Stereocontrolled synthesis of sulfur-linked analogues of the branched tetrasaccharide repeating-unit of the immunostimulant polysaccharide schizophyllan and of its β-(1 → 3)-branched, β-(1 → 6)-linked isomer

The branched, sulfur-linked tetrasaccharide S-(β-d-glucopyranosyl)-(1 → 3)-S-[(6-S-β-d-glucopyranosyl)-3,6-dithio-β-d-glucopyranosyl]-(1 → 3)-S-3-thio-d-glucopyranose (9) has been conveniently prepared by SN2 displacement of the triflate group in 1,2:5,6-di-O-isopropylidene-3-O-trifluoromethylsulfonyl-α-d-allofuranose with the sodium salt of 2,4-di-O-acetyl-3,6-di-S-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,3,6-trithio-β-d-glucopyranose (5). Conversely, reaction of the sodium salt of 5 with 1,2,3,4-tetra-O-acetyl-6-deoxy-6-iodo-β-d-glucopyranose afforded the positional isomer S-(β-d-glucopyranosyl)-(1 → 6)-S-[(3-S-β-d-glucopyranosyl)-3,6-dithio-β-d-glucopyranosyl]-(1 → 6)-S-6-thin-d-glucopyranose (12).