Uncommon Diterpenoids with Diverse Frameworks from the South China Sea Sponge Spongia officinalis and Their Anti-inflammatory Activities

Uncommon diterpenoids with diverse frameworks, including one unexpected iodinated oxa-6/6/6/6-tetracyclic diterpene (1) and its monobrominated 6/6/6-tricyclic analogue (2) and one novel isodolastane-type diterpene featuring an unusual aromatic 5/7/6-tricyclic ring system (3) as well as a related known dolastane-type diterpenoid (4), were isolated from the South China Sea sponge Spongia officinalis. Their structures, including absolute configurations, were established by extensive spectroscopic data analysis, X-ray diffraction analysis, and quantum mechanical-nuclear magnetic resonance and time-dependent density functional theory/electronic circular dichroism calculations. A plausible biosynthetic pathway of new compounds 1–3 was proposed. In bioassays, compounds 2 and 4 were found to exhibit significant anti-inflammatory effects on lipopolysaccharide-induced inflammation in BV-2 microglial cells.