A mixture of 7-chloro-10-[(2RS)-1-ethylamino-2-propyl]-2-phenothiazinecarbonitrile (5 g), iodethane (2.2 cc) and sodium carbonate (2 g) in dimethylformamide (50 cc) is charged, and the mixture is brought for 9 hours to 160° C. After cooling, the reaction mixture is poured into a mixture of distilled water (500 cc) and ethyl acetate (500 cc). The organic phase is separated after settling has taken place, washed with distilled water (4×300 cc), dried over magnesium sulphate, filtered and concentrated to dryness at 40° C. under reduced pressure (30 mm Hg; 4 kPa) to give 7-chloro-10-[(2RS)-1-diethylamino-2-propyl]-2-phenothiazinecarbonitrile (5 g).
