Structure of Porriolide, a New Metabolite from Alternaria porri

In our investigation on the isolation of bioactive compounds produced by Alternaria porri (Ellis) Ciferri, the causal fungus of black spot disease in the stone-leek and onion, we have already isolated altersolanol A and B, dactylariol, tentoxin, and porritoxin (1) from a culture medium of A. porri. In addition, 6-(3',3'-dimethylallyloxy)-4-methoxy-5-methylphthalide (2) has been reported as a metabolite of A. porri, although no particular inhibitory activity was apparent toward lettuce and stone-leek seedlings. This paper reports the isolation of a new metabolite named porriolide (3) from a culture medium of A. porri, and its phytotoxic effect on lettuce and stone-leek seedlings. Porriolide (3) as colorless needles, mp 231-232 degree C, had the molecular formula C sub(10)H sub(10)O sub(3), which was determined from its high-resolution mass spectrum. The IR spectrum showed absorption bands at 3343, a hydroxyl group; 1739, an alpha , beta -unsaturated gamma -lactone; and 1624/cm, a phenyl group. By comparing the super(1)H- and super(13)C-NMR spectral data of porriolide with those of 2, the former closely resembles the latter, except for the disappearance of protons corresponding to the dimethylallyloxy group in 2; namely, the C-1' to C-3' protons and a phenolic proton. Based on these spectral data, porriolide was identified as 6-hydroxy-4-methoxy-5-methylphthalide 3, which has been reported as a degradation product of 2, a metabolite of A. zinniae. Porriolide is the first reported isolation as a fungal metabolite.