Structurally Diverse Diterpenoids from the Sanya Bay Nudibranch Hexabranchus sanguineus and Its Sponge‐Prey Chelonaplysilla sp.

Abstract

AbstractInvestigation 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.