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In a 2 liter-flask were placed 100 g of diphosgene (trichloromethyl chloroformate), 1 liter of ethyl acetate and 109 g of p-aminophenol were added portionwise with stirring and cooling with ice-water. After completion of the addition, the mixture was warmed and refluxed for one hour. After cooling, 180 g of 50% aqueous dimethylamine solution were added dropwise while cooling with water and, after completion of the addition, stirring at room temperature was continued for 20 minutes. The resulting crystalline mass was filtered with suction, washed with water and thereafter dried to give 148 g of the end product as white crystal (yield: 82.1%). After recrystallization from a mixed solvent of acetone and methanol, the product showed of a melting point of 205°-6° C. The compound thus obtained was used in the following Preparation Examples 1 and 2.

1. additionwere added portionwise
2. additionAfter completion of the addition
3. temperaturethe mixture was warmed
4. temperaturerefluxed for one hour
5. temperatureAfter cooling
6. additionafter completion of the addition
7. stirringstirring at room temperature
8. filtrationThe resulting crystalline mass was filtered with suction
9. washwashed with water
10. customdried