To a stirred solution of 4-amino-3-fluorobenzoic acid (4.97 g, 32.04 mmol), tert-butyl piperazine-1-carboxylate (5.97 g, 32.04 mmol) and triethylamine (10 mL) in dichloromethane (100 mL) was added 1-propanephosphonic acid cyclic anhydride (20 mL, 50% solution in ethyl acetate, dropwise). The reaction mixture was stirred for 1 hour then was diluted with ethyl acetate, washed with potassium carbonate (aqueous), dried (magnesium sulfate) and concentrated under reduced pressure to give the title compound (9.5 g). MS (ESI) m/z: 324.5 [M+H]+.
