THF (20 mL) and water (15 mL) were added to tert-butyl cis(±)-4-amino-3-methoxypiperidine-1-carboxylate (described in Example (1e), 4.6 g, 20 mmol). The mixture was stirred, during which sodium bicarbonate (3.44 g, 40 mmol) and benzyl chloroformate (3.95 g, 22 mmol) were added at room temperature, followed by stirring for one hour. The reaction solution was diluted with water, followed by extraction with ethyl acetate. The organic layer was washed with brine, dried over anhydrous magnesium sulfate, and then concentrated under reduced pressure. The residue was purified by silica gel column chromatography (elution solvent: ethyl acetate/hexane=1/4, 1/2, 1/1) to obtain 6.89 g of the title compound as a colorless solid (95%).
