Enantioseparation and Stereoselective Fungicidal Activity of Fenoxanil Stereoisomer against Magnaporthe oryzae

Fenoxanil is a chiral amide fungicide mixture that comprises two enantiomeric pairs. This study describes the baseline separation of fenoxanil stereoisomers via ultrahigh-performance liquid chromatography coupled with tandem mass spectrometry using a Lux Cellulose-3 chiral column. The absolute configuration of the fenoxanil stereoisomers was confirmed via electronic circular dichroism. The elution sequence of the four fenoxanil stereoisomers was (1R,2R)-(+)-, (1R,2S)-(-)-, (1S,2R)-(+)-, and (1S,2S)-(-)-fenoxanil. The average recoveries of the fenoxanil stereoisomers in brown rice, rice husk, and rice plant ranged from 70.1% to 109.4%, with intraday and interday relative standard deviations of <9.4 and <11.7%, and a limit of quantitation of 0.05 mg/kg. Bioactivity assays against Magnaporthe oryzae revealed that (1R,2R)-(+)-fenoxanil is a highly bioactive stereoisomer, with 3.7- and 21.7-times higher bioactivity than the mix-fenoxanil (four stereoisomer mixture) and (1S,2S)-(-)-fenoxanil, respectively. The contribution rate of (1R,2R)-(+)-fenoxanil to the total bioactivity was 96.6%. The chiral carbon atom linked to the cyano group was identified as being crucial to the bioactivity of fenoxanil, with (1R,2RS)-fenoxanil exhibiting 2.2 times higher activity than the stereoisomer mixture. The findings of the study provide a basis for the reduction of fenoxanil use by using a single high-efficiency isomer, which is important for food safety and environmental protection.