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

Three new metabolites, microsphaeropsones A-C (1-3) with a unique oxepino[2,3-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 (8a), smaller amounts of emodin (5), the known aromatic xanthones 9a and 9b, the new 3,4-dihydrofusidienol A (8b), and the new aromatic xanthone 9c were isolated. The endophyte Seimatosporium species produced a new aromatic xanthone, seimatoxanthone A (10), and 3,4-dihydroglobosuxanthone A (12), closely related to alpha-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 I 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.