11.1 g (1.3 eq) tert-butyl methyl(3-oxopropyl)carbamate were added to a solution of 11 g (1 eq) N,N-dimethyl-4-phenylpiperidin-4-amine dihydrochloride in 110 ml methanol at 0° C. and the reaction mixture was stirred for 15 min at 0° C. 6.2 g (3 eq) sodium cyanoboron hydride were then added in portions and the mixture was stirred for 30 min at room temperature. The reaction mixture obtained was adjusted to a pH of 5-6 with acetic acid and stirred for 12 h at room temperature. The reaction course was monitored by thin-layer chromatography (20% MeOH/CHCl3). As the conversion was still not complete, 2.4 g sodium cyanoboron hydride were added and the reaction mixture obtained was adjusted to pH 5-6 with acetic acid and stirred for 60 min at room temperature.
