New destruxins from the entomopathogenic fungus Aschersonia sp.

Two new insecticidal cyclic depsipeptides, destruxins A4 (1) and A5 (2), were isolated by bioassay-guided fractionation of organic extracts from an undescribed species of entomopathogenic fungus from the genus Aschersonia. Another previously identified but related compound, homodestruxin B (3), was also isolated as a product of this fungus. The chemical structures of 1−3 were determined based on analysis of NMR and mass spectral data. In common with homodestruxin B (3), the novel compounds destruxins A4 (1) and A5 (2) have a MeIle moiety in place of the MeVal residue more typically found in destruxins. In addition, destruxin A5 has a β-MePro unit in place of the Pro residue found in most destruxins. LC50 values for 1 and 2 in an insecticidal assay against Drosophila melanogaster were estimated at 41 and 52 ppm, respectively. Homodestruxin B (3) showed no activity at 400 ppm in the same assay. Destruxins A4 (1) and A5 (2) are the first biologically active secondary metabolites reported from the fungal genu...