Triethylamine was added drop-wise to a stirred solution of 5-(Cbz-amino)-4-oxopentanoic acid (4.00 g; 15.0 mmol) and the product of 2a (3.65 g; 15.0 mmol) in ethyl acetate (50 mL). The stirred mixture was refluxed for 2 days under argon. After cooling to ambient temperature, 0.5 M HCl solution (50 mL) was added and the mixture was shaken thoroughly. The aqueous layer was extracted with ethyl acetate (2×20 mL). The combined ethyl acetate solutions were washed with saturated NaHCO3 solution (2×15 mL) and saturated NaCl solution (1×15 mL), then dried (MgSO4). After filtration and evaporation an amber oil was obtained that was purified by flash chromatography on a 60×55 mm silica gel 60 column eluted with ethyl acetate-hexane (1:1) (1000 mL), collecting 8×100 mL fractions. Evaporation of fractions 3-5 resulted in 4.92 g (77%) product (oil).
