Difunctional pyrido[2,3-b]pyrazine derivatives: Biological activity and preliminary mechanism

To develop new antifungal and antibacterial agents, three derivatives containing pyrido[2,3-b]pyrazine were synthesized through derivation and optimization. These derivatives exhibited excellent biological activities against 5 plant pathogenic fungal species (Rhizoctonia solani, Phomopsis sp., Botrytis cinerea, Sclerotinia sclerotiorum, and Magnaporthe oryzae) and 3 plant pathogenic bacterial species (Xanthomonas oryzae pv. oryzicola, Xanthomonas axonopodis pv. citri, and Pseudomonas syringae pv. actinidiae). Among them, the EC₅₀ values of compound X2 against the 5 plant pathogenic fungi were 3.15, 1.12, 0.69, 1.59, and 0.92 mg/L, respectively, which were significantly superior to those of fluopyram (192.29, 93.12, 42.23, 91.19, and 125.77 mg/L) and azoxystrobin (80.14, 25.14, 19.64, 36.81, and 59.51 mg/L). Its curative and protective activities against B. cinerea were 66.6 % and 60.3 %, respectively, which were significantly better than those of the positive controls fluopyram (27.0 % and 20.6 %) and azoxystrobin (60.1 % and 55.6 %). In addition, compound X2 can inhibit mycelial growth, damage cell membranes, and increase intracellular ROS levels, thereby leading to the death of pathogenic fungi. The EC₅₀ values of X3 against the 3 plant pathogenic bacteria were 0.13, 3.37, and 9.82 mg/L, respectively, which were significantly superior to those of thiodiazole‑copper and bismerthiazol. Its control effects on rice bacterial leaf streak were 40.79 % and 44.08 %, which were comparable to that of bismerthiazol (42.79 % and 46.35 %). Compound X3 can inhibit the growth of pathogenic bacteria by inhibiting a variety of virulence factors (exopolysaccharides, biofilms, and motility). Therefore, pyrido[2,3-b]pyrazine can serve as a good scaffold to discover new antibacterial agents and fungicides.