Synthesis of sialyl Le(x) ganglioside analogues modified at C-6 of the galactose residue to elucidate the mechanism of selection recognition

Article abstract of DOI:10.1016/S0008-6215(98)00013-5

Sialyl Lewis X ganglioside analogues modified at C-6 of the galactose residue, having 6-O-methoxy, 6-acetamido and 6-amino functional groups, have been synthesized. Treatment of 2-(trimethylsilyl)ethyl (methyl 5-acetamido- 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2- nonulopyranosylonate)-(2→3)-2-O-benzoyl-β-D-galactopyranoside with trimethyloxonium tetrafluoroborate or Tf₂O, followed by NAN₃, gave the 6- O-methyl and 6-azido derivatives, respectively, which were converted into their respective methyl 1-thioglycoside derivatives. The glycosyl donors obtained were coupled with 2-(trimethylsilyl)ethyl 2-acetamido-6-O-benzyl-2- deoxy-3-O-(4-methoxybenzyl)-β-D-glucopyranosyl-(1→3)-2,4,6-tri-O-benzyl- β-D-galactopyranosyl-(1→4)-2,3,6-tri-O-benzyl-β-D-galactopyranoside in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) to give their respective pentasaccharides. The glycosylation of the pentasaccharide acceptors derived from their precursors by removal of the 4-methoxybenzyl group, with phenyl 1-thioglycoside derivative of L-fucose using N- iodosuccinimide TfOH afforded the corresponding hexasaccharides, which were transformed in good yield, via reductive removal of the benzyl group, the simultaneous transformation of azide to amine and acetamide, O-deacylation and saponification of the methyl ester into the target compounds without the ceramide groups. On the other hand, the proper manipulation of the protecting group of the hexasaccharides, including a transformation of azide into acetamide and trifluoroacetamide, gave the hexasaccharide imidates, which were coupled with (2S,3R,4E)-2-azido-3-O-tert-butyldiphenylsilyl-4- octadecene-1,3-diol. Selective reduction of the azido group, N-acylation with octadecanoic acid, and the complete removal of the protecting groups gave the desired sialyl Le(x) ganglioside analogues.