Diisopropylethylamine → N-[[1-[(2-Furanylcarbonyl)amino]cyclohexyl]carbonyl]-L-valine ethyl ester

30 ml of dimethylformamide was added to a flask in which 5 g (27 mmol) of L-valine ethyl ester hydrochloride and 5 g (22.8 mmol) of 2-(2-furanyl)-3-oxa-1-azaspiro[4.5]dec-1-en-4-one were placed. Then, 3.53 g (27.4 mmol) of diisopropylethylamine was added thereto, and the mixture was stirred for 3 days. After the solvent was distilled off under reduced pressure, ethyl acetate was added thereto, and the mixture was successively washed with a 10% aqueous potassium hydrogensulfate solution and saturated brine, followed by drying with sodium sulfate. After the solvent was distilled off under reduced pressure, it was purified by silica gel chromatography to obtain 8.3 g (quantitative) of the title compound.