Design, Synthesis, and Fungicidal Activities of Novel Piperidyl Thiazole Derivatives Containing Oxime Ether or Oxime Ester Moieties

To explore the influence of the positions of the two nitrogen atoms on the thiazole ring and the isoxazoline ring on the activity, a series of novel piperidyl thiazole derivatives containing oxime ether and oxime ester moieties with two nitrogen atoms on the same or opposite sides have been designed, synthesized, and first evaluated for their fungicidal activities against Phytophthora capsici in vitro. The bioassay results showed that the target compounds possessed moderate to good fungicidal activities against P. capsici, among which oxime ether compound 11b shows the highest fungicidal activity in vitro (EC₅₀ = 0.0104 μg/mL) which is higher than dimethomorph (EC₅₀ = 0.1148 μg/mL) and diacetylenyl amide (EC₅₀ = 0.040 μg/mL). Compared with oxime ether compounds (the two nitrogen atoms are on the opposite sides), the activities of oxime ester compounds were significantly reduced. It is different from the commercial fungicide fluoxapiprolin, and the activities of the compounds with the two nitrogen atoms on the same side were significantly reduced compared to the compounds with the two nitrogen atoms on the opposite sides. Moreover, compounds 11b, 11d, 11e, and 11g showed moderate to good antifungal activities in vivo against Phytophthora capsici, Pseudoperonospora cubensis, and Phytophthora infestans. Scanning electron microscopy of compound 11b on the hyphae morphology showed that compound 11b might cause mycelial abnormalities of P. capsici.