To a solution of CBZ-L-tert-leucine (20.53 g, 77.4 mmol, 1 equiv) in THF (150 mL) was added 1,1′-carbonyldiimidazole (13.81 g, 85.14 mmol, 1.1 equiv) at 23° C. The resulting solution was stirred at 23° C. for 1 h. In a separate flask, n-butyllithium (101.59 mL of a 1.6 M solution in hexanes, 162.54 mmol, 2.1 equiv) was added to a solution of diisopropylamine (22.78 mL, 162.54 mmol, 2.1 equiv) in THF (100 mL) at −78° C. The reaction mixture was stirred for 15 min at −78° C., warmed to 0° C. for 5 min, then cooled back to −78° C. A solution of tert-butyl acetate (21.9 mL, 162.54 mmol, 2.1 equiv) in THF (10 mL) was added via cannula, and the resulting mixture was stirred at −78° C. for 10 min. The above imidazole solution was then added dropwise to the lithium enolate at −78° C. The resulting mixture was stirred at −78° C. for 1 h, quenched with 1 M HCl (100 mL), and extracted with EtOAc (2×100 mL). The combined organic layers were washed with brine (150 mL), dried over Na2SO4, and concentrated. The residue was purified by flash chromatography on silica gel (10% EtOAc in hexanes) to afford 4-benzyloxycarbonylamino-5,5-dimethyl-3-oxo-hexanoic acid tert-butyl ester as a pale yellow oil (12.06 g, 44%): IR (cm−1) 1717, 1508, 1265, 739; 1H NMR (CDCl3) δ 1.09 (s, 9H), 1.44 (s, 9H), 3.50 (s, 2H), 4.29 (d, 1H, J=9.3), 5.03 (s, 2H), 5.35-5.42 (m, 1H), 7.34 (s, 5H).
