General procedure for the synthesis of 4-BOC-3-morpholinecarboxaldehyde from tert-butyl 3-(hydroxymethyl)morpholine-4-carboxylate: to a stirred solution of oxalyl chloride (3.73 g, 2.56 mL, 29.00 mmol) in dichloromethane (DCM, 80 mL) was slowly added at -78 °C dimethyl sulfoxide (DMSO, 4.93 g, 4.47 mL, and 63.00 mmol). after 15 min, a solution of intermediate 90 (5.70 g, 26.26 mmol) in DCM (50 mL) was added. The reaction mixture was continued to be stirred at -78 °C for 2 hours. Subsequently, triethylamine (NEt3, 13.12 g, 18.71 mL, 129.70 mmol) was added and the reaction mixture was stirred at -78 °C for 30 min, then slowly warmed to room temperature and stirring was continued for 1 hour. After completion of the reaction, the mixture was concentrated in vacuum and the residue was partitioned between water (200 mL) and ethyl acetate (EtOAc, 200 mL). The aqueous phase was extracted with EtOAc (2 x 200 mL), and the combined organic phases were washed with brine (300 mL), dried over anhydrous magnesium sulfate (MgSO4), filtered, and the solvent removed in vacuum to afford 4-BOC-3-morpholinecarboxaldehyde (4.80 g, 84% yield) as a pale yellow crude solid. Nuclear magnetic resonance hydrogen spectroscopy (1H NMR, CDCl3) data: δ 9.58 (1H, s), 4.31 (2H, m), 3.62 (2H, br.m), 3.41 (1H, br.m), 3.11 (1H, br.s), 2.93 (1H, br.m), 1.40 (9H, s).
