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The naturally occurring cytotoxic saponin 28-O-β-d-glucopyranosylbetulinic acid 3β-O-α-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 α-l-rhamnopyranoside moieties at both C-3 and C-28 positions, was determined to be a potent cytotoxic agent (IC₅₀ 1.8−1.9 μM).