Step 1: At 0°C, DEAD (12.3 mL, 27.08 mmol, 40% toluene solution) was slowly added dropwise to THF containing ethyl 4-hydroxybenzoate (3.0 g, 18.05 mmol), N-Boc-4-hydroxypiperidine (4.72 g, 23.47 mmol), and triphenylphosphine (6.16 g, 23.47 mmol) ( 40 mL) solution. The reaction mixture was stirred at 0 °C to room temperature for 24 hours. Subsequently, the reaction solution was concentrated under reduced pressure. The residue was diluted with EtOAc, washed sequentially with 1N NaOH and saturated saline, and the organic phase was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (elution gradient: 0→30% EtOAc/hexane) to afford tert-butyl 4-(4-(ethoxycarbonyl)phenoxy)piperidine-1-carboxylate (5.4 g, 15.45 mmol, 85.6% yield) as a colorless oil.ESIMS m/z: 372.1 [M+Na]+ (calculated value C19H27NO5Na. 372.1).
