抄録
Rice blast disease, caused by infection of rice blast fungus Magnaporthe grisea (Hebert) Barr. (perfect stage of Pyricularia oryzae Cabara), has been the most serious pest for rice. The fungus mainly produces several phytotoxic compounds according to fermentation method, i. e., dihydropyriculol (1), pyriculol (2), and pyriculone (3) by a shaking culture; dihydropyriculariol (5) and pyriculariol (6) by an aeration and agitation culture; and tenuazonic acid by a stationary culture in a soy sauce-sucrose medium. Pyricuol (4) together with 2 were produced by a shaking culture with a potato dextrose medium. These salicylaldehyde-type phytotoxins are common metabolites of blast fungi of various host plants except a ginger. To develop novel methods to control the blast disease a better knowledge of the biosynthesis is necessary. Synthesis of both enantiomers of pyricuol (4) from (R)- and (8-lactates was achieved using [2,31-Wittig rearrangement and Stille coupling reactions as the key steps. This revealed that the natural 4 has an R-configuration. Racemic synthesis is also reported. On the other hand, optically active pyriculariol (6) was synthesized from L-rhamnose using microwave assisted Stille coupling reaction of the unprotected substrates as the key step. A comparison of the CD spectra and 1H NMR spectra of the corresponding MTPA esters found that natural pyriculariol (6) existed as a racemate. Biotransformation of the plausible biosynthetic intermediates 7-d2 and 8-d in deuterio forms was also examined. Not diol 7-d2 but aldehyde 8-d was converted by the fungus M. grisea into dihydropyriculariol (1-d) in soy sauce-sucrose medium under a shaking culture. Foliar application test of the synthetic compounds on rice leaves revealed that both a salicylaldehyde and a side chain fragments are necessary for the necrotic activity. In conclusion, the blast fungus M. grisea produced 1, 2, and 4 in optically active forms and 5 and 6 as racemates depending on the culture conditions. Additionally, it is interesting that both oxidation and reduction would be coexist at the later stage of biosynthesis of these phytotoxins.
