Traceless Staudinger ligation of glycosyl azides with triaryl phosphines: stereoselective synthesis of glycosyl amides.

Alpha-glycosyl amides can be synthesized from the corresponding O-benzyl-alpha-glycosyl azides using a traceless Staudinger ligation with diphenylphosphanyl-phenyl esters 4. All the phosphines employed and their phenol precursors are stable to air at 4 degrees C for months. Fast intramolecular trapping of the reduction intermediates results in the direct formation of the amide link, which, in turn, prevents epimerisation and allows retention of configuration at the anomeric carbon. Yields and alpha-selectivity are high when the reaction is performed in polar aprotic solvents. Removal of the benzyl ether protecting groups is achieved by catalytic hydrogenation. Alpha-glycosyl amides represent a class of virtually unexplored nonhydrolyzable monosaccharide derivatives that may find a useful application as sugar mimics. Conformational studies by NMR spectroscopy confirm that deprotected alpha-glycosyl amides in the gluco, galacto, and fuco series retain the normal pyranose conformation of the monosaccharide. The reaction of phosphines 4 with tetra-O-acetyl-glycosyl azides is nonstereoconservative, and beta-glycosyl amides are obtained in good yields and with complete stereoselectivity starting from both alpha and beta azides.