A solution of 1,2-diaminopropane (12.1 g, 163 mmol) in ethanol (20 ml) was added at room temperature over 90 min to a solution of ethyl chloroacetate (2.00 g, 16.3 mmol) in ethanol (70 ml), then after 2 h potassium carbonate (2.26 g, 16.3 mmol) was added. After another 2 h, insoluble material was removed by filtration, and the filtrate was evaporated. The residue was suspended in dichloromethane (50 ml) and treated with di-tert-butyl dicarbonate (24.9 g, 114 mmol), and the reaction mixture was stirred at room temperature for 16 h, then evaporated. The residue was chromatographed (SiO2; heptane-ethyl acetate gradient) to afford 984 mg of a 3:2 mixture of the desired 3-methyl-5-oxo-piperazine-1-carboxylic acid tert-butyl ester and the regioisomer 3-methyl-5-oxo-piperazine-1-carboxylic acid tert-butyl ester. 200 mg of this regioisomeric mixture was alkylated with t-(3-chloropropyl)piperidine as described in intermediate 17b to produce the title compound (120 mg) containing 2-methyl-5-oxo-4-(3-piperidin-1-yl-propyl)-piperazine-1-carboxylic acid tert-butyl ester (ca. 20%) as a regioisomeric impurity. Light yellow oil, MS (ISP)=340.3 (M+H)+.
