Synthesis of deoxy-L-fucose-containing sialyl Lewis X ganglioside analogues.

Sialyl Lex ganglioside analogs containing 2-, 3-, and 4-deoxyfucose in the place of L-fucose have been synthesized. Glycosylation of 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-beta-D-glucopyranosyl)-(1-->3)- 2,4,6-triO-benzyl-beta-D-galactopyranoside with the methyl 1-thioglycoside derivatives of the respective deoxyfucoses, using dimethyl(methylthio)sulfonium triflate (DMTST) as a promoter, gave the corresponding three protected 2-(trimethylsilyl)ethyl dideoxy-alpha-L-heoxpyranosyl-(1-->3)- O-2(acetamido-2-deoxy-beta-glucopyranosyl)-(1-->3)-beta-D-galactop yranosides. These were transformed by reductive ring-opening of their benzylidene acetal groups into the glycosyl acceptors 6, 8, and 10. Dimethyl(methylthio)sulfonium triflate promoted glycosylation of 6, 8, and 10 with methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha- D-galacto-2-nonulopyranosylonate)-(2-->3)-2,4,6-tri-O-benzoyl-1-th io-beta-D-galactopyranoside afforded the desired pentasaccharides, which were converted via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, and reaction with trichloroacetonitrile, into the corresponding alpha-trichloroacetimidates 14, 18, and 22. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol with 14, 18, and 22 in the presence of boron trifluoride etherate afforded the expected beta-glycosides, which were transformed in good yields, via selective reduction of the azido group, coupling with octadecanoic acid, O-deacylation, and deesterification, into the target compounds.