To a solution of (L)-leucine methyl ester hydrochloride (6.00 g, 0.033 mol) dissolved in acetonitrile (50 mL) was added (E)-4-bromo-3-(3-ethoxy-2,6-difluoro-phenoxy)-but-2-enoic acid ethyl ester (6.05 g) in acetonitrile (10 mL) and N,N-diisopropylethylamine (7.27 g, 0.056 mol) and the resulting mixture refluxed for 14 h. The reaction mixture was cooled to room temperature and then poured into ethyl acetate. The mixture was filtered to remove salts and the filtrate washed successively with saturated ammonium chloride, water, and brine. The solution was dried over sodium sulfate, filtered and evaporated. The residue was dissolved in tetrahydrofuran (10 mL) and then transferred to an Emry Optimizer microwave reaction vessel and heated at 160° C., for 4 h. The crude product obtained after aqueous work-up, was purified by ISCO flash chromatography (Teledyne Isco RediSep Flash Column 100 g, 5% to 60% ethyl acetate/hexanes) to afford (S)-2-[4-(3ethoxy-2,6-difluoro-phenoxy)-2-oxo-2,5-dihydro-pyrrol-1-yl]-4-methyl-pentanoic acid methyl ester (1.80 g, 28%) as an oil.
