The intermediate was synthesized by a general procedure based on US2010/0189644 and the disclosure of O'Donnell et al. The procedure was carried out as follows: benzophenone imine (8.6 g, 47.5 mmol), tert-butyl bromoacetate (9.3 g, 47.5 mmol), and acetonitrile (40 mL) were added to a round-bottomed flask equipped with a stirrer. After heating the reaction mixture to 70 °C, N,N-diisopropylethylamine (DIPEA, 8.3 mL, 47.5 mmol) was slowly added. A reflux condenser was installed and the reaction was maintained at 70 °C for 16 hours. The reaction progress was monitored by HPLC and TLC and after confirming the complete conversion of the reactants, the reaction mixture was cooled to room temperature. A 5:3 mixture of water/formic acid (1 mL) was added, followed by concentration of the reaction mixture under reduced pressure. The resulting solid was washed sequentially with 3:1 cold solution of water/ethanol (2 x 60 mL) and 1:1 cold solution of water/ethanol (30 mL). The solid was dried under high vacuum to afford tert-butyl N-diphenylmethylene-glycinate (14.9 g, 47.0 mmol, 99% yield) as a white solid. The product was characterized by 1H NMR (400 MHz, CDCl3) and mass spectrometry (LC/MS): 1H NMR δ 7.66 (2H, m), 7.47 (3H, m), 7.41 (1H, t, J=8 Hz), 7.34 (1H, t, J=8 Hz), 7.20 (2H, m), 4.13 (2H, s), 1.48 (9H, s); MS (LC/MS) m/z measured value 195.93, theoretical value 296.16 [M+H]+.
