An acetonitrile (150 mL) solution of di-tert-butyl dicarbonate (30.5 g, 0.14 mol) was slowly added to a reaction vial containing ethyl (S)-5-oxopyrrolidine-2-carboxylate (20 g, 0.127 mol) and di-tert-butyl dicarbonate (30.5 g, 0.14 mol) at 0 °C. 0.013 mol) in a reaction vial. The reaction mixture was stirred overnight at room temperature and then concentrated under vacuum. The residue was purified by fast chromatography (eluent: 30% ethyl acetate/hexane) to afford ethyl Boc-D-pyroglutamate (163A, 32.5 g, 100%) as an oil.1H NMR (CDCl3, 400 MHz) δ 4.58 (1H, dd, J = 3.0,9.5Hz), 4.22 (2H, q, J = 7.3Hz), 4.22 (2H, q, J = 7.3Hz), 4.22 (2H, q, J = 7.3Hz), 2.56-2.66 (2H, q, J = 7.3Hz). 2.56-2.66 (1H, m), 2.48 (1H, ddd, J = 3.8,9.6,13.1Hz), 2.25-2.35 (1H, m), 1.97-2.04 (1H, m), 1.48 (9H, s) and 1.28 (3H, t, J = 7.3Hz).
