(±)-Quassidine K, a pair of cytotoxic bis-β-carboline alkaloid enantiomers from Picrasma quassioides

(±)-Quassidine K (1), a pair of new bis-β-carboline alkaloid enantiomers, were isolated from Picrasma quassioides. Their structures were determined on the basis of detailed spectroscopic data analysis. The absolute configurations of (+)-S-quassidine K (1a) and (−)-R-quassidine K (1b) were determined by comparing with the reported experimental ECD spectra after chiral separation. The cytotoxicity assay showed activity against HeLa cells with IC50 values of 15.8 and 20.1 μM, respectively.