A mixture of 1-(4-fluoro-benzoyl)-(S)-piperidine-3-carboxylic acid (1.05 g, 4.17 mmol), tert-butyl carbazate (0.55 g, 4.17 mmol), HOBT (0.562 g, 4.17 mmol), EDCI.HCl (1.2 g, 6.25 mmol) in dry dichloromethane (8 mL) was kept under stirring overnight at ambient temperature, under nitrogen atmosphere. HCl 1N (30 mL) was then added and the phases separated. The organic layer was washed with HCl 1N (30 mL), with NaOH 1N (30 mL×2 times), then with water (30 mL). Evaporation of the organic solvent afforded a crude yellow oil which was purified by flash chromatography (silica gel, eluent: DCM/MeOH 70/1) to give 0.715 g of N′-[1-(4-fluoro-benzoyl)-(S)-piperidine-3-carbonyl]-hydrazinecarboxylic acid-tert-butyl ester.
