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

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