56.5 mg of (R)-3-((S)-2-(4,4-Dimethyl-3-(4-(3-phenylureido)-benzyl)-2,5-dioxoimidazolidin-1-yl)-2-(2-methylpropyl)acetylamino)-3-methylpropanol were dissolved in a mixture of 3 ml of ethyl acetate, 1 ml of toluene and 1 ml of water with 10.8 mg of potassium bromide. After addition of a catalytic amount of 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (=4-acetamido-TEMPO), a mixture of 0.5 ml of sodium hypochlorite solution (13% strength), 0.5 ml of saturated sodium hydrogencarbonate solution and 1 ml of water was added dropwise at 0° C. The mixture was stirred at 0° C. for 25 minutes. After reaction was complete, the mixture was treated with ethyl acetate, and the organic phase was washed with sodium thiosulfate solution and dried over sodium sulfate. After filtration, the solvent was removed on a rotary evaporator and the residue was purified by reversed phase HPLC (water/acetonitrile). Yield: 15 mg.
