In a nitrogen gas flow, 8.3 g of L-cysteine ethyl ester hydrochloride is added to a sodium ethoxide solution prepared by adding 2 g of metallic sodium to 80 ml of absolute ethanol. After stirring for 1 hours at room temperature, 80 ml of an ethanol solution containing 18.4 g of 2-(1-benzyloxycarbonyl-4-piperidyl)ethyl iodide is added dropwise to this reaction mixture. After stirring for 1 hour at room temperature, the reaction liquid is concentrated under reduced pressure. To the resulting residue, ice-cold water is added. After extracting with ether, the organic layer is washed with water, and dried over magnesium sulfate. Thereafter, the solvent is distilled off under reduced pressure to give 16.3 g of oily S-[2-(1-benzyloxycarbonyl-4-piperidyl)ethyl]-L-cysteine ethyl ester.
