To a stirred solution of methyl 1-(tert-butoxycarbonyl)-4-hydroxyproline (4.0 g, 16.3 mmol), Ph3P (5.14 g, 19.6 mmol), and 2,4-difluorophenol (2.55 g, 19.6 mmol) in THF (50 ml) was added DIAD (3.9 ml, 19.6 mmol), and the mixture was heated at reflux for 3 h. After cooling to room temperature, the mixture was concntrated in vacuo and the residue was chromatographed on silica gel with CHCl3-EtOAc (4:1) to give methyl 1-(tert-butoxycarbonyl)-4-(2,4-difluorophenoxy)pyrrolidine-2-carboxylate (5.82 g, quant) as yellow oil.
