PURPOSE: To obtain the titled substance in high yield, which is a synthetic intermediate for an antimicrobial agent in high yield, by treating cystosine with gaseous fluorine in the presence of hydrogen fluoride or a liquid solvent containing the hydrogen fluoride, and treating the reaction intermediate in the solvent at a specific temperature.

CONSTITUTION: Cytosine is fluorinated with gaseous fluorine in the presence of hydrogen fluoride or a liquid solvent containing the hydrogen fluoride. The resultant reaction intermediate is then treated in the solvent at a higher temperature than the fluorinating temperature to give the titled compound. The hydrogen fluoride may be used alone but water, chloroform, hexafluoroacetone hydrate or acetic acid, in an amount not to disturb the reaction may be present as a co-solvent. The molar amount of the gaseous fluroine necessary for the reaction if 1.0W2.0 times that of the cystosine. Preferably, the reaction temperature is usually 0°C or below for the fluorinating reaction and 5W20°C for the reaction in the latter stage.