Design, Synthesis, and Fungicidal Evaluation of Novel 1,3-Benzodioxole-Pyrimidine Derivatives as Potential Succinate Dehydrogenase Inhibitors

A series of novel 1,3-benzodioxole-pyrimidine derivatives were designed and synthesized. The <i>in vitro</i> bioassay indicated that compounds <b>4e</b>, <b>4g</b>, <b>4n</b>, <b>5c</b>, and <b>5e</b> displayed excellent fungicidal activities against test fungal strains. Especially, in the <i>in vitro</i> experiments, <b>5c</b> exhibited a broad spectrum of fungicidal activity against Botrytis cinerea, Rhizoctonia solani, Fusarium oxysporum, Alternaria solani, and Gibberella zeae with EC₅₀ values of 0.44, 6.96, 6.99, 0.07, and 0.57 mg/L, respectively, which were significantly more potent than those of positive control boscalid (EC₅₀: 5.02, >50, >50, 0.16, and 1.28 mg/L). <i>In vivo</i> testing on tomato fruits and leaves showed that <b>5c</b> displayed considerable protective and curative efficacy against A. solani. Scanning electron microscopy analysis indicated that <b>5c</b> possessed a strong ability to destroy the surface morphology of mycelia and seriously interfere with the growth of the fungal pathogen. In the <i>in vitro</i> enzyme inhibition assay, <b>5c</b> exhibited pronounced succinate dehydrogenase (SDH) inhibitory activity with an IC₅₀ value of 3.41 μM, equivalent to that of boscalid (IC₅₀: 3.40 μM). In addition, fluorescence quenching experiment further confirmed the strong interaction of <b>5c</b> with SDH. Through chiral resolution, <b>5c</b> was separated into two enantiomers. Among them, (<i>S</i>)-<b>5c</b> exhibited stronger fungicidal activity (EC₅₀: 0.06 mg/L) and SDH inhibitory (2.92 μM) activity than the <i>R</i>-enantiomer (EC₅₀: 0.17 mg/L and SDH IC₅₀: 3.68 μM), which was in accordance with the molecular docking study (CDOCKER Interaction Energy for (<i>R</i>)-<b>5c</b> and (<i>S</i>)-<b>5c</b>: −28.23 and −29.98 kcal/mol, respectively). These results presented a promising lead for the discovery of novel SDHIs as antifungal pesticides.