A mixture of 3.6 parts of 1-(2-chloroethyl)-4-ethyl-1,4-dihydro-5H-tetrazol-5-one, 6.4 parts of methyl 4-[N-(1-oxopropyl)N-phenylamino]-4-piperidinecarboxylate hydrochloride, 4 parts of sodium carbonate, 0.1 parts of potassium iodide and 240 parts of 4-methyl-2-pentanone is stirred and refluxed overnight with water-separator. The reaction mixture is cooled and poured onto water. The organic phase is separated, dried, filtered and evaporated. The residue is purified by column-chromatography over silica gel using a mixture of trichloromethane and methanol (97:3 by volume) as eluent. The pure fractions are collected and the eluent is evaporated. The residue is converted into the ethanedioate salt in 2-propanone. The salt is filtered off and dried, yielding 1.5 parts (13%) of methyl 1-[2-(4-ethyl-4,5-dihydro-5-oxo-1H-tetrazol-1-yl)ethyl]-4-[(1-oxopropyl)phenylamino]-4-piperidinecarboxylate ethanedioate (2:3); mp. 158.9° C.
