200 mg of 4-[4-((2S)-2-amino-2-tert-butoxycarbonyl-ethyl)-phenoxy]-butyric acid were dissolved in 2 ml of DMF and cooled to 0° C. 220 mg of 4-chlorobenzenesulfonyl chloride and 270 mg of diisopropylethylamine were added and the reaction mixture was stirred at 0° C. for 2 hours. The reaction mixture was cooled to −25° C. for 16 hours and then warmed to room temperature. The reaction was quenched by the addition of water, and the mixture was extracted three times with dichloromethane. The combined organic phases were dried with sodium sulfate, filtered and the solvent was removed in vacuo. The residue was chromatographed on silica gel eluting with dichloromethane/methanol/water/acetic acid (9/1/0.1/0.1). Yield 84 mg. MS (ES+): m/e=498.1 (M+H)+; 442.1.
