To a solution of 4-(7-bromo-1,2,3,4-tetrahydro-naphthalen-2-ylamino)-piperidine-1-carboxylic acid tert-butyl ester (610 mg, 1.5 mmol) and propionaldehyde (0.1 mL, 1.5 mmol) in dichloroethane (20 mL) under a nitrogen atmosphere was added sodium triacetoxyborohydride (795 mg, 3.75 mmol) in a single portion. The reaction was stirred at room temperature for 24 h then concentrated in vacuo. The residue was partitioned between EtOAc (75 mL) and 5% aq. KOH (50 mL). The aqueous phase was extracted twice more with EtOAc (2×30 mL). The combined organic layers were dried over MgSO4, filtered, and concentrated onto silica (10 g). This was placed on top of a flash column and eluted with 20% hexanes in acetone. The fractions containing product were pooled and concentrated to afford 4-[(7-bromo-1,2,3,4-tetrahydro-naphthalen-2-yl)-propyl-amino]-piperidine-1-carboxylic acid tert-butyl ester (587 mg).
