Ethyl trifluoromethanesulphonate → (4R,5S)-[(S)-2-(2,4-Difluoro-benzyl)-butyryl]-4-methyl-5-phenyl-oxazolidin-2-one

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A flask was charged with 54 g (156 mmol) of (4R,5S)-3-[3-(2,4-difluoro-phenyl)-propionyl]-4-methyl-5-phenyl-oxazolidin-2-one and 300 mL of dry THF, and the solution was cooled to −78° C. To this solution was added 172 mL (172 mmol) of 1 M NaHMDS in THF, and the mixture was stirred cold for 1 hour. Then 30.6 g (172 mmol) of ethyl triflate was added dropwise to the enolate solution, and the reaction mixture was warmed to −40° C. to −30° C. and stirred for 1.5 hour. The reaction mixture was quenched by the addition of aqueous brine. The THF was removed under reduced pressure and the residue was partitioned between EtOAc and water. The organics were collected, and EtOAc was removed under reduced pressure. The resulting crude oil was purified by column chromatography (3:1 hexanes/EtOAc eluent) to get 29.5 g of an impure oil that was used without further purification.

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