Cytotoxic Vobasine, Tacaman, and Corynanthe-Tryptamine Bisindole Alkaloids from <i>Tabernaemontana</i> and Structure Revision of Tronoharine

Abstract

Seven new indole alkaloids (<b>1</b>–<b>7</b>) comprising four vobasine, two tacaman, and one corynanthe-tryptamine bisindole alkaloid were isolated from the stem-bark extract of a Malayan <i>Tabernaemontana</i>. Two of the new vobasine alkaloids (<b>1</b>, <b>3</b>), as well as 16-epivobasine (<b>15</b>) and 16-epivobasenal (<b>17</b>), showed appreciable cytotoxicity toward KB cells (IC₅₀ ca. 5 μg/mL). The structure of the known <i>Tabernaemontana</i> alkaloid tronoharine (<b>8</b>) was revised based on newly acquired NMR data, as well as X-ray diffraction analysis