Abstract The reaction between an aldehyde R-CHO and diethyl 2-oxo-3-fluorobutan-1, 4- dioate as its sodium salt EtO 2 C-CO-CF-CO 2 Et−, Na+ mainly leads in THF to (Z)-2- fluoroalkenoates R-CH CF-CO 2 Et (Z/E&>-80/20), the Z-stereoselectivity depending on the bulk of the R group.