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-[alpha-L-arabinopyranosyL(1 -> 6)]-2-acetamido-2-deoxy-beta-D-glucopyranosyl aleanolic acid (1) and 3-O-[alpha-L-arabinopyranosyl(1 -> 2)-alpha-L-arabin opyran o syl (1 -> 6)] -2-acetamido-2-deoxy-beta-D-glucopyranosyl acacic acid lactone (2)) along with seven known saponins {3-O-[alpha-L-arabinopyranosyl(1 -> 6)]-2-acetamido-2-deoxy-beta-D-glucopyranosyl echinocystic acid (3), 3-0-[beta-D-xylopyranosyl (1 -> 2)-alpha-L-arabinopyranosyl(1 -> 6)]-2-acetamido-2-deoxy-beta-D-glucopyranosyl acacic acid lactone (concinnoside D) (4), 3-O-[beta-D-glucopyranosyl(1 -> 2)]-beta-D-glucopyranosyl oleanolic acid (5), 3-O-[alpha-L-arabinopyranosyl(1 -> 2)-alpha-L-arabinopyranosyl(1 -> 6)]-beta-D-glucopyranosyl oleanolic acid (6), 3-O-[beta-D-xylopyranosyl(1 -> 2)-alpha-L-arabinopyranosyl(1 -> 6)]-beta-D-glucopyranosyl oleanolic acid (7), 3-O-[alpha-L-arabinopyranosyl(1 -> 2)-alpha-L-arabinopyranosyl(1 -> 6)-[beta-D-glucopyranosyl(1 -> 2)]-beta-D-glucopyranoside echinocystic acid (8), and 3-O-[beta-D-xylopyranosyl(1 -> 2)alpha-L-arabinopyranosyl(1 -> 6)-[beta-D-glucopyranosyl(1 -> 2)]-beta-D-glucopyranoside echinocystic acid (9)). The structures of 1 and 2 were established on the basis of extensive 2D NMR (H-1-H-1 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 mu M, using the MTS assay.