Fraxitoxin, a New Isochromanone Isolated from Diplodia fraxini

A new isochromanone, named fraxitoxin, was isolated together with (–)-mellein and tyrosol from liquid cultures of Diplodia fraxini, a pathogen involved in the etiology of canker and dieback disease of Fraxinus spp. in Europe. It was characterized as 5-methoxy-3-methylisochroman-1-one using spectroscopic methods (essentially NMR and HR-EI-MS). Its absolute configuration (R) at C(3) was assigned by electronic circular dichroism (ECD) measurements and calculations. Phytotoxic activity of the compound was evaluated on ash, cork and holm oak leaves at concentration of 1 mg/ml by the leaf-puncture assay. Interestingly, fraxitoxin caused necrotic lesions only on ash leaves.