SYNTHETIC STUDIES ON SIALOGLYCOCONJUGATES .27. SYNTHESIS OF SIALYL-ALPHA(2-]6)-LEWIS-X

Synthesis of a positional isomer of sialyl Lewis X with regard to the substitution of the terminal galactose residue of the pentasaccharide by N-acetylneuraminic acid is described. Dimethyl(methylthio)sulfonium triflate-promoted coupling of 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl) (1 --> 3)-O-(2-acetamido-6-O-benzyl-2-deoxy-beta-D-glucopyranosyl) (1 --> 3)-O-(2,4,6-tri-O-benzyl-beta-D-galactopyranosyl)-(1 --> 4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside (1) with methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl 3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylonate)-(2 --> 6) 2,4-di-O-benzoyl-3-O-benzyl-1-thio-beta-D-galactopyranoside (2) gave the desired hexasaccharide 3. Compound 3 was converted into the alpha-trichloro-acetimidate 6, via reductive removal of the benzyl groups, O-acetylation, removal of the 2-(-trimethylsilyl)ethyl group, and treatment with trichloro-acetonitrile, which, on coupling with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1, 3-diol (7), gave the beta-glycoside 8. Finally, 8 was transformed, via selective reduction of the azide group, coupling with octadecanoic acid, O-deacylation, and hydrolysis of the methyl ester group, into the title ganglioside 11 in good yield.