Synthesis of rhamnogalacturonan I fragments

The partially deprotected trisaccharide 17 has been synthesized as an analogue of the repeating unit of the backbone of rhamnogalacturonan I. The trisaccharide 17 was obtained after prior selective derivatization of HO-3 and HO-4 of a rhamnopyranose cyanoethylidene glycosyl donor, followed by coupling with a tritylated galactopyranosyluronic acceptor (11), selective removal of the acetyl group at the O-2' position of the formed disaccharide 12, and glycosylation of the HO-2' position with methyl (ethyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-1-thio-beta-D-galactopyranosid)uronate (14) providing methyl (methyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-alpha-D-galactopyranosyluronate)-(1-->2)-(4-O-benzoyl-3-O-benzyl-alpha-L-rhamnopyranosyl)-(1-->4)-(allyl 2,3-di-O-benzyl-beta-D-galactopyranosid)uronate (15). Finally, palladium chloride catalyzed deallylation (16) and hydrogenolysis over Pd-C resulted in methyl (methyl alpha-D-galactopyranosyluronate)(1-->2)-(4-O-benzoyl-alpha-L-rhamnopyranosyl)-(1-->4)-alpha/beta D-galactopyranuronate (17).