New Eunicellin Diterpenes and 9,10-Secosteroids from the Gorgonian Muricella sibogae

Bioactivity‐guided isolation of the rare gorgonian Muricella sibogae (Nutting) yielded the two new eunicellin diterpenes sibogin A and B (1 and 2), the three new 9,10‐secosteroids sibogol A–C (6–8), together with the three known eunicellin diterpenes 3–5 and the five known 9,10‐secosteroids 9–13. Their structures were established by extensive spectral analysis (1D‐ and 2D‐NMR, IR, and MS). The cytotoxicity of the isolates 1–13 was evaluated in vitro against the selected tumor cell lines P388 and BEL‐7402. All the compounds showed only weak activity against P388 cell lines, with an inhibition rate ranging from 10 to 60% at a concentration of 50 μg/ml, whereas the were inactive against BEL‐7402 cell lines.