- Epimerization of cypermethrin stereoisomers in alcohols
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Isomerization induced by light, heat, and organic solvents has been shown to occur for some pyrethroid insecticides. Alcohols are popular solvents that are used in sample extraction, storage, and analysis. Thus, alcohol-induced epimerization may contribute to the incorrect interpretation of results from enantioselective chemical analysis and bioassay of pyrethroids like Cypermethrin. In this study, we investigated the relationship between the rate of epimerization of Cypermethrin stereoisomers: 1R-cis-αR and 1R-trans-αR and short-chain alkyl alcohol properties. In this study, complete epimerization of 1R-cis-αR produced an almost equal fraction of 1R-cis-αS, and that of 1R-trans-αR yielded 1R-trans-αS. For both stereoisomers, epimerization was most rapid in ethanol. The same stereoisomers underwent relatively rapid epimerization in methanol, n-propanol, 2-methyl1-propanol, and n-butanol but were stable in 2-butanol, suggesting that secondary alcohols have reduced reactivity, likely due to steric hindrance. We further evaluated epimerization of 1R-cis-αR and 1R-trans-αR stereoisomers of Cypermethrin as a function of water content in methanol. The presence of water in methanol generally increased the epimerization rate. For 1R-cis-αR, epimerization was most rapid with a water content of ≤2%, while for 1R-trans-αR, epimerization was most rapid with a water content of 10%. Results from this study clearly show that contact with commonly used primary alcohols may result in rapid abiotic epimerization, underscoring the importance of considering configurational stability in ensuring the analytical integrity and correct interpretation of bioassay data for stereoisomers of cypermethrin and similar pyrethroids. 2009 American Chemical Society.
- Nillos, Mae Grace,Qin, Sujie,Larive, Cynthia,Schlenk, Daniel,Gan, Jay
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experimental part
p. 6938 - 6943
(2010/07/15)
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- Chiral stability of synthetic pyrethroid insecticides
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Synthetic pyrethroids are chiral compounds consisting of multiple stereoisomers. Evaluation of enantioselectivity in environmental fate and ecotoxicity requires analytical methods that preserve stereoisomer integrity during analysis. In this study, we characterized the stability of stereoisomers from four commonly used pyrethroids, cis-bifenthrin (cis-BF), permethrin (PM), cypermethrin (CP), and cyfluthrin (CF), during gas chromatography (GC) analysis and sample preparation. Stereoisomers of cis-BF and PM were found to be stable, but those of CP and CF were unstable, under heat or in water. Isomer conversion occurred only at the aC in CP or CF, causing the analyte stereoisomer to convert to an epimer. At a GC inlet temperature of 260°C, about 9% conversion occurred for CP and CF. In organic solvents and sterile water, stereoisomers of cis-BF and PM were stable, but slow isomer conversion was observed for CP and CF in water at ambient temperature. However, isomer conversion for CP and CF was relatively insignificant (2-3%) when the GC inlet temperature was kept at ≤ 180°C or when on-column injection was used. Isomer conversion at the αC in water suggests that abiotic processes may also contribute to enantioselectivity observed in the environment for pyrethroids with the asymmetric αC.
- Liu, Weiping,Qin, Sujie,Gan, Jianying
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p. 3814 - 3820
(2007/10/03)
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