Trifluoroacetic acid → (2R)-5-(3-fluoro-phenyl)-3,4-dihydro-2H-pyrrole-2-carboxylic acid methyl ester

To a solution of (2R)-2-tert-butoxycarbonylamino-5-(3-fluoro-phenyl)-5-oxo-pentanoic acid methyl ester (0.57 g, 1.7 mmol) in 1.7 mL of CH2Cl2 at 0° C. was added dropwise 3.8 mL (49 mmol, 29 eq) of TFA. The mixture was stirred at 0° C. for 2 hr. After concentration under vacuum, the residue was dissolved in CH2Cl2 (30 mL), washed with 10% NaHCO3, water, and brine, dried (Na2SO4) and concentrated under vacuum to give (2R)-5-(3-fluoro-phenyl)-3,4-dihydro-2H-pyrrole-2-carboxylic acid methyl ester as a colorless oil. 1H NMR (CDCl3, 300 MHz) δ 7.77–7.70 (2H, m), 7.50 (1H, q), 7.27 (1H, tq), 5.05 (1H, tt), 3.90 (3H, s), 3.30–3.00 (2H, m), 2.55–2.30 (2H, m).