Full Stereocontrol in alpha-Glycosidation of 3-Deoxy-D-manno-2-octulosonic Acid (Kdo) Using Macrobicyclic Glycosyl Donors

We describe a method for the alpha-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 alpha-configuration at the C1 and C5 positions completely blocked nucleophilic attack to the beta-face of the anomeric center by sterically hindering the bicyclic system, realizing full alpha- selectivity during glycosidation.