Systematic syntheses of the isoglobo-series gangliosides, alpha-sialyl-(2 --> 3)- and -(2 --> 6)-isoglobopentaosyl ceramides (21 and 24, V3Neu5AciGb5Cer and V6Neu5AciGb5Cer) are described. 2-(Trimethylsilyl)ethyl 2-O-benzyl-4,6-O-benzylidene-alpha-D-galactopyranosyl-(1 --> 3)- 2,4,6,tri-O-benzyl-beta-D-galactopyranosyl-(1--> 4) -2,3,6-tri-O-benzyl-bet a-D- glucopyranoside (4), the core structure of the isoglobo-series gangliosides was prepared by the glycosylation of 2-(trimethylsilyl)ethyl 2,4,6-tri-O-benzyl-(1 --> 4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside (2) with methyl 3-O-acetyl-2-O-benzyl-4,6-O-benzylidene-1-thio-beta-D-galactopyranoside (1), and subsequent O-deacetylation. Coupling of 4 and methyl 3-O-acetyl-4,6-O-benzylidene-2-deoxy-2-phthalimido-1-thio-beta-D- galactopyranoside gave an isoglobotetraoside derivative 6, from which the phthaloyl and O-acetyl groups were removed. N-Acetylation then gave a tetrasaccharide acceptor 7. Dimethyl(methylthio)sulfonium triflate-promoted coupling of 7 with methyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D- galacto)-(2 --> 3)-2,4,6-tri-O-benzoyl-1-thio-beta-D-galactopyranoside or -(2 --> 6)-2,4-di-O-benzoyl-3-O-benzyl-1-thio-beta-D-galactopyranoside gave the pentasaccharide derivative 9 and 14 in good yields. Compounds 9 and 14 were converted into the corresponding alpha-trichloroimidates 12 and 17 which, on coupling with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol (18), gave the beta-glycoside 19 and 22 respectively, Finally, 19 and 22 were transformed, via selective reduction of azide group, condensation with octadecanoic acid, O-deacylation, and hydrolysis of the methyl ester group into 21 and 24, respectively.