Crude 2-{(2S,3S)-2-[(R)-2-(4-acetylamino-phenyl)-2-amino-acetylamino]-3-phenyl-butyrylamino}-thiazole-4-carboxylic acid methyl ester (≈0.175 mmol) was dissolved in dichloromethane (10 mL) and diisopropylethyllamine (72 μL, 0.41 mmol) was added. The resulting mixture was added to an ice cooled solution of diphosgene (13 μL, 0.109 mmol) in dichloromethane (10 mL). The reaction mixture was stirred for 15 minutes, diluted with ethyl acetate (100 mL) and washed with 0.2M aqueous hydrochloric acid, saturated aqueous sodium bicarbonate, brine and dried over sodium sulfate. The solution was filtered and the solvent removed in vacu. The residue was purified by chromatography over silica gel gradient eluted from 50% to 100% ethyl acetate in hexanes to afford 2-{(2S,3S)-2-[(R)-4-(4-acetylamino-phenyl)-2,5-dioxo-imidazolidin-1-yl]-3-phenyl-butyrylamino}-thiazole-4-carboxylic acid methyl ester (70 mg, 74%).
