Synthesis of disialoganglioside GD1 alpha and its positional isomer

The first total syntheses of disialoganglioside GD1 alpha and its alpha 2-6 positional isomer are described. Suitably protected pentasaccharide derivatives, derived from known pentasaccharide precursors, were selected as the glycosyl acceptors. Using our facile method of stereoselective alpha-glycosidation of sialic acid, these acceptors were coupled with methyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galact o-2- nonulopyranosid)onate in acetonitrile medium in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) to get the desired di-alpha-sialyl hexasaccharide derivatives in moderate yields. The hexasaccharide trichloroacetimidates upon coupling with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol gave the corresponding beta-linked sphingosine glycosides in high yields. These were subjected in sequence to selective reduction of the azido group and coupling of the thus formed amino group with octadecanoic acid, O-deacylation, and saponification of the methyl ester groups to obtain ganglioside GD1 alpha and its positional isomer having the main-chain residue 2-6 linked.