The phosgene is reacted in a toluene solution of N,N-diethylpropionamide to prepare a Schiff base hydrochloride. This was made into a dry diethyl ether solution, cooled to 0 ° C, stirred, and a solution of lithium dicyclohexylamide in anhydrous ether was added dropwise. After standing, the supernatant was decanted, distilled under reduced pressure, and a fraction of 60-62 ° C (12 kPa) was collected to obtain the product in a yield of 58.