Synthesis, Herbicidal Activity against Barnyard Grass, and Photolytic Behavior of Aryl 2,6-Dipyrimidinoxybenzoates

In this study, we investigated the characteristics and herbicidal potential of bispyribac phenolic esters, which belong to the 2-(pyrimidin-2-yloxy)benzoic acid (PYB) class of acetohydroxyacid synthase (AHAS-)-inhibiting herbicides. These herbicides are primarily used for controlling Poaceae and broadleaf weeds. Among them, bispyribac-sodium stands out as a representative in this class. Surprisingly, other bispyribac esters, including alkanol and phenol esters exhibit considerably reduced herbicidal activity compared to bispyribac-sodium. In contrast, oxime esters (e.g., pyribenzoxim) demonstrate high activity. To further understand and develop novel PYB herbicides, we synthesized and screened a series of bispyribac phenolic esters while investigating their photochemical behaviors. Several compounds displayed excellent herbicidal activity, with compounds Ia-19 and Ic showing impressive 90% effective dosages for fresh weight inhibition of barnyard grass, measuring 0.55 and 0.60 g a.i./hm², respectively. These values were approximately half of bispyribac-sodium or pyribenzoxim. The results indicate that the herbicidal activity of phenolic esters is influenced by both their binding ability to the AHAS enzyme and their decomposition into bispyribac acid. For instance, bispyribac phenol ester exhibited considerably reduced receptor affinity compared to bispyribac-sodium, and faced challenges in transforming into bispyribac acid, explaining its diminished herbicidal activity. However, introducing a photosensitive nitro group led to a complete transformation. This modification improved its affinity with AHAS and accelerated its decomposition into bispyribac acid, further accelerated by photocatalysis. Consequently, nitro-containing compounds displayed heightened herbicidal activity. The findings from this study open possibilities for structural optimization of phenolic esters through quantitative structure-activity relationship analysis, potentially regulating their activity-releasing period. Furthermore, the high activity of aromatic heterocyclic esters offers new insights into developing novel PYB herbicides.