Vobatensines A–F, Cytotoxic Iboga-Vobasine Bisindoles from Tabernaemontana corymbosa

Six new bisindole alkaloids of the iboga-vobasine type, vobatensines A–F (1–6), in addition to four known bisindoles (8–11), were isolated from a stem bark extract of a Malayan Tabernaemontana corymbosa. The structures of these alkaloids were determined based on analysis of the spectroscopic data and in the case of vobatensines A (1), B (2), and 16′-decarbomethoxyvoacamine (8) also confirmed by partial syntheses. Nine of these alkaloids (1–5, 8–11) showed pronounced in vitro growth inhibitory activity against human KB, PC-3, LNCaP, HCT 116, HT-29, MCF7, MDA-MB-231, and A549 cancer cells.