CN=[C:3]([OH:21])[C:4]1[C:5](=[C:9]([S:16][CH2:17][CH2:18][CH2:19][CH3:20])[CH:10]=[C:11]([N+:13]([O-:15])=[O:14])[CH:12]=1)[C:6]([OH:8])=[O:7].Cl>[OH-].[Na+]>[CH2:17]([S:16][C:9]1[CH:10]=[C:11]([N+:13]([O-:15])=[O:14])[CH:12]=[C:4]2[C:3]([O:7][C:6](=[O:8])[C:5]=12)=[O:21])[CH2:18][CH2:19][CH3:20]883 mg (3 mmols) of 3-n-butylthio-5-nitrophthalic acid N-methylimide are kept under reflux overnight in 9 ml of 1N sodium hydroxide solution and the mixture is acidified with 10 ml of 2N hydrochloric acid and, after 5 minutes under reflux, cooled. The mixture is extracted with tetrahydrofuran/toluene and the extracts are washed with saturated NaCl solution, dried over sodium sulfate and evaporated. The resulting acid is converted to the anhydride with 1 ml of acetic anhydride and 10 ml of toluene, by heating and evaporation. After recrystallisation from cyclohexane, 790 mg (94% of theory) of the title anhydride are obtained as a yellow product; melting point: 95°-97° C.
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