Benzofuran-2-carboxylic acid (310 mg, 1.9 mmol) was mixed with 3 ml of oxalyl chloride and refluxed for 1 h in the presence of a catalytic amount of DMF, then the excess oxalyl chloride was removed by vacuum. The residue was dissolved in 4 ml of dichloromethane and was added to a mixture of 400 mg (1.6 mmol) of 4′-amino-biphenyl-4-sulfonic acid, N,N-diisopropylethylamine (0.92 ml, 4 eq) and 4 ml of THF in an ice/water bath. The mixture was stirred at room temperature for 4 h. The reaction mixture was mixed with 2N HCl and in the process a suspension formed. The solid was collected by centrifugation and was washed with chloroform and 2N HCl to provide 540 mg of 4′-[(benzofuran-2-carbonyl)-amino]-biphenyl-4-sulfonic acid, obtained as pale brown solid.
