Cyclic Peroxide Acids and a new fatty acid from Okinawan Sponge Plakortis sp.

Bioassay-guided fractionation of a cytotoxic extract of an Okinawan sponge Plakortis sp. resulted in the isolation of a known cyclic peroxide methyl ester (1a) along with two new cyclic peroxide acids (1b and 2b) and a new aliphatic fatty acid (3). Compound 2b was confirmed to be an isomer of 1b by NMR and MS. Moreover, careful comparison of the spectroscopic data of 2b with that of peroxyplakoric acid A1 methyl ester (2a) suggested that compound 2b was identical with 2a. Treatment of 2b with TMS diazomethane yielded a compound identical with 2a with respect to NMR and MS. This data confirmed that the cyclic peroxidases 1b and 2b occurred naturally. Additionally, the structure of 3 was confirmed by 1D and 2D NMR experiments. A plausible biosynthetic pathway has been proposed that rationalizes transformations in the ring-opening of the cyclic peroxide. Compound 1a was evaluated to have IC50 8.7 mu g/mL against NBT-T2 cells while compound 3 was inactive. This result suggested that the cyclic peroxide moiety could be responsible for the cytotoxicity of the sponge constituents.