Crystal and molecular structures of a natural equimolecular mixture of twoepimeric diterpenes

Irroratin A (1), isolated from the aerial Darts 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-N MR spectral analysis. The two epimers were hound together by hydrogen bonds , and when in chloroform and methanol solution, the 20S-epimer predominates . Irroratin A exhibited potent cytotoxicity against several human cancer ce ll lines. (C) 2000 Elsevier Science Ltd. All rights reserved.