Synthesis of the thymidine building blocks for a nonhydrolyzable DNA analogue

The 3′-deoxy-3′-C-(2″-hydroxyethyl)thymidine derivatives 4 and 13 were efficiently synthesized from either thymidine via a known 3′-radical allylation, or from the branched-chain furanose 5 involving the removal of the 2′-hydroxyl after stereospecific nucleosidation. The 5′-deoxy-5′-thiothymidine 20 was prepared via a regiospecific Mitsunobu coupling, providing the third unit required for the synthesis of sulfide-linked DNA analogues.