Methyl (E)-4-Chloro-3-methoxy-2-butenoate: An Extremely Versatile Four Carbon Building Block

Methyl (E)-4-chloro-3-methoxy-2-butenoate (3a) is inexpensively prepared from methyl 4-chloroacetoacetate using thionyl chloride and methanol on an industrial scale. Many nucleophilic substitution reactions of chloride, which are impossible with the unprotected parent compound proceed readily with 3a.