SYNTHETIC STUDIES ON SIALOGLYCOCONJUGATES 48 - TOTAL SYNTHESIS OF GANGLIOSIDES GM1 AND GD1A

A stereocontrolled, facile total synthesis of gangliosides GM1. and GD1a, in connection with systematic synthesis of ganglio-series of ganglioside, is described. Glycosylation of 2-(trimethylsilyl)ethyl O-(2-acetamido-6-0-benzoyl-2-deoxy-beta-D-galactopyranosyl)-(1-->4)-O-[(methyl 5-acetamido-4,7,8,9-tetra-0-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylonate)-(2-->3)]-O-2,6-di-0-benzyl-beta-D-galacto-pyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside (4), with methyl 2,46-tri-O-benzyl-3-O-benzyl-1-thio-beta-D-galactopyranoside (8) or methyl O-(methyl 5-acetamido -4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranozylonate)-(2 -->3)-2,4,6-tri-O-benzoyl-1-thio-beta-D-galactopyranoside (9) by use of N-iodosuccinimide (NIS)-trifluoromethanesulfonic acid (TfOH) or dimethyl(methylthio) sulfonium triflate (DMTST) as a promoter, gave the corresponding beta-glycoside 10 and 18 in 66 and 62% yields, which were converted, via reductive removal of the benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, and subsequent imidate formation, into the alpha-trichloroacetimidates 13 and 21. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol (14) with 13 or 21 by use of trimethylsilyl trifluoromethanesulfonate gave the corresponding beta-glycoside 15 and 22, which on channeling through selective reduction of the azido group, coupling of the thus formed amino group with octadecanoic acid, 0-deacylation, and saponification of the methyl ester group, gave the tide gangliosides GM1 and GD1a.