Stereocontrolled Synthesis of Phosphorothioate DNA by the Oxazaphospholidine Approach

Stereocontrolled synthesis of oligodeoxyribonucleoside phosphorothioates (PS-ODNs) using nucleoside 3'-O-oxazaphospholidine derivatives as monomer units is described. A series of dialkyl(cyanomethyl)ammonium salts were developed and used as new activators for the condensation reactions of the diastereopure nucleoside 3'-O-oxazaphospholidines with 3'-O-protected nucleosides. In the presence of the new activators, the condensation reactions proceeded rapidly to give the corresponding dinucleoside phosphite triesters with high deastereoselectivity. After sulfurization and deprotection, diastereo-pure (Rp)- and (Sp)-dinucleoside phosphorothioates were obtained in excellent yields. The present methodology was also applied to the solid-phase synthesis of stereoregulated PSODNs. In addition, ab initio molecular orbital calculations were carried out to elucidate the mechanism of these diastereoselective phosphitylation and condensation reactions.