Absolute configuration of four chiral isomers of pyrisoxazole and their bioactivity

Abstract

Background: Pyrisoxazole is a fungicide that has two chiral carbon atoms and four isomers: (3S,5R)-, (3R,5S)-, (3S,5S)-, and (3R,5R)-pyrisoxazole.

Results: Pure crystals of four pyrisoxazole isomers were prepared by chiral separation and single-crystal cultivation. Their absolute configurations were established by X-ray single crystal diffraction analysis. Bioassays indicated that compound (3S,5R)-pyrisoxazole showed excellent fungicidal activity with a median effective concentration (EC₅₀ ) value of 0.14 μg mL^-1 and protective activity with an EC₅₀ value of 13.29 μg mL^-1 . These values are superior to the commercial fungicides boscalid and racemic pyrisoxazole.

Conclusions: The biological activity of racemic pyrisoxazole is due almost exclusively to the isomer (3S,5R)-pyrisoxazole; the other three isomers had very low activity. © 2020 Society of Chemical Industry.

Keywords: absolute configurations; bioactivity; chiral separation; pyrisoxazole; single crystal.