Synthesis of the trisaccharide moiety and a cholesteryl analog of phyteumosides

Abstract

A first synthesis of the trisaccharide moiety of the phyteumosides and its cholesteryl analog is described using easily accessible and regioselectively protected D-glucose, L-rhamnose and D-galactose building blocks via a linear glycosylation approach. The two glycosyl donors were prepared as thiophenyl (SPh) glycosides. Trichloroacetimidate (TCAI) coupling was employed for the first glycosylation step while thioglycoside activation was used for the second glycosylation to assemble the O-allyl trisaccharide. After removal of the anomeric allyl group, the trisaccharide was converted into a TCAI donor. Using cholesterol as an acceptor in combination with acetonitrile as a participating solvent to achieve β-selectivity in glycosylation, a phyteumoside analog was obtained.