[CH3:1][O:2][N:3]=[CH:4][C:5]1[CH:10]=[C:9]([NH2:11])[C:8]([Cl:12])=[CH:7][C:6]=1[Cl:13].[C:14]1(=O)[O:19][C:17](=[O:18])[C:16]2[CH2:20][CH2:21][CH2:22][CH2:23][C:15]1=2.O>C(O)(=O)C>[CH3:1][O:2][N:3]=[CH:4][C:5]1[CH:10]=[C:9]([N:11]2[C:17](=[O:18])[C:16]3[CH2:20][CH2:21][CH2:22][CH2:23][C:15]=3[C:14]2=[O:19])[C:8]([Cl:12])=[CH:7][C:6]=1[Cl:13]18 g (82 mmol) of 5-amino-2,4-dichlorobenzaldehyde (O-methyl)oxime were added to a solution of 12.5 g (82 mmol) of 3,4,5,6-tetrahydrophthalic anhydride in 300 ml of acetic acid, whereupon the mixture was stirred for 15 hours at reflux temperature. The reaction mixture was subsequently poured into 1 l water. The product was then extracted with 300 ml of methyl tert-butyl ether. The extract was washed with saturated aqueous sodium hydrogen carbonate solution, dried over magnesium sulfate and finally concentrated. The residue was purified by means of silica gel chromatography (eluent: hexane/ethyl acetate=4:1). Yield: 20.9 g; m.p.: 122-124° C.
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