Synthesis of deoxydinucleoside phosphates containing 4-thio substituted pyrimidine nucleobases

The preparations, by the hydrogenphosphonate approach in solution, of the five pyrimidine deoxy-dinucleoside monophosphates 14a, 14b, 16a, 16b and 17 containing a 4-thiosubstituted pyrimidine nucleobase at either their 3′- or 5′-end are described. These syntheses were conveniently accomplished using 4-thionucleoside derivatives having their sulfur function protected by S-pivaloyloxymethylation. The behaviour of unprotected 4-thionucleoside derivatives was also investigated and they were shown to give comparable overall yields.