Diterpenes from the Hainan soft coral Lobophytum cristatum Tixier-Durivault.

Two new prenylgermacrane-type diterpenoids, lobophytumins A and B (1 and 2), two new prenyleudesmane-type diterpenoids, lobophytumins C and D (3 and 4), and two new spatane-type diterpenoids, lobophytumins E and F (5 and 6), were isolated from the Hainan soft coral Lobophytum cristatum Tixier-Durivault. Their structures, including relative configuration, were elucidated by detailed analysis of spectroscopic data and by comparison with related known compounds. In addition, the absolute configuration of lobophytumin C (3) was tentatively assigned by comparing its specific rotation with that of the closely related model compound (-)-β-selinene (8). On the basis of biogenetic considerations, the absolute configurations of lobophytumins A, B, and D-F were also tentatively suggested. This is the first report of spatane-type diterpenoids from a soft coral source. The present work supports Faulkner's proposal of prenylgermacrene as the precursor of many diterpenes. In a bioassay, lobophytumins C and D (3 and 4) showed weak in vitro cytotoxicities against the tumor cell lines A-549 and HCT-116.