General procedure for the synthesis of N-Cbz-4-oxo-L-proline methyl ester from (2S,4S)-N-CBZ-4-hydroxyproline methyl ester: Dess-Martin periodinane (3.0 g, 7.1 mmol) was added batchwise to a solution of compound (2S,4S)-N-CBZ-4-hydroxyproline methyl ester (1.0 g , 3.6 mmol) in a solution of dichloromethane (DCM, 20 mL). After addition, the reaction mixture was stirred at room temperature for 1.0 hour. After completion of the reaction, the mixture was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate, v/v=5/1) to afford N-Cbz-4-oxo-L-proline methyl ester as a yellow oil (0.79 g, 79.5% yield). The structure of the compound was confirmed by the following spectral data: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.47 (d, 2H, J=8.24 Hz), 7.38 (d, 2H, J=8.24 Hz), 7.24 (m, 1H), 5.09 (s, 2H), 4.18 (t, 1H), 3.68 (s, 3H). 3.38 (m, 1H), 3.32 (m, 1H), 2.21 (m, 1H), 1.96 (m, 1H).
