Oxygenated Cembrane Diterpenes from the South China Sea Soft Coral Sinularia tumulosa

The chemical investigation of the rarely studied soft coral Sinularia tumulosa resulted in the discovery of five oxygenated cembrane diterpenes 1-5, including two new compounds situmulins A (1) and B (2). The structures of 1 and 2 were established through the extensive analyses of 1D and 2D NMR spectral data together with the comparisons with the known compounds. Furthermore, the absolute configuration of 1 was determined by the time-dependent density functional theory (TDDFT) ECD approach, while the relative configuration of 2 was assigned via the quantum mechanical-NMR (QM-NMR) calculations. It might be worth to point out it is the first time to disclose the presence of cembrane diterpenes in the title species. In the bioassays, compound 2 not only showed strong antibacterial activities against fish pathogenic bacteria Streptococcus parauberis FP KSP28 and Phoyobacterium damselae FP2244 with both minimum inhibitory concentration for 90 % (MIC₉₀ ) values of 25 μM, but also demonstrated substantial inhibitory efficacy against vancomycin-resistant Enterococcus faecium bacteria G1, G4, G7, G8 and G13 from several individuals, with MIC₉₀ values of 25, 50, 100, 50 and 25 μM, respectively.