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.