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Nanolobatone A, featuring an unprecedented tricyclo[10.3.0.01,2]pentadecane carbon skeleton, along with four new polyoxygenated and four unusual endoperoxide‐bridged casbane‐type diterpenoids were isolated from the Hainan soft coral Sinularia nanolobata. The structures of the new compounds were established by extensive spectroscopic analysis, X‐ray diffraction analysis, and time‐dependent density functional theory/electronic circular dichroism calculations. A plausible biosynthetic pathway of new isolates was proposed. Bioassays revealed that nanolobatone A showed weak antibacterial activity against the Gram‐positive bacteria Streptococcus pyogenes. [ABSTRACT FROM AUTHOR]

Investigation of the South China Sea nudibranch Hexabranchus sanguineus from Sanya Bay afforded, in addition to three known compounds, nine new diterpenoids of the 5,19‐cycloclerodane‐ (sanyanolides A–D), clerodane‐ (sanyanolide E) and subersin‐ (sanyanolides F–I) type. Remarkably, six diterpenoids aforementioned from H. sanguineus were also isolated from the sponge Chelonaplysilla sp. from the same water region, suggesting a trophic relationship between H. sanguineus and Chelonaplysilla sp. The structure and absolute configuration of new compounds were established by a combination of spectroscopic data, X‐ray diffraction analysis and/or time‐dependent density functional theory/electronic circular dichroism calculations. A plausible biogenetic relationship between these diterpenoids, along with the chemo‐ecological implications of their co‐occurrence in the two organisms investigated, was proposed and discussed. In in vitro bioassays, echinoclerodane A exhibited a potent inhibitory effect (IC50=2.81 μM) on LPS‐induced inflammatory response in RAW 264.7 macrophage cells. In addition, echinoclerodane A and oculatolide showed considerable antibacterial activities with MIC values ranging from 1.0 to 8.0 μg/mL. [ABSTRACT FROM AUTHOR]

A detailed chemical investigation of the nudibranch Glossodoris atromarginata collected from Weizhou Island, South China Sea, yielded a new spongian‐type diterpene 1, together with the four known‐related compounds 2–5. The structure of the new compound 1 was elucidated by the detailed spectroscopic analysis, the comparison of the spectroscopic data with the known diterpene isoagatholactone, and the 13C chemical shift calculation. In addition, evidence for the absolute stereochemistry of the known compound 2 was, for the first time, provided by the application of time‐dependent density functional theory electronic circular dichroism calculation. [ABSTRACT FROM AUTHOR]

Placobranchus ocellatus is well known to produce diverse and complex γ‐pyrone polypropionates. In this study, the chemical investigation of P. ocellatus from the South China Sea led to the discovery and identification of ocellatusones A–D, a series of racemic non‐γ‐pyrone polyketides with novel skeletons, characterized by a bicyclo[3.2.1]octane (1, 2), a bicyclo[3.3.1]nonane (3) or a mesitylene‐substituted dimethylfuran‐3(2H)‐one core (4). Extensive spectroscopic analysis, quantum chemical computation, chemical synthesis, and/or X‐ray diffraction analysis were used to determine the structure and absolute configuration of the new compounds, including each enantiomer of racemic compounds 1–4 after chiral HPLC resolution. An array of new and diversity‐generating rearrangements is proposed to explain the biosynthesis of these unusual compounds based on careful structural analysis and comparison with six known co‐occurring γ‐pyrones (5–10). Furthermore, the successful biomimetic semisynthesis of ocellatusone A (1) confirmed the proposed rearrangement through an unprecedented acid induced cascade reaction. [ABSTRACT FROM AUTHOR]