Sulfonic and oxanilic acid metabolites of acetanilide herbicides: Separation of diastereomers and enantiomers by capillary zone electrophoresis and identification by 1H NMR spectroscopy

Abstract

Biological degradation of acetanilide herbicides in soil results in the formation of the ethanesulfonic acid (ESA) and oxanilic acid (OXA) derivatives. These molecules exist in two (alachlor), four (acetochlor), and eight (metolachlor) stereoisomeric forms. Due to the hindered s-cis/s-trans isomerization of the amide bond, most of the diastereomers of the ESA and OXA compounds were separable by capillary zone electrophoresis (CZE). Using γ- cyclodextrin as chiral selector in CZE, complete separation of all four isomers of enantiomerically enriched (5S)-metolachlor OXA was achieved. The enantiomers of acetochlor ESA, acetochlor OXA, and racemic metolachlor OXA were partially separated. The proton nuclear magnetic resonance (1H NMR) spectra showed separated signals for all diastereomers in achiral environment and enantiomers with γ-cyclodextrin as chiral selector. The assignment of these signals to the corresponding absolute configurations was used for the identification of observed CZE peaks. Rate constants for the amide s-cis/s- trans isomerization were evaluated. The time scales for this isomerization at room temperature are seconds for the parent herbicides and several hours for the metabolites. Rotation of the phenyl ring with respect to the amide moiety was not observed. CZE allows the determination and quantification of the different isomers of ESA and OXA metabolites and will facilitate the investigation of the probable stereoselective degradation of acetanilide herbicides in the environment.