Active–latent glycosylation strategy toward Lewis X pentasaccharide in a form suitable for neoglycoconjugate syntheses

Glycosylation of 4-nitrophenyl 2-acetamido-6-O-tert-butyldiphenylsilyl-2-deoxy-1-thio-β-d-glucopyranoside with phenyl 2,3,4,6-tetra-O-benzoyl-1-thio-β-d-galactopyranoside in the presence of NIS and TfOH as catalyst gave the lactosamine derivative regiospecifically in high yield. Further 3-O-fucosylation with phenyl 2,3,4-tri-O-benzyl-1-thio-β-l-fucopyranoside using DMTST as promoter afforded the Lex trisaccharide intermediate. The latent glycosyl donor was transformed into its active form (p-acetamidothiophenyl) by reduction with zinc in acetic acid and N-acetylation. Glycosidation with p-nitrothiophenyl lactoside acceptor in the presence of NIS/TfOH as catalyst gave the Lex pentasaccharide in 71% yield.