in formula ( i ), b may be a guanine residue , an adenine residue , a 2 - amino - 6 - chloropurine residue , an xanthine residue , a hypoxanthine residue , a 2 , 6 - diaminopurine residue or a 2 - aminopurine residue , and preferably is guanin - 9 - yl , 2 - amino - 6 - chloropurin - 9 - yl , 2 - acetoamino - 6 - chloropurin - 9 - yl , 2 - acetoamino - 6 -( n , n - diphenylcarbamoyl ) oxypurin - 9 - yl , 2 - amino - 6 - benzyloxypurin - 9 - yl , 2 - amino - 6 -( methoxyethoxy ) purin - 9 - yl or adenin - 9 - yl . the compound of formula ( i ) according to the present invention may be prepared , for example , by the following reactions : ## str5 ## in formula ( a ) x represents a leaving group and b is defined as above . precisely , the hydroxyl group of compound ( 1 ) is converted into a leaving group such as a halogen or a sulfonyloxy group to form compound ( 2 ). this compound is then reacted with a purine derivative in the presence of a base such as potassium carbonate to prepare a compound according to the present invention . when compound ( 1 ) is halogenated , it may be reacted with carbon tetrabromide , carbon tetrachloride , bromine or n - bromosuccinimide in the presence of a trialkylphosphine or triphenylphosphine or it is halogenated using a halogenating agent such as thionyl chloride or phosphorus tribromide . typically , when triphenylphosphine and carbon tetrachloride are employed , 1 . 1 to 3 , preferably 1 . 2 to 2 equivalents of triphenylphosphine and 1 . 1 to 100 , preferably 1 . 5 to 5 equivalents of carbon tetrachloride are employed and reacted with the bicyclic compound of formula ( 1 ) in a solvent such as dichloromethane , dichloroethane or dimethylformamide at a temperature of - 20 ° to 40 ° c ., preferably 0 ° to 30 ° c . for the period of 5 minutes to 3 hours . in such a process , 0 . 1 to 2 equivalents of a base such as triethylamine or pyridine may be present . when thionyl chloride is used as a halogenating agent , a reaction may be conducted in the presence of 1 to 2 equivalents of a base such as triethylamine or pyridine using 1 to 2 equivalents of thionylchloride in a solvent such as dichloromethane at a temperature of 0 ° to 50 ° c . for a period of 5 minutes to 3 hours . in the case of conversion of the hydroxy group of the compound of formula ( 1 ) into a sulfonyloxy group , the reaction may be conducted in the presence of 1 to 2 equivalents of a base such as triethylamine or pyridine using 1 to 2 equivalents of methanesulfonyl chloride or p - toluenesulfonyl chloride in a solvent such as dichloromethane or ethyl acetate at a temperature of 0 ° to 50 ° c . for a period of 5 minutes to 10 hours . the reaction of compound ( 2 ) with the likes of a purine derivative may be conducted by adding compound ( 2 ) to the purine derivative in the presence of a base such as sodium potassium carbonate or sodium hydride in an amount of 1 to 3 , preferably 1 to 1 . 1 equivalents based on the amount of the purine derivative in a polar solvent such as dimethylformamide , acetonitrile or tetrahydrofuran at a temperature of 0 ° to 80 ° c . for a period of 1 to 72 hours . in this process , a crown ether such as 18 - crown - 6 may be added to facilitate the reaction . the starting material in the reaction mentioned above , namely , ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methanol ( compound ( 1 )) is a novel compound , and may be prepared by the multistep method infra : ## str6 ## in the formulas r 1 represents a lower alkyl group , and r 2 represents a lower alkyl or an aryl group . compound ( 4 ) may be obtained by reacting dialkyl malonate with epichlorohydrin in the presence of a base such as sodium alkoxide by the known method ( helv . chim . acta , 72 , 1301 ( 1989 )). in this reaction , by using an optically active epichlorohydrin , an optically active compound ( 4 ) is obtained and its absolute structure can be retained in compound ( 1 ) and compound ( 3 ). compound ( 4 ) can readily be converted into compound ( 5 ) by saponification with an alkali such as sodium hydroxide or potassium hydroxide followed by cyclization into a lactone using a suitable acid such as hydrochloric acid or sulfuric acid . compound ( 5 ) thus obtained leads directly to compound ( 1 ) through a reaction with 0 . 8 to 1 . 2 equivalents of a reducing agent such as diborane in a polar solvent such as tetrahydrofuran . alternatively , compound ( 5 ) is converted into compound ( 6 ) through a reaction with 1 to 10 equivalents of thionyl chloride and then into compound ( 1 ) through reaction with 0 . 5 to 2 equivalents of a reducing agent such as sodium borohydride in a polar solvent such as dioxane or diglyme . further , alternatively , compound ( 5 ) may be converted into compound ( 7 ) through a reaction with 1 to 2 equivalents of chloroformate in the presence of 1 to 2 equivalents of a base such as triethylamine or pyridine in a solvent such as tetrahydrofuran and then into compound ( 1 ) through reaction with 0 . 5 to 5 equivalents of a reducing agent such as sodium borohydride . another process can be used to obtain compound ( 1 ) as follows : ## str7 ## in the formulas above , r 1 is defined as above , and r 3 and r 4 are each hydrogen or an alkyl group . compound ( 4 ) is reacted with 1 to 10 equivalents of an amine such as ammonia in a solvent such as ethanol or methanol to prepare compound ( 8 ), to which 0 . 5 to 20 equivalents of a reducing agent such as sodium borohydride are added to prepare compound ( 9 ). then the amide is hydrolyzed with a suitable acid such as hydrochloric acid or sulfuric acid to cyclize the lactone ring , thereby preparing compound ( 1 ). still another alternative method of preparing compound ( 1 ) is as follows : ## str8 ## wherein r 1 is defined as above and m represents an alkali metal or an alkaline earth metal . compound ( 4 ) is reacted with 0 . 8 to 1 . 2 equivalents of a base such as sodium hydroxide in a solvent such as ethanol or methanol to prepare compound ( 10 ), with which 0 . 5 to 20 equivalents of a reducing agent such as sodium borohydride are reacted to prepare compound ( 11 ). then a suitable acid such as hydrochloric acid or sulfuric acid is added to cyclize the lactone ring to prepare compound ( 1 ). a cyclopropane derivative represented by formula ( i ) according to the present invention ( compound ( 3 )) can be converted into a compound represented by formula ( a ) ( compound ( 12 )) which has potent antiviral activity through a reaction with 0 . 5 to 20 equivalents of a reducing agent such as sodium borohydride in a suitable solvent such as ethanol , methanol , water or the like . ## str9 ## wherein b is defined as above . when b is 2 - amino - 6 - chloropurine , 2 - acetoamino - 6 - chloropurine , 2 - acetoamino - 6 -( n , n - diphenylcarbamoyl ) oxypurine , 2 - amino - 6 - benzyloxypurine or 2 - amino - 6 -( methoxyethoxy ) purine or the like , the base moiety can be converted into guanine by an appropriate means such as acid hydrolysis , alkaline hydrolysis and reduction . conversion into guanine is preferably conducted before the reduction of the lactone ring . the cyclopropane derivative of formula ( i ) and its precursor ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methanol include their racemates , optical isomers and stereoisomers . as described above , by using an optically active epichlorohydrin , any of the optical isomers can be prepared similarly as described above . alternatively , any of the intermediates or the final product may be resolved by optical resolution column chromatography or by fractional crystallization using diastereomeric salts . having generally described this invention , a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified . with regard to the relative configuration in the examples , the cyclopropane moiety is considered to be on a flat plane and the substituents positioned below the plane are represented by &# 34 ; α &# 34 ; while those positioned above the same by &# 34 ; β &# 34 ;. a 2 . 42 g ( 105 mmol ) amount of metallic sodium was dissolved in 200 ml of ethanol at 0 ° c . in an argon atmosphere . 16 . 7 g ( 110 mmol ) of diethyl malonate was added to the solution and then 7 . 8 ml ( 100 mmol ) of epichlorohydrin dissolved in 5 ml of ethanol was added dropwise at room temperature . the solution thus obtained was heated at 75 ° c . for 20 hours and then cooled to 0 ° c . the precipitate which formed was removed by filtration . the filtrate was concentrated under reduced pressure , and water was added to the residue , which was then extracted with dichloromethane . the organic layer was dried over anhydrous sodium sulfate , and the solvent was removed by distillation . the residue was purified by silica gel chromatography ( hexane : ethyl acetate = 5 : 1 to 1 : 1 ) to yield 12 . 0 g ( 70 mmol , 70 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 31 ( t , j = 7 . 1 hz , 3h ), 1 . 37 ( dd , j = 4 . 8 , 5 . 4 hz , 1h ), 2 . 08 ( dd , j = 4 . 8 , 8 . 0 hz , 1h ), 2 . 72 ( m , 1h ), 4 . 18 ( d , j = 9 . 6 hz , 1h ), 4 . 27 ( q , j = 7 . 1 hz , 2h ), 4 . 36 ( dd , j = 4 . 5 , 9 . 6 hz , 1h ) a 5 . 10 g ( 30 . 0 mmol ) amount of ethyl 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carboxylate was dissolved in 80 ml of 1n aqueous solution of sodium hydroxide and stirred at room temperature for 4 hours . after the solvent was concentrated to a volume of about 40 ml under reduced pressure , 10 ml of concentrated hydrochloric acid was added to adjust the ph to 1 or lower and the mixture was adsorbed on a resin ( sepabeads sp - 207 , mitsubishi kasei ) equilibrated with 0 . 1n hydrochloric acid . after washing the resin with 120 ml of 0 . 1n hydrochloric acid and 80 ml of water , the product was eluted with 300 ml of 20 % aqueous solution of methanol to yield 3 . 85 g ( 27 . 0 mmol , 90 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 56 ( dd , j = 4 . 5 , 5 . 6 hz , 1h ), 2 . 15 ( dd , 4 . 5 , 7 . 8 hz , 1h ), 2 . 96 - 3 . 03 ( m , 1h ), 4 . 31 ( d , j = 9 . 6 hz , 1h ), 4 . 47 ( dd , j = 5 . 0 , 9 . 6 hz , 1h ) a 134 . 7 mg ( 0 . 948 mmol ) amount of 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carboxylic acid was dissolved in 1 . 13 g of thionyl chloride , and stirred at 65 ° c . for 100 minutes . after cooling to room temperature , thionyl chloride was distilled off under reduced pressure to obtain 152 . 2 mg ( 0 . 984 mmol , 100 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 68 ( t , j = 5 . 4 hz , 1h ), 2 . 41 ( dd , j = 5 . 4 , 8 . 4 hz , 1h ), 3 . 04 - 3 . 11 ( m , 1h ), 4 . 24 ( d , j = 9 . 9 hz , 1h ), 4 . 44 ( dd , j = 4 . 7 , 9 . 9 hz , 1h ) a 76 . 1 mg ( 0 . 474 mmol ) amount of 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carbonyl chloride was dissolved in 2 ml of 1 , 4 - dioxane and 10 . 8 mg ( 0 . 284 mmol ) of sodium borohydride was added . after stirring for 3 hours at room temperature , phosphate buffer ( ph 7 ) was added and the mixture was extracted with ethyl acetate . the organic layer was dried over anhydrous sodium sulfate , and then the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 19 : 1 ) to yield 15 . 7 mg ( 0 . 123 mmol , 26 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 01 ( t , j = 4 . 8 hz , 1h ), 1 . 32 ( dd , j = 4 . 8 , 7 . 7 hz , 1h ), 2 . 26 - 2 . 33 ( m , 1h ), 3 . 69 ( d , 12 . 6 hz , 1h ), 4 . 05 ( d , j = 12 . 6 hz , 1h ), 4 . 18 ( d , j = 9 . 3 hz , 1h ), 4 . 34 ( dd , j = 4 . 7 , 9 . 3 hz , 1h ) a 72 . 5 mg ( 0 . 510 mmol ) amount of 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carboxylic acid and 77 . 4 mg ( 0 . 764 mmol ) of triethylamine were dissolved in 1 . 0 ml of tetrahydrofuran and cooled to - 18 ° c ., and then 83 . 0 mg ( 0 . 764 mmol ) of ethyl chloroformate in 0 . 5 ml of tetrahydrofuran was added . after stirring for 30 minutes at - 18 ° c ., 57 . 9 mg ( 1 . 53 mmol ) of sodium borohydride in 0 . 75 ml of water was added . after further stirring for 20 minutes at - 18 ° c ., 2 ml of 2n hydrochloric acid were added . tetrahydrofuran was distilled off under reduced pressure , and the residue was extracted with dichloromethane . after drying the organic layer over anhydrous sodium sulfate , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 19 : 1 ) to yield 35 . 9 mg ( 0 . 281 mmol , 55 %) of the above - named compound . this compound had physical data similar to that of example 1 . a 1 . 73 g ( 10 . 2 mmol ) amount of ethyl 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carboxylate was dissolved in 2m nh 3 / methanol solution . after stirring at room temperature for 15 minutes , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( ethyl acetate ) to obtain 1 . 12 g ( 6 . 62 mol , 65 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 25 ( t , j = 7 . 1 hz , 1h ), 1 . 80 ( dd , 4 . 5 , 9 . 7 hz , 1h ), 1 . 88 ( dd , 4 . 5 , 8 . 0 hz , 1h ), 2 . 19 - 2 . 30 ( m , 1h ), 3 . 67 - 3 . 78 ( m , 1h ), 3 . 90 - 3 . 99 ( m 1h ), 4 . 15 ( q , j = 7 . 1 hz , 2h ), 5 . 96 ( bs , 1h ), 8 . 25 ( bs , 1h ) a 94 . 9 mg ( 0 . 558 mmol ) amount at 1β - ethoxycarbonyl - 2α - hydroxymethylcyclopropan - 1 - carboxyamide was dissolved in 1 . 2 ml of t - butyl alcohol and 42 . 2 mg ( 0 . 558 mol ) of sodium borohydride was added . after heating to 83 ° c ., 0 . 2 ml of methanol was added and the mixture was heated under reflux for 1 hour . after cooling to room temperature , 0 . 279 ml of 2n hydrochloric acid was added and the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 9 : 1 ) to yield 70 . 3 mg ( 0 . 485 mmol , 87 %) of the above - named compound . 1 h - nmr ( cd 3 od ) δ : 0 . 88 ( dd , j = 4 . 9 , 8 . 9 hz , 1h ), 1 . 24 ( dd , j = 4 . 9 , 6 . 3 hz , 1h ), 1 . 40 - 1 . 50 ( m , 1h ), 3 . 50 ( d , j = 12 . 0 hz , 1h ), 3 . 61 ( dd , j = 7 . 4 , 11 . 7 hz , 1h ), 3 . 66 ( dd , j = 6 . 9 , 11 . 7 hz , 1h ), 3 . 76 ( d , j = 12 . 0 hz , 1h ) a 31 . 0 mg ( 0 . 214 mmol ) amount of 1β , 2α - bis ( hydroxymethyl ) cyclopropan - 1 - carboxyamide was admixed with 2 . 1 ml of 2n hydrochloric acid . after stirring at room temperature for 18 hours , 8 . 4 ml of water was added and the mixture was extracted with ethyl acetate . after drying the organic layer over anhydrous sodium sulfate , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 19 : 1 ) to yield 23 . 1 mg ( 0 . 166 mmol , 78 %) of the above - named compound . this compound had physical data similar to that of example 1 . a 258 . 7 mg ( 1 . 52 mmol ) amount of ethyl 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - carboxylate was dissolved in 5 ml of ethanol and 60 . 8 mg ( 1 . 52 mmol ) of sodium hydroxide in 5 ml of ethanol was added . after stirring for 16 hours , 287 . 5 mg ( 7 . 60 mmol ) of sodium borohydride was added and the mixture was heated under reflux for 3 hours . after cooling to room temperature , 4 . 56 ml of 2n hydrochloric acid was added . after distilling ethanol off under reduced pressure , 10 ml of 2n hydrochloric acid was further added . after stirring for 18 hours , the mixture was extracted with ethyl acetate . after drying the organic layer over anhydrous sodium sulfate , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 19 : 1 ) to yield 106 . 2 mg ( 0 . 829 mmol , 55 %) of the above - named compound . this compound had physical data similar to that of example 1 . a 15 . 7 mg ( 0 . 123 mmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methanol , 6 . 2 mg ( 0 . 062 mmol ) of triethylamine and 58 . 1 mg ( 0 . 221 mmol ) of triphenylphosphine were dissolved in 1 . 5 ml of dichloromethane , and the mixture was cooled to 0 ° c . and then 73 . 4 mg ( 0 . 221 mmol ) of carbon tetrabromide was added . after stirring for 100 minutes , phosphate buffer ( ph 7 ) was added and the mixture was extracted with dichloromethane . after drying the organic layer over anhydrous sodium sulfate , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( hexane : ethyl acetate = 5 : 1 ) to yield 13 . 3 mg ( 70 μmol , 57 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 28 ( t , j = 5 . 1 hz , 1h ), 1 . 49 ( dd , j = 5 . 1 , 7 . 8 hz , 1h ), 2 . 33 - 2 . 40 ( m , 1h ), 3 . 30 ( d , j = 11 . 3 hz , 1h ), 4 . 12 ( d , j = 11 . 3 hz , 1h ), 4 . 17 ( d , j = 9 . 3 hz , 1h ), 4 . 36 ( dd , j = 4 . 7 , 9 . 3 hz , 1h ) a 13 . 3 mg ( 69 . 6 μmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methyl bromide was dissolved in 1 . 4 ml of n , n - dimethylformamide and 11 . 8 mg ( 69 . 6 μmol ) of 2 - amino - 6 - chloropurine and 9 . 6 mg ( 69 . 6 μmol ) of potassium carbonate were added . after stirring for 17 hours at room temperature , insoluble materials were filtered off , and the filtrate was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 49 : 1 ) to yield 11 . 4 mg ( 41 . 1 μmol , 59 %) of the above - named compound . 1 h - nmr ( cd 3 od ) δ : 1 . 16 ( t , j = 4 . 8 hz , 1h ), 1 . 65 ( dd , j = 4 . 8 , 7 . 8 hz , 1h ), 2 . 70 - 2 . 76 ( m , 1h ), 4 . 20 ( d , j = 9 . 3 hz , 1h ), 4 . 29 ( dd , j = 4 . 5 , 9 . 3 hz , 1h ), 4 . 37 ( d , j = 15 . 0 hz , 1h ), 4 . 69 ( d , j = 15 . 0 hz , 1h ), 8 . 17 ( s , 1h ) a 31 . 3 mg ( 0 . 244 mmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methanol and 37 . 0 mg ( 0 . 366 mmol ) of triethylamine were dissolved in 1 ml of dichloromethane , and then 43 . 5 mg ( 0 . 366 mmol ) of thionyl chloride in 1 ml of dichloromethane was added . after stirring for 90 minutes , phosphate buffer ( ph 7 ) was added and the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( hexane : ethyl acetate = 2 : 1 ) to yield 29 . 5 mg ( 0 . 200 mmol , 82 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 16 ( t , j = 5 . 1 hz , 1h ), 1 . 47 ( dd , j = 5 . 1 , 8 . 0 hz , 1h ), 2 . 35 - 2 . 42 ( m , 1h ), 3 . 48 ( d , j = 12 . 0 hz , 1h ), 4 . 19 ( d , j = 9 . 3 hz , 1h ), 4 . 27 ( d , j = 12 . 0 hz , 1h ), 4 . 36 ( dd , j = 4 . 5 , 9 . 3 hz , 1h ) a 27 . 8 mg ( 0 . 190 mmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methyl chloride was dissolved in 1 . 9 ml of n , n - dimethylformamide and 32 . 2 mg ( 0 . 19 mmol ) of 2 - amino - 6 - chloropurine and 26 . 3 mg ( 0 . 190 mmol ) of potassium carbonate were added . after stirring for 21 hours at room temperature , stirring was further continued at 55 ° c . for 20 hours . insoluble materials were filtered off , and the filtrate was concentrated under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 49 : 1 ) to yield 41 . 5 mg ( 0 . 148 mol , 78 %) of the above - named compound . this compound had physical data similar to that of example 5 . a 33 . 0 g ( 0 . 258 mmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methanol and 39 . 1 mg ( 0 . 386 mmol ) of triethylamine were dissolved in 1 ml of dichloromethane and cooled to 0 ° c ., and then 35 . 5 mg ( 0 . 310 mmol ) of methanesulfonyl chloride in 1 ml of dichloromethane was added . after stirring for 21 hours , phosphate buffer ( ph 7 ) was added and the mixture was extracted with dichloromethane . after drying the organic layer over anhydrous sodium sulfate , the solvent was distilled off under reduced pressure . the residue was purified by silica gel column chromatography ( hexane : ethyl acetate = 1 : 1 ) to yield 35 . 5 mg ( 0 . 173 mmol , 67 %) of the above - named compound . 1 h - nmr ( cdcl 3 ) δ : 1 . 16 ( t , j = 5 . 1 hz , 1h ), 1 . 44 ( dd , j = 5 . 1 , 8 . 0 hz , 1h ), 2 . 47 - 2 . 54 ( m , 1h ), 3 . 08 ( s , 3h ), 4 . 13 ( d , j = 11 . 9 hz , 1h ), 4 . 21 ( d , j = 9 . 3 hz , 1h ), 4 . 38 ( dd , j = 4 . 8 , 9 . 3 hz , 1h ), 4 . 87 ( d , j = 11 . 9 hz , 1h ) a 35 . 5 mg ( 0 . 172 mmol ) amount of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methyl methanesulfonate was dissolved in 1 . 7 ml of n , n - dimethylformamide and 29 . 2 mg ( 0 . 172 mmol ) of 2 - amino - 6 - chloropurine and 23 . 8 mg ( 0 . 172 mmol ) of potassium carbonate were added . after stirring for 20 hours at room temperature , insoluble materials were filtered off , and the filtrate was distilled under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 49 : 1 ) to yield 37 . 5 mg ( 0 . 134 mmol , 78 %) of the above - named compound . this compound had physical data similar to that of example 5 . a 11 . 7 mg ( 41 . 8 μmol ) amount of 2 - amino - 6 - chloro - 9 -( 3 &# 39 ;- oxa - 2 &# 39 ; oxobicyclo 3 , 1 , 0 ! hexan - 1 &# 39 ;- yl ) methylpurine was dissolved in 0 . 5 ml of 80 % formic acid and the solution was heated at 100 ° c . for 2 hours . after cooling to room temperature , the solvent was distilled off under reduced pressure . after adjustment of the ph to 4 with potassium carbonate followed by purification by reverse c18 silica gel chromatography ( water : methanol = 3 : 1 ), 10 . 9 mg ( 41 . 8 μmol , 100 %) of the above - named compound were obtained . 1 h - nmr ( dmso - d 6 ) δ : 1 . 05 ( t , j = 4 . 8 hz , 1h ), 1 . 42 ( dd , j = 4 . 8 , 7 . 8 hz , 1h ), 2 . 57 - 2 . 64 ( m , 1h ), 4 . 11 ( d , j = 9 . 3 hz , 1h ), 4 . 13 ( d , j = 14 . 9 hz , 1h ), 4 . 25 ( dd , j = 4 . 5 , 9 . 3 hz , 1h ), 4 . 44 ( d , j = 14 . 9 hz , 1h ), 6 . 41 ( bs , 2h ), 7 . 66 ( s , 1h ) high resolution mass spectrum ( c 11 h 12 o 3 n 5 , m + + h ): a 20 . 9 mg ( 80 . 0 μmol ) amount of 9 -( 3 &# 39 ;- oxa - 2 &# 39 ;- oxobicyclo 3 , 1 , 0 ! hexan - 1 &# 39 ;- yl ) methylguanine was dissolved in 1 ml of ethanol , and 15 . 1 mg ( 0 . 400 mmol ) of sodium borohydride was added . after stirring at 80 ° c . for 90 minutes , 0 . 25 ml of 2n hydrochloric acid was added and ethanol was distilled off under reduced pressure . after adjustment of the ph to 4 with potassium carbonate followed by purification by reverse c18 silica gel chromatography ( water : methanol = 4 : 1 ), 19 . 7 mg ( 74 . 4 μmol , 93 %) of the above - named compound were obtained . 1 h - nmr ( dmso - d 6 ) δ : 0 . 40 ( t , j = 5 . 1 hz , 1h ), 0 . 88 ( dd , j = 4 . 8 , 8 . 7 hz , 1h ), 1 . 23 ( m , 1h ), 3 . 24 - 3 . 37 ( m , 2h ), 3 . 41 ( dd , j = 6 . 0 , 12 . 0 hz , 1h ), 3 . 58 ( dt , j = 12 . 0 , 6 . 0 hz , 1h ), 3 . 81 ( d , j = 14 . 1 hz , 1h ), 4 . 00 ( d , j = 14 . 1 hz , 1h ), 4 . 49 ( m , 1h ), 4 . 64 ( m , 1h ), 6 . 38 ( bs , 2h ), 7 . 71 ( s , 1h ), 10 . 49 ( bs , 1h ) high resolution mass spectrum c 11 h 16 o 3 n 5 , m + + h ): a 40 . 0 mg ( 1 . 00 mmol ) amount of 60 % sodium hydride which had previously been washed with hexane were suspended in 10 ml of n , n - dimethylformamide , and 210 . 5 mg ( 1 . 02 mmol ) of ( 3 - oxa - 2 - oxobicyclo 3 , 1 , 0 ! hexan - 1 - yl ) methyl methanesulfonate and 135 . 1 mg ( 1 . 00 mmol ) of adenine were added . after stirring at 60 ° c . for 3 hours , insoluble materials were filtered off , and the filtrate was distilled under reduced pressure . the residue was purified by silica gel column chromatography ( dichloromethane : methanol = 19 : 1 ) to yield 153 . 0 mg ( 0 . 624 mmol , 62 %) of the above - named compound . 1 h - nmr ( dmso - d 6 ) δ : 1 . 06 ( t , j = 4 . 7 hz , 1h ), 1 . 54 ( dd , j = 4 . 7 , 7 . 8 hz , 1h ), 2 . 59 - 2 . 67 ( m , 1h ), 4 . 10 ( d , j = 9 . 3 hz , 1h ), 4 . 22 ( dd , j = 5 . 0 , 9 . 3 hz , 1h ), 4 . 37 ( d , j = 15 . 0 hz , 1h ), 4 . 63 ( d , j = 15 . 0 hz , 1h ), 7 . 20 ( bs , 2h ), 8 . 10 ( s , 1h ), 8 . 14 ( s , 1h ) high resolution mass spectrum ( c 11 h 12 o 2 n 5 , m + + h ): a 36 . 1 mg ( 0 . 147 mmol ) amount of 9 -( 3 &# 39 ;- oxa - 2 &# 39 ;- oxobicyclo 3 , 1 , 0 ! hexan - 1 &# 39 ;- yl ) methyladenine was dissolved in 1 . 5 ml of ethanol , and 27 . 8 mg ( 0 . 736 mmol ) of sodium borohydride was added . after stirring at room temperature for 14 hours , 0 . 37 ml of 2n hydrochloric acid was added and ethanol was distilled off under reduced pressure . after adjustment of the ph to 4 followed by the purification by reverse c18 silica gel chromatography ( water : methanol = 4 : 1 ), 33 . 3 mg ( 0 . 134 mmol , 91 %) of the above - named compound were obtained . 1 h - nmr ( dmso - d 6 ) δ : 0 . 41 ( t , j = 5 . 1 hz , 1h ), 0 . 93 ( dd , j = 5 . 1 , 8 . 7 hz , 1h ), 1 . 32 ( m , 1h ), 3 . 23 - 3 . 44 ( m , 3h ), 3 . 58 ( m , 1h ), 4 . 02 ( d , j = 14 . 2 hz , 1h ), 4 . 19 ( d , j = 14 . 2 hz , 1h ), 4 . 56 ( d , j = 5 . 2 hz , 1h ), 4 . 74 ( d , j = 5 . 2 hz , 1h ), 7 . 20 ( bs , 2h ), 8 . 13 ( s , 1h ), 8 . 16 ( s , 1h ) high resolution mass spectrum ( c 11 h 16 o 2 n 5 , m + + h ): obviously , numerous modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein .