First total synthesis of ganglioside GT1a alpha

A first total synthesis of ganglioside GT1a alpha (IV3III6II3)Neu5AcGg4Cer) is described. The suitably protected sialyl alpha-(2-->6) ganglioside was glycosylated with the phenylthioglycoside of sialic acid in the presence of N-iodosuccinimide (NIS)-trimethylsilyl trifluoromethanesulfonate (TMSOTf), followed by further glycosylation with the methyl thioglycoside promoted by dimethyl(methylthio)sulfonium triflate (DMTST), to give the heptasaccharide. The oligosaccharide obtained was converted into the title ganglioside by the introduction of ceramide and then complete deprotection.