Design, Synthesis, and Antifungal Activities of Novel Pyrazole-4-carboxamide Derivatives Containing an Ether Group as Potential Succinate Dehydrogenase Inhibitors

A series of novel pyrazole-4-carboxamides bearing an ether group were designed and synthesized on the basis of the structure of commercial succinate dehydrogenase inhibitor (SDHI) fungicide flubeneteram via scaffold hopping and evaluated for their antifungal activities against five fungi. The bioassay results showed that most of the target compounds exhibited excellent in vitro antifungal activity against Rhizoctonia solani and some compounds exerted remarkable antifungal activities against Sclerotinia sclerotiorum, Botrytis cinerea, Fusarium graminearum, and Alternaria alternate. Particularly, compounds 7d and 12b displayed outstanding antifungal activity against R. solani, with an EC₅₀ value of 0.046 μg/mL, far superior to that of boscalid (EC₅₀ = 0.741 μg/mL) and fluxapyroxad (EC₅₀ = 0.103 μg/mL). Meanwhile, compound 12b also presented a broader fungicidal spectrum than other compounds. Moreover, in vivo anti-R. solani results showed that compounds 7d and 12b could significantly inhibit the growth of R. solani in rice leaves with excellent protective and curative efficacies. In addition, the results of the succinate dehydrogenase (SDH) enzymatic inhibition assay showed that compound 7d generated significant SDH inhibition, with an IC₅₀ value of 3.293 μM, which was about 2 times better than that of boscalid (IC₅₀ = 7.507 μM) and fluxapyroxad (IC₅₀ = 5.991 μM). Furthermore, scanning electron microscopy (SEM) analysis indicated that compounds 7d and 12b significantly destroyed the typical structure and morphology of R. solani hyphae. The molecular docking study revealed that compounds 7d and 12b could embed into the binding pocket of SDH and form hydrogen bond interactions with TRP173 and TRY58 at the activity site of SDH, which was in line with fluxapyroxad, indicating that they had a similar mechanism of action. These results demonstrated that compounds 7d and 12b could be promising candidates of SDHI fungicides, which deserved further investigation.