Novel 3,4-seco-3,19-dinorspongian and 5,17-epoxy-19-norspongian diterpenes from the marine sponge Spongia sp.

Twelve new norspongian diterpenes, dinorspongians A–F (1–6) and epoxynorspongians A–F (7–12), were isolated from the marine sponge Spongia sp. Compounds 1–6 were the first examples of dinorspongian diterpenes bearing the unprecedented 3,4-seco-3,19-dinorspongian diterpene skeleton. Compounds 7–12 were the new 19-norspongian diterpenes possessing a 5,17-epoxy unit firstly. The structures with absolute configurations of these compounds were elucidated by the methods of NMR, X-ray diffraction and quantum chemical calculation. The biogenetic routes of 1–12 were also proposed, which suggested that the 19-norspongian diterpenes might be the intermediates of 3,4-seco-3,19-dinorspongian diterpenes. It was the first time to discuss the biosynthetic relationship between these two types of norspongian diterpenes. In addition, all the compounds were evaluated for their cytotoxic activities against several cancer cell lines. Compound 11 showed moderate activities against the PC3 and PBL-2H3 cell lines with IC50 values of 24.8 and 27.2 μM, respectively.