To a solution of (4-methoxy-3-methyl-phenyl)-acetic acid (5.04 g, 27.97 mmol) and triethylamine (3.9 mL, 27.97 mmol) in anhydrous tetrahydrofuran (100 mL) under nitrogen at −78° C. was added trimethylacetyl chloride (3.44 mL, 27.97 mmol). The reaction mixture was stirred at −78° C. for 10 minutes, 0° C. for 1 hour, then re-cooled to −78° C. At the same time, to a separate flask charged with a solution of (R)-(+)-4-benzyl-2-oxazolidinone (4.96 g, 27.97 mmol) in anhydrous tetrahydrofuran (100 mL) under nitrogen at −78° C. was added dropwise a solution of n-butyllithium (14 mL, 28 mmol, 2 M in hexanes). The second reaction mixture was stirred at −78° C. for 20 minutes, then transferred via cannula into the first reaction flask containing the mixed anhydride at −78° C. The reaction mixture was stirred at 0° C. for 1 hour, then warmed to room temperature and stirred for 18 hours. The mixture was quenched with saturated aqueous ammonium chloride solution (200 mL), concentrated to about half of its original volume under reduced pressure to remove tetrahydrofuran. The remaining mixture was extracted with ethyl acetate (2×250 mL). The organic layers were separated, combined, dried over sodium sulfate and concentrated under reduced pressure. The residue was purified by chromatography over silica gel eluted with 1:1 ethyl acetate/hexanes to give (R)-4-benzyl-3-[2-(4-methoxy-3-methyl-phenyl)-acetyl]-oxazolidin-2-one as a pale yellow oil (8.5 g, 89%).
