8-acetoxy-4-(2-(diphenylmethoxy)ethyl)-3-oxo-3,4-dihydro-2H-1,4-benzooxazine (630 mg) was dissolved in THF (10 ml), and a 1.0M borane THF solution (4.5 ml) was added to the resultant solution at 0° C., followed by stirring at room temperature for 4 hours. After water was added to the reaction solution to terminate reaction, the reaction solution was extracted with ethyl acetate. The resultant organic layer was washed with water, saturated sodium bicarbonate water, water and saturated brine, dried over sodium sulfate, and then concentrated. The residue was purified by medium-pressure column chromatography (solvent:ethyl acetate/cyclohexane=1/5) to obtain the colorless oily object compound (550 mg, yield 90%).
