X-ray structure determination, absolute configuration and biological activity of phomoxanthone A

The dimeric xanthone phomoxanthone A (1a), a cytochalasin L-696,474 (2a), the corresponding 21-O-deacetyl-L-696,474 (2b), and 3-nitropropionic acid (3) were isolated from the endophytic fungus Phomopsis sp. The relative configuration of phomoxanthone A (1a) was elucidated by X-ray single-crystal analysis. The comparison of the calculated and measured CD spectra and CD calculation of model compounds revealed that the axial chirality along the biaryl axis dominates the CD spectra of the dimeric xanthone la, and can thus be determined by CD calculation as (a,S). Knowing the axial chirality, the absolute configurations of the stereogenic centers could be determined from the X-ray data of la as (5R,6R,10aR,5'R',6'R,10a'R). In addition, it was demonstrated that in both the solid state (KBr) and solution (methanol/dichloromethane, 4:1) 1a has the same conformation, i.e. similar dihedral angles along the biaryl axis. Comparison of solid-state CD measurement with calculation of CD data based on X-ray coordinates was introduced for the first time. Phomoxanthone A (1a) showed activity against bacteria and phytopathogenic fungi. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).