Herbicidal isoxazolyl imidazolidinones are prepared by reaction of an isoxazolyl-allylurea with ozone and reduction. The process comprises preparing an isoxazolyl imidazolidone of the formula
wherein R is C3-C6 branched alkyl or C1-C6 haloalkyl having 1-3 halo atoms; R1 is C1-C4 alkyl ;
comprising reacting with ozone an isoxazolylurea of the formula
wherein R2 and R3 independently represent hydrogen, phenyl, benzyl or C1-C7 alkyl; and reducing the resulting oxidized intermediate.