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

Under a nitrogen atmosphere, into a reaction vessel were charged 211.3 g (1.95 mol) of 1,4-phenylenediamine and 3900 ml of tetrahydrofuran. A solution prepared by dissolving 127.2 g (1.30 mol) of maleic anhydride in 600 ml of tetrahydrofuran was dropped into this under cooling with ice over a period of about 3.3 hours, then, the mixture was stirred at room temperature overnight. After completion of the reaction, the deposited crystal was isolated by filtration, washed twice with 280 ml of tetrahydrofuran, and dried to obtain 259.2 g of coarse (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoic acid as a yellow-orange powder. To 259.2 g of coarse (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoic acid was added 520 ml of water and the mixture was cooled to 0 to 10° C., and 237 ml of a 5N sodium hydroxide aqueous solution was dropped, then, 6.5 ml of a 1N sodium hydroxide aqueous solution was dropped. Thereafter, the solvent was distilled off under reduced pressure to obtain a residue to which 200 ml of 2-propanol was added, and the solvent was distilled off under reduced pressure again. To the resultant yellow-brown solid was added 800 ml of tetrahydrofuran and the mixture was stirred at room temperature overnight, the solid was isolated by filtration, and washed four times with 100 ml of tetrahydrofuran and dried at 45° C. for 5 hours, to obtain 297.7 g of sodium (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butenoate di-hydrate as a yellow powder. Yield: 86.7%.