General procedure for the synthesis of 1-Boc-3-piperidinecarboxaldehyde from tert-butyl 3-(hydroxymethyl)piperidine-1-carboxylate: a solution of dimethylsulfoxide (DMSO, 0.18 mL, 2.6 mmol) in dichloromethane (CH2Cl2, 5 mL) was slowly added dropwise at -78 °C to a solution of oxalyl chloride (2 M in CH2Cl2, 0.65 mL, 1.3 mmol ) in a solution of methylene chloride (4 mL). The reaction mixture was stirred at -68 °C for 15 min. Subsequently, tert-butyl 3-(hydroxymethyl)piperidine-1-carboxylate (0.22 g, 1.0 mmol) was added dropwise to a dichloromethane (4 mL) solution. After continued stirring at -78 °C for 15 min, triethylamine (Et3N, 6 mL) was added. The reaction mixture was warmed to room temperature and stirred for 16 hours. Upon completion of the reaction, water was added to quench the reaction and the mixture was extracted with ether. The organic layers were combined, dried over anhydrous sodium sulfate (Na2SO4) and concentrated under reduced pressure to give the yellow oily product 1-Boc-3-piperidinecarboxaldehyde (0.20 g, 92% yield). The crude product could be directly used in the subsequent reaction without further purification. Mass spectrum (ESI) m/z 214 (M + 1).
