New 3,4-seco-3,19-Dinor- and Spongian-Based Diterpenoid Lactones from the Marine Sponge Spongia sp.

Continuing chemical investigation of the Red Sea sponge Spongia sp. led to the isolation of four new 3,4-seco-3,19-dinorspongian diterpenoid lactones, secodinorspongins A−D (1−4), along with a classical spongian diterpenoid lactone, sponginolide (5). The chemical structures, including the absolute configurations of these compounds, were elucidated using the extensive spectroscopic study composed of 1D and 2D NMR data analyses, and a comparison between calculated-electronic-circular-dichroism (ECD) and experimental-circular-dichroism (CD) spectra. A plausible biosynthetic pathway of 1−4 was also proposed. Furthermore, the cytotoxicity, antibacterial and anti-inflammatory activities of 1−5 were evaluated. Compound 1 was found to exhibit inhibitory activity against the growth of Staphylococcus aureus (S. aureus), and 4 and 5 exhibited suppression of superoxide-anion generation and elastase release in fMLF/CB-induced human neutrophils.