synthesized when a living body is placed under a special condition; etc. The following compounds are specifically exemplified as purine bases: Adenine, 1-methyladenine, 2-methyladenine, N -(purine-6-yl-carbamoyl)- -threonine, N ⁶-(Δ²-isopentenyl)adenine, 2-methylthio- N ⁶-(Δ²-isopentenyl)adenine, 2-hydroxyadenine, N ⁶-methyladenine, N ⁶, N ⁶-dimethyladenine, N ⁶-( cis -4-hydroxyisopentenyl)adenine, N ⁶-( trans -4-hydroxyisopentenyl) adenine, 2-aminoadenine, guanine, 1-methylguanine, N ²-methylguanine, N ², N ²-dimethylguanine, 7-methylguanine, hypoxanthine, 1-methylhypoxanthine, xanthine, uric acid, 6-thiopurine, 6-chloropurine, 2,6-diaminopurine, etc. Further, the pyrimidine base means pyrimidine derivatives having close relation to a living body. The following compounds are exemplified as main pyrimidine bases: cytosine 4-amino-2-oxypyrimidine uracil 2,4-dioxypyrimidine thymine 2,4-dioxy-5-methylpyrimidine 5-methylcytosine 4-amino-2-oxy-5-methylpyrimidine oxymethylcytosine 4-amino-2-oxy-5-oxymethylpyrimidine The following compounds are specifically exemplified as other pyrimidine bases: 3-methylcytosine, N ⁴-acetylcytosine, N ⁴-methylcytosine, 2-thiocytosine, 3-methyluracil, 4-thiouracil, 5-hydroxyuracil, 5-hydroxymethyluracil, 5-methoxyuracil, 5-carboxymethyluracil, 2-thiothymine, 5-carboxymethyl-2-thiouracil, 5-(methoxycarbonylmethyl)-2-thiouracil, 5-( N -methylaminomethyl)-2-thiouracil, 5,6-dihydrouracil, 5,6-dihydrothymine, 5-(putrescinomethyl)uracil, S (+) 5-(4,5-dihydroxypentyl)uracil, 6-carboxyuracil, 5-fluorouracil, 5-iodouracil, etc. Examples of the protective group contained in base B and the protective group of the oxygen atom of the R¹ may include a trityl group, a monomethoxytrityl group, a dimethoxytrityl group, a silicon derivative group, a pivaloyl group, an alkyl group, an acyl group, etc. An example of nucleoside derivatives and synthetic methods thereof of the present invention is shown in schemes 1 and 2, and various embodiments of the present invention are described together therewith. (NOVEL COMPOUNDS) Specific examples of nucleoside derivatives represented by the general formula [I] of the present invention are shown in Table 1. Compounds B Protective group of B R¹ R² R³ A3 adenine residue DMTr- DMTrO- MsO- TsO- C3 cytosine residue Piv- PivO- " PivO- C5 " none " " " C6 " DMTr- " " " U2 uracil residue none " " " G2 guanine residue Piv- " " " A'2 adenine residue " " " " Specific examples of nucleoside derivatives represented by the general formula [II ] of the present invention are shown in Table 2. (The reaction of [I'] → [II']) The reaction for synthesizing nucleoside derivatives represented by the general formula [II'] from those represented by the general formula [I'] (hereinafter referred to as [I'] → [II'] reaction) is to provide nucleoside derivatives, wherein an amino group of the base residue and hydroxy groups at 2'- and 5'- positions of the ribose residue are protected and a mesyl group, a tosyl group or a trifluoromethanesulfonyl group is selectively introduced into 3'-position of the ribose residue. The resulting nucleoside derivatives are treated with a base and a reducing agent so as to effectively synthesize intermediates useful for the synthesis of dideoxynucleoside derivatives. A mesyl group is most preferable as a functional group which is to be selectively introduced into the 3'-position of the ribose and the reaction rate thereof is very high. In order to introduce the above mentioned functional group into the 3'-position of the ribose, first of all the nucleoside is treated in a conventional manner so as to introduce selectively a protective group such as pivaloyl group into the base residue (the 2'- and 5'-positions of the ribose), then the above mentioned functional group is introduced into the 3'-position of the ribose. Thus the synthetic methods of this invention of the [I'] → [II'] reaction are carried out by employing the novel nucleoside derivatives as intermediates which are prepared by introducing the above mentioned functional group into the 3'-position of the ribose. The [I'] → [II'] reaction may be carried out without isolating the compound of [I'] by continuously carrying out the processes of introducing the protective group such as pivaloyl group into the nucleoside and introducing the above mentioned functional group. Reaction examples of the [I'] → [II'] reaction and reaction conditions under which the reaction is carried out are as follows: i) Reaction examples Compound A3 → A6 Compound A5 → A6 Compound C3 → C4a Compound C6 → C7 Compound U2 → U3 Compound G2 → G3 Compound A'2 → A'3 ii) Reaction conditions a) Solvent It is preferable to use a mixed solvent containing an alcohol as the solvent, and for example, methanol, ethanol, propanol, water, benzene, toluene, hexane, chloroform, dichloroethane, N , N -dimethylformamide, dimethylsulfoxide, ether, tetrahydrofuran, dioxane, ethyl acetate, acetonitrile, etc. may be used. Further, heavy water (D₂O, T₂O), or a heavy hydrogen- or tritium-labelled alcohol may be used. b) Reagents As reagents, a metal alkoxide or another alkaline substance as the base, and a reducing agent may be used in an amount of 1 to 20 mole equivalents, respectively. Preferred metal alkoxides include alkoxides such as methoxide, ethoxide, propoxide and t-butoxide of an alkali metal, an alkaline earth metal, etc., particularly magnesium, lithium, potassium, sodium or the like. Magnesium methoxide, sodium methoxide and potassium hydroxide may most advantageously be used. Other preferable alkaline substances include sodium hydroxide, an alkylaluminum compound, an alkyl lithium compound, etc. Preferred reducing agents include a metal borohydride complex containing an alkali metal or alkaline earth metal represented by sodium borohydride (NaBH₄), sodium cyanoborohydride, lithium cyanoborohydride, tetraalkylammonium borohydride, diborane, an aluminum-hydrogen complex represented by lithium aluminum hydride (LiAlH₄), an organotin hydride represented by tributyltin hydride. Further, these compound may be those heavy hydrogen- or tritium-labelled. c) Other conditions Reaction time is preferably 5 minutes to 20 hours, and reaction temperature is 0 to 100°C. Further, it is preferable to carry out the reaction in an inert gas such as nitrogen. (The reaction of selectively eliminating the Piv- group alone in the compound [I'']) Reaction examples of the reaction for selectively eliminating the Piv- group of B alone in the nucleoside derivatives represented by the formula [ I'' ] and reaction conditions under which the reaction is carried out are as follows: i) Reaction example C3 → C5 ii) Reaction conditions a) Solvent Water, methanol, ethanol, benzene, toluene, hexane, ethyl acetate, tetrahydrofuran, chloroform, ether, etc. may be used as the solvent. b) Reagent An alkaline compound such as a magnesium alkoxide, sodium hydroxide or potassium hydroxide is generally be used as the reagent. A magnesium alkoxide is the most preferable. When other alkaline compounds are used, temperature condition and reaction time must be controlled so that the pivaloyl group of the base alone is eliminated. c) Other reaction conditions Reaction time is 5 to 20 hours, and reaction temperature is 0 to 100°C. (Reactions in each step shown in schemes) Preferable and acceptable reaction conditions under which each of the reaction steps exemplified in schemes 1 - 5 is carried out are mentioned below. Various protective groups such as trityl groups, monomethoxytrityl groups, dimethoxytrityl groups, silicon derivative groups, alkyl groups, pivaloyl groups and acyl groups may be used for the hydroxyl group at the 5'-position of ribose and the amino group of the base in each step shown in scheme 1. However, the protective groups may be present or absent although it is required to protect the hydroxyl group at the 5'-position of the ribose in the mesylation and tosylation steps. Further, it is preferable in order to increase solubility of the nucleoside derivatives in organic solvents that either the hydroxyl group at the 5'-position of the ribose or the amino group of the base is protected. As for synthetic methods of Compound C2 shown in scheme 2, it makes reference to Kazuo Kamaike, Fumihiko Uemura, S. Yamakage, S. Nishino, and Shunichi Ishido, "Simple synthetic procedure for 2'- and -3'-tetrahydropyranyl ribonucleoside derivatives involving regioselective acylation with acyl chlorides, and synthesis of ribonucleotide oligomers" Nucleic Acid Research, Symposium Series No. 16, 1985; Nucleosides & Nucleotides, 6 , 699 (1987). In order to synthesize Compound C3 with protection of Compound C1, it is preferable to protect the amino group of the base and the ribose 5'- and 2'-positions at the same time using pivaloyl groups as shown in scheme 2. However, acetyl groups and benzoyl groups may each also be used in place of pivaloyl groups under an adjusted reaction condition. Further, it is also possible to synthesize Compound C8a from Compound C1 through a route shown in scheme 6. It is difficult to apply the (A3 → A6)-type reaction (scheme 1) using the 2'-tosylated compound to the steps for synthesizing C4a and C7 (scheme 2) because of the existence of the carbonyl group in the base. When the base does not have the carbonyl group which participates in the 2'-position or the carbonyl group of the base is protected, the 2'-tosylated compounds may be used. A method where the keto group of the base is opened and the oxygen is methylated to make an enol type may, for example, be utilized for protecting the carbonyl group of the base. Compound C4a [1-(3-deoxy-β- - threo -pentofuranosyl) cytosine] and biological properties thereof are disclosed in the report by W. Kreis, K. A. Watanabe, and J. J. Fox [Helv. Chem. Acta, 61 , 1011 (1978)]. Compound C11 (2'-azido-2',3'-dideoxycytidine) and biological properties thereof are disclosed in the report by Shunji Izuta, Shigeru Kimura, Kenji Takenuki, and Mineo Saneyoshi, (Nucleic Acid Research, Symp. Ser., No. 17, 1986, p. 154). 1) Synthesis of Compounds A1 and A4 Compound A1 and Compound A4, starting compounds disclosed in scheme 1 may be obtained by a known method, for example by tosylating the nucleoside in the route shown in the following scheme 7. The 2'- O -tosyl compound (Compound A1) and the 3'-O-tosyl compound (Compound A4) may readily be separated by fractional crystallization. Heretofore, a method of obtaining an intermediate (corresponding to Compound A6) for synthesis of dideoxynucleoside derivatives from the 3'- O -tosyl compound (Compound A4) has been known (F. Hansske and M. J. Robins, J. Am. Chem. Soc., 105 , 6736 (1983)). However, in this method the large scale preparation of the 3'- O -tosyl compound (Compound A4) is tedious. On the other hand, synthetic method of the invention allows the use of the 2'- O -tosyl compound in addition to the 3'- O -tosyl compound as the starting materials. Though the starting material used in scheme 7 is adenosine, such nucleosides as specifically exemplified below may further be used in the present invention in place of adenosine: 1-methyladenosine, 2-methyladenosine, N ⁶-methyladenosine, N ⁶, N ⁶-dimethyladenosine, 2-hydroxyadenosine, N -[(9-β- -ribofuranosylpurin-6-yl)carbamoyl]- L -threonine, N -[(9-β- -ribofuranosyl purin-6-yl)- N -methylcarbamoyl]threonine, N -[(9-β- -ribofuranosyl-2-methylthiopurin-6-yl)carbamoyl]threonine, N ⁶-(Δ²-isopentenyl)adenosine, 2-methylthio- N ⁶-(Δ²-isopentenyl)adenosine, N ⁶-( cis -4-hydroxyisopentenyl)adenosine, 2-methylthio- N ⁶-( cis -4-hydroxyisopentenyl)adenosine, zeatosine, cytidine, 2-thiocytidine, 3-methylcytidine, N ⁴-acetylcytidine, 5-methylcytidine, guanosine, 1-methylguanosine, N ²-methylguanosine, N ², N ²-dimethylguanosine, 7-methylguanosine, N ², N ²,7-trimethylguanosine, wyosine, wybutosine, peroxywybutosine, Q nucleoside, Q*nucleoside (the compound where the hydroxyl hydrogen atom at the 5-position of the cyclopentene of Q nucleoside is replaced by a galactosyl residue depicted below). inosine, 1-methylinosine, xanthosine, uridine, 3-methyluridine, 5-methyluridine, 5-methyl-2-thiouridine, 4-thiouridine, 5-carboxymethyluridine, 5-carboxymethyl-2-thiouridine, 5-(carboxymethylaminomethyl)uridine, 5-(carboxymethylaminomethyl)-2-thiouridine, 5-(methoxycarbonylmethyl)uridine, 5-(methoxycarbonylmethyl)-2-thiouridine, 5-(methylaminomethyl)-2-thiouridine, 3-(3-amino-3-carboxypropyl)uridine, 5-hydroxyuridine, 5-methoxyuridine, methyl uridine-5-oxyacetate, 5-hydroxymethyluridine, dihydrouridine, 5-methyl-5,6-dihydrouridine, pseudouridine, 1-methyl-3-(3-amino-3-carboxypropyl)pseudouridine, orotidine, etc. 2) Compound A1 → Compound A2 Compound A2 may be obtained by reacting Compound A1 with dimethoxytrityl chloride in a solvent such as pyridine or triethylamine, preferably at a temperature of 65 to 85°C for 1 to 3 hours. General reaction conditions for obtaining Compound A2 from Compound A1 are as follows. a) Solvent As the solvent, a solvent mainly consisting of pyridine or triethylamine is used. However, an aprotic organic solvent may be used in combination therewith. Examples of the aprotic organic solvent are benzene, toluene, hexane, chloroform, dichloroethane, dichloromethane, N',N-dimethylformamide, dimethylsulfoxide, ether, tetrahydrofuran, dioxane, ethyl acetate, acetonitrile, etc. However, a neutral solvent is used when 3,4-dihydro-2H-pyran is used as a reagent. b) Catalyst As a catalyst, 4-dimethylaminopyridine, an alkali metal or alkaline earth metal alkoxide (sodium methoxide, etc.), one of various Grignard reagents ( t -butylmagnesium chloride, etc.) or the like is used in an amount of 0.1 to 10 mole equivalents. However, when the pyran is used as a reagent, benzenesulfonic acid, toluenesulfonic acid, camphorsulfonic acid, hydrochloric acid, sulfuric acid, a strongly acidic ion exchange resin or the like is used as an acid catalyst. c) Reagent As a reagent, trityl chloride, monomethoxytrityl chloride, dimethoxytrityl chloride, benzyl chloride, benzyl bromide, monomethoxybenzyl chloride, monomethoxybenzyl bromide, dimethoxybenzyl chloride, dimethoxybenzyl bromide, β-methoxyethoxymethyl chloride, methoxymethyl chloride, benzoyl chloride, acetyl chloride, pivaloyl chloride, t -butyldimethylsilyl chloride, dimethylphenylsilyl chloride, 3,4-dihydro-2 H -pyran or the like is used. d) Other conditions Reaction time is 10 minutes to 20 hours, reaction temperature is 0 to 100°C, and the reaction is carried out at normal pressure while humidity is avoided. 3) Compound A2 → Compound A3 Compound A3 may be obtained by reacting Compound A2 with mesyl chloride in a solvent such as pyridine or triethylamine, preferably at room temperature for 1 to 3 hours. General reaction conditions for obtaining Compound A3 from Compound A2 are as follows. a) Solvent As the solvent, a solvent mainly consisting of pyridine or triethylamine is used. However, an aprotic organic solvent may be used in combination therewith. Examples of the aprotic organic solvent are benzene, toluene, hexane, chloroform, dichloroethane, dichloromethane, N , N -dimethylformamide, dimethylsulfoxide, ether, tetrahydrofuran, dioxane, ethyl acetate, acetonitrile, etc. b) Catalyst As a catalyst, 4-dimethylaminopyridine, an alkali metal or alkaline earth metal alkoxide (sodium methoxide, etc.), one of various Grignard reagents ( t -butylmagnesium chloride, etc.) or the like is used in an amount of 0.1 to 10 mole equivalents. c) Reagent As the reagent, an acid chloride, acid anhydride, acid imidazolide or the like of methanesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid or the like may be used. d) Other conditions Reaction time is 10 minutes to 20 hours, reaction temperature is 0 to 100°C, and the reaction is carried out at normal pressure while humidity is avoided. 4) Compound A3 → Compound A6 Compound A6 may be obtained by reacting Compound A3 with magnesium methoxide, sodium methoxide or the like in the presence of a reducing agent such as sodium borohydride in a benzene-methanol mixed solvent at a temperature of 55 to 75°C for 3 to 7 hours. General reaction conditions are the same with those of the [I'] → [II'] reaction. 5) Compound A4 → Compound A5 Compound A5 may be obtained by reacting Compound A4 with dimethoxytrityl chloride in a solvent such as pyridine, preferably at room temperature for 2 to 4 hours. General reaction conditions for obtaining Compound A5 from Compound A4 are the same with general reaction conditions for obtaining Compound A2 from Compound A1. 6) Compound A5 → Compound A6 Compound A6 may be obtained by reacting Compound A5 with magnesium methoxide, sodium methoxide or the like in the presence of sodium borohydride or the like as a catalyst in a benzene-methanol mixed solvent, preferably at 55 to 75°C for 5 to 10 minutes. General reaction conditions are the same with those of the [I'] → [II'] reactions. 7) Compound A6 → Compound A7a Compound A7a may be obtained by reacting Compound A6 with mesyl chloride in a solvent such as pyridine, preferably at room temperature for 1 to 3 hours. General reaction conditions for obtaining Compound A7a are the same with general reaction conditions for obtaining Compound A3 from Compound A2. 8) Compound A7a → Compound A8 Compound A8 may be obtained by reacting Compound A7a with sodium azide in a solvent such as dimethylformamide or diethylformamide, preferably at a temperature of 100 to 120°C for 3 to 5 hours. General reaction conditions for obtaining Compound A8 from Compound A7a are as follows. a) Solvent As the solvent, a solvent mainly consisting of N , N -dimethylformamide, N , N -diethylformamide or dimethylsulfoxide is used, and solvents as exemplified below may be used in combination therewith. That is, benzene, toluene, hexane, chloroform, dichloroethane, dichloromethane, ether, tetrahydrofuran, dioxane, ethyl acetate, pyridine, acetonitrile, methanol, ethanol, propanol, water, etc. may be added. b) Reagent An alkali metal aside compound may be used as the reagent, and sodium aside or lithium aside is preferably used. c) Other conditions Preferably, reaction time is 10 minutes to 20 hours, and reaction temperature is 0 to 150°C. 9) Compound A8 → Compound A9 Compound A9 may be obtained by treating Compound A8 in the presence of an acid catalyst, preferably at room temperature for 2 to 4 hours to carry out hydrolysis. General reaction conditions for obtaining Compound A9 from Compound A8 are as follows. a) Solvent Water, methanol, ethanol, propanol, pyridine, N , N -dimethylformamide, dimethylsulfoxide, benzene, toluene, hexane, chloroform, dichloroethane, dichloromethane, ether, tetrahydrofuran, dioxane, ethyl acetate, acetonitrile, etc. may be used as the solvent. b) Catalyst Acetic acid, formic acid, hydrochloric acid, p -toluenesulfonic acid, sulfonic acid, camphorsulfonic acid, zinc bromide, zinc chloride, an acidic ion exchange resin, etc. may be used as the catalyst Amount of the catalyst is 0.001 mole% to a large excessive amount. c) Other reaction conditions Reaction time is 5 minutes to 20 hours and reaction temperature is -20 to 100°C. 10) Compound A6 → Compound A7b Acquisition of Compound A7b from Compound A6 may be attained by tosylating R 3' according to a known method, for example using TsCl and Et₃N-pyridine. 11) Compound A7b → Compound A10 The same reaction conditions as in the case of synthesis of Compound A8 from Compound A7a may be used, provided that an alkali metal iodo compound such as sodium iodide or lithium iodide, or an alkyl or aryl quaternary ammonium iodo compound such as tetra- n -butylammonium iodide is used in place of the aside compound as the reagent is used. 12) Compound A10 → Compound A11 Compound A11 is synthesized by reacting Compound A10 with a reducing agent such as tributyltin hydride in a solvent such as toluene, preferably in the presence of azobisisobutyronitrile or light of a mercury lamp as catalysts at room temperature for 30 minutes to 6 hours. General reaction conditions for obtaining Compound A11 from Compound A10 are as follows. a) Solvent Benzene, toluene, cyclohexane, hexane, methanol, ethanol, ethyl acetate, etc. may be used as a solvent. b) Catalyst As catalysts, a radical-generating agent such as azobisisobutyronitrile in an amount of 0.01 to 1 mole equivalent, light (preferably, that of a mercury lamp), etc. may be used. c) Reducing agent A metal borohydride complex containing an alkali metal or alkaline earth metal represented by sodium borohydride (NaBH₄), sodium cyanoborohydride, lithium cyanoborohydride, tetraalkylammonium borohydride, diborane, an aluminum-hydrogen complex represented by lithium aluminum hydride (LiAlH₄), an organotin hydride represented by tributyltin, etc. may preferably used as a reducing agent. Further, these compounds may be those heavy hydrogen- or tritium-labelled. A reducing agent may be used preferably in an amount of 1 to 10 mole equivalent. d) Other reaction conditions Reaction time is preferably 5 minutes to 24 hours, and reaction temperature is preferably room temperature to 120°C. Further, it is preferable to degas the solvent and to carry out the reaction under an inert gas stream. 13) Compound A11 → Compound A12 Compound A12 may be obtained by treating Compound A11 in the presence of an acid catalyst, preferably at room temperature for 2 to 4 hours to carry out hydrolysis. General reaction conditions for obtaining Compound A12 from Compound A11 are the same with reaction conditions for obtaining Compound A9 from Compound A8. 14) Compound C3 → Compounds C4a, C4b and C4c A method of synthesizing Compound C4a from Compound C3 and reaction conditions thereof are the same with the method and reaction conditions for obtaining Compound A6 from Compound A3. General reaction conditions are the same with those of the [I'] → [II'] reaction. Because of a strong hygroscopic nature and difficulty of purification thereof, Compound C4a may be crystallized in methanol as picrate (C4b) or hydrochloride (C4c). 15) Compound C3 → Compound C5 Compound C5 may be obtained by reacting Compound C3 with magnesium methoxide, preferably in a methanol-benzene solvent at a temperature of 20 to 30°C for 30 minutes to 2 hours. General reaction conditions for obtaining Compound C5 from Compound C3 are as follows: a) Solvent Water, methanol, ethanol, benzene, toluene, hexane, ethyl acetate, tetrahydrofuran, chloroform, ether, etc. may be used as the solvent. b) Reagent An alkaline compound such as a magnesium alkoxide, sodium hydroxide or potassium hydroxide is generally be used as the reagent. A magnesium alkoxide is the most preferable. When other alkaline compounds are used, temperature condition and reaction time must be controlled so that the pivaloyl group of the base alone is eliminated. c) Other reaction conditions Reaction time is 5 to 20 hours, and reaction temperature is 0 to 100°C. 16) Compound C5 → Compound C6 Compound C6 may be obtained by reacting Compound C5 with DMTrCl in a pyridine solvent according to a known method to protect the base. 17) Compound C6 → Compound C7 The method and reaction conditions for synthesizing Compound C7 from Compound C6 are the same with the method and reaction conditions for obtaining Compound A6 from Compound A3. General reaction conditions are the same with the [I'] → [II'] reaction. 18) Compound C4a or C7 → Compound C8 Compound C8a or C8b may be obtained by reacting Compound C4a or C7 with DMTrCl or PivCl in a pyridine solvent according to a known method to protect the base and the ribose 5'-position. 19) Compound C8 → Compound C9 Compounds C9a to C9e may be obtained by reacting Compound C8a or C8b with mesyl chloride, tosyl chloride, phenyl chlorothionocarbonate (chlorothionoformate) or 1,1'-thiocarbonyldiimidazole in a solvent such as pyridine, preferably at room temperature for 1 to 3 hours. General reaction conditions of the step for obtaining Compound C9 are the same with the general reaction conditions for obtaining Compound A3 from Compound A2. 20) Compound C9 → Compound C10 The method of synthesizing Compound C10 from Compound C9 and reaction conditions therefor are the same with the method and reaction conditions for synthesizing Compound A8 from Compound A7. 21) Compound C10 → Compound C11 The method and reaction conditions of synthesizing Compound C11 from Compound C10 are the same with the method and reaction conditions for synthesizing Compound A9 from Compound A8. 22) Compound C9c → Compound C12 Compound C12 may be obtained by reacting Compound C9c, preferably, with sodium methoxide in a methanol solvent at room temperature for 24 hours. General reaction conditions for obtaining Compound C12 from Compound C9c are as follows: a) Solvent A sole solvent or a mixed solvent of an alcohol such as methanol, ethanol, i -propanol or t -butanol, N , N -dimethylformamide, N , N -diethylformamide and dimethylsulfoxide may preferably used as the solvent. Benzene, ethyl acetate, hexane, tetrahydrofuran, dioxine, etc. may also be used. b) Reagent A metal alkoxide such as sodium methoxide or potassium t -butoxide, sodium hydroxide, potassium hydroxide, an alkali metal hydride such as sodium hydride, DBN(1,5-diazabicyclo[4,3,0)-nonene-5), DBU(1,8-diazabicyclo[5,4,0)-unde-7-cene), or a metal alkylamide such as lithium diethylamide is used in an amount of 0.01 to 10 mole equivalents as the reagent. c) Other reaction conditions Reaction time is 10 minutes to 24 hours, reaction temperature is room temperature to 140°C, and the reaction is preferably carried out an under dry, inert gas atmosphere. 23) Compound C12 → Compound C13 Compound C13 may preferably be obtained by treating Compound C12 in a methanol solvent in the presence of hydrogen gas and palladium catalysts at room temperature for 30 minutes to one hour. General reaction conditions for obtaining Compound C13 from Compound C12 are as follows. a) Solvent Methanol, ethanol, i -propanol, water, etc. may be used as the solvent. b) Catalyst As the catalysts, a metal catalyst such as palladium, platinum, rhodium, ruthenium or nickel, may be used in an amount of 0.01 to 10 weight %. c) Other reaction conditions It is preferable that reaction time is 10 minutes to 24 hours, reaction temperature is 0 to 100°C and hydrogen gas is used at 1 to 100 atm. 24) Compound U2 → U3, G2 → G3, A'2 → A'3 Preferable reaction conditions are the same with those of the A3 → A6 reaction, and general reaction conditions are the same with those of the [I'] → [II'] reaction. (The Best Embodiment for Carrying out the Invention) The present invention is further specifically described below according to examples and reference example. Reference example 1 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-2'- O -tosyladenosine (Compound A2). 2'-O-Tosyladenosine (Compound A1) (4.21 g, 10 mmol) and 4,4'-dimethoxytrityl chloride (7.45 g, 22 mmol) were added to anhydrous pyridine (50 ml), and stirred at room temperature for 20 hours. After ice cooling, 50% aqueous pyridine was added, the mixture was extracted with chloroform, and the organic layer was washed successively with aqueous sodium bicarbonate and water, and dried over magnesium sulfate. After distillating away the chloroform, the pyridine was removed by co-evaporation with toluene, and the residue was subjected to silica gel column chromatography [eluant : toluene-ethyl acetate-triethylamine (9:1:0.1) → toluene-ethyl acetate-triethylamine-methanol (9:1:0.1:0.2)]. Fractions containing the desired substance were combined and concentrated to give Compound A2 (7.74 g, yield 75%). Physical properties of Compound A2 Powders (amorphous) Elementary analysis C H N S Calculated (%) 69.06 5.40 6.82 3.12 (for C₅₉H₅₅N₅O₁₀S) Found (%) 68.94 5.39 6.74 3.21 Specific rotations [α] 24 / D = -39.2° (c = 0.5, CHCl₃) IR spectrum (KBr) 3450 1253 705 2960 1178 583 1608 1035 553 cm⁻¹ 1510 828 ¹H-NMR spectrum CDCl₃, δ 2.29 (3H, s, C-CH₃) 4.65 (1H, m, H-3') 3.31 (1H, dd, H-5') 5.73 (1H, t, H-2') 3.44 (1H, dd, H-5'') 6.07 (1H, d, H-1') 3.77 (12H, s, O-CH₃) 7.73 (1H, s, H-2) 4.19 (1H, m, H-4') 7.81 (1H, s, H-8) Example 1 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(3- O -mesyl-2- O -tosyl-β- -ribofuranosyl)adenine (Compound A3) Mesyl chloride (1.45 ml, 18.6 mmol) was added to a solution of the monotosyl compound (Compound A2) (9.50 g, 9.3 mmol) in anhydrous pyridine (40 ml) at room temperature, the mixture was stirred for one hour. After cooling, 50% aqueous pyridine was added to decompose an excess of the mesyl chloride. The reaction mixture was extracted with ether, and the ether layer was washed successively with water, aqueous sodium bicarbonate and water, and dried over magnesium sulfate. After removal of the ether, the pyridine was removed under reduced pressure by co-evaporation with toluene, and the residue was subjected to silica gel column chromatography [eluant : benzene-ethyl acetate-triethylamine (95:5:1) → benzene-ethyl acetate-triethylamine-methanol (95:5:1:1).] The fractions containing the desired product were combined and concentrated to afford 8.35 g of Compound A3 (yield 81%). A part thereof was dissolved in dichloromethane, and the solution was dropwise added to pentane to prepare an analytical sample. Physical properties of Compound A3 Powders (amorphous) Elementary analysis C H N S Calculated (%) 66.36 5.91 5.95 5.45 (for C₆₀H₅₇N₅O₁₂S₂·C₅H₁₂) Found (%) 66.26 6.01 5.80 5.41 Specific rotations [α] 20 / D = -34.1° (c = 0.6, CHCl₃) IR spectrum (KBr) 3440 1470 1180 705 2980 1370 1070 670 1610 1300 1034 580 1510 1255 830 553 cm⁻¹ ¹H-NMR Spectrum CDCl₃, δ 2.28 (3H, s, Ts-CH₃) 4.45 (1H, q, H-4') 3.11 (3H, s, Ms-CH₃) 5.44 (1H, dd, H-3') 3.45 (1H, dd, H-5') 5.89 (1H, dd, H-2') 3.54 (1H, dd, H-5'') 6.08 (1H, d, H-1') 3.78 (12H, s, O-CH₃) 7.69 (1H, s, H-2) 7.78 (1H, s, H-8) Example 2 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(3-deoxy-β- - threo -pentofuranosyl)adenine (Compound A6) Method 1 From Compound A3 Disulfonate (Compound A3) (110 mg, 0.1 mmol) was dissolved in a mixture of benzene (1 ml) and methanol (1 ml), and magnesium methoxide (86 mg, 1 mmol) and sodium borohydride (19 mg, 0.5 mmol) were added thereto at room temperature. The mixture was stirred at 65 - 70°C under an atmosphere of dry nitrogen for 4 hours. After cooling, acetone (0.5 ml) was added to inactivate an excess of the reducing agent, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed successively with an aqueous ammonium chloride and water, and dried over magnesium sulfate. The organic solvent was distilled away and the residue was subjected to silica gel column chromatography [eluant : benzene-ethyl acetate-triethylamine (9:1:0.1) → benzene-ethyl acetate-triethylamine-methanol (9:1:0.1:0.1)]. Fractions containing the desired substance were concentrated to give Compound A6 (63 mg, yield 73%). From Compound A5 In a manner similar to Method 1, monotosyl compound (Compound A5) (102 mg, 0.1 mmol) was treated with magnesium methoxide (1 mmol)-sodium borohydride (0.5 mmol) at 65°C for 7 minutes, and Compound A6 was obtained after the column chromatography (85 mg, yield 99%). Physical properties of Compound A6 Powders (amorphous) Elementary analysis C H N Calculated (%) 72.66 5.79 8.15 (for C₅₂H₄₉N₅O₇·0.2H₂O) Found (%) 72.84 6.08 7.87 Specific rotations [α] 22 / D = +9.2° (c = 0.9, CHCl₃) IR spectrum (KBr) 3440 1254 705 2970 1180 585 1610 1037 cm⁻¹ 1510 830 ¹H-NMR spectrum CDCl₃, δ 2.15 (1H, m, H-3') 2.47 (1H, m, H-3'') 3.19 (1H, dd, H-5') 4.53 (2H, br s, H-2') and OH) 3.56 (1H, dd, H-5'') 6.06 (1H, d, H-1') 3.78 (12H, s, O-CH₃) 8.02 (1H, s, H-2) 4.34 (1H, m, H-4') 8.24 (1H, s, H-8) Example 3 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(3-deoxy-2- O -mesyl-β- - threo -pentofuranosyl)adenine (Compound A7) A solution of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(3-deoxy-β- - threo -pentofuranosyl)adenine (Compound A6) (544 mg, 0.64 mmol) in anhydrous pyridine (4 ml) was cooled with ice water, methanesulfonyl chloride (0.2 ml, 2.6 mmol) was added, and the mixture was stirred at room temperature for 5 hours. After cooling, 50% aqueous pyridine was added to decompose an excess of the reagent, and the mixture was diluted with chloroform. The organic layer was washed successively with water, aqueous sodium bicarbonate and water and dried over magnesium sulfate. The organic solvents were removed under reduced pressure. The residue was subjected to silica gel column chromatography [ eluant : chloroform-methanol-triethylamine (99:1:1) ] to provide 556 mg of the mesylate compound (Compound A7) (yield 93%). A part of the product was dissolved in dichloromethane and the solution was dropwise added to pentane to form a precipitate, which was then used as an analytical sample. Physical properties of Compound A7 Powders (amorphous) Elementary analysis C H N S Calculated (%) 68.93 6.08 7.11 3.26 (for C₅₃H₅₁N₅O₉S·0.7C₅H₁₂) Found (%) 68.89 6.11 6.94 3.32 Specific rotations [α] 22 / D = +8.2° (c = 0.9, CHCl₃) IR spectrum (KBr) 3420 1367 824 2950 1250 750 1600 1173 700 1505 1030 580 cm⁻¹ ¹H-NMR spectrum CDCl₃, δ 2.47 (1H, m, H-3') 3.78 and 3.79 (12H, s, O-CH₃) 2.62 (1H, m, H-3'') 4.34 (1H, m, H-4') 2.52 (3H, s, C-CH₃) 5.32 (1H, m, H-2') 3.38 (1H, dd, H-5') 6.34 (1H, d, H-1') 3.44 (1H, dd, H-5'') 8.00 (1H, s, H-2) 8.02 (1H, s, H-8) Example 4 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(3-deoxy-2- O -tosyl-β- - threo -pentofuranosyl)adenine (Compound A7b) To a solution of the deoxy compound (Compound A6) (457 mg, 0.53 mmol) in anhydrous pyridine (3 ml) was added toluenesulfonyl chloride (1.00 g, 5.3 mmol), and the mixture was stirred at room temperature for 15 hours. The reaction mixture was cooled, treated with 50% aqueous pyridine, and extracted with chloroform. The organic layer was washed successively with water, aqueous sodium bicarbonate and water, and dried over magnesium sulfate. After distilling away the chloroform, the pyridine was removed as the toluene azeotrope, and the residue was subjected to silica gel column chromatography [ eluant : toluene-ethyl acetate-triethylamine (95:5:1)]. The fractions containing the desired substance were combined and concentrated to give 241 mg of the tosylated compound (Compound A7b) (yield 45%) Physical properties of Compound A7b Powders (amorphous) TLC Rf = 0.70 (silica gel : chloroform-ethyl acetate methanol (8:2:0.2)) IR spectrum (KBr) 3420 1367 1030 675 2940 1292 895 580 1600 1246 822 550 cm⁻¹ 1503 1172 ¹H-NMR spectrum CDCl₃, δ 2.44 (1H, m, H-3') 3.77 and 3.80 (12H, s, O-CH₃) 2.56 (1H, m, H-3'') 4.24 (1H, m, H-4') 2.15 (3H, s, C-CH₃) 4.88 (1H, m, H-2') 3.32 (1H, dd, H-5') 6.03 (1H, d, H-1') 3.41 (1H, dd, H-5'') 7.80 (1H, s, H-2) 7.95 (1H, s, H-8) Example 5 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(2-( R )-azido-2,3-dideoxy-β- - glycero -pentofuranosyl)adenine (Compound A8) The 3'-deoxy-2'-O-mesylated compound (Compound A7) (523 mg, 0.56 mmol) was dissolved in N , N -dimethylformamide (12 ml), and sodium aside (1.09 g, 16.8 mmol) was added thereto, followed by stirring at 105 - 110°C for 5 hours. The reaction mixture was cooled and extracted with ether containing a small amount of chloroform. The extract was washed several times with water and dried over magnesium sulfate. The organic solvents were removed under reduced pressure, and the residue was subjected to silica gel column chromatography [eluant : chloroform-ethyl acetate-triethylamine (97:3:1)] to give 382 mg of the azide compound (Compound A8) (yield 78%). Physical properties of Compound A8 Powders (amorphous) Elementary analysis C H N Calculated (%) 70.18 5.55 12.59 (for C₅₂H₄₈N₈O₆·0.5H₂O) Found (%) 70.26 5.53 12.45 Specific rotations [α] 20 / D = -37.5° (c = 0.4, CHCl₃) IR spectrum (KBr) 3420 1505 823 2950 1248 700 2100 (N₃) 1173 580 cm⁻¹ 1600 1030 ¹H-NMR spectrum CDCl₃, δ 2.13 (1H, ddd, H-3') 4.53 (1H, m, H-4') 2.41 (1H, m, H-3'') 4.82 (1H, m, H-2') 3.33 (1H, dd, H-5') 5.98 (1H, d, H-1') 3.41 (1H, dd, H-5'') 7.94 (1H, s, H-2) 3.77 (12H, s, O-CH₃) 8.03 (1H, s, H-8) Example 6 Synthesis of 9-(2-azido-2,3-dideoxy-β- - glycero -pentofuranosyl)adenine (Compound A9) A mixture of the DMTr-protected 2'-azide compound (Compound A8) (182 mg, 0.21 mmol) and 80% acetic acid (6.8 ml) was stirred at room temperature for 4 hours. Acetic acid was removed under reduced pressure by co-evaporation with toluene-ethanol, and a small amount of ethanol was added to the residue to deposit the free 2'-azide compound (Compound A9) as crystals (26 mg, yield 43%). The mother liquor where the crystals had been removed was concentrated, and the residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (95:5)]. The fractions containing the desired product were combined and concentrated to give Compound A9 (19 mg, yield 32%; total 75%). Physical properties of Compound A9 Colorless crystals Melting point 203 - 204°C (dec.) Elementary analysis C H N Calculated (%) 43.47 4.38 40.56 (for C₁₀H₁₂N₈O₂) Found (%) 43.47 4.38 40.60 Specific rotations [α] 22 / D = -66.0° (c = 0.6, DMF) IR Spectrum (KBr) 3400 3140 2120 (N₃) 1695 1610 1480 1422 1295 1174 1120 1096 1068 1010 952 795 721 632 543 cm⁻¹ ¹H-NMR Spectrum DMSO-d₆, δ 2.14 (1H, m, H-3') 4.85 (1H, m, H-2') 2.46 (1H, m, H-3'') 5.25 (1H, t, OH) 3.54 (1H, m, H-5') 6.01 (1H, d, H-1') 3.72 (1H, m, H-5'') 7.36 (2H, br s, NH₂) 4.32 (1H, m, H-4) 8.16 (1H, s, H-2) 8.40 (1H, s, H-8) Example 7 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(2-( R )-iodo-2,3-dideoxy-β- - glycero -pentofuranosyl)adenine (Compound A10) To a solution of the 2'-tosylated compound (Compound A7b) (160 mg, 0.16 mmol) in N , N -dimethylformamide (1 ml) was added lithium iodide (212 mg, 1.60 mmol) and the mixture was stirred at 105 - 110°C for 5 hours. After cooling, the reaction mixture was diluted with ether containing a small amount of chloroform. The solution was washed several times with water and dried over magnesium sulfate. The drying agent was removed by filtration and the filtrate was concentrated under reduced pressure. The resulting residue was subjected to silica gel chromatography [silica gel 10 g; eluant: toluene - ethyl acetate - triethylamine (30:1:0.3)] and preparative TLC [(silica gel 200 x 200 x 2 mm; developing solvent: benzene - ethyl acetate - methanol - triethylamine (9:1:0.15:0.1)] to afford 80 mg of the 2'-iodo compound (Compound A10) (yield 52%). An analytical sample was prepared by dropwise adding a dichloromethane solution of the iodo compound to pentane, followed by collecting the resulting precipitates by filtration and drying them. Physical properties of Compound A10 Powders (amorphous) Elementary analysis C H N Calculated (%) 65.07 5.27 7.09 (for C₅₂H₄₈N₅O₆I·0.3C₅H₁₂) Found (%) 64.95 5.18 6.81 IR Spectrum (KBr) 3445 1604 1465 1174 703 2955 1583 1294 1031 584 cm⁻¹ 2870 1507 1250 826 ¹H-NMR Spectrum CDCl₃ δ 2.35 (1H, m, H-3') 4.99 (1H, dt, H-2') 2.58 (1H, m, H-3'') 6.42 (1H, d, H-1') 3.39 (1H, dd, H-5') 6.77-6.86, 7.15-7.41 (27H, m, arom, NH) 3.45 (1H, dd, H-5'') 7.90 (1H, s, H-2) 3.77, 3.78 and 3.79 (12H, s, OMe) 8.03 (1H, s, H-8) 4.66 (1H, m, H-4') Example 8 Synthesis of N ⁶, O 5' -bis(4,4'-dimethoxytrityl)-9-(2,3-dideoxy-β- - glycero -pentofuranosyl)adenine (Compound A11) The 2'-iodo compound (Compound A10) (217 mg, 0.22 mmol) was dissolved in previously degassed toluene (5 ml). Tributyltin hydride (91 µl, 0.34 mmol) and α , α'-azobisisobutyronitrile (5 mg, 0.03 mmol) were added thereto under an atmosphere of nitrogen, and the mixture was stirred at 80 o C for 20 minutes. After cooling, the reaction mixture was concentrated under reduced pressure, and the residue was purified by silica gel chromatography [silica gel 15 g; eluant: toluene - ethyl acetate - triethylamine (50:1:0.5)] to give 156 mg of the dideoxy compound (Compound A11) (yield 83%). Physical properties of Compound A11 Amorphous Elementary analysis C H N Calculated (%) 73.02 5.97 8.19 (for C₅₂H₄₉N₅O₆·0.85H₂O) Found (%) 73.32 6.05 7.89 IR Spectrum (KBr)cm⁻¹ 3445 1605 1297 1073 705 2980 1507 1250 1032 585 2860 1463 1170 826 ¹H-NMR Spectrum (CDCl₃) δ 2.05 (2H, m, H-3') 6.27 (1H, dd, H-1') 2.49 (2H, m, H-2') 6.76-6.72 (8H, each, d, arom H) 3.32 (2H, d, H-5') 6.88 (1H, s, NH) 3.76 (12H, s, OMe) 7.18-7.43 (18H, m, arom) 4.33 (1H, m, H-4') 7.94 (1H, s, H-2) 8.04 (1H, s, H-8) Reference example 2 Synthesis of 9-(2,3-dideoxy-β- - glycero -pentofuranosyl)adenine (dideoxyadenosine) (Compound A12) The dimethoxytrityl-protected 2',3'-dideoxy compound (Compound A11) (65 mg, 0.077 mmol) was dissolved in chloroform (385 µl,) and a solution of zinc bromide (ZnBr₂ 141 mg, 0.62 mmol) in methanol (182 µl) - chloroform (676 µl) was added thereto, followed by stirring at room temperature for about 4 hours. After completion of the reaction methanol (2 ml) was added and chloroform was distilled off under reduced pressure with such precaution that the reaction solution is not evaporated to dryness. Water was added to the resulting methanol solution, the resulting precipitate of trityl alcohol was removed by filtration and the filtrate was passed through a HP-20 column (10 x 130 mm) (eluant: water-30% methanol). The fractions containing the desired substance were concentrated under reduced pressure, and the residue was purified using Domex 1-X2 (OH⁻ form, eluant: methanol) and recrystallised from ethanol to afford the desired dideoxyadenosine (Compound A12) as colorless crystals. Yield 8 mg (44%). Compound A12 is the known compound, and its physical properties are disclosed in M. J. Robins, J. R. McCarthy, R. K. Robins, Biochem., 5 , 224 (1966). Physical properties of Compound A12 Melting point 185 - 186°C NMR Spectrum (D₂O) δ from DSS 2.04 (1H, m, H-3') 4.36 (1H, m, H-4') 2.23 (1H, m, H-3'') 6.29 (1H, dd, H-1') 2.52 (1H, m, H-2') 8.18 (1H, s, H-2) 2.57 (1H, m, H-2'') 8.30 (1H, s, H-8) 3.66 (1H, dd, H-5') 3.82 (1H, dd, H-5'') Example 9 Synthesis of N ⁴, O 2' , O 5' -tripivaloyl-3'-O-methanesulfonylcytosine (Compound C3) Pivaloyl chloride (19.8 ml, 160 mmol) was added all at once to a solution of cytidine (972 g, 40 mmol) in pyridine (140 ml) under ice cooling, followed by stirring at room temperature for 90 minutes. Then, methanesulfonyl chloride (186 ml, 240 mmol) was added under ice cooling, followed by stirring at room temperature for one hour. After cooling, an aqueous 50% pyridine solution was added to decompose an excess of the reagent, and the mixture was extracted with ether. The organic layer was washed successively with aqueous sodium bicarbonate and aqueous saturated sodium chloride, and dried over magnesium sulfate. After evaporating the solvent under reduced pressure, the remaining pyridine was removed by coevaporation with toluene, and the residue was subjected to silica gel column chromatography [eluant: benzene-ethyl acetate (2:1)] to give 18.3 g of the desired 3'-mesylated compound (Compound C3) (yield 80%). An analytical sample was prepared by dropwise adding a dichloromethane solution of Compound C3 to n-pentane and drying the resulting precipitates. Physical properties of Compound C3 Amorphous Elementary analysis C H N S Calculated (%) 52.02 6.88 7.28 5.55 (for C₂₅H₃₉N₃O₁₀I·0.2H₂O) Found (%) 51.93 6.80 7.21 5.55 IR Spectrum (KBr)cm⁻¹ 3450 2705 1632 1315 1142 530 3380 1736 1557 1280 975 3000 1720 1483 1180 928 2960 1670 1370 1140 845 ¹H-NMR Spectrum (CDCl₃) δ 1.26 (9H, s, C(CH₃)₃) 5.30 (1H, t, H-3') 1.27 (9H, s, C(CH₃)₃) 5.49 (1H, dd, H-2') 1.29 (9H, s, C(CH₃)₃) 6.01 (1H, d, H-1') 3.07 (3H, s, SO₂CH₃) 7.47 (1H, d, H-5) 4.44 (2H, d, H-5', H-5'') 4.52 (1H, m, H-4') 7.82 (1H, d, H-6) 8.19 (1H, br s, NH) Reference example 3 Synthesis of 1-(3-deoxy-β- - threo -pentofuranosyl) cytosine (Compound C4a, C4b and C4c) Compound C3 (241 g, 4.20 mmol) was dissolved in a mixture of benzene (4 ml) and methanol (4 ml), and magnesium methoxide (1.19 g, 13.8 mmol) and sodium borohydride (321 mg, 8.49 mmol) were added at room temperature, followed by stirring for 10 hours. After cooling, aceton (2 ml) was added to decompose an excess of the reagent, an aqueous ammonium chloride solution was added, and the deposited precipitates were removed by filtration. The filtrate was partitioned between ethyl acetate and water, and the ethyl acetate layer was further washed twice with water. The thus obtained water layer was passed through a column packed with HP-20 (Dowex Co., styrene-divinylbenzene polymer, 50 ml) to adsorb by-products, and the desired C4a product was eluted with water (about 1 ℓ). Then, an activated carbon (50 ml, Wako Junyaku Co., Ltd., for column choromatography) was added to this aqueous solution, followed by stirring for 20 minutes. The activated carbon was separated by filtration, and after washing with water the adsorbed organic substance was eluted with methanol to provide the desired 3'-deoxy threo compound (C4a) along with its erythro isomer (786 mg, yield 83%, threo : erythro = 93:7). Since Compound C4a has a strong hygroscopic property and is hard to be purified, it was crystallized from methanol as the picrate (C4b) or as the hydrochloride (C4c). Physical properties of Compound C4a Amorphous IR Spectrum (KBr)cm⁻¹ 3380 1490 1123 2960 1395 1063 1647 1289 786 1605 1200 600 ¹H-NMR Spectrum (DMSO-d₆) δ 1.74 (1H, ddd, H-3') 5.67 (1H, d, H-5) 2.27 (1H, ddd, H-3'') 5.88 (1H, d, H-1') 3.55 (1H, m, H-5') 7.68 (1H, d, H-6) 3.99-4.04 (1H, m, H-4') 6.9-7.1 (2H, br s, NH₂) 4.24-4.29 (1H, m, H-2') 5.08 (1H, m, OH) 5.18 (1H, d, OH) Physical properties of Compound C4b Melting point 208 - 209°C (dec.) IR Spectrum (KBr)cm⁻¹ 3600 1695 1550 1120 820 3350 1675 1335 1082 790 3130 1610 1315 1035 740 2980 1575 1265 910 710 623 ¹H-NMR Spectrum (DMSO-d₆) δ 1.75 (1H, m, H-3') 6.05 (1H, d, H-5) 2.26 (1H, m, H-3'') 8.13 (1H, d, H-6) 3.60 (2H, m, H-5',5'') 8.59 (2H, s, arom) 4.08 (1H, m, 4') 9.45 (1H, br s, N⁺-H) 4.40 (1H, m, H-2') 5.19 (1H, d, H-1') Physical properties of Compound C4c Melting point 183.5 - 184.5° (dec.) Elementary analysis C H N S Calculated (%) 41.00 5.35 15.94 13.45 (for C₁₉H₁₃N₃O₄·HCl) Found (%) 41.00 5.38 15.90 13.55 Specific rotations [α] 26 / D + 151° (c = 0.5, H₂O) IR Spectrum (KBr)cm⁻¹ 3460 1700 1382 1072 780 3240 1675 1284 982 700 3040 1635 1118 930 609 2750 1540 1092 810 590 Example 10 Synthesis of 2',5'-di- O -pivaloyl-3'- O -methanesulfonylcytidine (Compound C5) Compound C3 (667 mg, 1.16 mmol) was dissolved in a mixture of benzene (1 ml) and methanol (1 ml), and magnesium methoxide (150 mg, 1.74 mmol) was added to this solution, followed by stirring at room temperature for 30 minutes. The reaction mixture was cooled, an aqueous saturated ammonium chloride solution was added, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed twice with saturated aqueous sodium chloride and dried over magnesium sulfate. The solvents were evaporated under reduced pressure and the residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (50:1)] to give Compound C5 (495 mg, yield 87%). An analytical sample was prepared by dropwise adding a dichloromethane solution of Compound C5 to pentane and drying the resulting precipitates. Physical properties of Compound C5 Amorphous Elementary analysis C H N S Calculated (%) 49.07 6.38 8.58 6.55 (for C₂₀H₃₁N₃O₉S) Found (%) 48.95 6.34 8.49 6.50 Specific rotations [α] 20 / D +41.8° (c = 0.3, CHCl₃) IR Spectrum (KBr)cm⁻¹ 3370 1650 1180 930 3000 1484 1145 790 2960 1369 1040 532 1736 1285 975 ¹H-NMR Spectrum (CDCl₃) δ 1.24 (9H, s, C(CH₃)₃) 5.72 (1H, d, H-1') 1.26 (9H, s, C(CH₃)₃) 5.87 (1H, d, H-5) 3.08 (3H, s, SO₂CH₃) 7.38 (1H, d, H-6) 4.47-4.35 (3H, m, H-4', H-5', H-5'') 5.40 (1H, t, H-3') 5.49 (1H, dd, H-2') Example 11 Synthesis of N ⁴-(4,4'-dimethoxytrityl)-2',5'-di- O -pivaloyl-3'- O -methanesulfonylcytidine (Compound C6) 4,4'-Dimethoxytrityl chloride (685 mg, 2.02 mmol) was added to a solution of Compound C5 (825 mg, 1.69 mmol) in anhydrous pyridine (4 ml) and the mixture was stirred at room temperature for 3 hours. After cooling, aqueous 50% pyridine was added to decompose an excess of the reagent, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed with aqueous sodium bicarbonate and dried over magnesium sulfate. The organic solvents were evaporated, the remaining pyridine was removed as the toluene azeotrope and the residue was subjected to silica gel column chromatography [eluant: chloroform-ethyl acetate-triethylamine (80:20:1)] to give 1.280 g of the desired compound C6 (yield 96%). An analytical sample was prepared by dropwise adding a dichloromethane solution of Compound C6 to pentane and drying the resulting precipitates. Physical properties of Compound C6 Amorphous Elementary analysis C H N S Calculated (%) 61.49 6.29 5.25 4.00 (for C₄₁H₄₉N₃O₁₁S·0.5H₂O) Found (%) 61.61 6.21 5.25 3.97 Specific rotations [α] 20 / D +4.0° (c = 0.5, CHCl₃) IR Spectrum (KBr)cm⁻¹ 3430 1662 1497 1180 779 2990 1653 1365 1144 702 2950 1635 1280 1032 587 1735 1506 1256 829 525 ¹H-NMR Spectrum (CDCl₃) δ 1.14(9H, s, C(CH₃)₃) 5.32(1H, br t, H-3') 1.26(9H, s, C(CH₃)₃) 5.53(1H, dd, H-2') 3.05(3H, s, S-CH₃) 5.73(1H, d, H-1') 3.80(3H, s, O-CH₃) 6.82(5H, br d) 4.32-4.42(3H, m) 7.08-7.33(10H, m) 5.05(1H, d, H-5) Example 12 Synthesis of N ⁴-(4,4'-dimethoxytrityl)-1-(3-deoxy-β- - threo -pentofuranosyl)cytosine (Compound C7) Compound C6 (50 mg, 0.063 mmol) was dissolved in a mixture of benzene (0.5 ml) and methanol (0.5 ml), and magnesium methoxide (27 mg, 0.313 mmol) and sodium borohydride (12 mg, 0.317 mmol) were added, followed by stirring for 10 hours. After cooling acetone (0.1 ml) was added to decompose an excess of the reagents, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed with aqueous sodium bicarbonate and dried over magnesium sulfate. The organic solvents were evaporated, the remaining pyridine was removed as the toluene azeotrope, and the residue was subjected to silica gel column chromatography (eluant: chloroform-methanol-triethylamine (100:2:1)) to afford the desired 3'-deoxy threo compound C7 and its epimer, erythro compound (total 30 mg) (yield 90%, threo : erythro = 93:7). The compound is C7 difficult for separation from its epimer, and was used, without purification, for further reaction. Example 13 Synthesis of N ⁴-(4,4'-dimethoxytrityl)-1-(3-deoxy-β- - threo -pentofuranosyl)cytosine (Compound C7) Reaction was carried out in a manner similar to that in Example 12 using Compound C6 (236 mg, 0.298 mmol) and magnesium methoxide (129 mg, 1.494 mmol). Tetra-n-butylammonium borohydride (383 mg, 1.488 mmol) was used in place of sodium borohydride. As the result, 142 mg of the desired 3'-deoxy threo compound C7 and an erythro isomer were obtained (yield 90%, threo compound : erythro = 93:7). Physical properties of Compound C7 Amorphous IR Spectrum (KBr)cm⁻¹ 3400 1250 830 2950 1180 782 1635 1115 702 1505 1030 585 NMR Spectrum (CDCl₃) δ 2.02 (1H, m, H-3') 4.28 (1H, m, H-4') 2.39 (1H, m, H-3'') 4.57 (1H, m, H-2') 3.63 (1H, dd, H-5') 5.07 (1H, d, H-5) 3.79 and 3.80 (6H, each, s, OCH₃) 5.90 (1H, d, H-1') 3.88 (1H, dd, H-5'') 6.80-6.85, 7.12-7.30 (m, arom) 7.57 (1H, d, H-6) Example 14 Synthesis of N ⁴, O 5' -bis(4,4'-dimethoxytrityl)-1-(3-deoxy-β- - threo -pentofuranosyl)cytosine (Compound C8a) The mono DMTr compound (Compound C7, 530 mg, 1.0 mmol) was dissolved in anhydrous pyridine (5 ml), and 4,4'-dimethoxytrityl chloride (407 mg, 1.2 mmol) was added thereto, followed by stirring at room temperature for 2 hours. After cooling, 50% aqueous pyridine was added to decompose an excess of the reagent, and the mixture was extracted with ether. The organic layer was washed successively with aqueous sodium bicarbonate and water, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure. The pyridine was removed as the toluene azeotrope. The residue was subjected to silica gel column chromatography [eluant: benzene-ethyl acetate-triethylamine-methanol (50:50:1:1)], and the fractions containing the desired substance were combined and concentrated to give 594 mg of the di-DMTr compound (Compound C8a) (yield 71%). Physical properties of Compound C8a Amorphous Elementary analysis C H N Calculated (%) 73.45 6.16 5.04 (for C₅₁H₅₁N₃O₈) Found (%) 73.20 6.46 4.79 Specific rotations [α] 25 / D = +17.1° (c = 1.1, CHCl₃) TLC Rf = 0.53 (Silica gel; CHCl₃ - MeOH (9:1)) IR spectrum (KBr)cm⁻¹ 3420 1643 1180 702 2960 1510 1034 584 2850 1252 830 ¹H-NMR Spectrum (CDCl₃) δ 2.04 (1H, m, H-3') 4.15 (1H, m, H-4') 2.20 (1H, m, H-3'') 4.68 (1H, m, H-2') 3.28 (1H, dd, H-5') 4.82 (1H, d, H-5) 3.41 (1H, dd, H-5'') 5.98 (1H, d, H-1') 3.73, 3.74 and 3.75 (12H, each s, O-CH₃) 6.72-7.34 (m, arom) 7.90 (1H, d, H-6) Example 15 Synthesis of N ⁴, O 5' -dipivaloyl-1-(3-deoxy-β- - threo -pentofuranosyl)cytosine (Compound C8b) The diol compound (Compound C4a, 180 mg, 0.799 mmol), prepared according to the method (Example 10), was dissolved in anhydrous pyridine (2 ml), and pivaloyl chloride (230 µl, 1.87 mmol) was added thereto, followed by stirring at room temperature for 6 hours. After cooling, water was added to decompose an excess of the reagent, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed successively with aqueous sodium bicarbonate and aqueous saturated sodium chloride, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure. The pyridine was removed as the toluene azeotrope and the residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (50:1)] to provide 250 mg of the desired dipivaloylated compound (Compound C8b) (yield 79%) and its erythro isomer (25 mg, yield 8%). Physical properties of Compound C8b Colorless crystals Melting point 150 - 151°C Elementary analysis C H N Calculated (%) 57.71 7.39 10.63 (for C₁₉H₂₉O₆N₃·HCl) Found (%) 57.64 7.34 10.58 Specific rotations [α] 24 / D = +134° (c = 0.5, CHCl₃) IR Spectrum (KBr)cm⁻¹ 3450 1648 1395 1128 920 3000 1618 1310 1100 803 2900 1558 1278 1035 625 1716 1485 1157 1015 ¹H-NMR Spectrum (CDCl₃) δ 1.26 (9H, s, C(CH₃)₃) 4.52 (1H, dd, H-5'') 1.29 (9H, s, C(CH₃)₃) 4.88 (1H, ddd, H-2') 1.8-2.1 (1H, br s, OH) 5.97 (1H, d, H-1') 1.95 (1H, ddd, H-3') 7.46 (1H, d, H-5) 2.43 (1H, ddd, H-3'') 8.10 (1H, d, H-6) 4.21 (1H, dd, H-5') 8.20 (1H, br s, NH) 4.42-4.45 (1H, m, H-4') Example 16 Synthesis of N ⁴, O 5' -bis(4,4'-dimethoxytrityl)-1-(3-deoxy-2- O -mesyl-β- - threo -pentofuranosyl)cytosine (Compound C9a) Methanesulfonyl chloride (0.132 ml, 1.7 mmol) was added to a solution of Compound C8a (470 mg, 0.56 mmol) in anhydrous pyridine (3 ml) and the mixture was stirred for 4.5 hours. After cooling, ice water was added to decompose an excess of the reagent, and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed successively with water, aqueous sodium bicarbonate and water, and dried over magnesium sulfate. The organic solvents were evaporated, the remaining pyridine was removed as the toluene azeotrope, and the residue was subjected to silica gel column chromatography [eluant: benzene-ethyl acetate-triethylamine-methanol (50:50:1:3)] to give 406 mg (yield 79%) of the mesylated compound (Compound C9a). An analytical sample was prepared by dropwise adding a dichloromethane solution of this mesylated compound to pentane and drying the resulting precipitates. Physical properties of Compound C9a Amorphous Elementary analysis C H N S Calculated (%) 69.56 6.66 4.27 3.26 (for C₅₂H₅₃N₃O₁₀S·C₅H₁₂) Found (%) 69.43 6.69 4.14 3.27 Specific rotations [α] 25 / D = +27.6° (c = 1.0, CHCl₃) TLC Rf = 0.58 (Silica gel; CHCl₃ - MeOH (9:1)) IR spectrum (KBr)cm⁻¹ 3450 1660 1177 827 530 2970 1510 1032 705 2860 1250 906 587 ¹H-NMR Spectrum (CDCl₃) δ 2.28 (1H, ddd, H-3') 4.17 (1H, m, H-4') 2.49 (1H, ddd, H-3'') 5.31 (1H, q, H-2') 2.88 (3H, s, C-CH₃) 4.96 (1H, d, H-5) 3.30 (2H, br s, H-5', 5'') 6.11 (1H, d, H-1') 3.74, 3.75, 3.76 and 6.75-7.36 (m, arom.) 3.77 (12H, each s, O-CH₃) 7.56 (1H, d, H-6). Example 17 Synthesis of N ⁴, O 5' -bis(4,4'-dimethoxytrityl)-1-(3-deoxy-2- O -tosyl-β- - threo -pentafuranosyl)cytosine (Compound C9b) Compound C8a (250 mg, 0.30 mmol) was dissolved in anhydrous tetrahydrofuran (2 ml), and a t -butylmagnesium chloride-THF solution (1.7 M, 360 µl, 0.61 mmol) and then p-toluenesulfonyl chloride (172 mg, 0.90 mmol) were added, followed by stirring at room temperature for 22 hours. The reaction mixture was poured in aqueous 2% sodium bicarbonate, and extracted with toluene. The extract was washed successively with aqueous saturated sodium bicarbonate and water, and dried over magnesium sulfate. Magnesium sulfate was removed by filtration, the filtrate was concentrated under reduced pressure, and the residue was purified by silica gel chromatography [10 g, eluant: chloroform - methanol - triethylamine (50:1:0.5)] to give 290 mg (yield 97%) of the 2'-tosylated compound (Compound C9b) as colorless glasses. Physical properties of Compound C9b Amorphous IR Spectrum (KBr)cm⁻¹ 3430 1652 1172 825 2960 1505 1030 699 2860 1245 895 580 ¹H-NMR Spectrum (CDCl₃) δ 2.17 (1H, m, H-3') 4.92 (1H, m, H-2') 2.41 (1H, m, H-3'') 5.00 (1H, d, H-5) 2.44 (3H, s, Ts) 6.00 (1H, d, H-1') 3.25 (2H, m, H-5) 6.76-7.38 (31H, m, arom, NH) 3.77, 3.75, 3.74 (12H, each s, OMe) 7.55 (1H, d, H-6) 4.14 (1H, m, H-4') Example 18 Synthesis of N ⁴, O 5' -dipivaloyl-1-(3-deoxy-2- O -tosyl-β- - threo -pentofuranosyl)cytosine (Compound C9c) Compound C8b (1.684 g, 4.28 mmol) was dissolved in dry tetrahydrofuran (15 ml), and sodium hydride (purity 60%, 257 mg, 6.92 mmol) was added under ice cooling, followed by stirring for 15 minutes. Further, p -toluenesulfonyl chloride (1.63 g, 8.39 mmol) was added and the mixture was stirred at room temperature for 3 hours. Ice water was added to quench the reaction and the mixture was extracted with ether containing a small amount of chloroform. The organic layer was washed successively with aqueous sodium bicarbonate and aqueous saturated sodium chloride, and dried over magnesium sulfate. The organic solvents were evaporated and the residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (100:1)] to afford 1.73 g (yield 74%) of the desired tosylated compound C9c. Physical properties of Compound C9c Colorless crystals Melting point 175 - 176°C Elementary analysis C H N S Calculated (%) 56.82 6.42 7.65 5.83 (for C₂₆H₃₅N₃O₈S) Found (%) 56.78 6.48 7.55 5.70 Specific rotations [α] 24 / D = +109° (c =0.6, CHCl₃) IR Spectrum (KBr)cm⁻¹ 3450 1733 1626 1312 902 3000 1718 1558 1180 814 2950 1673 1486 1026 663 ¹H-NMR Spectrum (CDCl₃) δ 1.23 (9H, s, C(CH₃)₃) 5.24 (1H, ddd, H-2') 1.31 (9H, s, C(CH₃)₃) 5.99 (1H, d, H-1') 2.26 (1H, ddd, H-3') 7.25-7.6 (4H, m, arom) 2.42 (3H, s, ArMe) 7.60 (1H, d, H-5) 2.58 (1H, ddd, H-3'') 7.87 (1H, d, H-6) 4.22 (1H, dd, H-5') 4.35-4.43 (2H, m, H-5'', H-4') Example 19 Synthesis of N ⁴, O 5' -dipivaloyl-1-(3-deoxy-2- O -phenoxythiocarbonyl-β- - threo -pentofuranosyl)cytosine (Compound C9d) Compound C8b (100 mg, 0.25 mmol) was dissolved in acetonitrile (2 ml), and N , N -dimethylaminopyridine (80 mg, 0.66 mmol) and phenyl chlorothionocarbonate (40 µl, 0.33 mmol) were added, followed by stirring at room temperature for 20 hours. Water was added to the reaction mixture and extracted with chloroform, and the organic layer was washed successively with water, aqueous saturated sodium bicarbonate and water, and dried over magnesium sulfate. The drying agent was removed by filtration, the filtrate was concentrated under reduced pressure, and the residue was purified by preparative TLC [200 x 200 x 2 mm, developing solvent: chloroform-methanol (9:1)] to give 73 mg of Compound C9d as colorless glasses (yield 53%). Physical properties of Compound C9d Amorphous IR Spectrum (film) cm⁻¹ 3420 1638 1270 3000 1557 1205 1732 1488 1125 1715 1398 799 1671 1310 735 ¹H-NMR Spectrum (CDCl₃) δ 1.26 (9H, s, piv) 6.03 (1H, m, H-2') 1.30 (9H, s, piv) 6.28 (1H, d, H-1') 2.29 (1H, m, H-3') 6.98 (1H, d, H-5) 2.75 (1H, m, H-3'') 7.24-7.48 (5H, m, Ph) 4.35 (2H, m, H-5') 7.98 (1H, d, H-6) 4.48 (1H, m, H-4') 8.13 (1H, br s, amide) Example 20 Synthesis of N ⁴, O 5' -dipivaloyl-1-(3-deoxy-2- O -imidazoylthiocarbonyl-β- - threo -pentofuranosyl)cytosine (Compound C9e) Compound C8b (80 mg, 0.20 mmol) and 1,1'-thiocarbonyldiimidazole (50 mg, 0.28 mmol) were dissolved in acetonitrile (1 ml) and the solution was stirred at room temperature for 20 hours. The organic solvent was evaporated and the residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (50:1)] to afford 83 mg of the desired compound C9e (yield 81%). Physical properties of Compound C9e Amorphous ¹H-NMR Spectrum (CDCl₃) δ 1.21 (9H, s, C(CH₃)₃) 6.20 (1H, d, H-1') 1.28 (9H, s, C(CH₃)₃) 6.25 (1H, m, H-2') 2.19 (1H, ddd, H-3') 7.01, 7.41 (2H, each, m, arom.) 2.83 (1H, ddd, H-3'') 7.55 (1H, d, H-5) 4.14 (1H, dd, H-5') 8.10 (1H, br s, NH) 4.47-4.53 (2H, m, H-4', H-5'') 8.12 (1H, d, H-6) 8.12 (1H, s, arom.) Example 21 Synthesis of N ⁴, O 5' -bis(4,4'-dimethoxytrityl)-1-[(2 R )-2-azido-2,3-dideoxy-β- - glycero -pentofuranosyl]cytosine (Compound C10) The 3'-deoxy-2'-O-mesyl compound (Compound C9a) (287 mg, 0.32 mmol) was dissolved in N , N -dimethylformamide (4 ml), and sodium azide (102 mg, 1.58 mmol) was added thereto, followed by stirring at 115 - 120°C for 4 hours. The reaction mixture was cooled and extracted with ether, and the extract was washed several times with water and dried over magnesium sulfate. The ether was removed and the residue was subjected to silica gel column chromatography [eluant: benzene-ethyl acetate-triethylamine-methanol (50:50:1:0.2)] to give 251 mg of the azide compound (Compound C10) (yield 93%). Physical properties of Compound C10 Amorphous Elementary analysis C H N Calculated (%) 71.31 5.87 9.79 (for C₅₁H₅₀N₆O₇) Found (%) 71.11 5.86 9.52 Specific rotations [α] 25 / D = 31.0° (c = 0.9, CHCl₃) TLC Rf = 0.66 Silica gel [benzene-ethyl acetate-methanol (1:1:0.1)] IR spectrum (KBr)cm⁻¹ 3430 2110(N₃) 1248 1033 585 2980 1655 1175 825 2850 1505 1102 702 ¹H-NMR Spectrum (CDCl₃) δ 1.77 (1H, dd, H-3') 4.27 (1H, d, H-2') 2.04 (1H, m, H-3'') 4.72 (1H, d, H-5) 3.30 (1H, dd, H-5') 5.89 (1H, br s, H-1') 3.54 (1H, dd, H-5'') 6.7-7.3 (m, arom.) 3.73 and 3.74 (12H, each s, CH₃) 7.84 (1H, d, H-6) 4.35 (1H, m, H-4') Reference example 4 Synthesis of 1-[(2 R )-2-azido-2,3-dideoxy-β- - glycero -pentofuranosyl]cytosine (Compound C11) A mixture of the DMTr-protected 2'-azido compound (Compound C10) (171 mg, 0.20 mmol) and 80% acetic acid (3 ml) was stirred at 50 - 55°C for 75 minutes. The acetic acid was removed under reduced pressure and azeotropy using toluene-ethanol, and a small amount of dichloromethane was added to the residue to deposit the free 2'-azide compound (Compound C11) as crystals (42 mg, yield 84%). An analytical sample was obtained by recrystallization from isopropyl alcohol. Physical properties of Compound C11 Melting point 171 - 172°C (dec.) Elementary analysis C H N Calculated (%) 42.55 4.84 33.08 (for C₉H₁₂N₆O₃·0.1H₂O) Found (%) 42.48 4.75 33.02 Specific rotations [α] 25 / D = -42.6° (c = 0.3, DMF) IR spectrum (KBr)cm⁻¹ 3480 2130(N₃) 1620 1105 705 3300 1680 1500 970 595 3100 1656 1290 853 2970 1639 1250 780 ¹H-NMR Spectrum (DMSO-d₆) δ 1.81 (1H, ddd, H-3') 5.14 (1H, t, OH) 2.00 (1H, ddd, H-3'') 5.70 (1H, d, H-5) 3.56 (1H, ddd, H-5') 5.72 (1H, d, H-1') 3.78 (1H, ddd, H-5'') 7.14 (2H, br d, NH₂) 4.20 (1H, m, H-4') 7.96 (1H, d, H-6) 4.32 (1H, br d, H2') Reference example 5 Synthesis of 1-(2,3-dideoxy-β- - glycero -pent-2'-enofuranosyl)cytosine (Compound C12) A 25 weight % sodium methoxide-methanol solution (175 µl) was added to a solution of the tosylated compound C9c (84 mg, 0.153 mmol) in methanol (2 ml), and the mixture was stirred under reflux for 24 hours. After cooling, aqueous saturated ammonium chloride was added to quench the reaction, and the by-products were extracted with ethyl acetate and removed. An activated carbon (manufactured by WAKO JUNYAKU Co., Ltd., for chromatography, 10 ml) was added to the resulting water layer to adsorb the organic matters. The resulting activated carbon was washed with water. Elution with methanol gave 2.5 mg of the unsaturated compound C12 (yield 7.8%). The tosylated compound C9c (53 mg, 0.097 mmol) and potassium tert -butoxide (54 mg, 0.48 mmol) were dissolved in tert -butyl alcohol (1 ml), and the solution was stirred at 60°C for 30 minutes. The reaction mixture was poured in aqueous saturated ammonium chloride and extracted with ethyl acetate. The organic layer was washed with aqueous saturated sodium chloride, dried over magnesium sulfate, and concentrated under reduced pressure. Saturated aqueous ammonia-methanol (1 ml) was added to the residue and the mixture was stirred at room temperature for 24 hours. The methanol was evaporated under reduced pressure and the residue was purified by silica gel column chromatography to give 16 mg of the desired unsaturated compound C12 (yield 79%). The same procedure as described in Method 2 was carried out except that sodium hydride (oil, purity 60%, 19 mg, 0.48 mmol) was used in place of potassium tert -butoxide. As the result, 10 mg of the desired unsaturated compound was obtained (yield 49%). Physical properties of Compound C12 ¹H-NMR Spectrum (DMSO-d₆) δ 3.57 (2H, m, H-5', H-5'') 6.89 (1H, m, H-1') 4.76 (1H, m, H-4') 7.14 (2H, br d, NH₂) 4.95 (1H, t, OH) 7.68 (1H, d, H-6) 5.70 (1H, d, H-5) 5.87 (1H, m, H-3') 6.33 (1H, m, H-2') Reference example 6 Synthesis of 1-(2,3-dideoxy-β- - glycero -pentofuranosyl)cytosine (dideoxycytidine) (Compound C13) The reduction was carried out according to the method of Horwitz et al. (J. P. Horwitz, J. Chua, M. Noel, and J. T. Donatti, J. Org. Chem., 32 , 817 (1967)). The unsaturated compound C12 (50 mg, 0.239 mmol) and 5% palladium-carbon (10 mg) in methanol (3 ml) was stirred under a hydrogen atmosphere for one hour. After removal of the insoluble materials by filtration, the solvent was evaporated under reduced pressure and the resulting residue was purified by silica gel column chromatography [eluant: chloroform-methanol (8:2)] to obtain 36 mg of the desired dideoxycytidine C13 (yield 71%). Physical properties of Compound C13 ¹H-NMR Spectrum (DMSO-d₆) δ 1.72-1.89 (3H, m, H-2', H-3', H-3'') 2.21-2.30 (1H, m, H-2'') 3.54 (1H, m, H-5') 7.10 (2H, br d, NH₂) 3.68 (1H, m, H-5'') 7.92 (1H, d, H-6) 4.01 (1H, m, H-4') 5.05 (1H, t, OH) 5.72 (1H, d, H-5) 5.93 (1H, dd, H-1') Example 22 Synthesis of 1-(3-deoxy-β- - threo -pentofuranosyl)uracil (Compound U3) Pivaloyl chloride (9.08 g, 75 mmol) was added to a solution of uridine (7.32 g, 30 mmol) in dry pyridine (100 ml) at 5°C, and the mixture was then stirred at room temperature for 30 minutes. After the reaction mixture had been cooled to 5°C, mesyl chloride (13.8 g, 120 mmol) was added thereto and stirred at room temperature for one hour. The reaction mixture was cooled again and the reaction was quenched with 50% aqueous pyridine. The product was extracted with ether containing a small amount of chloroform. The extract was washed with water, aqueous sodium bicarbonate, and then water, and dried over magnesium sulfate. The organic solvent was removed. The pyridine was removed by toluene azeotropic evaporation. The residue was dissolved in methanol (100 ml) and cooled to 5°C. A solution of potassium hydroxide (6.72 g, 120 mmol) in methanol (80 ml) was added thereto with stirring, and then immediately sodium borohydride (2.28 g, 60 mmol) was added thereto. After the reaction mixture had been stirred at room temperature for 24 hours, the insoluble crystals were collected by filtration with through a Celite pad and washed with methanol. The filtrate and the washings were combined and cooled to 5°C. A mixture of concentrated hydrochloric acid (9.8 ml, ca. 118 mmol) and methanol (37 ml) was added thereto. The precipitate was collected by filtration through a Celite pad and washed with methanol. The filtrate and washings were combined, concentrated to 40 ml, and cooled. Then a mixture of concentrated hydrochloric acid (2.1 ml) and methanol (10 ml) was added again and the insoluble materials were collected by filtration and washed with methanol. The filtrate and washings were concentrated and the water was removed three times by ethanol azeotropic co-evaporation. The residue was subjected to silica gel column chromatography [eluant: chloroform-methanol (99:1)] and the objective compound (Compound U3) (3.82 g, 56%) was eluted with the same solvent system (97:3 → 9:1). An analytical sample was obtained by recrystallization from isopropyl alcohol. Physical properties of Compound U3 Melting point 146 - 147°C Elementary analysis C H N Calculated (%) 47.37 5.30 12.28 (for C₉H₁₂N₂O₅) Found (%) 47.18 5.26 12.38 Specific rotations [α] 20 / D = +97.7° (c = 1.14, DMF) IR Spectrum (KBr) cm⁻¹ 3430, 3270, 2960, 1620, 1452, 1436, 1407, 1278, 1197, 1122, 1065, 1032, 992, 921, 877, 810, 753, 718, 695, 633, 572, 540 ¹H-NMR spectrum (DMSO-d₆) δ 11.78 (1H, br s, H-3) 4.33 (1H, m, H-2') 7.74 (1H, d, H-6) 4.00 (1H, m, H-4') 5.87 (1H, d, H-1') 3.60 (1H, dd, H-5') 5.55 (1H, dd, H-5) 3.55 (1H, dd, H-5'') 5.35 (1H, br s, OH-2) 2.24 (1H, m, H-3') 5.05 (1H, br s, OH-5') 1.75 (1H, m, H-3'') Example 23 Synthesis of 9-(3-deoxy-β- - threo -pentofuranosyl) guanine (Compound G3) Guanosine (5.0 g, 17.7 mmol) was suspended in dry pyridine (50 ml). To the suspension was added pivaloyl chloride (9.8 ml, 79.6 mmol) under ice cooling, and the mixture was then stirred at room temperature for 45 minutes. After ice cooling, methanesulfonyl chloride (4.1 ml, 53.0 mmol) was added dropwise to the reaction mixture, after which it was first stirred at 0°C for 1 hour and then at room temperature for 3 hours. The reaction mixture was cooled with iced water and a small amount of water was added thereto and the mixture was stirred for 30 minutes. The reaction mixture was extracted with chloroform-ether and the organic layer was washed with saturated aqueous sodium bicarbonate and then with water, and dried over magnesium sulfate. The drying agent was removed by filtration and the filtrate was evaporated. The residue was dissolved in toluene and the solvent was evaporated under reduced pressure. This procedure was repeated several times to completely remove the remaining pyridine.. The residue was dissolved in methanol (120 ml). Sodium methoxide (7.6 g, 140.7 mmol) and sodium borohydride (2.7 g, 71.4 mmol) were added to the solution, after which it was first stirred at room temperature for 1 hour and then at 50°C for 5 hours. After ice cooling, a small amount of acetone was added to decompose an excess of NaBH₄. Methanol (100 ml) was added to the mixture, after which it was concentrated to 1/2 volume. This procedure was repeated to remove methyl borate thus produced as a by-product. The reaction mixture was again cooled with ice, and concentrated hydrochloric acid was gradually added to neutralize. Then, water was added and the mixture was concentrated under reduced pressure. The residue from which alcohol had been removed completely was suspended in water (about 150 ml), and applied to HP-20 (DIAION: 600 ml made up with water, Mitsubishi Chemical Industries Ltd.) column. Water (about 1.5 ℓ ) was passed through the column to elute inorganic salts, and the product was then eluted with 5% methanol-water followed with 5% methanol - 0.1M ammonia water. Fractions containing compound G3 were concentrated to dryness under reduced pressure and the residue was recrystallized from methanol to give colorless crystals (1.31 g, 27.8%). The mother liquor was dried up under reduced pressure, again subjected to HP-20 column chromatography (300 ml) and followed by the same procedures mentioned above to give compound G3 (0.55 g, 11.6%). Total amount of the obtained products was 1.86 g (39%). Physical properties of compound G3 Melting point > 265°C (235°C sintered) Elementary analysis C H N Calculated (%) 44.05 5.03 25.69 (for C₉H₁₂N₂O₅) Found (%) 43.86 4.85 25.49 Specific rotations [α] 20 / D = +8.7° (c = 1.10, DMF) IR Spectrum (KBr) cm⁻¹ 3450, 3190, 2950, 2760, 1055, 1642, 1605, 1539, 1648, 1406, 1391, 1362, 1327, 1177, 1069, 1045, 782, 685, 640 ¹H-NMR spectrum (DMSO-d₆) δ 10.82 (1H, br s, NH-1) 4.42 (1H, m, H-2') 7.83 (1H, s, H-8) 4.03 (1H, m, H-4') 6.64 (2H, s, NH₂-2) 3.61 (1H, m, H-5') 5.86 (1H, d, H-1') 3.54 (1H, m, H-3') 5.39 (1H, br d, OH-2') 2.28 (1H, m, H-3') 5.06 (1H, br t, OH-5') 1.96 (1H, m, H-3'') Example 24 Synthesis of 9-(3-deoxy-β- - threo -pentofuranosyl) adenine (Compound A'3) Adenosine (5.0 g, 18.7 mmol) was suspended in dry pyridine (50 ml) and cooled to -15°C. To the suspension was added pivaloyl chloride (7.0 ml, 56.8 mmol), and the mixture was first stirred at -15°C for 1.5 hour, and then at 0°C for 2 hours. Methanesulfonyl chloride (4.3 ml, 55.6 mmol) was added dropwise to the reaction mixture under ice cooling. The mixture was stirred at room temperature for 3 hours, then cooled with ice again. A small amount of water was added and stirred for 30 minutes. The reaction mixture was extracted with chloroform-ether. The organic layer was washed with saturated aqueous sodium bicarbonate solution and then with water, and dried over magnesium sulfate. The drying agent was removed by filtration and the filtrate was concentrated in vacuo to dryness to give amorphous substances. The remaining pyridine was removed by repeated co-evaporation with toluene. The residue was dissolved in methanol (150 ml), cooled to -15°C, after which sodium methoxide (8.1 g, 150 mmol) and sodium borohydride (2.5 g, 66.1 mmol) were added. The mixture was first stirred at room temperature for 14 hours and then at 50°C for 3 hours. After ice cooling, a small amount of acetone was added to the reaction mixture to decompose an excess of NaBH₄. Concentrated hydrochloric acid was gradually added to neutralize the solution. Ethanol was added to the mixture, which was then concentrated under reduced pressure to remove water completely. The residue was suspended in chloroform-methanol and applied to a silica gel column (250 g). Elution with chloroform - methanol (8:1 → 5:1) gave compound A'3 as oils, which were then crystallized from ethanol-ether. Yield: 2.7 g (57%). The physical properties of the product was identical with those reported earlier [R. Mengel and H. Wiedner, Chem. Ber., 109 , 1395 (1976); A. Nilass and J. Chattopadhyaya, Synthesis, 1986 , 196)]. Physical properties of compound A'3 Melting point 190 - 192°C Elementary analysis C H N Calculated (%) 47.47 5.26 27.68 (for C₁₀H₁₃N₅O₃·0.1H₂O) Found (%) 47.36 5.22 27.67 Specific rotations [α] 20 / D = -24.3° (c = 1.08, DMF) IR Spectrum (KBr) cm⁻¹ 3370, 3230, 1680, 1672, 1611, 1575, 1490, 1428, 1385, 1341, 1316, 1220, 1115, 1105, 1093, 1056, 999, 799, 721, 652, 590 ¹H-NMR spectrum (DMSO-d₆) δ 8.30 (1H, s, H-8) 4.51 (1H, m, H-2') 8.14 (1H, s, H-2) 4.09 (1H, m, H-4') 7.24 (2H, bs, NH₂) 3.66 (1H, m, H-5') 6.15 (1H, d, H-1') 3.58 (1H, m, H-5'') 5.42 (1H, d, OH-2') 2.29 (1H, m, H-3') 5.17 (1H, t, OH-5') 2.02 (1H, m, H-3'') Industrial utility The dideoxynucleoside derivatives among the novel nucleoside derivatives of the present invention are useful as an antiviral agent. Those except for the dideoxynucleoside derivatives among the novel nucleoside derivatives of the present invention are useful as intermediates for synthesizing the dideoxynucleosides derivatives. Further, according to the synthetic processes of the present invention, above-mentioned and known intermediates can be synthesized short-step reactions using inexpensive reagents.

Claims: Nucleoside derivatives represented by the following general formula [I]: wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or a imidazolyl group optionally having a protective group; R¹ and R³ are a PivO- group, a TsO- group, a DMTrO- group or a benzoyl group; R² is a MsO- group, a TflO-group or a TsO- group. The nucleoside derivatives of claim (1) wherein B is an adenine base residue optionally having a protective group. The nucleoside derivatives of claim (1) wherein B is a guanine base residue optionally having a protective group. The nucleoside derivatives of claim (1) wherein B is a uracil base residue optionally having a protective group. The nucleoside derivatives of claim (1) wherein B is a cytosine base residue optionally having a protective group. The nucleoside derivatives or any one of claims (1) to (5) wherin R² is a MsO- group and R¹ and R³ are a PivO- group. The nucleoside derivatives of any one of claims (1) to (5) wherein R¹ is a DMTrO- group, R² is a MsO- group and R³ is a TsO- group. Nucleoside derivatives represented by the following general formula [II]: wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or a imidazolyl group optionally having a protective group; R¹ is OR⁴ wherein R⁴ is a protective group or a hydrogen atom; R³ is an azido group, a halogen atom or a hydrogen atom; R³' is a MsO- group, a TsO- group, a a or a hydroxyl group 1 provided that when B is a cytosine residue optionally having a protective group, at least one of R¹ and R³' is other than hydroxyl group and R³ is not an azido group; further, when B is an adenine residue optionally having a protective group or a uracil residue optionally having a protective group, at least one of R¹ and R³' is other than hydroxyl group. The nucleoside derivatives of claim (8) wherein B is an adenine base residue optionally having a protective group. The nucleoside derivatives of claim (8) wherein B is a guanine base residue optionally having a protective group. The nucleoside derivatives of claim (8) wherein B is a uracil base residue optionally having a protective group. The nucleoside derivatives of claim (8) wherein B is a cytosine base residue optionally having a protective group. The nucleoside derivatives of any one of claims (8) to (12) wherein R³ is a hydrogen atom and R³' is a hydroxyl group. The nucleoside derivatives of any one of claims (8) to (12) wherein R³ is an azide group and R³' is a hydrogen atom. The nucleoside derivatives of any one of claims (8) to (12) wherein R³ is a halogen atom and R³' is a hydrogen atom. A process for synthesizing a nucleoside derivative represented by the formula [II']: (wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or an imidazolyl group optionally having a protective group; R¹ is OR⁴ wherein R⁴ is a protective group or a hydrogen atom; R³ is a hydrogen atom; and R³' is a hydroxyl group) which comprises treating a compound represented by the formula [I']: (wherein B is a purine base residue optionally having a protective group, a pyrimidine base residue optionally having a protective group or an imidazolyl group optionally having a protective group; R¹ and R³ are PivO- group, a TsO- group, a DMTrO- group, a benzoyl group or a hydroxyl group; and R² is a MsO- group, a TflO- group or a TsO- group) with a base and a reducing agent. The process of claim (16) wherein the base is a metal alkoxide. The process of claim (17) wherein the metal alkoxide is an alkali metal or an alkaline earth metal alkoxide. The process of claim (18) wherein the metal alkoxide is a magnesium methoxide. The process of claim (17) wherein the reducing agent is a metal borohydride complex whose metal is an alkali metal or alkaline earth metal. The process of claim (20) wherein the metal borohydride complex is a sodium borohydride. A process for selectively eliminating the Piv- group of B alone in the nucleoside derivatives represented by the formula [I'']; (wherein B is a purine base residue protected by a Piv- group, a pyrimidine base residue protected by a Piv- group or an imidazolyl group protected by a Piv- group; R¹ and R³ are a PivO- group; R² is a MsO-group, TflO- group, TsO- group, hydroxyl group or a hydrogen atom) which comprises treating the nucleoside derivatives [I''] with an alkaline compound. The process of claim (22) wherein the alkaline compound is a magnesium alkoxide. The process of claim (23) wherein the magnesium alkoxide is magnesium methoxide.

Inventors: KAMANA MASAJIRO, KUZUHARA HIROYOSHI, NISHIKAWA MASAHIRO, YAMASAKI NORITSUGU, KAWANA, MASAJIRO, KUZUHARA, HIROYOSHI, NISHIKAWA, MASAHIRO, YAMASAKI, NORITSUGU
Applicants: RIKAGAKU KENKYUSHO
Application year: 1988
Main classification: C07H19
IPC codes: C07D473, C07H19, C07D405