To a stirred suspension of 2-amino-5-chloro-pyridine (1.01 g) in dichloroethane (20 ml) was added portionwise triphosgene (831 mg). Triethylamine (2.22 ml) was then added dropwise and the mixture was stirred at 50° C. for 1 h. The mixture was then concentrated in vacuo to afford a beige solid containing a mixture of triethylammonium chloride and 5-chloro-2-isocyanato-pyridine. This solid was then added to a stirred solution of (RS)-tert-butyl 3-(4-aminophenyl)pyrrolidine-1-carboxylate (350 mg, CAS 908334-28-1) and N,N-diisopropylethylamine (0.68 ml) in dichloroethane (6 ml) and the resulting mixture was stirred at 60° C. overnight. The mixture was then diluted with dichloromethane and washed with water. The phases were separated and the organic phase was dried over sodium sulphate and concentrated in vacuo. The residue was purified by column chromatography (SiO2; gradient: heptane/EtOAc) to give (RS)-3-{4-[3-(5-chloro-pyridin-2-yl)-ureido]-phenyl}-pyrrolidine-1-carboxylic acid tert-butyl ester (212 mg, 38%) as an off-white solid. MS (ISP): 419.2 ([{37Cl}M+H]+), 417.2 ([{35Cl}M+H]+).
