Highly stereoselective beta-anomeric glycosidation of a 2'-deoxy syn-5'-configured 4'-spironucleoside

A direct enantioselective pathway that delivers exclusively the beta-anomer of a 4'-spironucleoside has been successfully developed. The key starting material is the enantiomerically pure dihydroxy lactone 19, which has proven amenable to conversion to glycal 22 via the chloro acetonide. This intermediate is then capped as the 3,5-O-(tetraisopropyldisiloxane-1,3-diyl) glycal. The latter can enter into N-iodosuccinimide-promoted glycosidation with persilylated thymine. Only the beta anomer is formed. Ensuing deiodination and desilylation proceed quantitatively to furnish the targeted, previously elusive anomer.