Cytotoxic Dihydrothiophene-Condensed Chromones from Marine-Derived Fungus Penicillium oxalicum

Two new dihydrothiophene-condensed chromones and a new natural chromone, namely oxalicumones A–C (1–3), respectively, were isolated from a culture broth of a marine-derived fungus, Penicillium oxalicum. The structures of 1–3 and acetylated derivatives of 1 (4–7) were elucidated on the basis of spectroscopic methods and chemical reactions. The absolute configuration of 1 and 2 were established by using the modified Mosher ester method and circular dichroism data of an in situ formed [Rh₂(OCOCF₃)₄] and [Mo₂(OAc)₄] complex. (R)-MTPA ester of 1 showed cytotoxicity against A375, SW-620, and HeLa carcinoma cell lines with IC₅₀ values of 8.9, 7.8, and 18.4 µM, respectively. Compound 1 displayed cytotoxicity against A375 and SW-620 cell lines with IC₅₀ values of 11.7 and 22.6 µM, respectively. The structure–biological activity relationship of 1 was discussed.