Discovery of Novel Pyrazole β-Ketonitrile Derivatives as Broad Spectrum SDHI Fungicides by Introducing a Flexible Motif

Scaffold optimization plays a critical role in the exploration of fungicides with improved potency and an expanded spectrum. In this study, a series of novel pyrazole β-ketonitrile derivatives were rationally designed and identified as potent broad-spectrum succinate dehydrogenase inhibitor fungicides, through modifying the molecular flexibility via a flexible methylene motif. After stepwise modifications, compounds A21, A24, and A33 displayed a broad spectrum of antifungal activities in vitro against Fusarium graminearum, Sclerotinia sclerotiorum, and Rhizoctonia solani. Especially, compound A33 (EC₅₀ = 0.356 μg/mL) exhibited comparable in vitro fungicidal activities against F. graminearum to the positive control pydiflumetofen (EC₅₀ = 0.104 μg/mL). In the greenhouse assay, compound A33 displayed moderate in vivo protective effects against rice blast at 100 μg/mL, significant in vivo protection against cucumber powdery mildew at 50 μg/mL, and moderate in vivo protective effects against wheat scab at 200 μg/mL. The succinate dehydrogenase (SDH) inhibitory assay revealed that A21, A24, and A33 are potent SDH inhibitors with inhibitory concentration values of 0.123, 0.0317, and 0.0709 μM, respectively. Docking results demonstrated that H-bonds and cation–π interactions between A33 and residues of Trp173, Tyr91, and Arg46 are important for the binding of compounds within SDH. Our findings revealed that the improvement of flexibility in the pyrazole β-ketonitrile scaffold is an effective approach to explore novel potent broad-spectrum SDHIs.