Melotenuines A-E, cytotoxic monoterpenoid indole alkaloids from Melodinus tenuicaudatus.

Five new monoterpenoid indole alkaloids, melotenuines A-E (1-5), along with 18 known indole alkaloids, were isolated from the twigs and leaves of Melodinus tenuicaudatus. The structures of the new alkaloids were determined by a combination of MS, NMR and ECD analysis. Melotenuine A (1) represents the first example of aspidosperma-meloscandonine type bisindole alkaloids characterized by a methylene bridge between the two monomers, while melotenuine B (2) possessed a rare eburnamine-melsocandonine skeleton. All of the new indole alkaloids were evaluated for in vitro cytotoxicities against five human cancer cell lines. Among them, alkaloid 4 showed specific cytotoxicity against HL-60 cell line with IC₅₀ value (5.15 ± 0.16 μM) comparable with that of positive control.