Synthesis and Cytotoxicity of Bidesmosidic Betulin and Betulinic Acid Saponins

The naturally occurring cytotoxic saponin 28-O-beta-D-glucopyranosylbetulinic acid 3 beta-O-alpha-L-arabinopyranoside (3) was easily synthesized along with seven bidesmosidic saponins starting from the lupane-type triterpenoids betulin (1) and betulinic acid (2). As highlighted by the preliminary cytotoxicity evaluation against A549, DLD-1, MCF7, and PC-3 human cancer cell lines, the bidesmosidic betulin saponin 22a, bearing alpha-L-rhamnopyranoside moieties at both C-3 and C-28 positions, was determined to be a potent cytotoxic agent (IC50 1.8-1.9 mu M).