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.