3) The stereocontrolled cyclization of (E)-3-(2-oxooxazolidin-3-ylcarbonyl)-2-propenoic acid methyl ester (XIX) with 1,1-di(methylsulfanyl)ethylene (XX) catalyzed by a chiral reagent prepared in situ with diisopropoxytitanium dichloride and the chiral dioxolane (XXI), gives [1S-(1alpha,2beta)]-3,3-bis(methylsulfanyl)-2-(2-oxooxazolidin-3-ylcarbonyl)cyclobutane-1-carboxylic acid methyl ester (XXII), which is treated with magnesium methoxide in methanol yielding the dicarboxylic ester (XXIII). The reduction of (XXIII) with LiAlH4 in ether affords the corresponding dimethanol compound (XXIV), which is silylated with tert-butyldiphenylsilyl chloride (BPS-Cl), triethylamine and 4-(dimethylamino)pyridine (DMAP) in dichloromethane giving the bis(silyl) ether (XXV). The reaction of (XXV) with N-chlorosuccinimide (NCS) and AgNO3 in acetonitrile/water yields the cyclobutanone (XXVI), which is reduced with diisobutylaluminum hydride (DIBAL) in toluene affording the silylated [1S-(1alpha,2beta,3beta)]-3-hydroxycyclobutyl-1,2-dimethanol (XXVII). The reaction of (XXVII) with methanesulfonyl chloride and triethylamine gives the expected mesylate (XXVIII), which is condensed with 6-O-(2-methoxyethyl)guanine (XXIX) by means of LiH in DMF to afford the corresponding adduct (XXX). Finally, this compound is deprotected by treatment with HCl in methanol.
5) The thermal rearrangement of 3-bromo-4,6-dimethyl-2-oxo-2H-pyran-5-carboxylic acid ethyl ester (XXXIX) with NaOH in refluxing water gives (rac)-trans-3-methylenecyclopropane-1,2-dicarboxylic acid (XL), which was submitted to optical resolution with (R)-alpha-methylbenzylamine (XLI) yielding, after crystallization, the corresponding (R,R)-enantiomeric salt (XLII). The decomposition of the salt and esterification of the free acid with trimethyl orthoformate affords (1R-trans)-3-methylenecyclopropane-1,2-dicarboxylic acid dimethyl ester (XLIII), which is reduced with LiAlH4 in THF to (1R-trans)-3-methylenecyclopropane-1,2-dimethanol (XLIV). The esterification of (XLIV) with benzoic anhydride and triethylamine in ethyl acetate affords the corresponding dibenzoate (XLV), which is epoxidized with m-chloroperbenzoic acid (MCPBA) in dichloromethane to afford the oxirane (XLVI). The isomerization of (XLVI) by means of LiI in ethyl acetate gives (2S-trans)-2,3-bis(benzoyloxymethyl)cyclobutanone (IX), which is reduced with lithium trisiamylborohydride in THF yielding [1S-(1alpha,2beta,3beta)]-3-hydroxycyclobutane-1,2-dimethanol 1,2-dibenzoate ester (X). The reaction of (X) with p-toluenesulfonyl chloride in pyridine affords the corresponding tosylate (XI), which is condensed with 6-O-benzylguanine (XII) by means of K2CO3 and 18-crown-6 in DMF yielding the substituted guanine (XIII). Finally, this compound is deprotected by treatment first with sodium methoxide in hot methanol, and then with 3N HCl.
6) The stereoselective cyclization of bis[(-)-menthyl] fumarate (XLVII) with 1,1-bis(methylsulfanyl)ethylene (XX) by means of diisobutylaluminum chloride in hexane gives (1S-trans)-3,3-bis(methylsulfanyl)cyclobutane-1,2-dicarboxylic acid di-(-)-menthyl ester (XLVIII), which is reduced with LiAlH4 in THF yielding (1S-trans)-3,3-bis(methylsulfanyl)cyclobutane-1,2-dimethanol (XXIV). The silylation of (XXIV) with tert-butyldimethylsilyl chloride (TBMS-Cl) and imidazole in DMF affords the bis-silylated diol (II), which is treated with AgNO3 and N-chlorosuccinimide (NCS) in acetonitrile/water to give the cyclobutanone (L). The reaction of (L) with O-methylhydroxylamine in pyridine yields the corresponding O-methyloxime (LI), which is selectively reduced with NaBH4 in THF to the cyclobutylamine (LII). The condensation of (LII) with 4-chloro-6-hydroxy-5-nitropyrimidine-2-amine (LIII) by means of triethylamine in DMF affords the substituted 5-nitro-2,4-diaminopyrimidine (LIV), which is reduced with H2 over Pd/C in formic acid yielding the corresponding triaminopyrimidine (LV). Finally, this compound, without isolation, is cyclized with formic acid at 130 C.
4) The reaction of the previously described chiral cyclobutanone (IX) with hydroxylamine gives the corresponding oxime (XXXI), which is reduced with H2 over PtO2 yielding [1R-(1alpha,2beta,3alpha)]-2,3-di(cyclohexylcarbonyloxymethyl) cyclobutane-1-amine (XXXII). The condensation of (XXXII) with 4,6-dichloropyrimidine-2-amine (XXXIII) affords (XXXIV), which is further condensed with 4-chlorophenyldiazonium chloride (XXXV) to give the 5-azo-pyrimidine derivative (XXXVI). The hydrogenation of the azo group of (XXXVI) with Zn/HCl yields the 2,5-diaminopyrimidine derivative (XXXVII), which is cyclized with triethyl orthoformate to afford [1R-(1alpha,2beta,3alpha)]-2-amino-9-[2,3-bis (cyclohexylcarbonyloxymethyl)cyclobutyl]-6-chloropurine (XXXVIII). Finally, this compound is converted into the final product in the usual way.
1) The cyclization of ketene diethylketal (I) with diethyl fumarate (II) in hot tert-butanol gives trans-3,3-diethoxycyclobutane-1,2-dicarboxylic acid diethyl ester (III), which is saponified with KOH in hot methanol to the corresponding racemic diacid (IV). The condensation of (IV) with (R)-(-)-2-phenylglycinol (V) by means of dicyclohexylcarbodiimide (DCC) in dichloromethane followed by fractionated crystallization of the resulting diastereomeric mixture yields the diamide (VI). The hydrolytic reduction of (VI) by sequential treatments with trimethylsilyl chloride and imidazole, dinitrogen tetraoxide and finally with LiBH4 affords (S,S)-trans-3,3-diethoxycyclobutane-1,2-dimethanol (VII), which is benzoylated with benzoyl chloride in pyridine to the dibenzoate (VIII). The cleavage of the ketal group of (VIII) with H2SO4 in acetonitrile gives the cyclobutanone (IX), which is reduced with LS-Selectride in THF yielding [1S-(1alpha,2beta,3beta)]-3-hydroxycyclobutane-1,2-dimethanol 1,2-dibenzoate (X). The reaction of (X) with p-toluenesulfonyl chloride affords the corresponding tosylate (XI), which is condensed with 6-O-benzylguanidine (XII) by means of K2CO3 and 18-crown-6 in hot DMF giving [1R-(1alpha,2beta,3alpha)]-6-O-benzyl-9-[2,3-di(benzoyloxymethyl) cyclobutyl]guanine (XIII). Finally, this compound is treated with sodium methoxide in hot methanol. 2) The reaction of 6-chloropurine-2-amine (XIV) with 47% HI gives 6-iodopurine-2-amine (XV), which is condensed with [1S-(1alpha,2beta,3beta)]-3-(trifluoromethylsulfonyloxy)cyclobutane-1,2-dimethanol 1,2-dibenzoate (XVI) (obtained by treatment of the previously described cyclobutanol [X] with trifluromethanesulfonyl anhydride [Tf2O]) by means of benzyltriethylammonium hydroxide and pyridine in dichloromethane to afford the condensed iodopurine (XVII). The reaction of (XVII) with sodium methoxide hydrolyzes the benzoyl groups affording the 6-O-methylguanine derivative (XVIII), which is finally treated with aqueous HCl at 95 C.
7) [14C]-labeled lobucavir has been synthesized as follows: The reaction of [1R-(1alpha,2beta,3alpha)]-2,3-di(benzyloxymethyl)cyclobutan-1-amine (LVI) with 4,6-dichloropyrimidine-2-amine (XXXIII) by means of triethylamine in acetonitrile gives [1R-(1alpha,2beta,3alpha]-4-[2,3-di(benzyloxymethyl)cyclobutylamino]-6-chloropyrimidine-2-amine (LVII), which is condensed with 4-chlorophenyldiazonium chloride (XXXV) to afford the azo derivative (LVIII). The reduction of (LVIII) with Zn/acetic acid yields the corresponding 2,5-diaminopyrimidine derivative (LIX), which is cyclized with [14C]-labeled triethyl orthoformate to give [8-14C]-[1R-(1alpha,2beta,3alpha)]-9-[2,3-di(benzyloxymethyl) cyclobutyl]-6-chloropurine-2-amine (LX). The reaction of (LX) with sodium methoxide in methanol yields the corresponding 6-O-methylguanine derivative (LXI), which is debenzylated by hydrogenolysis with H2 over Pd(OH)2 in ethanol/cyclohexane affording [8-14C]-[1R-(1alpha,2beta,3alpha)]-9-[2,3-di(hydroxymethyl)cyclobutyl]-6-O-methylguanine (LXII). Finally, this compound is demethylated with aqueous HCl.
A new practical asymmetric synthesis of lubocavir has been described: The diastereoselective cycloaddition of (-)-dimenthyl fumarate (I) with ketene dimethylacetal (II) by means of diisobutylaluminum chloride (DIBAC) in toluene gives the cyclobutane (III), which is reduced with LiAlH4 yielding the cyclobutyl dicarbinol (IV). The reaction of (IV) with benzoyl chloride affords the dibenzoate (V), which is deprotected in acidic medium to give the cyclobutanone (VI). The stereoselective reduction of (VI) with DIBAC yields the chiral cyclobutanol (VII), which is treated with triflic anhydride to afford the activated triflate (VIII). The regioselective condensation of (VIII) with 2-amino-6-iodopurine benzyltriethylammonium salt (IX) gives the expected addition product (X), which is debenzoylated with NaOMe in methanol yielding the 2-amino-6-methoxypurine (XI). Finally, this compound is converted to lobucavir by refluxing with aqueous hydrochloric acid.