A solution of N,N′-dicyclohexylcarbodiimide (DCC, 3.34 g; 16.2 mmol) in dry dichloromethane (20 mL) was added using a syringe to a stirred mixture of 5-(Boc-amino)-4-oxopentanoic acid (3.70 g; 16.0 mmol), t-butyl 3-hydroxypropanoate (2.31 g; 15.8 mmol), and 4-dimethylaminopyridine (DMAP, 50 mg) in dichloromethane (40 mL) at ambient temperature under argon. The mild exothermic reaction was moderated with an ice-water bath. After stirring 2 days at ambient temperature, the mixture was suction-filtered on a glass-sinter filter. The residue was washed with dichloromethane (3×20 mL). The combined filtrates were evaporated and the residue was purified by flash chromatography on a 45×55 mm silica gel 60 column eluted with ethyl acetate-hexane (1:1) (500 mL), collecting 7×50 ml fractions. After evaporation of fractions 2-4, 5.45 g (96%) product was obtained (pale yellow oil).
