Homopetasinic acid (1) was isolated from fungi of the Diaporthe sp. strain 1308-05. NMR spectroscopic structural analysis revealed a petasol (3) substructure and a (4E,6E)-7-carboxy-3-hydroxy-2-methylhepta-4,6-dienoate side chain. The absoluteconfiguration of the petasol moiety was established by the specificrotation value after basic hydrolysis. The (2′S,3′S)-configuration of the side chain was