Synthesis of Seven-Membered Cross-Conjugated Cyclic Trienes by 8 pi Electrocyclic Reaction

A method for the synthesis of 3-methylene-1,4-cycloheptadiene derivatives via an 8 pi electrocyclization reaction was developed. The triene substrate bearing a phosphate or carbamate group was prepared from gamma,delta-unsaturated esters and alpha,beta-unsaturated aldehydes in four steps. Upon treatment with NaHMDS or KHMDS, the substrate formed a heptatrienyl anion, which underwent electrocyclization and subsequent beta-elimination of the leaving group. The product could be converted into a tropylium salt in two steps.