Biscembranoids and their probable biogenetic precursor from the Hainan soft coral Sarcophyton tortuosum

Five novel biscembranoids, ximaolides A-E (2-6), and their proposed biogenetic precursor, methyl tortuosoate (1), were isolated from the Hainan soft coral Sarcophyton tortuosum. The structures of compounds 1-6 were elucidated by spectroscopic methods, mainly NMR techniques. The relative stereochemistry of biscembranoids 2 and 6 was secured by X-ray diffraction analysis, whereas the relative configurations of chiral centers in compounds 1, 3, 4, and 5 have been suggested by both biogenetic considerations and NOESY experiments.