ε-  (CAS no.: ), which could be called as 2-Oxohexamethylenimine, could be produced through the following synthetic routes.

A. 1-Triisopropylsilyloxy-1-azidocyclohexane : A 2-L, two-necked, round-bottomed flask is equipped with a magnetic stirrer, argon inlet, and a rubber septum. The flask is charged with freshly-distilled 1-triisopropylsilyloxycyclohexene (25.47 g, 100 mmol) and anhydrous dichloromethane (1.0 L). Azidotrimethylsilane (68.5 mL, 500 mmol) is added via syringe, immediately followed by anhydrous Dowex® 50 × 8-100 (24.98 g) in a single portion from a dry flask. The suspension is stirred vigorously at ambient temperature for ca. 48 hr. The reaction mixture is filtered to recover the Dowex® resin and solvent is removed under reduced pressure to afford a clear, colorless oil. The crude oil is applied to a 5 × 13-cm column of silica gel (120 g, 230-400 mesh packed with hexanes). The column is quickly eluted with hexanes (750 mL) and fractions are collected in 25-mL test tubes. The fractions containing the desired product are identified by thin layer chromatography, combined, and concentrated under reduced pressure to afford the 1-triisopropylsilyloxy-1-azidocyclohexane (26.75 g, 90%) as a colorless oil.

B. ε-Caprolactam : The reaction apparatus (see Fig. 1) is charged with 1-triisopropylsilyloxy-1-azidocyclohexane (25.9 g, 87.1 mmol) and cyclohexane (435 mL). The clear, colorless solution is purged with nitrogen for a 15 min period to exclude oxygen. The solution is cooled to ca. 0°C and irradiated (≥200 nm) for 3.5 hr. The solvent is removed under reduced pressure to afford a pale yellow oil. The crude oil is dissolved in dichloromethane (ca. 10 mL) and applied to a 6 × 6-cm column of silica gel (88 g, 230-400 mesh packed with 1 : 1 ethyl acetate/hexanes). An additional 1 cm of silica gel is added to the top of the column and the mixture is stirred with a glass rod to homogenize the layer, rinsed with eluent, and the column is repacked. The column is eluted with 1 : 1 ethyl acetate/hexanes (200 mL), then with 1 : 9 methanol/ethyl acetate (500 mL), and fractions are collected in 25-mL test tubes. The fractions containing the desired product are identified by thin layer chromatography, combined, and concentrated under reduced pressure to afford ε-caprolactam (8.20 g, 83%) as an off-white solid.