Novel SDH Inhibitors as Antifungal Leads: From Azobenzene Derivatives to the 1,2,4-Oxadiazole Compounds

Azo-incorporating was reported to be an effective strategy for increasing SDH inhibitory activity but for poor in vivo control effects. Herein, the azo-incorporated compounds were structurally optimized to retain a preferential conformation by replacing the azo bond with their bioisosteres. Interestingly, the 1,2,4-oxadiazole compound D2 displayed a broad fungicidal spectrum as well as fluxapyroxad. More excitedly, compound D2 showed excellent antifungal activities against rice sheath blight disease both in vitro (EC50 = 0.001 μg/mL) and in vivo (EC50 = 1.08 μg/mL, EC95 = 4.67 μg/mL). In addition, an extra π–π interaction was found between the 1,2,4-oxadiazole ring of compound D2 and the phenyl ring of residue D_Y586, which might interpret the enhanced potency of compound D2 against Rhizoctonia solani. Further structural optimizations of the 1,2,4-oxadiazole compounds gave several analogues that made a breakthrough in controlling rice blast disease. These 1,2,4-oxadiazole compounds, derived from azobenzene derivatives, could be antifungal leads especially against R. solani and Magnaporthe grisea, exemplifying an interesting mode of pesticide discovery and providing theoretical guidance for innovation of the SDHI fungicide.