Synthetic Studies on Sialoglycoconjugates 41: A Facile Total Synthesis of Ganglioside GM2

A stereocontrolled, facile total synthesis of ganglioside GM2 is described. Coupling of 2- (trimethylsilyl)ethyl O-(2,6-di-O-benzyl-(β-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-D-glucopyranoside (2), prepared from 2-(trimethylsilyl)ethyl β-lactoside (1) by selective 3′,4′-O-isopropylidenation, O-benzylation, and subsequent removal of the isopropylidene group, with methyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy -2-thio-D-glycero-D-galacto -2-nonulopyranosid)onate (4) using N-iodosuccini-midc (NIS), gave the trisaccharide (5), which on condensation with methyl 6-O-benzoyl -2-dcoxy-3,4-O-isopropylidene-2-phthalimido-l-thio-β-D-galactopyranoside (11), gave the protected ganglioside GM 2 oligosaccharide 12. Compound 12 was transformed, via O-deisopropylidenation, O-acetylation, removal of the phthaloyl group, N-acetylation, removal of the benzyl groups followed by (O-acetylation, selective removal of the 2-(rximethylsilyl)ethyl group, and subsequent imidate formation, into the final glycosyl donor 19. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-l,3-diol (20) with the α-trichloroacetimidate 19 gave the β-glycoside 21, which on channeling through selective reduction of the azide group, coupling of the amino group with octadecanoic acid, O-deacylation and saponification of the methyl ester group, gave the title ganglioside.