Boron trifluoride etherate → Diethyl 5-oxoazepane-1,4-dicarboxylate

To a cold solution (−30° C.) of 1-carbethoxypiperidin-4-one (3.0 ml, 26.5 mmol) in anhydrous ether, were simultaneously added a solution of boron trifluoride etherate (3.4 ml, 26.5 mmol) in diethyl ether (2.8 ml) and a solution of ethyl diazodicarboxylate (3.6 ml, 34.5 mmol) in diethyl ether (2.8 ml) over 1.5 hours via a syringe-pump. Upon completion of additions, the reaction mixture was allowed to stir for an additional hour at −30° C. and then warmed to room temperature. The reaction mixture was washed with 30% potassium carbonate and the organic phase was dried over Na2SO4 and concentrated in vacuo to give diethyl 5-oxoazepane-1,4-dicarboxylate as a crude yellow oil.