Novel Glycosidation Method Using 2,6-Anhydro-2-thio Sugars for Stereocontrolled Synthesis of 2,6-Dideoxy-α- and -β-glycosides

Powerful and highly stereocontrolled O-glycosidation methods using several kinds of 2,6-anhydro-2-thio sugars as glycosyl donors have been developed for the synthesis of both 2,6-dideoxy-α- and -β-glycosides which frequently occur in biologically important natural products. Both glycosidations of phenyl 3,4-di-O-acetyl-2,6-anhydro-l,2-dithio-D-altropyranoside (2) and 3,4-di-O-acetyl-2,6-anhydro-l-fluoro-2-thio-D-altropyranoside (3) with alcohols exclusively gave the corresponding 2,6-anhydro-2-thio-a-glycosides. In contrast, the glycosidations of 1,3,4-tri-O-acetyl-2,6-anhydro-2-thio-D-altropyranose (4) with alcohols afforded the corresponding 2,6-anhydro-2-thio-β-glycosides with high stereocontrol. Furthermore, a novel method for the controlled block synthesis of 2,6-dideoxy oligosaccharides by the combined use of the activated 2,6-anhydro-2-thio sugar 23 and the deactivated 2,6-anhydro-2-sulfinyl sugar 24, both of which have the same thiophenyl leaving group at the anomeric positions, has been demonstrated. The 2,6-anhydro-2-thio-α- and -β-glycosides obtained by the present methods were effectively converted into the corresponding 2,6-dideoxy-α- and -β-glycosides by both hydrogenolysis using Raney-Ni as a catalyst and reductive desulfurization using Bu₃SnH and AIBN.