18.84 g (75 mmols) of 3,5-dinitrophthalic acid N-methylimide [prepared according to Example 2] are initially introduced into 250 ml of ethyl acetate and treated with 30.4 g (220 mmols) of ground, anhydrous potassium carbonate, and 14 g (78 mmols) of 3,4-dichlorothiophenol are then added dropwise. After the addition of 95 ml of tetrahydrofuran, the mixture is stirred overnight and then evaporated to dryness. The residue is kept under reflux overnight in 100 ml of water, the mixture is extracted at 25° C. with petroleum ether and filtered and the aqueous filtrate is acidified with 300 ml of 2N HCl and heated under reflux. After 2.5 hours, the mixture is cooled and extracted with tetrahydrofuran/toluene and the extract is washed with saturated NaCl solution, dried over sodium sulfate and evaporated. The residue is warmed to the reflux temperature with 100 ml of toluene and 7.66 g (75 mmols) of acetic anhydride and, after cooling, the mixture is evaporated. After recrystallisation from methylene chloride/n-pentane, 22.2 g (80% of theory) of 3-(3,4-dichlorophenylthio)-5-nitrophthalic anhydride are obtained; melting point: 154°-7° C.
