Design, Synthesis, and Biological Activity of Silicon-Containing Carboxamide Fungicides

Bioisosteric silicon replacement has proven to be a valuable strategy in the design of bioactive molecules for crop protection and drug development. Twenty-one novel carboxamides possessing a silicon-containing biphenyl moiety were synthesized and tested for their antifungal activity and succinate dehydrogenase (SDH) enzymatic inhibitory activity. Among these novel succinate dehydrogenase inhibitors (SDHIs), compounds 3a, 3e, 4l, and 4o possessing appropriate clog P and topological polar surface area values showed excellent inhibitory effects against Rhizoctonia solani, Sclerotinia sclerotiorum, Botrytis cinerea, and Fusarium graminearum at 10 mg/L in vitro, and the EC₅₀ values of 4l and 4o were 0.52 and 0.16 mg/L against R. solani and 0.066 and 0.054 mg/L against S. sclerotiorum, respectively, which were superior to those of Boscalid. Moreover, compound 3a demonstrated superior SDH enzymatic inhibitory activity (IC₅₀ = 8.70 mg/L), exhibiting 2.54-fold the potency of Boscalid (IC₅₀ = 22.09 mg/L). Docking results and scanning electron microscope experiments revealed similar mode of action between compound 3a and Boscalid. The new silicon-containing carboxamide 3a is a promising SDHI candidate that deserves further investigation.