GM1 is a common ganglioside pentasaccharide present on mammalian cell surface. It has been shown to play important roles in cellular communications and initiation of β-amyloid aggregation. In order to synthesize GM1, an efficient synthetic route was developed via a [3+2] strategy. The GM3 trisaccharide acceptor bearing an azido propyl group at the reducing end was prepared using the traditional acetamide protected sialyl thioglycosyl donor, which gave better stereoselectivity than sialyl donors protected with trichloroacetamide or oxazolidinone. The glycosylation of the axial 4-hydroxyl group of GM3 by the disaccharide donor was found to be highly dependent on donor protective groups. Donor bearing the more rigid benzylidene group gave low glycosylation yield. Replacing the benzylidene with acetates led to productive coupling and formation of the fully protected GM1 pentasaccharide. Deprotection of the pentasaccharide produced GM1 functionalized with the amino propyl side chain, which will be a valuable probe for biological studies.