Synthesis of a model compound related to an anti-ulcer pectic polysaccharide

A stereocontrolled synthesis of the model compound for an anti-ulcer active polysaccharide (Bupleuran 2IIc) is described. Glycosidation of the disaccharide acceptor, 2-O-acetyl-3-O-benzyl-4-O-(p-methoxybenzyl)-α-L-rhamnopyranosyl-(1→4)-2,3,6-tri-O-benzyl-α-D-galactopyranosyl trichloroacetimidate, with the disaccharide receptor, allyl 3,4-di-O-benzyl-α-L-rhamnopyranosyl-(1→4)-2,3,6-tri-O-benzyl-β-D-galactopyranoside, using silver triflate (AgOTf) as a promoter gave the desired tetrasaccharide derivative, which was transformed into the acidic tetrasaccharide, corresponding to a segment of the rhamnogalacturonan (Bupleuran 2IIc) polysaccharide, propyl α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA, via removal of the corresponding ether and ester protecting groups, followed by oxidation. Copyright (C) 2000 Elsevier Science Ltd.