The chiral herbicide beflubutamid (II): Enantioselective degradation and enantiomerization in soil, and formation/degradation of chiral metabolites

Beflubutamid is a chiral soil herbicide currently marketed as racemate against dicotyledonous weeds in cereals. Biotests have shown that (-)-beflubutamid is at least 1000× more active than (+)-beflubutamid. Potential substitution of the racemate by (-)-beflubutamid should therefore be further considered. Here, we investigated the degradation behavior in soils and formation and degradation of two chiral metabolites. Laboratory incubation experiments were performed with an alkaline and an acidic soil. The compounds were analyzed by enantioselective GC-MS. Degradation rate constants were determined by kinetic modeling. In the alkaline soil, degradation of beflubutamid was slightly enantioselective, with slower degradation of the herbicidally active (-)-enantiomer. In the acidic soil, however, both enantiomers were degraded at similar rates. In contrast, degradation of a phenoxybutanamide metabolite was highly enantioselective. Chiral stability of beflubutamid and its metabolites was studied in separate incubations with the pure enantiomers in the same soils. In these experiments, (-)-beflubutamid was not converted to the nonactive (+)-enantiomer and vice versa. Significant enantiomerization was, however, observed for the major metabolite, a phenoxybutanoic acid. With regard to biological activity and behavior in soils, enantiopure (-)-beflubutamid definitively has the potential to substitute for the racemic herbicide.