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.

BACKGROUND: To study the effect of changing the piperidine ring of oxathiapiprolin on the fungicidal activity, we designed and synthesized novel piperazine thiazole derivatives containing oxime ether or oxime ester moieties, and studied their fungicidal activities against Phytophthora capsici in vitro.RESULTS: These derivatives showed moderate to good fungicidal activities against Phytophthora capsici, two oxime ether derivatives showed higher fungicidal activity in vitro than dimethomorph (EC 50 = 0.1331 ∼g mL −1 ) and comparable to oxathiapiprolin (EC 50 = 0.0042 ∼g mL −1 ). Oxime ester derivatives showed significantly reduced activities compared with oxime ether derivatives. Most of these derivatives showed broad-spectrum fungicidal activity against the other eight kinds of fungi. Moreover, four derivatives exhibited good antifungal activities in vivo against Phytophthora capsici, Pseudoperonospora cubensis, and Phytophthora infestans. The hyphae morphology study showed that compound 10d might cause mycelial abnormalities of Phytophthora capsici. CONCLUSION:The activity of 10b against Phytophthora infestans was better than that of mandipropamid, and compound 10d exhibited higher fungicidal activities against Pseudoperonospora cubensis and Phytophthora infestans than mandipropamid. These two derivatives emerged as promising candidates for antifungal drugs.

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To study the influence of changing the piperidine ring on the fungicidal activity, at the same time based on the successful experience of opening the oxazoline ring, a series of novel ethylenediamine bridged thiazole derivatives containing oxime ether and oxime ester moieties were designed, synthesized, and first evaluated for their fungicidal activities against phytophthora capsaicae in vitro. The bioassays results showed that the target compounds possessed moderate fungicidal activities against phytophtho‐ra capsaicae. Compound 28b showed 53.6% fungicidal activity at 50 μg/ml. Oxime ether and oxime ester derivatives showed the similar level of activities. However, the activities of these compounds were lower than that of oxathiapiprolin, suggesting the piperidine ring was essential to maintain the fungicidal activities of these compounds. For the oxime ether derivatives, 16b, 22a, 25b, and 28b, the substituents on ethylenediamine have certain influence on the activity. Increasing the chain length of substituents is beneficial to the fungicidal activity. Meanwhile, these compounds were tested their fungicidal activities against other 9 fungi, and these compounds showed broad spectrum fungicidal activities. Some compounds exhibited more than 70% fungicial activities against specific fungi.

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Two isopropyl (3-methyl-1-oxo-1-((1-((4-(prop-2-yn-1-yloxy)phenyl)thio)propan-2-yl)amino)butan-2-yl)carbamate diastereomers were isolated. Fungicidal activities indicated that the isolated four chiral compounds possessed excellent activity against P. capsici with the EC50 value of 4a (1.30 μg/mL), 4b (0.078 μg/mL), 4c (1.85 μg/mL), and 4d (44.4 μg/mL). Among them, compound 4b exhibited remarkably high activities against Phytophthora capsici, which is better than that of positive control dimethomorph. Its R and S isomers showed that chiral influences the activity against P. capsici.