To a cooled solution (0° C.) of 4-{2-[(3-bromo-4-chloro-benzoyl)-methyl-amino]-phenyl}-butyric acid (1.07 g, 2.60 mmol) in anhydrous DCM (10 mL), oxalyl chloride (0.34 mL, 3.90 mmol) was added dropwise. The mixture was stirred at room temperature for 4 hours, concentrated in vacuo and redissolved in anhydrous dichloromethane (5 mL). A solution of t-butanol (960 mg, 13 mmol) in anhydrous DCM (5 mL) was added and the mixture was stirred at room temperature overnight. After dilution with DCM, the mixture was washed with sat NaHCO3 and brine, dried over Na2SO4, filtered and concentrated. Purification by silica gel column chromatography (EtOAc/Hexanes: 1/4 with gradient up to EtOAc/Hexanes 1/3) afforded 1.1 g of 4-{2-[(3-bromo-4-chloro-benzoyl)-methyl-amino]-phenyl}-butyric acid tert-butyl ester as a colorless oil. MS [M+H]+ (fragment): 411.7 (f; tR=3.10 min, (method 1)
