Synthesis and anti-phytopathogenic activity of 8-hydroxyquinoline derivatives

Phytopathogenic fungi have become a serious threat to the quality of agricultural products, food security and human health globally, necessitating the need to discover new antifungal agents with de novo chemical scaffolds and high efficiency. A series of 8-hydroxyquinoline derivatives were designed and synthesized, and their antifungal activity was evaluated against five phytopathogenic fungi. In vitro assays revealed that most of the tested compounds remarkably impacted the five target fungi and their inhibitory activities were better than that of the positive control azoxystrobin. Compound 2, in particular, exhibited the highest potency among all the tested compounds, with an EC₅₀ of 0.0021, 0.0016, 0.0124, 0.0059 and 0.0120 mM respectively against B. cinerea, S. sclerotiorum, F. graminearum, F. oxysporum and M. oryzae, followed by compound 5c. The morphological observations of optical microscopy and scanning electron microscopy revealed that compounds 2 and 5c caused mycelial abnormalities of S. sclerotiorum. Futhermore, the results of in vivo antifungal activity of compounds 2 and 5c against S. sclerotiorum showed that 5c possessed stronger protective and curative activity than that of 2, and the curative effects of 5c at 40 and 80 μg mL^-1 (84.18% and 95.44%) were better than those of azoxystrobin (77.32% and 83.59%). Therefore, compounds 2 and 5c are expected to be novel lead structures for the development of new fungicides.