2-[1-(6-tert-Butoxycarbonylamino-pyridin-3-yl)-propyl]-malonic acid diethyl ester (3.23 g, 8.19 mmol) was dissolved in a mixture of dichloromethane (12 mL) and ethanol (24 mL, 99.5%). To this solution was added dropwise a solution of potassium hydroxide (0.528 g, 87%, 8.20 mmol) in ethanol (16 mL, 99.5%) at 0° C. over 0.5 h. Stirring was continued for 16 h while the mixture was allowed to warn up to room temperature. After concentration under reduced pressure to 5-10 mL, water (50 mL) was added and the resulting emulsion was stirred for 0.5 h, and then filtered. The filtrate was washed twice with a mixture of ethyl acetate and diethyl ether (2:1, vol/vol). The aqueous layer was acidified by addition of a solution of citric acid in water (10%) to reach a pH of 4 to 5, and extracted three times with ethyl acetate. The combined organic layers were washed with brine, dried over magnesium sulphate, and the solvent was removed under reduced pressure to yield 2-[1-(6-tert-butoxycarbonylamino-pyridin-3-yl)-propyl]-malonic acid monoethyl ester (2.47 g, 82%)
