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.

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.