A solution of tert-butyl ((1S)-3-bromo-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)carbamate (1.17 g, 3.2 mmol) in DMF (16 mL) was treated with 2,6-dichlorobenzoic acid (794 mg, 4.2 mmol) followed by cesium fluoride (1.18 g, 7.4 mmol).