A solution of 2.5 g (16 mmol) of sodium dihydrogen phosphate dihydrate in 25 ml of water was added dropwise at 10-15° C. to a solution of 21.3 g (60 mmol) of 5-(4-chloro-5-difluoromethoxy-1-methyl-1H-pyrazol-3-yl)-2,4-dichlorobenzaldehyde in 120 ml of acetonitrile. 6 ml of a 30% by weight hydrogen peroxide solution and then, in the course of two hours, a solution of 8.7 g (96 mmol) of sodium chlorite in 80 ml of water were added dropwise to this mixture. The reaction mixture was stirred for a further hour and then acidified with 3 N hydrochloric acid. The solids content was separated from the resulting suspension and dried. Yield: 13.7 g; m.p. 159-160° C.
