A solution of (2R,3S)-2-((tert-butoxycarbonyl)amino)-3-((tert-butyldimethylsilyl)oxy)butanoic acid (615a) (14.52 g, 43.5 mmol) in THF (100 mL) was treated with 1,1′-carbonyldiimidazole (7.41 g, 45.7 mmol) and stirred at RT for 30 min and then at 60° C. for 2 h. The mixture was cooled to RT and then potassium ethyl malonate (Aldrich) (14.82 g, 87 mmol) and magnesium chloride (Aldrich) (8.29 g, 87 mmol) were added. The resulting suspension was stirred at 50° C. for 20 h. The suspension was concentrated and then extracted into EtOAc (2×100 mL) from saturated aq. brine (500 mL) containing 5 M aq. HCl (40 mL, 200 mmol). The combined organic extracts were washed with saturated aq. NaHCO3 (2×100 mL), dried (Na2SO4), and concentrated to give a pale yellow gum. The crude product was purified by flash chromatography on silica gel (160 g) eluting with DCM to give (4R,5S)-ethyl 4-((tert-butoxycarbonyl)amino)-5-((tert-butyldimethylsilyl)oxy)-3-oxohexanoate (165) (11.49 g, 28.5 mmol, 65% yield) as a colorless oil. LC-MS m/z 426.0, [M+Na]+.
