Chiral herbicides

Zipper C, M Bunk, AJB Zehnder, H-PE Kohler (1998) Enantioselective uptake and degradation of the chiral herbicide dichloroprop [(R5)-2-(2,4-dichlorophenoxy)propionic acid] by Sphingomonas herbicidov-orans MH. J Bacteriol 180 3368-3374. 

Zipper C, K Nickel, W Angst, H-PE Kohler (1996) Complete microbial degradation of both enantiomers of the chiral herbicide Mecoprop [(R,S)-2-(4-chloro-2-methylphenoxy)]propionic acid in an enantioselective manner by Sphingomonas herbicidovorans sp. nov. Appl Environ Microbiol 62 4318-4322. 

Cai X, Liu W, Sheng G (2008) Enantioselective degradation and ecotoxicity of the chiral herbicide diclofop in three freshwater algae cultures. J Agric Food Chem 56 2139-2146... 

Enantioselective Hydrogenation of A -Arylimines in the Synthesis of the Chiral Herbicide, ( S)-Metolachlor... 

Zipper C., SuterM. J.-F., Haderlein S. B., Gruhl M., and Kohler H.-P. E. (1998) Changes in the enantiomeric ratio of (R) to (S) mecoprop indicate in situ biodegradation of this chiral herbicide in a polluted aquifer. Environ. Sci. Technol. 32, 2070-2076. 

In view of the structural novelty of this series of chiral herbicides, it was imperative to determine to what extent herbicidal activity is specifically linked to its molecular structure, and to define its structure-activity relationship requirements as a preliminary step toward designing even more potent representatives for this new series of herbicides. The... 

Andre, C., Berthelot, A., Thomassin, M., Guillaume, Y. C. (2006). Enantioselective aptameric molecular recognition material design of a novel chiral stationary phase for enantioseparation of a series of chiral herbicides by capillary electrochromatography. Electrophoresis 27, 3254-3262. 

FIGURE 14.13 Illustration of the chiral herbicide mecoprop only the R enantiomer has herbicidal activity and is now marketed in the enantiomeric pure form. 

Natural systems may be quite complex. For example, the enantiomerization of phenoxyalkanoic acids containing a chiral side chain has been studied in soil using H20 (Buser and Muller 1997). It was shown that there was an equilibrium between the R- and 5-enantiomers of both 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and 2-(2,4-dichlorophenopxy)propionic acid (DCPP) with an equilibrium constant favoring the herbicidally active / -enantiomers. The exchange reactions... 

Buser H-R, MD Muller (1997) Conversion reactions of various phenoxyaUcanoic acid herbicides in soil 2 elucidation of the enantiomerization process of chiral phenoxy acids from incubation in a DjO/soil system. Environ Sci Technol 31 1960-1967. 

Schneiderheinze, J.M., Armstrong, D.W., Berthod, A. (1999). Plant and soil enantioselective biodegradation of racemic phenoxyalkanoic herbicides. Chirality 11, 330-337. 

Kumada s use of a ferrocene moved away from the C2-symmetrical motive, as planar chirality can result from the two ferrocene rings having different substituents. The development of this class of ligand is well documented [5, 125-127]. The best-known uses of these ligands are for reductions of carbon-heteroatom multiple bonds, as in the synthesis of the herbicide, Metolachlor [128, 129]. 

A reexamination of the chiral discrimination of PPA herbicides was done on a TE CSP using the perturbation method to calculate the solute distribution isotherms [86]. The effect of temperature was well described by the bi-Langmuir model and enabled confirmation of the previous results by the same authors [85]. 

Guillaume, Y.C. et al.. Chiral discrimination of phenoxypropionic acid herbicides on teicoplanin phase effect of mobile phase modifier, Chromatographia, 55, 143, 2002. 

Andre, C. and Guillaume, Y.C., Reanalysis of chiral discrimination of phen-oxypropionic acid herbicides on a teicoplanin phase using a bi-Langmuir approach, Chromatographia, 58, 201,2003. 

Metolachlor is the active ingredient of Dual , one of the most important grass herbicides for use in maize and a number of other crops. In 1997, after years of intensive research. Dual Magnum, with a content of approximately 90% (I S)-diastereomers and with the same biological effect at about 65% of the use rate, was introduced into the market. This chiral switch was made possible by the new technical process that is briefly described below. The key step of this new synthesis is the enantioselective hydrogenation of the isolated MEA imine, as depicted in Figure 1.3. 

Metolachlor is applied as a mixture of eight different stereoisomers, only four of which have herbicidal activity (10). This implies that the other four isomers are applied as contaminants, with no additional benefit to crop production. Whether the chirality of metolachlor influences its degradation rate is unknown. This lack of information is mainly due to the difficulty of separation and analysis of its isomers. Because the stereochemistry of compounds plays an important role in their biological activity and degradation pathways, it is valuable to investigate the influence of stereochemistry on the rate of metolachlor degradation in soil. 

Figure 20. A Structure of immobilized dihydroquinidine carbamate serving as chiral anion exchanger type CSP. B Chromatogram showing a direct resolution of the herbicide 2-(2,4-dichlorophenoxy)propanoic acid (Dichlorprop). Reprinted with permission from ref 140.

Scheme 7. Resolution of racemic (1-methoxy)-2-propylamine to give the (S)-amine, an intermediate in the synthesis of a chiral corn herbicide.

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