Cytotoxic Oleanane-Type Saponins from <i>Albizia inundata</i>

Abstract

Bioassay-guided fractionation of a CH₂Cl₂−MeOH extract of the aerial parts of <i>Albizia inundata</i> resulted in the isolation of two new natural oleanane-type triterpene saponins {3-<i>O</i>-[α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl oleanolic acid (<b>1</b>) and 3-<i>O</i>-[α-l-arabinopyranosyl(1→2)-α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl acacic acid lactone (<b>2</b>)} along with seven known saponins {3-<i>O</i>-[α-l-arabinopyranosyl(1→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl echinocystic acid (<b>3</b>), 3-<i>O</i>-[β-d-xylopyranosyl (l→2)-α-l-arabinopyranosyl(l→6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl acacic acid lactone (concinnoside D) (<b>4</b>), 3-<i>O</i>-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranosyl oleanolic acid (<b>5</b>), 3-<i>O</i>-[α-l-arabinopyranosyl(1→2)-α-l-arabinopyranosyl(l→6)]-β-d-glucopyranosyl oleanolic acid (<b>6</b>), 3-<i>O</i>-[β-d-xylopyranosyl(1→2)-α-l-arabinopyranosyl(l→6)]-β-d-glucopyranosyl oleanolic acid (<b>7</b>), 3-<i>O</i>-[α-l-arabinopyranosyl(l→2)-α-l-arabinopyranosyl(1→6)-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranoside echinocystic acid (<b>8</b>), and 3-<i>O</i>-[β-d-xylopyranosyl(l→2)-α-l-arabinopyranosyl(1→6)-[β-d-glucopyranosyl(l→2)]-β-d-glucopyranoside echinocystic acid (<b>9</b>)}. The structures of <b>1</b> and <b>2</b> 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 <b>1</b>, <b>3</b>, <b>6</b>, and <b>7</b> 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.