A stirred solution of 5-Fluoro-4-hydroxy-3-[(S)-2-(2-oxo-2H-pyridin-1-yl)-propionylamino]-pentanoic acid tert-butyl ester (135 mg, 0.38 mmol) in anhydrous DCM (5 ml) was treated with 1,1,1-triacetoxy-1,1-dihydro-1,2-benziodoxol-3(1H)-one (193 mg, 0.46 mmol) at 0° C. The resulting mixture was kept at 0° C. for 1.5 h, diluted with ethyl acetate, then poured into a 1:1 mixture of saturated aqueous sodium hydrogen carbonate and saturated aqueous sodium thiosulphate. The organic layer was removed and the aqueous layer re-extracted with ethyl acetate. The combined organic extracts were dried (Na2SO4) and concentrated. The residue was purified by flash chromatography (1% methanol in ethyl acetate) to afford the title compound as a colorless gum (125 mg, 93%): 1H NMR (400 MHz, CDCl3) δ 1.2-1.4 (9H, 2×s), 2.6-2.9 (2H, m), 4.7-4.9 (1H, m), 4.9-5.3 (2H, m), 5.6-5.7 (1H, m), 6.2-6.3 (1H, m), 6.5-6.6 (1H, m), 7.2-7.4 (1H, m), 7.5-7.6 (1H, m), 8.0-8.2 (1H, m); 13C NMR (100 MHz, CDCl3) δ 16.57, 16.87, 28.24, 28.29, 36.44, 36.60, 52.50, 52.62, 52.90, 53.09, 82.31, 82.44, 83.51, 83.72, 85.33, 85.55, 107.35, 107.44, 120.45, 120.49, 134.42, 134.67, 140.22, 140.36, 162.85, 162.98, 169.93, 169.99, 170.59, 170.63, 202.58, 202.64, 202.75, 202.80; 19F NMR (376 MHz, CDCl3) δ −231.94, −231.96, −232.48, −232.49.
