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 <i>β</i>-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 <b>A21</b>, <b>A24</b>, and <b>A33</b> displayed a broad spectrum of antifungal activities <i>in vitro</i> against <i>Fusarium graminearum</i>, <i>Sclerotinia sclerotiorum</i>, and <i>Rhizoctonia solani</i>. Especially, compound <b>A33</b> (EC₅₀ = 0.356 μg/mL) exhibited comparable <i>in vitro</i> fungicidal activities against <i>F. graminearum</i> to the positive control pydiflumetofen (EC₅₀ = 0.104 μg/mL). In the greenhouse assay, compound <b>A33</b> displayed moderate <i>in vivo</i> protective effects against rice blast at 100 μg/mL, significant <i>in vivo</i> protection against cucumber powdery mildew at 50 μg/mL, and moderate <i>in vivo</i> protective effects against wheat scab at 200 μg/mL. The succinate dehydrogenase (SDH) inhibitory assay revealed that <b>A21</b>, <b>A24</b>, and <b>A33</b> 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 <b>A33</b> 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 <i>β</i>-ketonitrile scaffold is an effective approach to explore novel potent broad-spectrum SDHIs.