A pair of tirucallane C27-triterpenoid cyclopentenone epimers from the stem barks of Aphanamixis grandifolia

Two novel tirucallane C27-triterpenoid epimers, aphagranins A (1) and B (2), featuring an unprecedented enolized cyclopentenone presented in the side-chain at C-17, were isolated from the stem barks of Aphanamixis grandifolia. Extensive spectroscopic analyses helped the establishment of the structures of the two isolates, whose absolute configurations were determined using density functional theory (DFT) calculations of optical rotation, and electronic circular dichroism (ECD). Remarkable discrepancies in the inhibitory activities against the growth of six lines of human cancer cells (MCF-7, A549, HepG2, Bel-7402, SGC-7901, and BGC-823) were found for the two epimers: with IC50 less than 10 μM, aphagranin A exhibited much stronger antiproliferative activity than aphagranin B, showing no such activities with IC50 over 20 μM.