Acetic Acid → Tert-butyl [2-fluoro-5-(methylamino)phenyl]carbamate

To a solution of the above-mentioned crude product of tert-butyl [2-fluoro-5-(formylamino)phenyl]carbamate in tetrahydrofuran (100 mL) was added borane dimethylsulfide complex (11.7 mL, 111 mmol), and the mixture was stirred at room temperature for 1.5 hr. To the reaction mixture were successively added methanol (20 mL) and acetic acid (10 mL), and the mixture was stirred at room temperature for 30 min. The reaction mixture was concentrated under reduced pressure. To the obtained residue was added saturated aqueous sodium hydrogen carbonate solution (100 mL), and the mixture was extracted with ethyl acetate (100 mL, 30 mL). The combined organic layer was washed with saturated brine (20 mL), and dried over anhydrous sodium sulfate. The insoluble material was filtered off, and the filtrate was concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography (hexane/ethyl acetate=100/0→90/10), and the fractions containing the object product were concentrated under reduced pressure to give the title compound (7.80 g, 73%) as a yellow oil.