Uncommon Diterpenoids from the South China Sea Soft Coral Sinularia humilis and Their Stereochemistry

The chemical investigation of the South China Sea soft coral Sinularia humilis has resulted in the isolation of a library of diverse diterpenoids, including four new cembranoids, namely, humilisins A–D (1–4), two new uncommon diterpenoids possessing a tetradeca­hydrocyclo­penta­[3′,4′]­cyclobuta­[1′,2′:4,5]­cyclonona­[1,2-b]­oxirene ring system, namely, humilisins E and F (5 and 6), and eight known related compounds (7–14). Humilisin A (1) is the first cembranoid with an ether linkage between C-3 and C-7. The structures and absolute configurations of 1–8 were determined by extensive spectroscopic data analyses, chemical reactions, and a series of quantum chemical calculations including quantum mechanical–nuclear magnetic resonance (QM-NMR), time-dependent density functional theory–electronic circular dichroism (TDDFT-ECD), and optical rotatory dispersion (ORD) methods. In bioassay, compound 6 displayed anti-inflammatory activity in lipopolysaccharide (LPS)-stimulated BV-2 microglia cells.