Stereoselective synthesis of dithymidine phosphorothioates using d-xylose derived chiral auxiliaries

Different sized catalysts 1, 2, 5 and 7 were prepared and used as the activators in the coupling reactions of phosphoramidites with nucleosides. d-Xylose derived chiral auxiliary 22a has been synthesized and applied for the stereoselective synthesis of dithymidine phosphorothioates 27. The mechanisms of the coupling reactions are discussed.