Two New Diterpenoids Formed by Transannular Diels–Alder Cycloaddition from the Soft Coral Sarcophyton tortuosum, and Their Antibacterial and PPAR-β Agonist Activities

Two new cembrane-derived tricyclic diterpenes belonging to the sarcophytin family, namely 4<i>a</i>-hydroxy-chatancin (<b>1</b>) and sarcotoroid (<b>2</b>), together with two known related ones (<b>3</b> and <b>4</b>), were isolated from the soft coral <i>Sarcophyton tortuosum</i> collected off Ximao Island in the South China Sea. The structures of the new compounds were elucidated by extensive spectroscopic analysis, a quantum mechanical nuclear magnetic resonance (QM-NMR) method, a time-dependent density functional theory electronic circular dichroism (TDDFT-ECD) calculation, X-ray diffraction analysis, and comparison with the reported data in the literature. A plausible biosynthetic pathway of compounds <b>1</b>-<b>4</b> was proposed, involving undergoing a transannular Diels-Alder cycloaddition. In the bioassay, the new compound <b>1</b> displayed significant inhibitory activities against the fish pathogens <i>Streptococcus parauberis</i> KSP28, oxytetracycline-resistant <i>Streptococcus parauberis</i> SPOF3K, and <i>Photobacterium damselae</i> FP2244, with MIC values of 9.1, 9.1, and 18.2 μg/mL, respectively. Furthermore, by conducting a luciferase reporter assay on rat liver Ac2F cells, compounds <b>1</b>, <b>3</b>, and <b>4</b> were evaluated for peroxisome proliferator-activated receptor (PPAR) transcriptional activity, and compound <b>3</b> showed selective PPAR-β agonist activity at a concentration of 10 μΜ.