Stereochemical Investigations of Isochromenones and Isobenzofuranones Isolated from Leptosphaeria sp. KTC 727

Two new metabolites, (R)-3,4-dihydro-4,6,8-trihydroxy-4,5-dimethyl-3-methyleneisochromen-1-one (1) and (R)-7-hydroxy-3-((S)-1-hydroxyethyl)-5-methoxy-3,4-dimethylisobenzofuran-1(3H)-one (2), were isolated along with two structurally known related compounds (3 and 4) from the culture broth of Leptosphaeria sp. KTC 727 (JCM 13076 = MAFF 239586). These structures were disclosed mainly with 1H and 13C NMR spectroscopic analyses. The relative configuration of 2 was established by NOE studies. The absolute configuration of this molecule was determined by a combination of the modified Mosher’s method and CD spectra after derivatizations. The theoretical CD profiles also supported these assignments. Structural correlations enabled us to establish the absolute configurations of metabolites 1, 3, and 4, in which configurations of the latter two had not been established. Compound 2 exhibited the strongest antifungal activity among them, inhibiting the hyphal growth of Cochliobolus miyabeanus at about 0.5 μg/mL.