To a solution of (3-chloro-propylamino)-(4-fluoro-phenyl)-acetic acid methyl ester (14.34 g) in DMF (40 mL) was added BOC2O (11.44 g) followed by triethylamine (15.9 g) and the resultant mixture was stirred overnight at 60° C. A further 0.25 equivalents of BOC2O was added and the reaction mixture was stirred a further 18 hours. The reaction mixture was diluted with EtOAc (200 mL) and water (150 mL), the phases were separated and the organic phase was washed with 0.5 N HCl solution, water, brine and dried over sodium sulfate. Filtration of the drying agent and evaporation of the volatiles in vacuo afforded crude [tert-butoxycarbonyl-(3-chloro-propyl)-amino]-(4-fluoro-phenyl)-acetic acid methyl ester (10.91 g, MS (ES+): 360.1 (MH+)) as a brown oil which was used in the subsequent reaction without further purification.
