7 g (1 equiv.) of N,N-dimethyl-4-phenylpiperidine-4-amine were added in portions to a solution of 6.5 g (1.5 equiv.) of tert-butyl methyl(2-oxoethyl)carbamate in 60 ml of methanol. The reaction mixture was cooled to 0° C., 3.97 g (2.5 equiv.) of sodium cyanoborohydride were added in portions, and stirring was then carried out for 10 minutes at room temperature. The resulting reaction mixture was adjusted to a pH of ˜5 by means of acetic acid and was stirred for 12 hours at room temperature. The progress of the reaction was monitored by means of thin-layer chromatography (20% MeOH/CHCl3). Because the reaction was not yet complete, 1.5 g of sodium cyanoborohydride and acetic acid were added and the reaction mixture was stirred for a further 35-40 minutes.
