Iridium-promoted deoxyglycoside synthesis: stereoselectivity and mechanistic insight.

Herein, we devised a method for stereoselective -glycosylation using an Ir(i)-catalyst which enables both hydroalkoxylation and nucleophilic substitution of glycals with varying substituents at the C3 position. In this transformation, 2-deoxy-α--glycosides were acquired when glycals equipped with a notoriously poor leaving group at C3 were used; in contrast 2,3-unsaturated-α--glycosides were produced from glycals that bear a good leaving group at C3. Mechanistic studies indicate that both reactions proceed the directing mechanism, through which the acceptor coordinates to the Ir(i) metal in the α-face-coordinated Ir(i)-glycal π-complex and then attacks the glycal that contains the -glycosidic bond in a -addition manner. This protocol exhibits good functional group tolerance and is exemplified with the preparation of a library of oligosaccharides in moderate to high yields and with excellent stereoselectivities.