A solution of 4-amino-piperidine-1-carboxylic acid tert-butyl ester (427 mg; 2.13 mmol), bis(4-fluorophenyl) sulfone (493 mg; 1.94 mmol) and potassium carbonate (1.01 g; 7.76 mmol) in dimethylsulfoxide (5 mL) is heated over night at 100° C. The temperature is then increased to 120° C. and the mixture is reacted at this temperature for 24 hours. The reaction is then diluted with dichloromethane (100 mL), filtered and the filtrate is washed with aq. 1N hydrochloric acid (2×20 mL), and water (2×20 mL). The organic layer is dried over magnesium sulfate, filtered and concentrated under reduced pressure. The residue obtained is finally purified by column chromatography on silica gel (dichloromethane:methanol 100:0 (200 mL) and then 98:2 (150 mL)) to afford the desired product (653 mg; 1.50 mmol).
