Study on the diastereoselective synthesis of dithymidine phosphorothioates through a d-xylose derived chiral auxiliary and development of a novel catalyst

1,2-Di-O-isopropylidene-3-C-cyanoethyl-5-deoxy-5-isopropylamino-d-xylofuranose 16a was synthesized. The use of 16a as a chiral auxiliary led to the diastereoselective formation of dithymidine phosphorothioate. The chiral auxiliary was easily removed by treatment with concentrated ammonia. 2-Mesityl-4,5-dicyanoimidazole displays higher diastereoselectivity than the 2-bromo-4,5-dicyanoimidazole in the coupling reaction.