Cembrane- and Casbane-Type Diterpenes from the South China Sea Soft Coral Sinularia nanolobata

Five new cembrane-type diterpenes (1–5), one new casbane-type diterpene (7), and ten known related ones (6, 8–16) were isolated from the South China Sea soft coral Sinularia nanolobata. The structures of new compounds were elucidated through comprehensive spectroscopic analysis, NMR calculation with DP4+ probability analysis, time-dependent density functional theory–electronic circular dichroism (TDDFT-ECD) calculations, and comparison with the reported spectroscopic data of known analogs. Structurally, compounds 2–5 featured rare five- to seven-membered oxygen-containing heterocyclic rings with diverse joints, reflecting remarkable chemical diversity of secondary metabolites. In bioassays, all isolates were evaluated for their cytotoxic and anti-inflammatory effects. Although no significant cytotoxic or anti-inflammatory activity was observed, the structural novelty of these compounds expands the chemical diversity of this class of natural products, providing valuable leads for further structural modifications and biological investigations.