Asymmetric synthesis of C4′α-carboxylated 2′-deoxynucleosides.: Preparation of oxetanone derivatives and influence of solvent on the stereochemistry of base introduction

A synthesis of methyl 5-O-benzyl-3-O-tert-butyldimethylsilyl-4 alpha-methoxycarbonyl-2-deoxyribofuranoside from dimethyl L-tartrate is presented. Coupling of this substance with standard purine and pyrimidine bases, promoted by tin tetrachloride, provides the corresponding 4'alpha-methoxycarbonylnucleosides with moderate yield and selectivity. A series of related bicyclic donors, derived by forming a beta-lactone between the 3-OH and the 4 alpha-carboxy group, were prepared and assayed in coupling reactions. In nonpolar solvents good endo-selectivity is observed whereas in acetonitrile the exoanomers are preferentially obtained. Methods for the removal of the 5'-O-benzyl ethers of the nucleosides are presented.