Synthetic studies toward pyruvate acetal-containing saccharides: En route to the efficient synthesis of Rhizobium-related exopolysaccharide fragments

The disaccharide building block benzyl O-{2,3-di-O-benzoyl-4,6-O-[(R)-1-(methoxycarbonyl) ethylidene]-β-d-galactopyranosyl}-(1→3)-2-O-benzoyl-4,6-O-[(S)-1-(methoxycarbonyl)ethylidene]-α-d-glucopyranoside (13), related to a Rhizobium exopolysaccharide, was prepared by coupling various 4,6-O-[(R)-1-(methoxycarbonyl)ethylidene]-d-galactosyldonors (benzoyl-protected chloride 1, pivaloyl-protected chloride 2, and benzoyl-protected fluorides 3 and 4, and trichloroacetimidate 5) with benzyl 2-O-benzoyl-4,6-O-[(S)-1-(methoxycarbonyl)ethylidene]-α-d-glucopyranoside (10) and the corresponding 2,3-O-tetraisopropyldisiloxane-protected glucoside 12. The best results, with respect to β-selectivity and yield of the coupling, were obtained with 5 and 10 in dichloromethane. The β-linked (13) and α-linked (14) disaccharides were efficiently converted via the 1-OH derivatives 17 and 21 into the corresponding trichloroacetimidates 18 and 22. The latter were used for the synthesis of the disaccharide ligands 4,6-(R)-pyruvate-β-d-Galp-(1→3)-4,6-(S)-pyruvate-β-d-Glep-O(CH2)5NH2 (20), and 4,6-(R)-pyruvate-α-d-Galp-(1→3)-4,6-(S)-pyruvate-β-d-Glcp-O(CH2)5NH2 (24). The corresponding tri- and tetra-saccharide derivatives 4,6-(R)-pyruvate-β-d-Galp-(1→3)-4,6-(S)-pyruvate-β-d-Glcp-(1→4)-β-d-Glcp-O(CH2)5NH2 (28) and 4,6-(R)-pyruvate-β-d-Galp-(1→3)-4,6-(S)-pyruvate-β-d-Glcp-(1→4)-β-d-Glcp-(1→4)-β-d-Glcp- O(CH2)5NH2 (36) were obtained similarly.