Synthesis of Oligodeoxynucleoside Phosphoro-Monothioates and Phosphorodithioates by a Phosphotriester Method

Abstract

A phosphotriester method for the synthesis of dithymidine phosphoromonothoates and phosphorodithioates with new S-protecting groups has been investigated. Four of the S-protecting groups possesed catalytic activity, however side reactions occurred during deprotection. The best S-protecting group was 4-chloro-2-nitlobenzyl which could be removed with a minimum of side reactions (0.3 %). The coupling reagent PyFNOP (14) gave protected dithymidine phosphoromonothioate in 96 % yield after 15 min coupling. Furthermore PyFNOP chemoselectively activates oxygen in nucleoside phosphorodithioate monomers 9 and can be used for the synthesis of oligodeoxynucleoside phosphorodithioates with mixed base sequences.