Full Stereocontrol in α-Glycosidation of 3-Deoxy-d- manno -2-octulosonic Acid (Kdo) Using Macrobicyclic Glycosyl Donors.

We describe a method for the α-selective glycosidation of 3-deoxy-d- manno -2-octulosonic acid (Kdo) using a macrobicyclic Kdo donor as the precursor of a bridgehead oxocarbenium ion, whose stereoselectivity is not affected by the substrate structure and reaction conditions. Strapping Kdo via tethering in the α-configuration at the C1 and C5 positions completely blocked nucleophilic attack to the β-face of the anomeric center by sterically hindering the bicyclic system, realizing full α-selectivity during glycosidation.