7 g (1 eq) N,N-dimethyl-4-phenylpiperidin-4-amine were added in portions to a solution of 6.5 g (1.5 eq) tert-butyl methyl(2-oxoethyl)carbamate in 60 ml methanol. This reaction mixture was cooled to 0° C., 3.97 g (2.5 eq) sodium cyanoboron hydride were added in portions and then the mixture was stirred for 10 min at room temperature. The reaction mixture obtained was adjusted to a pH of ˜5 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, 1.5 g sodium cyanoboron hydride and acetic acid were added and the reaction mixture was stirred for a further 30 to 45 min. Once the conversion was complete, the methanol was distilled off, 100 ml saturated NaHCO3 solution were added and the mixture obtained was extracted with chloroform (2×200 ml) and the combined organic phases were dried over Na2SO4. Following removal of the solvent under reduced pressure, the residue was purified by column chromatography (silica gel; 5% MeOH/CHCl3). 8 g (64%) of product were obtained in the form of an oil.
