3-(2-(N-tert-Butoxycarbonylamino)ethoxy)-5-phenylisoxazole (0.8 g) was dissolved in tetrahydrofuran (16 ml), butyllithium (1.6M hexane solution, 3.7 ml) was added dropwise thereto at -70° C. under a nitrogen atmosphere, and the resulting mixture was stirred at the same temperature for 10 minutes. After acetone (0.3 ml) was added dropwise to the reaction mixture and the mixture was stirred for 10 minutes, the temperature of the mixture was raised to 0° C. The reaction mixture was poured into ice-cold water and the pH of the mixture was adjusted to a value of 6 with an aqueous potassium dihydrogenphosphate solution. The mixture was extracted with ethyl acetate and the organic layer was washed with a saturated aqueous NaCl solution, followed by evaporation of the solvent under reduced pressure. The residue was purified by silica gel column chromatography (eluent: hexane/ethyl acetate=3/1) to obtain the title compound (0.42 g, 44%) as a colorless oil.
