In a 250 mL round-bottomed flask, (S)-4-benzyl-3-(pent-4-enoyl)oxazolidin-2-one, Intermediate S8A (3.35 g, 12.92 mmol) was added to THF (40 mL) to give a colorless solution. The mixture was cooled in −78° C. bath for 10 min. Sodium bis(trimethylsilyl)amide (14.21 mL, 14.21 mmol) was added slowly to the reaction mixture. The reaction mixture turned a light orange color. The reaction mixture was stirred at −78° C. for 2 h. Then tert-butyl 2-bromoacetate (5.04 g, 25.8 mmol) was added slowly. The reaction mixture was stirred at −78° C. for 3 hr. The reaction was quenched with saturated NH4Cl. The mixture was diluted with brine and extracted with Et2O. The organic layer was dried over MgSO4, filtered and concentrated to give a crude material that was purified on silica gel column (120 g ISCO) eluting with a gradient from 0-50% EtOAc/hexane. [The product showed weak UV absorption at 254 nm]. Tubes with product were collected and concentrated to afford (R)-tert-butyl 3-((S)-4-benzyl-2-oxooxazolidine-3-carbonyl)hex-5-enoate, Intermediate S8B (3.2 g, 8.40 mmol, 65.0% yield) as a white solid. 1H NMR (400 MHz, chloroform-d) δ 7.40-7.21 (m, 5H), 5.89-5.66 (m, 1H), 5.15-5.03 (m, 2H), 4.73-4.61 (m, 1H), 4.34-4.23 (m, 1H), 4.16 (d, J=5.1 Hz, 2H), 3.35 (dd, J=13.4, 3.1 Hz, 1H), 2.88-2.67 (m, 2H), 2.52-2.38 (m, 2H), 2.28-2.13 (m, 1H), 1.44 (s, 9H).
