Synthesis norflurazon

The molecular target site of triketone herbicides is the enzyme -hydroxyphenylpyruvate dioxygenase (HPPD). Inhibition of this enzyme disrupts the biosynthesis of carotenoids and causes a bleaching (loss of chlorophyll) effect on the foliage similar to that observed with inhibitors ofphytoene desaturase (e.g. norflurazon). However, the mechanism of action of HPPD inhibitors is different. Inhibtion of HPPD stops the synthesis of homogen tisate (HGA), which is a key precursor of the 8 different tocochromanols (tocopherols and tocotrienols) and prenyl quinones. In the absence of prenylquinone plastoquinone, phytoene desaturase activity is interrupted. The bleaching of the green tissues ensues as if these compounds inhibited phytoene desaturase. 

Phytoene desaturase bleaching herbicides. Bleaching herbicides inhibit the synthesis of carotenoids in plants [30,31], A number of phytoene desaturase herbicides such as norflurazon (Solicam , Zorial ), flurochloridone (Racer ), fluri-done (Sonar ), and diflufenican (Fenican , Legacy ) have been commercialized. 

Trifluoromethylaniline, directly or indirectly, is the starting material for the synthesis of all four of the herbicides norflurazon, flurochloridone, fluridone, and diflufenican. Norflurazon is prepared from 3-trifluoromethylphenyl hydrazine, which is obtained from the diazotization and reduction of 3-trifluoromethylaniline [35], Diflufenican is prepared from the reaction of 3-trifluoromethylphenol and... 

The biochemical mode of action of fluridone, similar to that of norflurazon, is the inhibition of carotenoid synthesis (Waldrep and Taylor, 1976 Buenida et al.. 1978 Devlin el al., 91%). 

It can be established from these investigations that unlike triazines and diuron, norflurazon damages the photosynthetic system of the plants. The main action is the blocking of carotenoid synthesis, as a result of which carotenoid precursors (phytoene and phytofluene) are accumulated because of the inhibition of dehydrogenation reactions. 

An involvement of carotenoids in the thylakoid insertion of light-harvesting proteins has also been concluded from a pulse-chase measurement of the rate of protein insertion into membranes that have reduced amounts of carotenoids, due to norflurazon treatment (Dahlin and Timko, 1997). It should be kept in mind that carotenoids may also control other biosynthetic steps in the synthesis and assembly of light-harvesting complexes such as the import of the protein precursors into plastids (Dahlin, 1993 Dahlin and Franz6n, 1997). 

As pointed out previously for chlorophyll destruction and ethane evolution with algae in the presence of norflurazon (Tables I,III), peroxidative reactions may show up as a consequence of a primary inhibition of carotene synthesis, which itself does not induce peroxidation-initiating radicals. 

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