Synthetic Studies on Sialoglycoconjugates 91: Total Synthesis of Gangliosides GD1C and GT1A

Abstract

A first total synthesis of gangliosides GD1c and GT1a containing Neu5Acα(2→8) Neu5Acα(2→3)Gal residue in their non-reducing terminal is described. Condensation of methyl O-[methyl 5-acetamido-8-O-(5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylono-1¹,9-lactone) -4,7- di-O-acetyl-3,5-dideoxy-D-glycero-α-D-galcto-2-nonulopyranosyranosylanate]-(2→3)-2,4,6-tri-O-benzoyl-1-thio-β-D-gala-ctopyranoside (1) with 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-β-D-galactopyranosyl)- (1→4) -O -(2,3,6-tri-O-benzyl-β-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-D-glucopyranoside (2) or 2-(trimethylsilyl)ethyl O-(2-acetamido-6-O-benzyl-2-deoxy-β-D-galactopyranosyl)-(1→4)-(9-[methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→3)]-O-(2,6-di-O-benzyl-β-D-galactopyranosyl) - (1→4) - 2,3,6-tri-O-benzyl-β-D-glucopyranoside (3) in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) gave the corresponding hexa-and heptasaccharide derivatives 4 and 5, respectively. These oligosaccharides were converted into the α-trichloroacetimidates 10 and 11 via reductive removal of the benzyl groups and/or benzylidene group, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group and treatment with trichloroacetonitrile, which, on coupling with 2-azidosphingosine derivatives 12 or 13, gave the β-glycosides 14 and 15, respectively. Finally, 14 and 15 were transformed, via selective reduction of the azido group, coupling with octadecanoic acid and removal of all protecting groups, into the title gangliosides GD1c 18 and GT1a 19.