To a solution of tert-butyl [(1S)-2-[((1S)-3-(benzyloxy)-2-oxo-1-{[(3S)-2-oxopyrrolidin-3-yl]methyl}propyl)amino]-1-(cyclohexylmethyl)-2-oxoethyl]carbamate (200 mg, 0.38 mmol) in dioxane (4 mL) was added 4 M hydrochloric acid/dioxane (4 mL), and the solution was stirred at an ambient temperature for 2 h before removing the solvents in vacuo, azeotroping the residue with toluene (2 × 10 mL), and drying in vacuo for 1 h.