2-(4-Fluorobenzyl)-1-(methoxycarbonyl)piperidine-4-carboxylic acid (4.428 g, 14.99 mmol) (reference compound 43) was dissolved in methyl THF (100 mL) and di(1H-imidazol-1-yl)methanone (3.65 g, 22.49 mmol) added. The suspension was stirred at room temperature overnight under nitrogen (flask 1). In a separate flask potassium 3-ethoxy-3-oxopropanoate (4.59 g, 26.99 mmol) was suspended in methyl THF (100 mL) and magnesium chloride (2.57 g, 26.99 mmol) added. The suspension was stirred at 50° C. under nitrogen for 3 h (flask 2). The slightly yellow suspension in flask 1 was now added to the white suspension in flask 2. The resulting white suspension was stirred under nitrogen at room temperature overnight. The mixture was acidified to pH 1 with 3.8 M HCl and MTBE added. The phases were separated and the organic phase extracted with water, sat NaHCO3 and water. Evaporated the solvents to yield a yellow oil. The diastereoisomers were separated on Biotage (0%=>70% EtOAc in heptane, 7 CV; Biotage® KP-SIL 340 g column). Mixed fractions were repurified on Biotage (30%=>65% EtOAc in heptane, 10 CV; Biotage® KP-SIL 50 g column). Trans-methyl 4-(3-ethoxy-3-oxopropanoyl)-2-(4-fluorobenzyl)piperidine-1-carboxylate (0.458 g, 10.36%) and cis-methyl 4-(3-ethoxy-3-oxopropanoyl)-2-(4-fluorobenzyl)piperidine-1-carboxylate (2.531 g, 57.3%) were isolated. Cis-isomer: 1H NMR (600 MHz, cdcl3) δ 1.20-1.25 (m, 3H), 1.61-1.68 (m, 1H), 1.77-1.91 (m, 3H), 2.64-2.73 (m, 2H), 2.82-2.87 (m, 1H), 2.94-3.00 (m, 1H), 3.43 (s, 2H), 3.60 (s, 3H), 3.90 (dd, 1H), 4.11-4.17 (m, 3H), 6.91-6.96 (m, 2H), 7.08-7.13 (m, 2H). MS m/z 366 (M+H)+. Trans-isomer: 1H NMR (600 MHz, cdcl3) δ 1.15-1.26 (m, 3H), 1.39-1.94 (m, 4H), 2.59-3.00 (m, 4H), 3.32-3.66 (m, 5H), 3.96-4.28 (m, 3H), 4.50 (d, br., 1H), 6.88-6.95 (m, 2H), 6.99-7.18 (m, 2H). MS m/z 366 (M+H)+
