Synthesis and biological activities of three sulfated sialyl Lex ganglioside analogues for clarifying the real carbohydrate ligand structure of L-selectin

Sulfated sialyl Lex ganglioside analogues at C-6 of d-galactose, N-acetyl-d-glucosamine, and of both d-galactose and N-acetyl-d-glucosamine residues have been synthesized, in order to clarify the structure of the real carbohydrate ligand of L-selectin. Coupling of the suitably protected N-acetyl-d-glucosaminyl-β(1 → 3)-lactose derivatives 13 and 16 with the sialyl α(2 → 3)-d-galactopyranosyl trichloroacetimidates 10 and 12 (glycosyl donors), via glycosylation of 2-(trimethylsilyl)ethyl 4,6-O-benzylidene-β-d-galactopyranoside (1) with the phenyl 2-thioglycoside derivative (2) of N-acetylneuraminic acid (Neu5Ac) using N-iodosuccinimide/TfOH, O-benzoylation, removal of the benzylidene group affording 5, selective 6-O-levulinoylation, O-benzoylation, removal of the 2-(trimethylsilyl)ethyl group, and imidate formation, or via O-acetylation of 5, removal of the 2-(trimethylsilyl)ethyl group, then imidate formation, gave the pentasaccharides 18–20. The glycosylation of the pentasaccharide acceptors (21–23) derived from 18–20 by removal of the 4-methoxybenzyl group, with phenyl 1-thioglycoside derivative 27 of l-fucose using dimethyl(methylthio)sulfonium triflate (DMTST) afforded the corresponding hexasaccharides 28–30, which were transformed in good yields, via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, imidate formation, coupling with (2S,3R,4E)-2-azido-O-benzoyl-4-octadecene-1,3-diol (35) in the presence of boron trifluoride etherate, selective reduction of the azido group, coupling with octadecanoic acid, selective removal of the levulinoyl groups, treatment with sulfur trioxide-pyridine complex, then removal of the protecting groups, into the desired sulfated sialyl Lex ganglioside analogues 50–52.