Absolute Stereochemistry of Novel Isochromanone Derivatives from Leptosphaeria sp KTC 727

A novel isochromanone, (S)-8-hydroxy-6-methoxy-4,5-dimethyl-3-methylene-isochromen-1-one (1), known 2 and previously reported metabolites from Leptosphaeria sp. KTC 727 (JCM 13076 = MAFF 239586) were isolated from the same source by culturing for a relatively long period. The results of the present study disclose their structures involving the absolute stereochemistry. The planar structures of these molecules were established by ESIMS and NMR spectral analyses. The absolute configuration of 1 was established by comparing its electronic circular dichroism (ECD) spectrum with that of structurally-related known compound 3. The relative stereochemistry of 2 was revealed by a combination of nuclear Overhauser effect (NOE) experiments and thermodynamic discussions. Successful transformation of 1 to 2 led us to assign the configuration of 2 after comparing their ECD spectra. These compounds exhibited weak antifungal activities against Cochliobolus miyabeanus.