SOLUTION-PHASE SYNTHESIS OF PHOSPHOROTHIOATE OLIGODEOXYNUCLEOSIDES BY THE PHOSPHOTRIESTER METHOD

A ''phosphorothioate triester method'' was investigated for the solution-phase synthesis of phosphorothioate oligonucleosides, Using fully protected 3'-phosphorothiolate thymidine bearing O-cyanoethyl and S-2,4-dichlorobenzyl groups as phosphorothioate protecting groups, decathymidine nonaphosphorothioate was efficiently assembled through a blockwise procedure, Two side reactions occurred during the deprotection steps: breakage of internucleoside linkages (1.8% per linkage) and formation of phosphate diester linkages (0.9%). Substitution of the dichlorobenzyl group by the more labile 4-nitrobenzyl S-protecting group reduced the extent of internucleoside bond breakage by one-half.