To a suspension of benzyl 4-oxo-1-piperazine carboxylate (11.7 g, 50.3 mmol) in ether (100 ml) at -78 °C was slowly added ethyl diazoacetate (6.84 ml, 65.3 mmol) followed by dropwise addition of boron trifluoride ether complex (6.30 ml, 50.3 mmol). After 1 hr of reaction, the reaction was gradually warmed up to room temperature to give a clarified yellow solution. Saturated aqueous potassium carbonate solution was added slowly dropwise until no gas was produced. The organic phase was separated and washed with saturated aqueous potassium carbonate solution (2 x 50 ml), dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give ethyl 1-Cbz-5-oxoazepane-4-carboxylate as a yellow oil (16.5 g, quantitative yield). The product can be used directly in the next reaction without further purification.LCMS analysis showed the following: calculated value MH+ (320); measured value 85% (MH+) m/z 320, retention time Rt=1.92 min.1H NMR (500MHz, CDCl3) δ 7.27-7.40 (5H, m), 5.05-5.19 (2H, m). 4.13-4.30 (2H, m), 3.84-3.98 (1H, m), 3.67-3.84 (2H, m), 3.34-3.57 (2H, m), 2.62-2.93 (2H, m), 1.98-2.14 (2H, m), 1.18-1.34 (3H, m).
