To a solution of 1-tert-butoxycarbonyl-4-ethoxycarbonylpiperidin-4-ylacetic acid (1.6 g) and triethylamine (0.71 ml) in dimethylformamide (20 ml) was added diphenylphosphoryl azide (1.1 ml) at room temperature, and the mixture was stirred at 70° C. for 30 min and then at 90° C. for 10 min. Benzyl alcohol (0.58 ml) was added at the same temperature, and the mixture was stirred for 20 hours. After completion of the reaction, water was added and the mixture was extracted with ethyl acetate and washed with 1N aqueous sodium hydroxide solution. The organic layer was dried over anhydrous magnesium sulfate and the solvent was evaporated. The obtained residue was purified by silica gel column chromatography and dried under reduced pressure to give the title compound (1.35 g).
