The first total synthesis of tumor-associated glycolipid antigen, sialyl dimeric Lex, is described. Regioselective glycosylation of the suitably protected Lewis X (Lex) pentasaccharide derivative6with phenyl 4-O-acetyl-6-O-benzyl-2-deoxy-3-O-(4-methoxy-benzyl)-2-phthalimido-1-thio-β-d-glucopyranoside (5) gave the hexasaccharide7, which was converted,viaremoval of the phthaloyl groups and selectiveN-acetylation, into the hexasaccharide acceptor9. Dimethyl(methylthio)sulfonium triflate (DMTST) promoted glycosylation of9with methylO-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-d-glycero-α-d-galacto-2-nonulopyranosylonate)-(2→3)-2,4,6-tri-O-benzoyl-1-thio-β-d-galactopyranoside (10) afforded regioselectively the expected octasaccharide11, which was converted into13viaO-acetylation and removal of the methoxybenzyl group. Fucosylation of13with the methyl thioglycoside14was performed by use ofN-iodosuccinimide (NIS)-trifluoromethanesulfonic acid(TfOH) as a promoter to give the desired nonasaccharide15. After replacing the benzyl groups of15by the acetyl groups, the 2-(trimethylsilyl)ethyl group at the reducing end was selectively transformed into the α-trichloroacetimidate18. Coupling of18with (2S, 3R, 4E)-2-azido-3-O-tert-butyldiphenylsilyl-4-octadecene-1,3-diol (19) gave the corresponding β-glycoside20, which was transformed,viaselective reduction of the azide group, coupling with octadecanoic acid,O-desilylation,O-deacylation, and hydrolysis of the methyl ester group, into the title ganglioside1in good yield.