Borneol-Containing Cinnamic Acid Derivatives from <i>Pinus yunnanensis</i> as CYP51 Inhibitors: Design, Synthesis, and Mechanistic Study for Fungicidal Development

A cinnamic acid derivative bearing borneol <b>SX-6</b>, isolated and identified from <i>Pinus yunnanensis</i> exudates, was used as a lead compound to conduct structural optimization for the innovation of fungicidal agents. Here, thirty-five <b>SX-6</b> derivatives were designed, synthesized, and evaluated for their fungicidal activities. Among them, <b>SX-6-6</b> provided the EC₅₀ values of 3.29, 8.22, and 6.68 mg/L against <i>Botrytis cinerea</i>, <i>Sclerotinia sclerotiorum</i>, and <i>Curvularia mebaldsii</i>, respectively, whose activities are superior or similar to those of hymexazol. Besides, photoreaction was used to transfer <b>SX-6-6</b> to its <i>Z</i>-isomer <b>SX-6-6i</b>, which possessed EC₅₀ values of 2.97 mg/L against <i>S. sclerotiorum</i>. <i>In vivo</i> experiments proved that <b>SX-6-6</b> and <b>SX-6-6i</b> have excellent protective effects on <i>B. cinerea</i> and <i>S. sclerotiorum</i> at 200 mg/L, respectively. The action mechanism studies reveal that <b>SX-6-6</b> displays fungicidal activities due to 14α-demethylase (CYP51) inhibition, which suppresses ergosterol synthesis, impairs fungal cell membrane function, and causes cell death. In addition, acute toxicity studies showed that <b>SX-6-6</b> was safe for rodents at a dose of 1 g/kg. This is the first report on the application of plant protection using active derivatives from <i>P. yunnanensis</i>, offering a novel structural precursor for the design of fungicidal agents.