Excursion to the World of Heptacyclic Compounds Made of Azulenes and Acetylenedicarboxylates

Azulenes and acetylenedicarboxylates react under acid catalysis (Bronsted or Lewis) and form (2aRS,8aSR)-2a,8a-dihydrocyclopenta[cd]azulene-1,2-dicarboxylates as intermediate products, which then dimerize by central bond-formation between C(2a1) and C(2′a1) and various peripheral C,C′-atoms of the dihydroazulene fragments, depending on the substituents present. The reactions are often accompanied by the formation of side-products, such as 2-(azulen-1-yl)fumarates and -maleates and others caused by H-shifts of the primary intermediates. H-Shifts between the two tetrahydrocyclopenta[cd]azulene parts of the heptacyclic structures were also found.