Selective herbiddes

More than 25 different substituted urea herbicides are currently commercially available [30, 173]. The most important are phenylureas and Cycluron, which has the aromatic nucleus replaced by a saturated hydrocarbon moiety. Benzthiazuron and Methabenzthiazuron are more recent selective herbiddes of the class, with the aromatic moiety replaced by a heterocyclic ring system. With the exception of Fenuron, substituted ureas (i.e., Diuron, Fluometuron, Fig. 10, Table 3) exhibit low water solubilities, which decrease with increasing molecular volume of the compound. The majority of the phenylureas have relatively low vapor pressures and are, therefore, not very volatile. These compounds show electron-donor properties and thus they are able to form charge transfer complexes by interaction with suitable electron acceptor molecules. Hydrolysis, acylation, and alkylation reactions are also possible with these compounds. 

Triketones with Heterocyclic Substituents. Triketones represent a well-studied, but still developing, family of herbicides. Some with pyiidyl substituents are among the most active. Much of the research in this area has been reviewed. The triketones are inhibitors of the plant enzyme 4-hydroxyphenylpymvate dioxygenase (HPPD), which plays a key role in the biosynthesis of plastoquinone and tocopherol. Compound 11.6, which is known as nicotinoyl syncarpic acid, is shown as a typical structure of this type. Its potent herbiddal activity led to synthetic work that has yielded many related stmctures in an effort to improve selectivity in the herbicidal action. 

Thies, F, Backhaus, T, Bossmann, B, Grimme, LH, Xenobiotic biotransformation in unicellular green algae. Involvement of cytochrome P450 in the activation and selectivity of the pyridazinone pro-herbidde metflura-zon. Plant Physiol. 112, 361-370 (1996). 

Matsui, J. Fujiwara, K. Ugata, S. Takeuchi, T. Sohd-phase extraction with a dibutyl-melamine-imprinted polymer as triazine herbidde-selective sorbent. J. Chromatogr. A. 

Phenoxy herbiddes are used against a wide variety of broad-leaved weeds to protect crops such as sorghmn, maize, cereal grains, fruit trees, and some vegetables. Phenoxy compoimds have varying degrees of selectivity for certain types of broad-leaved weeds and crops. These compoimds have very little effect on grasses. 

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