Baphicacanthcusines A–E, Bisindole Alkaloids from the Leaves of Baphicacanthus cusia (Nees) Bremek

Four pairs of stereoisomeric indole alkaloids, (±)-baphicacanthcusines A–D (1–4), and one new indole alkaloid, baphicacanthcusine E (5), together with nine known compounds were identified from the leaves of Baphicacanthus cusia. (±)-1 and -2 possess an unprecedented skeleton in which two indole moieties are bridged by a phenylpropane unit. (±)-3 represents the first natural dispiro-oxazolidinone bisoxindoles. The absolute configurations in 1–5 were assigned based on quantum chemical calculations, including the calculated chemical shift with DP4plus analysis, the calculated optical rotation values, and the calculated electronic circular dichroism spectra. A plausible biosynthetic pathway for 1–5 was proposed. Compounds (±)-1, (−)-2, and 11 exhibited cytotoxicity against MCF-7 cells with IC₅₀ values of 20.0–78.5 μM.