1.41 g (4.51 mmol) 2-dibenzylamino-3-hydroxy-3-methyl-butyric acid and 0.54 ml (4.95 mmol) 2,5-difluoronitrobenzene in 46 ml tetrahydrofuran were treated at 0° C. with 19.8 ml (9.9 mmol) potassium bis(trimethylsilyl)amide (0.5 M in toluene). The solution was stirred overnight at room temperature and was then poured on ice/water. Extraction with ethylacetate and chromatography on silicagel with diethylether/heptane (30:70 to 75:25) and yielded 0.40 g (20%) 2-dibenzylamino-3-(4-fluoro-2-nitro-phenoxy)-3-methyl-butyric acid as yellow solid, MS m/e (%): 453.4 (M+H+, 100).
