In a 11 three-necked flask equipped with a dropping funnel and a stirring means were placed 11.0 g of N-(2-hydroxyethyl)-a-phenylmaleimide and 500 ml of dioxane, and 7.70 ml of triethylamine was added at 0° C. under stirring. A dioxane solution of 11.5 g of 3,5-dinitrobenzoyl chloride was added dropwise thereto at 0° C. and stirring was continued at room temperature overnight. After completion of the reaction, 11 of water was added to the reaction solution and the resulting crystalline substance was collected by filtration. It was recrystallized twice from ethyl acetate to afford 6.10 g of N-(2-(3,5-dinitrobenzoyl)oxyethyl)-α-phenylmaleimide. This compound was subjected to reduction of a nitro group without any further purification. This compound had a melting point of 179.1-180.7° C.
