Stereocontrolled synthesis of phosphorothioate RNA by the oxazaphospholidine approach.

Stereoregulated diribonucleoside phosphorothioates were synthesized by the use of 2'-O-TBDMS-protected ribonucleoside 3'-O-oxazaphospholidine derivatives as monomers and N-(cyanomethyl)ammonium salts as activators. Diastereoselectivity of the condensation reaction was found to be highly dependent on the substituent groups of the oxazaphospnolidine ring as well as the structure of the activators. By the use of the optimized oxazaphospholidine monomers and activators, diastereopure diribonucleoside phosphorothioate bearing Rp and Sp configurations were obtained in good yields.