A flask was charged with 4-(3-cyclobutylureido)benzoic acid (51.3 mmol, 12.023 g), triethyl-amine (129 mmol, 18 mL, 13.07 g) and dichloromethane (75 mL) and cooled to 0° C. tert-Butyl piperazine-1-carboxylate (51.3 mmol, 9.56 g) was then added and the reagents were stirred together for 10 minutes at 0° C. before 1-propanephosphonic acid cyclic anhydride (64.2 mmol, 38.0 mL, 40.8 g, 50% solution in ethyl acetate) was slowly added. The reaction mixture was allowed to warm slowly to room temperature. After 5 hours stirring, the reaction was poured into a flask containing water (100 mL) and brine (20 ml). The mixture was then stirred and left to stand overnight. The resulting precipitate was collected by vacuum filtration, washed with cold water and dried under vacuum to yield the title compound as a white powder (19.6 g). MS (ESI) m/z 403.5 [M+H]+
