4-(4-Benzyloxy-phenylamino)-piperidine-1-carboxylic acid tert-butyl ester (40a, 3.0 g, 7.84 mmol) was dissolved in CH2Cl2 (50 mL) and treated with isovaleraldehyde (0.84 mL, 7.84 mmol). The reaction was stirred for 30 minutes, then cooled to 0° C., treated with NaBH(OAc)3, and stirred overnight. The reaction was diluted with EtOAc (400 mL), washed with saturated bicarbonate solution and brine, dried over Na2SO4, and concentrated. The residue was chromatographed on silica gel eluting with 6% MeOH/CH2Cl2 to give 3.3 g (93%) of the desired product.
