The present investigation of the South China Sea soft coral Sarcophyton trocheliophorum resulted in the discovery of six new polyoxygenated diterpenes, namely sartrocheliols A–E (1, 3, 5–8) along with four known ones, 2, 4, 9, and 10. Based on extensive spectroscopic data analysis, sartrocheliol A (1) was identified as an uncommon capnosane diterpene, while sartrocheliols B–E (3, 5–8) were established as cembrane diterpenes. They displayed diverse structural features not only at the distinctly different carbon frameworks but also at the various types of heterocycles, including the epoxide, γ-lactone, furan, and pyran rings. Moreover, their absolute configurations were determined by a combination of quantum mechanical-nuclear magnetic resonance (QM-NMR) approach, modified Mosher’s method, and X-ray diffraction analysis. In the anti-tumor bioassay, compound 4 exhibited moderate cytotoxic activities against A549, H1975, MDA-MB-231, and H1299 cells with the IC50 values ranging from 26.3 to 47.9 μM.

Two novel diterpenoids, sinuaustones A (1) and B (2), featuring an unprecedented tricyclo[9.3.1.03,15]tetradecane carbon framework were isolated from the South China Sea soft coral Sinularia australiensis.

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A detailed chemical investigation of the Sanya Bay nudibranch Hexabranchus sanguineus yielded thirteen new sesquiterpenoids, namely sanyagunins AÀ H, sanyalides AÀ C, and sanyalactams A and B, along with eleven known related ones. Sanyalactams A and B feature an unprecedented hexahydrospiro[indene-2,3'-pyrrolidine] core. The structures of new compounds were established by a combination of extensive spectroscopic data analysis, quantum mechanicalnuclear magnetic resonance methods, the modified Mosher's method, and X-ray diffraction analysis. Based on analysis of NOESY correlations and the modified Mosher's method, the stereochemistry of two known furodysinane-type sesquiterpenoids were revised. A plausible biogenetic relationship between these sesquiterpenoids wasproposed and discussed, and a chemo-ecological relationship of the title animal and its possible sponge preys has been analyzed. In bioassays, sanyagunin B showed moderate antibacterial activity, whereas 4α-formamidogorgon-11-ene exhibited potent cytotoxicity with IC 50 values ranging from 0.87 to 1.95 μM.

Fifteen new diterpenoids, namely xishaklyanes A-O (1–15), along with three known related ones (16–18), were isolated from the soft coral Klyxum molle collected from Xisha Islands, South China Sea. The stereochemistry of the new compounds was elucidated by a combination of detailed spectroscopic analyses, chemical derivatization, quantum chemical calculations, and comparison with the reported data. The absolute configuration of compound 18 was established by the modified Mosher’s method for the first time. In bioassay, some of these compounds exhibited considerable antibacterial activities on fish pathogenic bacteria, and compound 4 showed the most effective activity with MIC of 0.225 μg/mL against Lactococcus garvieae.