1-(tert-Butoxycarbonyl)-4-[(2-hydroxymethylphenyl)amino]piperidine (716 mg) was dissolved in tetrahydrofuran (7 ml), added with phthalimide (349 mg), triphenylphosphine (736 mg) and diethyl azodicarboxylate (0.43 ml) and stirred at room temperature for 21 hours. The reaction mixture was added with water (10 ml) and extracted twice with dichloromethane (10 ml). The organic layer was dried over anhydrous magnesium sulfate, and then the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (elution solvent: hexane:ethyl acetate=4:1) to obtain crude 1-(tert-butoxycarbonyl)-4-[(2-phthalimidylmethylphenyl)amino]piperidine (420 mg).
