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To a solution of 3.8 g of 5,5-dimethyl-3-{4-[(trifluoro-methyl)thio]phenyl}imidazolidine-2,4-dione obtained in stage a) below in 7 mL of concentrated sulphuric acid is added dropwise at −5° C. a mixture of nitric acid (0.75 mL) in 3 mL of concentrated sulphuric acid. After stirring for 4 hours at 0° C. (ice bath) the mixture is poured onto 150 g of ice and the pH is raised to 10 by addition of concentrated aqueous ammonia. The aqueous phase is then extracted with twice 150 mL of ethyl acetate and the combined organic phases are dried over magnesium sulphate, filtered and concentrated under reduced pressure. The residue is purified by chromatography on a column of silica, eluting with a mixture of dichloromethane and methanol (95/5 by volume) to give 1.8 g of 5,5-dimethyl-3-{3-nitro-4-[(trifluoromethyl)thio]phenyl}imidazolidine-2,4-dione, the characteristics of which are as follows:

1. extractionThe aqueous phase is then extracted with twice 150 mL of ethyl acetate
2. dry with materialthe combined organic phases are dried over magnesium sulphate
3. filtrationfiltered
4. concentrationconcentrated under reduced pressure
5. customThe residue is purified by chromatography on a column of silica
6. washeluting with a mixture of dichloromethane and methanol (95/5 by volume)