Tabernaesines A–I, Cytotoxic Aspidosperma–Aspidosperma-Type Bisindole Alkaloids from Tabernaemontana pachysiphon

Twenty-one aspidosperma–aspidosperma alkaloids, including the new tabernaesines A–J (1–9), were obtained from Tabernaemontana pachysiphon. The structures and absolute configurations were elucidated using HRMS and NMR experiments. Compounds 1–9 possessed a rare spiro heterocycle moiety between the monomeric units, while compounds 4 and 5 were characterized by an indole ring fused with an (N,N-diethyl)­methyl amino group. Compounds 1, 5–7, 15, and 16 exhibited moderate cytotoxic potency against various human cancer cell lines at IC50 2.5–9.8 μM.