Oxalyl chloride (3.3 mL, 2M in CH2Cl2, 6.42 mmol) was stirred in dichloromethane (3 mL) in a three-necked flask. The reaction mixture was cooled to −78° C., and dimethyl sulfoxide (0.91 mL, 12.85 mmol) was added so that the internal temperature did not exceed −65° C. The reaction mixture was then stirred for 15 minutes. The (3R,4S)-3-hydroxymethyl-4-phenyl-pyrrolidine-1-carboxylic acid tert-butyl ester (713 mg, 2.57 mmol) in dichloromethane (6 mL) was added dropwise keeping the internal temperature below −65° C. and then stirred for 15 minutes. Diisopropylethylamine (4.5 mL, 25.7 mmol) was added keeping the internal temperature below −65° C. and then stirred for 20 minutes. The cooling bath was removed and the reaction mixture was stirred at room temperature for 2 h. The reaction mixture was concentrated to give a colorless oil, which was purified by flash silica gel chromatography eluting with ethyl acetate: hexanes (1:4) to give 496 mg (70%) of (3R,4S)-3-formyl-4-phenyl-pyrrolidine-1-carboxylic acid tert-butyl ester.
