A three neck flask of 1 l equipped with a dropping funnel and a stirrer was charged with 11.1 g of N-(2-hydroxyethyl)-α-phenylmaleimide and 500 ml of dioxane, and 7.70 ml of triethylamine was added while stirring at 0° C. A dioxane solution of 11.5 g of 3,5-dinitrobenzoyl chloride was dropwise added thereto at 0° C., and the mixture was stirred at a room temperature all the night through. After finishing the reaction, the reaction solution was added to 1 l of water to filter the resulting crystal. This was recrystallized in ethyl acetate twice to obtain 6.10 g of N-(2-(3, 5-dinitrobenzoyl) oxyethyl)-α-phenylmaleimide. This compound was subjected to reduction of a nitro group without refining furthermore. This compound had a melting point of 179.1 to 180.7° C.
