Formation of Substituted Benzo[a]heptalenes via Bergman Cyclization of Vicinal Di(ethynyl)-heptalenes

Abstract

By Hafner's synthesis, dimethyl heptalene-4,5-dicarboxylates are easily available from azulenes and dimethyl acetylene-dicarboxylate. Treatment with Takai reagent leads to 4-acetylheptalene-5-carboxylates, which by the procedure of Negishi et al. are further transformed into 4-ethynyl-heptalene-5-carboxylates. Reduction to heptalene-5-methanols, followed by Swern oxidation yields the corresponding heptalene-5-carbaldehydes. Treatment with trimethylsilyldiazomethane in the presence of butyllithium gives 4,5-di(ethynyl)-heptalenes, which on heating in chlorobenzene in the presence of cyclohexa-1,4-diene are transformed into benzo[a]heptalenes.