Stereoselective Degradation and Transformation Products of a Novel Chiral Insecticide, Paichongding, in Flooded Paddy Soil.

Paichongding is a chiral neonicotinoid insecticide currently marketed as racemate against sucking and biting insects. Under anaerobic condition, all paichongding stereoisomers underwent appreciable degradation in soil during 100 days of incubation, with estimated t1/2 values between 0.18 and 3.15 days. Diastereoselectivity in paichongding degradation was observed, with enantiomers (5S,7R)- and (5R,7S)-paichongding being more preferentially degraded in soils than enantiomers (5R,7R)- and (5S,7S)-paichongding. The half-lives of (5R,7R)- and (5S,7S)-paichongding were 3.05 and 3.15 days, respectively, as compared to 0.18 day for (5R,7S)- and (5S,7R)-paichongding. A total of nine intermediates were identified, of which depropylated paichongding was the predominant metabolite and appeared to be stable and recalcitrant to further degradation. Paichongding is degraded via denitration, depropylation, nitrosylation, demethylation, hydroxylation, and enol-keto tautomerism, producing chiral and biologically active products. These findings could have implications for environmental risk and food safety evaluations.