PURPOSE: To obtain the title substance being an intermediate for the synthesis of pentafluoroethylpyrimidine economically by using a polyfloroalkylcarbonylacetic acid lower alkyl ester as a starting material and mixing and reacting this with a formamidine salt in the presence of an alkali alcoholate.

CONSTITUTION: A polyfluoroalkylcarbonylacetic acid lower alkyl ester of formula I (R is trifluoromethyl or pentafluoroethyl; and R1 is a 1-4C lower alkyl) is used as a starting material, and this is mixed and reacted with a formamidine salt of formula II (X is an acetate or a carbonate) in the presence of an alkali alcoholate or in the presence of ammonium carbonate or an alkali metal carbonate in a solvent such as toluene or methanol to obtain a compound of formula III. It is suitable that the compound of formula I used is especially 4,4,5,5,5- pentafluoro-3-oxopentanoic acid ethyl ester or trifluoroacetoacetic acid ethyl ester.