Synthesis of a thio-linked Lewis A (Lea) epitope

The synthesis of heptyl (α-l-fucopyranosyl)-(1→4)-S-[(β-d-galactopyranosyl)-(1→3)]-1,4-dithio-β-d-glucopyranoside (2), as thio-linked Lewis A analogue was based on thexyldimethylsilyl 3-O-allyl-2-O-benzoyl-6-O-(4-methoxybenzyl)-4-thio-β-d-glucopyranoside (15) which was readily obtained from d-galactose. Reaction of 15 with O-3,4-di-O-acetyl-2-O-(4-methoxybenzyl)-α-l-fucopyranosyl trichloroacetimidate (8) as fucosyl donor afforded the α-(1→4)-thio-linked disaccharide. Replacement of the 4-methoxybenzyl groups by acetyl groups and removal of the 3a-O-allyl group afforded as 3a-O-unprotected acceptor thexyldimethylsilyl (2,3,4-tri-O-acetyl-α-l-fucopyranosyl)-(1→4)-S-6-O-acetyl-2-O-benzoyl-4-thio-β-d-glucopyranoside (19), which gave with 2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl trichloroacetimidate as galactosyl donor (20) the trisaccharide. Transformation into a trichloroacetimidate as glycosyl donor, glycosylation of heptylmercaptan, and then removal of the O-acyl protective groups afforded target molecule 2.