A mixture of 1.98 g (6.5 mmol, 1 eq) of 6-amino-3-chloro-2-(4-methylpiperazine-1-sulfonyl)phenol and 2.20 g (48.8 mmol, 2 eq) of 3,4-diethoxy-3-cyclobutene-1,2-dione was placed in solution in 20 ml of ethanol. The reaction medium was heated at 50° C. for 16 hours. The insoluble material was filtered off, washed with ethanol and dried under vacuum at 45° C. 2.05 g of 3-[4-chloro-2-hydroxy-3-(4-methylpiperazine-1-sulfonyl)phenylamino]-4-ethoxycyclobut-3-ene-1,2-dione were obtained in the form of a yellow solid. Yield=74%.
