RECENT ADVANCES IN GLYCOSYLATION REACTIONS

Several novel solutions to the efficient and selective preparation of glycosides are presented. Commercial tris(4-bromophenyl)ammoniumyl hexachloroantimonate, a one-electron transfer homogeneous reagent, nicely activates in acetonitrile various ethyl or phenyl S-glycosides having either participating or non-participating substituents at C-2 to give beta-O-glycosides in good yield. In a variation on the theme, phenyl S-glycosides react with alcohols under mild electrochemical conditions to give the corresponding O-glycosides. Azidoxanthates, prepared via a two-step sequence from various galactals, are efficient galactosyl donors for the steroselective synthesis of protected precursors of biologically important galactosamine-containing oligosaccharides. We have also discovered that anomeric enol-ethers, or the corresponding carbonates, are remarkably efficient glycosyl donors. Finally, a "non-orthodox" strategy based on the idocyclization of appropriate acyclic enol-ethers resulted in a novel approach to KDO-containing disaccharides. This last synthesis critically benefited from the use of the Tebbe reagent for the preparation of the key starting enol ethers.