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

Two new cembrane-derived tricyclic diterpenes belonging to the sarcophytin family, namely 4-hydroxy-chatancin () and sarcotoroid (), together with two known related ones ( and ), were isolated from the soft coral 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 - was proposed, involving undergoing a transannular Diels-Alder cycloaddition. In the bioassay, the new compound displayed significant inhibitory activities against the fish pathogens KSP28, oxytetracycline-resistant SPOF3K, and 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 , , and were evaluated for peroxisome proliferator-activated receptor (PPAR) transcriptional activity, and compound showed selective PPAR-β agonist activity at a concentration of 10 μΜ.