P-Phenylenediamine → (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoic acid

Under a nitrogen atmosphere, into a reaction vessel were charged 15.00 g (138.7 mmol) of 1,4-phenylenediamine and 255 ml of tetrahydrofuran. A solution prepared by dissolving 9.07 g (92.5 mmol) of maleic anhydride in 45 ml of tetrahydrofuran was dropped into this under cooling with ice over a period of about 3 hours, then, the mixture was stirred at room temperature overnight. After completion of the reaction, the deposited crystal was isolated by filtration, washed with 30 ml of tetrahydrofuran, and dried at 40° C. to obtain 18.14 g of coarse (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoic acid as an orange powder. To 18.14 g of coarse (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoic acid was added 36 ml of water and the mixture was cooled to 0 to 10° C., and 17.59 ml of a 5N sodium hydroxide aqueous solution was dropped. Thereafter, the solvent was distilled off under reduced pressure to obtain a residue to which 270 ml of ethanol was added, and the mixture was stirred at 70° C. for 1 hour, cooled down to room temperature, then, the solid was filtrated and washed twice with 25 ml of methanol. The resultant solid was dried at 45° C. for 2 hours, to obtain 16.2 g of sodium (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoate as a pale brown-white powder. Yield: 76.8%