Curvularin-Type Metabolites from the Fungus Curvularia sp. Isolated from a Marine Alga

Two new curvularin‐type macrolides, curvulone A (1) and B (2), and two known ones (3a, 4) of the rare 15R series have been isolated from the fungus Curvularia sp., which has been isolated from the marine alga Gracilaria folifera. Their structures were determined by extensive 2D NMR experiments and supported by the single‐crystal X‐ray analysis of 1. The structural elucidation of 1, which has a benzo[b]furanone moiety as part of a 12‐membered macrolactone, has led to a revision of the structure of the previously reported (11S,15S)‐11β‐hydroxy‐12‐oxocurvularin. The absolute configuration of curvulone A was established independently by the solid‐state TD‐DFT ECD method and by measuring the anomalous dispersion effect. The absolute configuration of curvulone B was determined by TD‐DFT ECD calculations on the computed solution conformers. Two different solid‐state conformers of 4 were identified by X‐ray analysis of the single crystals obtained from different solvents; TD‐DFT ECD calculations were performed to reproduce the experimental ECD spectra. All four metabolites were biologically active against fungal, bacterial and algal test organisms. The 12‐membered macrolide curvulone A (1) with a unique benzo[b]furanone moiety and the open‐chain precursor curvulone B (2) have been isolated from the fungusCurvularia sp. Their absolute configurations are of the rare 15R type and were determined by a combination of single‐crystal X‐ray analysis and CD calculations (by the solid‐state ECD TD‐DFT method).