Convergent synthesis of glycoalkaloids solasonine and its saponin derivative.

We present a practical and convergent synthesis of glycoalkaloids solasonine 1 and its saponin derivative 2, incorporating a {3--α-L-rhamnopyranosyl-(1→2)--[β-D-glucopyranosyl-(1→3)]-β-D-galactopyranoside} moiety. The key features of this strategy include the following: (1) AuCl-BuCN cooperative catalysis enabling 1,2- stereoselective glycosidation of 2-branched trisaccharide trichloroacetimidate donors with steroidal aglycons, in the absence of neighboring group participation; (2) "cyanide effect" mediated regioselective benzoylation of the 4- and 6-hydroxyl groups of galactopyranosyl disaccharide; and (3) an effective approach to prevent orthoester byproduct formation.