A 70% methanol extract of UV-irradiated rice leaves (400 g) was separated by chromatographic methods to give UV-induced compound 1 (2.1 mg) which showed a possible molecular ion at m/z 300 in the GC/MS analysis. Its structure was determined by NMR and MS methods. The 1H- and 13C-NMR spectra of 1 were identical to those of 10-oxodepressin (2), a casbane-type diterpene derived from the soft coral, Sinularia depressa. The specific rotation of 1 was positive, whereas that of 2 was negative. We therefore established 1 as ent-10-oxodepressin. The accumulation of 1 was also induced by an inoculation of the rice blast fungus. Compound 1 inhibited spore germination (IC50 30 ppm) and germ tube growth (IC50 10 ppm) of the rice blast fungus. We thus concluded that 1 was a novel rice phytoalexin.

Rice phytoalexins are regarded as one of the most important weapons against pathogenic microorganisms. We attempted to identify novel phytoalexins and their derivatives using GC/MS and LC/MS analyses. Diterpene derivatives, 9β-pimara-7,15-diene-3β,6β,19-triol, 1, stemar-13-en-2α-ol, 2, and 1α,2α-dihydroxy-ent-12,15-cassadiene-3,11-dione, 3, were isolated from UV-irradiated rice leaves by chromatographic methods. These structures were confirmed by 1D- and 2D-NMR and MS analyses. Interestingly, all three compounds were accumulated following an infection by the rice blast pathogen Magnaporthe oryzae. Compounds 1 and 2 exhibited weak antifungal activity and may be the biosynthetic intermediates of rice phytoalexins momilactones and oryzalexin S, respectively. Compound 3 exhibited relatively high inhibitory activity against the fungal mycelial growth of M. oryzae to the same extent as the known phytoalexin phytocassane A. We conclude that 3 is a member of the cassane-type phytoalexin family and propose the name phytocassane F.