Synthesis of rhamnogalacturonan I fragments

The partially deprotected trisaccharide 17 has been synthesized as an analo gue of the repeating unit of the backbone of rhamnogalacturonan I. The tris accharide 17 was obtained after prior selective derivatization of HO-3 and HO-4 of a rhamnopyranose cyanoethylidene glycosyl donor, followed by coupli ng with a tritylated galactopyranosyluronic acceptor (11), selective remova l of the acetyl group at the O-2' position of the formed disaccharide 12, a nd 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 me thyl (methyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-alpha-D-galactopyranosylur onate)-(1-->2)-(4-O-benzoyl-3-O-benzyl-alpha-L-rhamnopyranosyl)-(1-->4)-(al lyl 2,3-di-O-benzyl-beta-D-galactopyranosid)uronate (15). Finally, palladiu m chloride catalyzed deallylation (16) and hydrogenolysis over Pd-C resulte d in methyl (methyl alpha-D-galactopyranosyluronate)(1-->2)-(4-O-benzoyl-al pha-L-rhamnopyranosyl)-(1-->4)-alpha/beta D-galactopyranuronate (17).