Synthesis and Evaluation of Novel 1-Methyl-1<i>H</i>-pyrazol-5-amine Derivatives with Disulfide Moieties as Potential Antimicrobial Agents

Sulfur-containing compounds have diverse biological functions and are crucial in crop protection chemistry. In this study, a series of novel 1-methyl-1<i>H</i>-pyrazol-5-amine derivatives incorporating disulfide moieties were synthesized and evaluated for their antimicrobial properties. <i>In vitro</i> bioassays demonstrated that compound <b>7f</b> displayed potent antifungal activity against <i>Valsa mali</i>, with an EC₅₀ value of 0.64 mg/L, outperforming allicin (EC₅₀ = 26.0 mg/L) but lower than tebuconazole (EC₅₀ = 0.33 mg/L). <i>In vivo</i> experiments confirmed that compound <b>7f</b> could effectively inhibit <i>V. mali</i> infection on apples at a concentration of 100 mg/L, similar to the positive control tebuconazole. Mechanistic studies revealed that compound <b>7f</b> could induce hyphal shrinkage and collapse, trigger intracellular reactive oxygen species accumulation, modulate antioxidant enzyme activities, initiate lipid peroxidation, and ultimately cause irreversible oxidative damage to the cells of <i>V. mali</i>. Additionally, compound <b>7b</b> exhibited notable antibacterial activity, particularly against <i>Pseudomonas syringae</i> pv. <i>actinidiae</i>, with a MIC₉₀ value of 1.56 mg/L, surpassing the positive controls allicin, bismerthiazol, and streptomycin sulfate. These findings suggest that 1-methyl-1<i>H</i>-pyrazol-5-amine derivatives containing disulfide moieties hold promise as potent candidates for the development of novel antimicrobial agents.