Xanthones and Oxepino[2, 3‐b]chromones from Three Endophytic Fungi

Three new metabolites, microsphaeropsones A–C ( 1 – 3 ) with a unique oxepino2,3‐b]chromen‐6‐one (ring‐enlarged xanthone) skeleton, were isolated from the endophytic fungus Microsphaeropsis species, co‐occurring with their putative biogenetic anthraquinoide precursors citreorosein ( 4 ) and emodin ( 5 ). From another Microsphaeropsis species, large amounts of fusidienol A ( 8 a ), smaller amounts of emodin ( 5 ), the known aromatic xanthones 9 a and 9 b , the new 3,4‐dihydrofusidienol A ( 8 b ), and the new aromatic xanthone 9 c were isolated. The endophyte Seimatosporium species produced a new aromatic xanthone, seimatoxanthone A ( 10 ), and 3,4‐dihydroglobosuxanthone A ( 12 ), closely related to α‐diversolonic ester ( 13 ) from Microdiplodia sp.. The structures were determined mainly by extensive 1D and 2D NMR experiments and supported by X‐ray single‐crystal analysis of 1 and the oxidation product 7 . The absolute configurations of the microsphaeropsones A–C ( 1 – 3 ) were established by comparison of the electronic and vibrational circular dichroism (ECD and VCD) spectra of 1 with time‐dependent DFT (TDDFT) and DFT calculations by using either the solid‐state structures or DFT‐optimized geometries as inputs. Preliminary studies indicated that 1 , 2 , and enone 7 showed antibacterial, fungicidal, and algicidal properties.