Twenty-four steroid-based natural products, 9,10-secosteroids (1–10), 1,4-dien-3-one steroids (11–19), and 4-en-3-one steroids (20–24), containing varying side-chains, were isolated from the South China Sea gorgonian Muricella sibogae. The structures of one new 9,10-secosteroid, sibogol D (1), and two new 1,4-dien-3-one steroids, sibogols E and F (11 and 12), were elucidated by extensive spectroscopic analyses and by comparison with the literature data. Cytotoxicities for all the isolates were evaluated against four selected tumor cell lines, HL-60, HCT116, K562, and P388. Compounds 3, 9, and 13 exhibited potent cytotoxic activities against the HL-60 cell line, with IC₅₀ values ranging from 1.27 to 10.05 μM, and compound 3 was also cytotoxic against HCT116 with an IC₅₀ value of 5.8 μM. The bioassay results also indicated a potential relationship between structural patterns and activity. The newly presented series of 1,4-dien-3-one and 4-en-3-one types of steroids relating to the unique 9,10-secosteroids in biogenesis were found in this species for the first time, which is of considerable chemotaxonomic significance.

Four new polyhydroxylated steroids, 1–4, and the racemic form of cyclopentenone 9, together with four known steroids, 5–8, one known cyclopentenone derivative, 10, and one known butenolide derivative, 11, were isolated from the soft coral Sinularia acuta collected from Weizhou Island of Guangxi Province, P. R. China. Their structures were elucidated on the basis of spectroscopic analyses and by comparison of the corresponding data with those previously reported. The cytotoxicities of the isolates 1–11 in vitro against the selected tumor cell lines HL-60, HeLa, and K562 were evaluated. Compounds 2 and 5 showed potent cytotoxicities against HL-60 cell lines with IC₅₀ values of 7.3 and 9.9 μM, respectively. Compounds 5 and 6 showed moderate activities against K562 cell lines with IC₅₀ values of 10.9 and 11.7 μM, respectively, while compounds 1, 2, and 6 showed weak activities against HeLa cell lines with respective IC₅₀ values of 44.8, 27.1, and 18.2 μM. This is the first report on chemical and bioactivity research of S. acuta.

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.

Nine achiral tetraprenylated alkaloids, including three new compounds, named malonganenones I–K (1–3, resp.), together with six known analogs, 4–9, were isolated from the gorgonian Euplexaura robusta collected from Weizhou Island of Guangxi Province, China. The structures of compounds 1–3 were elucidated by extensive spectral analyses, especially of their 1D- and 2D-NMR data. Compounds 1, 4, 6, and 7 showed moderate cytotoxicities against K562 and HeLa tumor cell lines with IC₅₀ values ranging from 0.35 to 10.82 μM. Compound 6 also showed moderate inhibitory activity against c-Met kinase at a concentration of 10 μM.