Plakortinic acids C and D: a pair of peroxide-polyketides possessing a rare 7,8-dioxatricyclo[4.2.2.0^2,5]dec-9-ene core from a two-sponge association of Plakortis symbiotica-Xestospongia deweerdtae

Plakortinic acids C (3) and D (4), two unprecedented peroxide-polyketides with 7,8-dioxatricyclo[4.2.2.0^2,5]dec-9-ene scaffold, as well as known biogenetically related congeners, plakortinic acids A (1) and B (2), were isolated from a two-sponge association of Plakortis symbiotica-Xestospongia deweerdtae. Upon chemical derivatization, the structures and relative configurations of 3 and 4 were characterized by analysis of HRESIMS and NMR spectroscopic data, molecular modeling studies, and chiroptical comparisons with known natural products and published values of [α]_D of related synthetic analogs. A mixture of methyl ester derivatives 5 and 6 displayed negligible cytotoxicity against a panel of 60 cell lines of various human cancers at a concentration of 10 μM.