Cytotoxic Oleanane-Type Saponins from Albizia inundata

Bioassay-guided fractionation of a CH2Cl2−MeOH extract of the aerial parts of Albizia inundata resulted in the isolation of two new natural oleanane-type triterpene saponins {3-O-[α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl oleanolic acid (1) and 3-O-[α-l-arabinopyranosyl(1→2)-α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl acacic acid lactone (2)} along with seven known saponins {3-O-[α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl echinocystic acid (3), 3-O-[β-d-xylopyranosyl (l→2)-α-l-arabinopyranosyl(l→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl acacic acid lactone (concinnoside D) (4), 3-O-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranosyl oleanolic acid (5), 3-O-[α-l-arabinopyranosyl(1→2)-α-l-arabinopyranosyl(l→6)]-β-d-glucopyranosyl oleanolic acid (6), 3-O-[β-d-xylopyranosyl(1→2)-α-l-arabinopyranosyl(l→6)]-β-d-glucopyranosyl oleanolic acid (7), 3-O-[α-l-arabinopyranosyl(l→2)-α-l-arabinopyranosyl(1→6)-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranoside echinocystic acid (8), and 3-O-[β-d-xylopyranosyl(l→2)-α-l-arabinopyranosyl(1→6)-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranoside echinocystic acid (9)}. The structures of 1 and 2 were established on the basis of extensive 2D NMR (1H−1H COSY or DQF-COSY, HSQC, HMBC, TOCSY, and HSQC-TOCSY) spectroscopic, ESIMS, and chemical methods. Saponins 1, 3, 6, and 7 showed cytotoxicity against human head and neck squamous cells (JMAR, MDA1986) and melanoma cells (B16F10, SKMEL28) with IC50 values in the range 1.8−12.4 μM, using the MTS assay.