Isolation and identification of an undescribed isocoumarin derivative from endophytic fungus Thermothielavioides terrestris YB4

An undescribed isocoumarin derivative, named (+/-)-isothielavic acid (1), was isolated from the metabolites of endophytic fungus Thermothielavioides terrestris YB4 together with two known compounds 4(S)-thielavic acid (2) and thymine (3). The structure of (+/-)-isothielavic acid (1) was elucidated by comprehensive analysis of NMR spectroscopy and HRESIMS data as well as single-crystal X-ray diffraction experiment. Bioassay results indicated the weak antibacterial activity of compound 1 in vitro.