0.5 cm3 of 5N aqueous sodium hydroxide was added to a stirred solution of 0.34 g of methyl (3R,4R)-4-[3-(6-methoxyquinolin-4-yl)propyl]-1-[2-(thien-2-ylthio)ethyl]piperidine-3-carboxylate in 3 cm3 of methanol and then the mixture was heated at a temperature in the region of 60° C. for 20 hours. After evaporating the solvents under reduced pressure (5 kPa) at a temperature in the region of 40° C., the residue obtained was taken up in 5 cm3 of water and then acidified by addition of 1 cm3 of 35% hydrochloric acid. The mixture was again evaporated under reduced pressure under the same conditions as above; the residue obtained was taken up in 5 cm3 of a dichloromethane/methanol (90/10 by volume) mixture. The insoluble material was filtered off and then the solvent was evaporated under reduced pressure (5 kPa) at a temperature in the region of 40° C. 0.35 g of (3R,4R)-4-[3-(6-methoxyquinolin-4-yl)propyl]-1-[2-(thien-2-ylthio)ethyl]piperidine-3-carboxylic acid dihydrochloride was obtained in the form of a solid with a beige color which melts with softening in the region of 150° C.
