A solution of the crude (3S)-3-({N-[(4-methoxy-1H-indol-2-yl)carbonyl]-l-leucyl}amino)-2-oxo-4-[(3S)-2-oxopyrrolidin-3-yl]butyl oxo(phenyl)acetate in methanol (40 mL) was placed under an atmosphere of N2 and treated with potassium carbonate (7 mg, 0.05 mmol) with vigorous stirring.