To a solution of tert-butyl ((1S)-3-(benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)carbamate (200 mg, 0.53 mmol) in dioxane (5 mL) was added 4 M hydrochloric acid/dioxane (5 mL), and the solution was stirred at an ambient temperature for 4 h before removing the solvents in vacuo, azeotroping the residue with toluene (2 × 10 mL), and drying in vacuo for 1 h.