Resistance buster compounds for MBI-D insensitive rice blast fungus - Inquiry on effective compounds among derivatives of MBI-D fungicides

Effective molecules against resistant strains of Pyricularia oyzae to the melanin biosynthesis inhibitors (MBI-D) were designed by modifying the amine and acid parts of carpropamid {(1RS,3SR)-2,2-dichloro-N-[(R)-1-(4-chlorophenyl) ethyl]-1-ethyl-3-methylcyclopropanecarboxamide}. Substitution of the 1-phenylethylamine moiety with a 2-phenylethylamine increased the inhibitory activity against MBI-D-resistant strains. Reduction of the bulkiness of the amine part by replacing the benzene ring of 2-phenylethylamine with thiophene was effective to improve the activity. Among the derivatives, 5-chloro-3-ethylthiophene compounds showed the highest efficacy. Through the studies regarding structure-activity relationship of the compounds with five-membered heterocyclic rings, the discovery of new resistance buster compounds could be prospective.