A mixture of (±)-1,1-dimethylethyl 4-oxo-2-(phenylmethyl)-1-piperidine carboxylate (0.1 mol) and N-(4-phenyl-4-piperidinyl)acetamide (0.1 mol) in 2-propanol (500 ml) was hydrogenated at 50° C. with platinum (4 g) as a catalyst in the presence of titanium (IV)isopropoxide (28.5 g) and thiophene (4%; 1 ml). After uptake of hydrogen the catalyst was filtered off and the filtrate was evaporated. The residue was purified by column chromatography over silica gel (eluent: CH2Cl2/CH3OH 97/3). The pure fractions were collected and the solvent was evaporated, yielding 40 g (74.1% ) of (±)-1,1-dimethylethyl 4-[4-(acetylamino)-4-phenyl-1-piperidinyl]-2-(phenylmethyl)-1-piperidinecarboxylate (intermediate 19).
