4-(4-Benzyloxy-phenylamino)-piperidine-1-carboxylic acid tert-butyl ester (9.51 g, 24.9 mmol) was dissolved in CH2Cl2 (150 mL) and treated with isovaleraldehyde (2.7 mL, 24.9 mmol). The reaction was stirred for 30 minutes, then cooled to 0° C., treated with NaBH(OAc)3, allowed to warm to room temperature as the ice melted, and stir overnight. The reaction was diluted with CH2Cl2 (300 mL), washed twice with saturated sodium bicarbonate solution and once with brine, dried over Na2SO4, and concentrated. The crude material was chromatographed on silica gel eluting with 4% MeOH/CH2Cl2 to give 9.59 g (85%) of the desired product.
