Synthetic studies on sialoglycoconjugates, part 118.: A highly efficient total synthetic route to α-series gangliosides:: GM1α, GD1α, and GT1α

A highly efficient total synthetic route to alpha -series gangliosides GM1 alpha, GD1 alpha and GT1 alpha is described. The suitably protected gangliotriose (GgOse3) derivatives, i.e., 2-(trimethylsilyl)ethyl (2-acetamido-2-deoxy-3-O-p-methoxybenzyl-beta -D-galactopyranosyl)-(1-->4)-(2,3,6-tri-O-benzyl-beta -D-galactopyranosyl)-(1 -->4)-2,3,6-tri-O-benzyl-beta -D-glucopyranoside (8) and the corresponding III3-levulinoyl derivative (9), were regioselectively glycosylated with the phenyl 2-thioglycoside of N-acetylneuraminic acid (Neu5Ac) promoted by N-iodosuccinimide (NIS)-trimethylsilyl trifluoromethanesulfonate (TMSOTf) or trifluoromethanesulfonic acid (TfOH) in acetonitrile, to give the desired alpha -Neu5Ac-(2-->6)-gangliotriose (III(6)Neu5AcGgOse3) derivatives as the major products (11 and 12). The p-methoxybenzyl (MPM) group in 11 or the levulinoyl group in 12 was selectively removed, and the resulting 2-(trimethylsilyl)ethyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha -D-galacto-2-nonulopyranosylonate)-(2-->6)-(2-acetamido-2-deoxy-beta -D-galactopyranosyl)-(1-->4)-(2,3,6-tri-O-benzyl-beta -D-galactopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-beta -D-glucopyranoside (13), a key glycosyl acceptor, was systematically glycosylated with the galactose donor (14), alpha -Neu5Ac-(2-->3)-galactuse donor (15) and alpha -Neu5Ac-(2-->8)-alpha -Neu5Ac-(2-->3)-galactose donor (20) to give the protected GM1 alpha (16, 70%), GD1 alpha (17, 80%) and GT1 alpha (21, 87%) oligosaccharides, respectively, which can be converted to the target gangliosides by the introduction of ceramide and then complete deprotection.