Under nitrogen atmosphere, (S)-4-benzyl-2-azolidinone (800 mg, 4.5 mmol) was dissolved in a dry reaction vial of anhydrous tetrahydrofuran (30 mL) and cooled to -78 °C in a dry ice/acetone bath. A hexane solution of 1.6 M n-butyllithium (5.0 mmol, 3.1 mL) was added slowly and dropwise over 3 min. After stirring for 30 min, propionyl chloride (5.0 mmol, 0.43 mL) was added dropwise over 3 min. The reaction solution was slowly warmed to room temperature over 14 h. The reaction was subsequently quenched by the addition of saturated ammonium chloride solution (10 mL) and water (30 mL). The aqueous layer was extracted with ethyl acetate (3 x 40 mL), the organic phases were combined and dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to afford (R)-(-)-benzyl-3-propionyl-2-oxazolidinone (oil, 1.0 g, 96% yield) with >95% purity by HPLC.1H NMR (500 MHz, CDCl3): δ7.33 (dd, J1=7.4 Hz, J2=7.6 Hz, 2H), 7.29 (d, J=6.9 Hz, 1H), 7.21 (d, J=7.4 Hz, 2H), 4.67 (m, 1H), 4.18 (m, 2H), 3.31 (dd, J1=13.3 Hz, J2=3.2 Hz, 1H), 2.96 (m, 2H), 2.77 (dd, J1=13.3 Hz, J2=3.7 Hz, 1H), 1.21 (t, J=7.4 Hz, 3H).
