Discovery and photosynthesis of sinuaustones A and B, diterpenoids with a novel carbon scaffold isolated from soft coral Sinularia australiensis from Hainan

Two novel diterpenoids, sinuaustones A (1) and B (2), featuring an unprecedented tricyclo[9.3.1.0 3,15 ]tetradecane carbon framework, along with a new spatane-type diterpenoid 3 and two related known compounds 4 and 5, were isolated from the South China Sea soft coral Sinularia australiensis . The structures of the new compounds were established by detailed spectroscopic, X-ray diffraction analysis and quantum chemical computation methods. Photosynthesis of 1 and 2 starting from co-isolated 5 was completed giving insight into the plausible biosynthetic pathway of 1 and 2, as well as their structural relationship with co-occurring 3–5. In the in vitro bioassay, compounds 1, 2, 3, and 5 exhibited inhibitory effects on lipopolysaccharide (LPS)-induced inflammatory responses in RAW264.7 cells with IC 50 values of 20.9, 75.1, 9.1, and 30.4 μM, respectively.