Stereoselective synthesis of (Z)-methyl 3-perfluoroalkyl-4-substitutedphenylbutenoates

Abstract

The stereoselective synthesis of (Z)-methyl 3-perfluoroalkyl-4-substituted-phenylbut-3-enoates 8a–o has been investigated. The reaction of substituted benzyltriphenylphosphonium bromides 5a–e with methyl 2-perfluoroalkylnoates 2a–c in the presence of K₂CO₃ at room temp. gives two adducts, methyl 3-perfluoroalkyl-4-substituted-phenyl-2-triphenylphosphoranylidenebut-3-enoates, 6a–o and methyl 3-perfluoroalkyl-4-substituted-phenyl-4-triphenylphosphoranylidenebut-2-enoates, 7a–o. Compounds 6a–o exists as pairs of E/Z isomers, the ratios are 2 : 1 for R_F= CF₃, and 3 : 1 for R_F= C₂F₅ or C₃F₇. Compounds 7a–o are transformed into 6a–o when heated. (Z)-Methyl 3-perfluoroalkyl-4-substituted-phenylbut-3-enoates 8a–o can be obtained stereoselectively in high yield when an aqueous methanol solution of mixed 6a–o and 7a–o was heated for 10–20 h.