Crystal and molecular structures of a natural equimolecular mixture of two epimeric diterpenes

Irroratin A (1), isolated from the aerial parts of Isodon irrorata (Forrest.) (Labiatae), was shown to be an equimolecular mixture of two C-20 epimers of a new ent-kaurene diterpene both in the crystalline state and pyridine solution, based on X-ray, LC/MS/MS, FABMS as well as extensive 1D- and 2D-NMR spectral analysis. The two epimers were bound together by hydrogen bonds, and when in chloroform and methanol solution, the 20S-epimer predominates. Irroratin A exhibited potent cytotoxicity against several human cancer cell lines.