6.38 g of sodium triacetoxyborohydride (30.1 mmol) was added to a solution of 4.68 g of (1,1-dimethyl-2-oxoethyl)carbamic acid t-butyl ester obtained in Reference Example 3 (25.0 mmol), 5.0 g of 2-benzyloxyaniline (25.0 mmol) and 1.44 ml of acetic acid (25.0 mmol) in methylene chloride (250 ml) under ice-cooling, and the mixture was stirred at room temperature for 16 hours. A saturated sodium bicarbonate aqueous solution was added to the reaction mixture, followed by extraction with methylene chloride. Then, the organic layer was washed with brine and dried over anhydrous sodium sulfate. After filtration, the solvent was evaporated under reduced pressure, and the residue was purified by silica gel column chromatography (elution solvent:hexane/ethyl acetate=5/1) to obtain 8.50 g of the title compound (yield: 91%).
