2.9 g of potassium tert.butylate (content 97%, 0.025 mols) are suspended in 169 g of dry acetonitrile (water content <0.01%) and cooled to 0-5° C. Subsequently, at 0-5° C., 36.2 g of 3-methylamino-4,4,4-trifluorobut-2-enoic acid isopropylester (content 98.3%, 0.17 mols) are added and maintained at this temperature for 30 minutes, whereby a red-brown solution is produced. Then, over the course of 15 minutes, at 0-5° C., 27.3 g of 5-isocyanato-2-chloro-benzoic acid 1-allyloxycarbonyl-1-methyl-ethyl-ester (content 99%, 0.084 mols) are dispensed in, whereby a dark brown solution is obtained. The latter is adjusted to pH 7 with 3.0 g of 1 N hydrochloric acid [content of desired product in the solution is 15.5%, which corresponds to a yield of 91.2%]. The solvent is subsequently distilled off in a vacuum, and the residue (68 g) is recrystallised in 200 g of isopropyl alcohol, whereby the inorganic salts are removed by hot filtration. After drying in a vacuum drying chamber at 60° C. for 12 hours, 32.8 g (82% of theory) of the desired product are isolated in a content of 98.7%.
