Synthesis of the α- d-GlcpA-(1→3)-α- l-Rhap-(1→2)- l-Rha trisaccharide isolated from the cell wall hydrolyzate of the green alga, Chlorella vulgaris

The title trisaccharide was synthesized from 6- O-acetyl-2,3,4-tri- O-benzyl-α- d-glucopyranosyl chloride ( 10), ethyl 2,4-di- O-benzyl-1-thio- ( 5) and benzyl 3,4-di- O-benzyl-α- l-rhamnopyranoside ( 9). The disaccharide 11 obtained from compounds 5 and 10 was used as the glycosyl donor to glycosylate the rhamnopyranoside derivative 9 having free OH-2 using the NIS–AgOTf-mediated glycosylation methodology. Zemplén deacetylation of the trisaccharide 12 resulted in the 6′′-OH derivative ( 13), which was selectively oxidized with CrO 3 to the uronic acid derivative 14. The benzyl groups were removed by catalytic hydrogenolysis to furnish the target trisaccharide ( 1). Preparation of the title trisaccharide is reported.