Atropisomeric bi(9,10-dihydro)phenanthrene and phenanthrene/bibenzyl dimers with cytotoxic activity from the pseudobulbs of Pleione bulbocodioides.

Bulbocodioidins E-H (1-4), four pairs of undescribed racemic bi(9,10-dihydro)phenanthrene and phenanthrene/bibenzyl atropisomers, along with four known compounds (5-8) were isolated from the ethanol extract of the pseudobulbs of Pleione bulbocodioides. Their structures were established by HRESIMS and comprehensive NMR spectroscopic data analysis. Their absolute configurations were elucidated by comparison of their experimental and calculated ECD (electronic circular dichroism) curves. Furthermore, compound 4a displayed cytotoxic activity against colon cancer (HCT-116), liver cancer (HepG2), and breast cancer (MCF-7) cell lines with IC₅₀ values of 7.6, 3.8 and 3.4 μM, respectively. Compound 6 showed cytotoxic activity against breast cancer cell lines (MCF-7) with IC₅₀ value of 5.4 μM.