To a stirred suspension of 58.5 parts of 4-(phenylamino)-1-(2-phenylethyl)-4-piperidinecarboxylic acid, sodium salt in 300 parts of hexamethylphosphoric triamide are added dropwise 29.2 parts of iodoethane at room temperature (slightly exothermic reaction: the temperature rises to 34° C.). Upon completion, stirring is continued at room temperature for 48 hours. The reaction mixture is poured onto 1000 parts of water and the product is extracted with methylbenzene. The extract is washed with water, dried, filtered and evaporated. The residue is crystallized twice: first from petroleumether and then from 35 parts of 2,2'-oxybispropane, yielding 45.2 parts of ethyl 4-(phenylamino)-1-(2-phenylethyl)-4-piperidinecarboxylate; mp. 65.4° C.
