Triethylamine → N,N'-Bis(1-methylethyl)urea

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Triethylamine (0.75 mL, 5.5 mmol) was added to a suspension of tetronic acid (500 mg, 5.00 mmol) in dichloromethane (25 mL) to obtain a homogeneous solution. The solution was cooled to 0° C., mixed with 4-N,N-dimethylaminopyridine (200 mg, 1.7 mmol), 3-cyclohexne-1-carboxylic acid (0.65 mL, 5.6 mmol), and then, diisopropylcarbodiimide (1.0 mL, 6.3 mmol), followed by stirring for 30 min. The resulting mixture was removed from the ice bath, stirred for 14 hours at room temperature, and filtered to remove thus-produced insoluble diisopropyl urea; the reaction solution was washed twice with 5% hydrochloric acid (20 mL) and the water layer was extracted twice with dichloromethane (10 mL). The organic layers were combined, dried over sodium sulfate anhydride, and concentrated; the thus-obtained residue was purified by silica gel column chromatography (silica gel, ethyl acetate:methanol=15:1). The resultant was arranged to be a sodium salt by adding an excess amount of sodium hydrogencarbonate and distilled water (100 mL), and then washed with dichloromethane; the water layer was acidified by adding 5% hydrochloric acid and extracted four times with dichloromethane (20 mL). The resultant was dried over sodium sulfate anhydride and concentrated to give pale yellow crystals (394 mg, 38%).

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