Hymenosetin, a 3-decalinoyltetramic acid antibiotic from cultures of the ash dieback pathogen, Hymenoscyphus pseudoalbidus

A tetramic acid antibiotic with strong activities against MRSA was isolated from virulent strain of the important phytopathogenic fungus, Hymenoscyphus pseudoalbidus. •A tetramic acid antibiotic, Hymenosetin, was obtained from the ash dieback fungus.•Its structure was elucidated by NMR spectroscopy and mass spectrometry.•The absolute configuration was assigned by CD spectroscopy and HETLOC data.•Hymenosetin showed strong antibiotic activities, even against MRSA.•Hymenosetin was devoid of phytotoxic effects against the host plant, Fraxinus. A 3-decalinoyltetramic acid, for which the trivial name hymenosetin is proposed, was isolated from crude extracts of a virulent strain of the ash dieback pathogen, Hymenoscyphus pseudoalbidus (=“Chalara fraxinea”). This compound was produced only under certain culture conditions in submerged cultures of the fungus. Its planar structure was determined by NMR spectroscopy and by mass spectrometry. The absolute stereochemistry was assigned by CD spectroscopy and HETLOC data. Hymenosetin exhibited broad spectrum antibacterial and antifungal activities (including strong inhibition of MRSA), as well as moderate cytotoxic effects. So far, the metabolite proved inactive in assays for evaluation of phytotoxicity, whereas viridiol, another secondary metabolite known from H. pseudoalbidus, was regarded as phytotoxic principle of the pathogen against its host, Fraxinus excelsior. Further studies will show whether hymenosetin constitutes a defence metabolite that is produced by the pathogenic fungus to combat other microbes and fungi in the natural environment.