Reaction Steps
To a solution of diisopropylamine (1.8 mL, 12.6 mmol) in 30 mL THF at −78° C. was added n-butyl lithium (1.6 M solution in hexanes, 7.6 mL, 12.1 mmol), and the mixture stirred 10 minutes at which time oxazolidinone 137 (3.2 g, 10.1 mmol) in 10 mL THF was added dropwise. The solution was stirred for 30 minutes, t-butyl bromoacetate (1.8 mL, 12.1 mmol) was added quickly dropwise at −50° C., and the mixture was allowed to warm slowly to 10° C. over 3 hours. The mixture was partitioned between Et2O and sat. NH4Cl (aq), the phases were separated, and the organic phase dried (MgSO4), and concentrated. The residue was purified by flash chromatography (16:1 to 8:1 hexanes:EtOAc) to provide 2.65 g (61%) of ester 138 as a colorless crystalline solid, mp=84–86° C. [α]D23+17.1 (c=1.00, CHCl3); 1H NMR (CDCl3): δ 7.34 (m, SH), 5.62 (d, J=7.3 Hz, 1H), 4.73 (quint, J=6.8 Hz, 1H), 4.29 (m, 1H), 2.67 (dd, J=9.8, 16.4 Hz, 1H), 2.40 (dd,J=5.1, 16.4 Hz, 1H), 1.69 (m, 1H), 1.38 (s, 9H), 1.28 (m, 7H), 1.08 (m, 1H), 0.88 (m, 9H); 13C NMR (CDCl3) δ 176.45, 171.22, 152.71, 133.64, 128.86, 125.86, 80.83, 78.87, 55.33, 40.02, 38.21, 37.59, 36.31, 30.86, 29.29, 28.22, 23.14, 20.41, 14.36, 14.26. Anal. Calcd for C25H37NO5: C, 69.58; H, 8.64; N, 3.25. Found: C, 69.37; H, 8.68; N, 3.05.
