Bleaching herbicides

Bleaching Herbicides. Membrane-based modes of herbicidal action relevant to photosynthesis (37) include those of inhibitors of carotenoid biosynthesis, eg, norflura2on, diftmon, y -phenoxyben2amines inhibitors of chlorophyll biosynthesis, eg, oxadia2on, DTP or... 

A second class of herbicides primarily affects ( -carotene desaturase. These herbicides are apparent feedback inhibitors of PD as well. This class of compounds includes dihydropyrones like LS 80707 [90936-96-2] (56) and 6-methylpyridines (57,58). The third class consists of the ben2oylcyclohexane-diones, eg, 2-(4-chloro-2-nitroben2oyl)-5,5-dimethyl-cyclohexane-I,3-dione. This class of atypical bleaching herbicides induces phytoene accumulation when appHed either pre- or post-emergence. However, it does not inhibit phytoene desaturase activity in vitro (59). Amitrole also has been considered a bleaching herbicide, though its main mode of action is inhibition of amino acid synthesis. 

SCHULZ A, CRT o, BEYER P and KLEINIG H (1993) SC-0051, a 2-benzoyl-cyclohexane-l,3-dione bleaching herbicide, is a potent inhibitor of the enzyme /<-hydroxyphenylpyruvate dioxygenase , Terr, 318, 162-6. 

Bleaching applications Bleaching fastness Bleaching herbicides... 

Takahashi, M., Kawamura, S., Miyakado, M., Sanemitsu, Y., Tanaka, S. (1993) Uptake and translocation of bleaching herbicidal compounds in radish seedlings. Pestic. Sci. 39, 159-177. 

The first significant commercial impact of fluorine in agriculture occurred with the introduction several decades ago of an aromatic trifluoromethyl group in several classes of herbicides, such as dinitroaniline, diphenyl ether, and bleaching herbicides. These early classes of herbicides continue to be used today, though their sales are steadily declining as newer, more effective materials are replacing them. 

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. 

Structure-activity studies have shown the crucial role of the trifluoromethyl group in optimum biological activity of bleaching herbicides [38], Isoxaflutole (Balance , Merlin ) is a root or foliar uptake systemic herbicide with broad-spectrum control in corn and sugarcane of both grass and broadleaf weeds [39], Isoxaflutole is rapidly converted in plants and in soil to the diketonitrile form, which is the biologically active species (Fig. 7) [40],... 

Sandmann, G. 1993. Spectral determination of carotenoid precursors in Scenedesmus cells treated with bleaching herbicides. In Boger, P., Sandmann, G. (Eds ), Target Assays for Modern Herbicides and Related Phytotoxic Compounds. Lewis Publishers, Boca Raton, FL, 3-8... 

Several 2-aryl- and 2-phenylmethyl-3,5-isoxazolidine-dlones were synthesized and found to be bleaching herbicides with good tolerance by soybeans. The most active member, 2-(2-chlorophenyl)methyl-4,4-dimethyl-3,5-isoxazolidinedione, failed to perform in the field due to its instability in soil. To improve the chemical stability by molecular modifications, a series of 3-isoxazolidinones were prepared and found to be highly active bleaching herbicides with excellent soybean tolerance. Synthesis and structure-activity relationships are discussed. One of the most active compounds,... 

An interesting structure-activity observation is that the 3-isoxazolidinones (8) are only slightly more active than their synthetic precursor hydroxamic acids (9) (Table X). For example, the difference in activity between FMC 57020 and its precursor hydroxamic acid toward these 4 species of weeds is very small. They both show a bleaching herbicidal response with excellent soybean tolerance. They also demonstrate a parallel substituent effect, i.e. they both follow the same relative activity order among different substituents such as those shown in Table X. 

Sandman et. l. (8) have found the N-phenyl-2-phenoxynicotin-amides to be powerful inhibitors of phytoene desaturase. Several bleaching herbicides that inhibit the phytoene to phytofluene transformation have the same 3-trifluoromethylphenyl group (8, 9). This group includes norflurazon, metflurazon, fluridone, 7luometuron and fluorochloridone. In the nicotinamide series the same 3-trifluoromethylphenyl group gives optimum herbicide activity. 

As can be seen from Table IV, the nitrophenyl analogs of the nicotinamides are inactive. Interestingly, the 2-(3-trifluoromethylphenyl ) benzamides (compound IV minus the nitro group) were moderate herbicides (less than 50% weed control at 1 lb). These compounds are also bleaching herbicides as are the well known 3-phenoxybenzamides. The 3-phenoxybenzamides are also known to be inhibitors of phytoene desaturase (11). 

A great diversity in molecular structure is observed among herbicides which inhibit carotene biosynthesis as is exemplified by the structures of norflurazon, fluridone and difunone (shown below). Nonetheless, many of these compounds, which comprise a subset of the larger group known as bleaching herbicides, appear to inhibit the same step in the biosynthetic pathway to the carotenoids (1 ). The inhibited step is the desaturation of 15-cis phytoene to 15- cis phytofluene (Figure 1) and the build-up of phytoene in plants and in cell-free systems which have been treated with these herbicides is well documented (2-4). 

Other bleaching herbicides such as the experimental phenylfuranone herbicide, dlfunon, (20,23) and derivatives, or fluridone (21) are thought to act on... 

Besides the bleaching cause by either carotene inhibition or peroxidation more targets for "bleaching herbicides" are conceivable, e.g., interference with chlorophyll formation itself. DTP, a substituted pyrazole, was reported to induce chlorosis by blocking... 

In addition to the bleaching herbicides, electron transport inhibitors binding at the B protein (Figure... 

Carotenoid Biosynthesis and Phytotoxic Effects of Bleaching Herbicides 4.1.2.1 Targets for Bleaching Herbicides... 

Most commercial so-called bleaching herbicides inhibit the synthesis of carotenoids by interfering with carotenoid biosynthesis at the level of phytoene desatur-ase [170, 171, 172]. Errzyme kinetics with several inhibitors have revealed a reversible binding to the en2yme and non-competitive inhibition [173],... 

As a methodology to search for a new lead, we focused on the chemical approach using A,S-heterocycles. This approach led to the evolution of 5-methylene-thiazolidines as the lead to bleaching herbicides. Further structural modifications based on the parent compound resulted in the creation of a new family of bleaching herbicides, 2-(W-difluoroacelylimino)-S-methyl-3-(3-trifiuoromethylphenyl)-l,3-thiazoline, (S-3085) widi potent preemergence herbicidal activity and selectivity on cotton crops. On die basis of biochemical studies, 3-phenylthiazolines have been characterized as a new inhibitor of phytoene desaturase. Facile syndieses of 1,3-thiazolines and the manufacturing process toward S-3085 will be reported in future publications. 

Desaturation is easily inhibited and is a major target site for bleaching herbicides. The short chromophore acyclic intermediates such as phytoene which accumulate are unable to quench the energy of Chl or 02/ and thus cannot protect the plant against photosensitized... 

Carotenoids also protect the cell from damage due to photooxidation catalyzed by light-absorbing pigments such as chlorophylls. This effect was noted in comparisons of normal carotenoid-containing bacteria with mutants in which the carotenoids are replaced by the more saturated and colorless carotenoid, phytoene (8) (Ramage, 1972). Many bleaching herbicides inhibit carotenoid biosynthesis the plants usually die because of photooxidation (Britton, 1991). 

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