A methylene chloride (15 mL) solution of carbonyl diimidazole (498 mg, 3.07 mmol) was added at 0° C. to a solution of (R)-3-(4-amino-3-hydroxy-phenyl)-2-benzyloxycarbonylamino-propionic acid methyl ester (1.17 g, 3.07 mmol), diisopropylethylamine (1.60 mL, 9.21 mmol), and methylene chloride (85 mL). The mixture was stirred at 0° C. for 4 h. The solvents were removed in vacuo and the residue was purified by silica gel chromatography using ethyl acetate/hexanes as eluent to afford the title compound as a white solid (51%). 1H-NMR (CDCl3, 500 MHz) δ 9.07 (s, 1H), 7.37-7.29 (m, 5H), 6.96 (s, 1H), 6.90 (d, J=8.0 Hz, 1H), 6.87 (d, J=8.0 Hz, 1H), 5.36 (d, J=8.0 Hz, 1H), 5.11 (d, J=12.0 Hz, 1H), 5.07 (d, J=12.5 Hz, 1H), 4.65 (dd, J=13.5, 5.5 Hz, 1H), 3.74 (s, 3H), 3.17 (dd, J=14.0, 5.5 Hz, 1H), 3.07 (dd, J=14.0, 6.0 Hz, 1H). 13CNMR (CDCl3, 125 MHz) δ 171.9, 155.7, 155.5, 144.1, 136.2, 130.8, 128.6, 128.42, 128.38, 128.2, 125.1, 111.1, 109.8, 67.2, 55.1, 52.6, 38.3. Mass spec.: 371.18 (MH)+.
