To a solution of N,N-di-Boc-4-bromo-2,6-difluoro-phenylamine (9.0 g, 22.0 mmol) obtained in Step A, palladium acetate (49 mg, 0.218 mmol), S-Phos (181 mg, 0.441 mmol), and tert-butoxy potassium (4.45 g, 39.7 mmol) in toluene (100 ml), 1-ethyl-piperazine (4.20 ml, 33.1 mmol) was added, followed by stirring at 60° C. for 14 hours. After cooling to room temperature, 100 ml of water was added, followed by extraction with ethyl acetate (100 ml×2). The organic layer was dried over sodium sulfate, and the solvent was distilled off under reduced pressure. This was purified by silica gel column chromatography (dichloromethane/methanol=100/1 to 10/1), whereby the desired compound was obtained (2.4 g, 32%).
