Summary. The O, S-dimethylphosphorochloridate 10 reacts with 5'-0- monomethoxytritylthymidine 11 in pyridine giving nucleoside-3'-thiolophosphodzster 13 in high yields: in acetonitrile and z the presence of I-methylimidazole, this reaction leadsto the nucleoside-3'-phosphotriester 14-without formation of diester 13. From the latter, the Tp and TpT have been prepared.
