Design, Synthesis, and Evaluation of Nematicidal and Antifungal Activities of Novel Azoxy Compounds Incorporating an Ester Moiety

Bursaphelenchus xylophilus is a parasitic nematode that causes serious ecological and economic harm to pine trees. Taking ZSZLT-240 (LC₅₀ = 0.889 μg·mL^-1) as the lead compound, 26 azoxy compounds were designed and synthesized. Most of the azoxy compounds showed excellent nematicidal activity, especially Ib, IIa, and IIIh, with LC₅₀ values of 0.424, 0.267, and 0.803 μg·mL^-1, respectively, significantly better than avermectin (LC₅₀ = 16.4 μg·mL^-1) and emamectin benzoate (LC₅₀ = 13.3 μg·mL^-1). Scanning electron microscopy (SEM) showed that the nematodes treated with compounds Ib and IIIh were severely shrivelled, and there were obvious local depressions and ruptures. Furthermore, the azoxy compounds that had good nematicidal activity against B. xylophilus also showed good inhibition against Meloidogyne incognita, and compounds Ic, Ii, and IIa showed good root knot inhibition activity. In addition, compound Ib showed good fungicidal activity against Rhizoctonia solani and Botryosphaeria dothidea, with EC₅₀ values of 2.00 and 1.80 μg·mL^-1, respectively. SEM showed that the mycelium of R. solania and B. dothidea treated by compound Ib was dry and varied in thickness, and the mycelium of R. solania even appeared fracture. A 3D-QASR model was established according to the bioactivity against B. xylophilus, and it was advantageous to the promotion of activity when the 4-substituted group is more electronegative and smaller. In conclusion, the novel azoxy compounds, especially Ib, IIa, and IIIh, have great potential in the prevention and treatment of B. xylophilus.