Starting from the pyrimidine-biphenyl compound 1 as a lead structure, a series of di-substituted pyrimidine -biphenyl compounds were designed and synthesized by introducing disubstituents to its δ-ring. All the target compounds were characterized by ¹H NMR, ¹³C NMR, ¹⁹F NMR, and HRMS. The results of herbicidal activity evaluation in the greenhouse assay showed that most of the compounds exhibited excellent herbicidal activity against Echinochloa crusgalli, Digitaria san- guinalis, and Amaranthus retroflexus in post-emergence treatments. Notably, N-(2-((4,6-dimethoxypyrimidin-2-yl)oxy)benzyl) -3'-methoxy-5'-methyl-[1,1'-biphenyl]-2-amine (4f) demonstrated an average pre- and post-emergence weed control of 81% against four tested weeds, exhibiting broad-spectrum herbicidal activities. The study of molecular mode of action revealed that the target of 4f was acetohydroxyacid synthase (AHAS). 4f established extensive hydrophobic interactions with the residues within the hydrophobic cavity of AHAS, particularly enhancing the interactions with residues M352, V485, and the cofactor FAD. Consequently, 4f (IC₅₀ = 98.1 μM) exhibited higher AHAS inhibitory activity compared to the lead structure 1 (IC₅₀ = 157 μM). These findings suggested that 4f was worthy of further investigation.

Key words: acetohydroxyacid synthase inhibitor, biphenyl, Suzuki cross coupling, herbicidal activity, molecular docking