Cladoxanthones A and B, Xanthone-Derived Metabolites with a Spiro[cyclopentane-1,2′-[3,9a]ethanoxanthene]-2,4′,9′,11′-tetraone Skeleton from a Cladosporium sp.

Cladoxanthones A (1) and B (2), two xanthone-derived metabolites featuring a new spiro[cyclopentane-1,2′-[3,9a]ethanoxanthene]-2,4′,9′,11′(4a′H)-tetraone skeleton, were isolated from cultures of the ascomycete fungus Cladosporium sp., together with the known mangrovamide J (3). Their structures were elucidated primarily by NMR experiments. The absolute configurations of 1 and 2 were assigned by X-ray crystallography using Cu Kα radiation. Compound 1 could be generated from the hypothetical precursors related to α-methylene ketone and dihydro-xanthone via a Diels–Alder reaction, while 2 could be an oxidative coupling product resulting from 1 and 3. Compounds 1 and 2 showed weakly cytotoxic effects.