A mixture of 6.44 g of methyl (3R,4R)-4-[3-(6-methoxyquinolin-4-yl)-3-oxopropyl]piperidine-3-carboxylate in 100 cm3 of acetonitrile, 3.43 g of 2-(3-fluorophenylthio)ethyl-1-chloride, 8.85 g of potassium carbonate, and 1.24 g of potassium iodide was heated at a temperature in the region of 65° C. for 48 hours. After cooling, the insoluble material was filtered off. The filtrate was concentrated under reduced pressure (5 kPa) at a temperature in the region of 40° C. The oily residue was purified by chromatography at atmospheric pressure on a column of silica gel (particle size 20-45 μm; diameter 6 cm; 250 g), elution was carried out with a dichloromethane/ethyl acetate/methanol (50/50/3 by volume) mixture, and 50-cm3 fractions were collected. The fractions from 19 to 25 were collected. These fractions were combined and then concentrated under reduced pressure (5 kPa) at approximately 40° C. 2.1 g of methyl (3R,4R)-1-[2-(3-fluorophenylthio)ethyl]-4-[3-(6-methoxyquinolin-4-yl)-3-oxopropyl]piperidine-3-carboxylate were obtained in the form of a mobile brown oil. Methyl (3R,4R)-4-[3-(6-methoxyquinolin-4-yl)-3-oxopropyl]-piperidine-3-carboxylate was prepared as described in Example 5.
