A solution of 10.8 g of (3R,4R)-4-[3-oxo-3-(6-methoxyquinolin-4-yl)propyl]-1-(tert-butyloxycarbonyl)piperidine-3-acetic acid in 460 cm3 of anhydrous methanol to which 4.3 cm3 of concentrated sulfuric acid (d =1.83) had been added was heated at a temperature in the region of 65° C. with stirring for 2 hours. After cooling to approximately 20° C., the reaction mixture was evaporated under reduced pressure (5 kPa) at a temperature in the region of 40° C. and then the residue obtained was taken up in 200 cm3 of water and rendered alkaline by addition of sodium hydrogencarbonate until a pH in the region of 8-9 was obtained. The mixture was extracted with 4 times 200 cm3 of ethyl acetate. The aqueous phase was basified to a pH in the region of 11 by addition of the necessary amount of sodium carbonate. The mixture was extracted with 2 times 200 cm3 of ethyl acetate. The organic extracts were combined, dried over magnesium sulfate, filtered, and evaporated under reduced pressure (5 kPa) at a temperature in the region of 40° C. 6.84 g of methyl (3R,4R)-4-[3-oxo-3-(6-methoxyquinolin-4-yl)propyl]piperidine-3-acetate were obtained in the form of an oil with a brown color.
