Phytocassanes A, B, C and D, novel diterpene phytoalexins from rice, Oryza sativa L.

Four novel phytoalexins, designated phytucassanes A, B, C and D, were isolated from rice (Oryza sativa L.) leaves infected with Magnaporthe grisea (previously designated Pyricularia oryzae) and from rice stems infected with Rhizoctonia solani. The structures of the phytocassanes, classified as cassane-type diterpenes, were determined by spectroscopic methods. Phytocassanes had high antifungal activity against the pathogenic fungi, M. grisea and R solani. The ED50 values of phytocassanes A, B, C and D in prevention of spore germination of M. grisea were 20, 4, 7 and 25 μg/ml, respectively. Furthermore, large amounts of phytocassanes were produced in rice infected with the pathogenic fungi.