A thiopyranchromenone and other chromone derivatives from an Endolichenic fungus, Preussia africana

The first example of a naturally occurring thiopyranchromenone, preussochromone A (1), and five other new chromone derivatives, preussochromones B-F (2-6), were isolated from solid cultures of an endolichenic fungus, Preussia africana. The structures of 1-6 were established primarily by NMR experiments, and 2 and 4 were further confirmed by X-ray crystallography. The absolute configurations of 1 and 2 were determined by the application of electronic circular dichroism (ECD), whereas those of C-5 in 3, C-6 in 4, and the 6,7-diol in 5 were deduced via the CD data of the in situ formed Rh?(OCOCF?)?] complex, the modified Mosher method, and Snatzke's method, respectively. Compounds 1 and 3 showed significant cytotoxicity against A549 cells.