To obtain the subject compound useful as an intermediate for various syntheses by reacting a specific aldehyde with a specific acid chloride in the presence of an alkali metal chloride.

This compound of formula III is efficiently obtained, while preventing side reactions, by reacting an aldehyde such as H(CF2)2CHO of formula I (Rf is a 1-20C fluoroalkyl group) with an acid chloride such as C2H2 COCl of formula II (R is a 1-20C alkyl, alkenyl, or fluoroalkyl) at 0.8-3.0 molar times based on the aldehyde in the presence of a 0.1-10wt.% alkali metal chloride such as cesium chloride or potassium chloride, preferably in a 2-20% polar solvent such as tetraglyme at -20 to 150°C, under normal pressure (if necessary under higher pressure).