2-Diethoxyphosphoryl-2-fluoroacetic acid was converted into the lithium α lithiocarboxylate dianion by treatment with n-butyllithium in hexane/tetrahydrofuran at −70°C. The Horner reaction between this new dianion and carbonyl compounds gave various 2-fluoro-2-alkenoic acids. Application of the method to the cis,trans caronaldehyde ethyl esters led to the 2-fluoroethenyl pyrethroid derivatives (Z,cis, E-cis, Z-trans,