Cytotoxic monoterpenoid indole alkaloids from Tabernaemontana bovina

Chemical investigation of the native medicinal plant Tabernaemontana bovina led to the isolation of five previously unreported monoterpenoid indole alkaloids tabernovinaines A-E (1-5) together with twenty-seven known analogs (6-32), including a bisindole alkaloid 1 with the (E)-4-aminobut-3-en-2-one fragment, as well as a unique cage skeleton 2 containing 6/5/8/6/6 ring system. The chemical structures of these unreported compounds were elucidated using mass spectrometry, NMR spectroscopy, circular dichroism, density functional theory calculations, and derivatizations. The activity evaluation shows that the bisindole alkaloid 1 revealed a potential cytotoxic effect by inducing HepG2 cell apoptosis and damaging clonal sphere expansion.