PURPOSE: To obtain the titled compound useful as an intermediate for synthesizing antiviral activator simply at low cost in high yield, by reacting a 5-formyluracil nucleoside with a specific phosphorane, hydrolyzing the reaction product.

CONSTITUTION: A 5-formyluracil nucleoside shown by the formula I (R1 is H, or OH) is reacted with a phosphorane shown by the formula (R2)3P=CXCOOR3 (R2 and R3 are alkyl, or aryl; X is H or halogen), to give a 5-alkoxy or aloxycarbonylvinyluracil nucleotide shown by the formula II. This compound is then hydrolyzed preferably with an alkali at 0°CW room temperature for serveral hours, to give the desired compound shown by the formula III. The compound shown by the formula III wherein group X is halogen is a novel compound.