Belamchinenin A, an unprecedented tricyclic-fused triterpenoid with cytotoxicity from Belamcanda chinensis

Belamchinenin A (1), an unprecedented 6/5/6-fused tricyclic triterpenoid with a novel carbon skeleton, has been isolated from the rhizomes of Belamcanda chinensis. Compound 1 features a half-caged tricyclic nucleus with a flexible geranyl side chain. Its structure was determined by the interpretation of spectroscopic data. The absolute configuration of tricyclic nucleus system of 1 was unequivocally assigned by ECD calculation. Geometries optimization indicated that the tricyclic nucleus system is rigid. Compound 1 showed cytotoxic activities against five cells with IC₅₀ values from 2.29 to 4.47 μM.