Structural Features of Herbicidal Imidazolinones

The structural features of imidazolinones important for target site and herbicide activity have been summarized [11-14]. The orientation of the imidazolinone ring ortho to the acid equivalent is critical. Derivatives of the acid equivalent are herbicidally active if they can be metabolized to the acid either in the soil or in the plant. Likewise, tricyclic derivatives such as 4 are pro-herbicides that must be metabolized to the acid-imidazolinone form. 

The commercial herbicides are a mixture of R and S isomers at the chiral center where the methyl and isopropyl substituents are placed, but the (R)-isomer is approximately ten-fold more potent both as an enzyme inhibitor and as an herbicide. Substituents other than methyl and isopropyl are substantially weaker enzyme inhibitors an herbicides [14]. 

The aromatic ring component illustrates the relative contributions of enzyme inhibition and physicochemical properties to herbicidal activity. The benzene imidazolinones are approximately ten-fold more potent than the corresponding pyridine derivatives as enzyme inhibitors but are less potent as herbicides. 

The primary factor that determines the biological activity of the imidazolinones besides the inhibition of AHAS is their ability to translocate to meristematic tissue. AH AS, the target site for these herbicides, functions primarily in rapidly dividing tissue and decreases rapidly as tissue matures [15]. Thus, the difference in herbicidal activity among the six commercial imidazolinones depends on differences in their ability to be absorbed and translocate within the plant. Imazaquin 

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