Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step

The reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° yields, beside the expected heptalene‐ and azulene‐1,2‐dicarboxylates, tetracyclic compounds of type ‘anti’ ‐ V and tricyclic compounds of type E ( cf. Schemes 2–4 and 8–11 ). The compounds of type ‘ anti ’‐ V represent Diels ‐ Alder adducts of the primary tricyclic intermediates A with ADM. In some cases, the tricyclic compounds of type E also underwent a consecutive Diels ‐ Alder reaction with ADM to yield the tetracyclic compounds of type ‘ anti ’‐ or ‘ syn ’‐ VI ( cf. Schemes 2 and 8–11 ). The tricyclic compounds of type E , namely 4 and 8 , reversibly rearrange via [1,5]‐C shifts to isomeric tricyclic structures ( cf . 18 and 19 , respectively, in Scheme 6 ) already at temperatures > 50°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di‐π‐methane reaction. The observed heptalene‐ and azulene‐1,2‐dicarboxylates as well as the tetracyclic compounds of type ‘ anti ’'‐ V are formed from the primary tricyclic intermediates A via rearrangement (→heptalenedicarboxylates), retro ‐ Diels ‐ Alder reaction (→ azulenedicarboxylates), and Diels ‐ Alder reaction with ADM. The different reaction channels of A are dependent on the substituents. However, the main reaction channel of A is its retro ‐ Diels ‐ Alder reaction to the starting materials (azulene and ADM). The highly reversible Diels ‐ Alder reaction of ADM to the five‐membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)‐controlled, in contrast to the at 200° irreversible ADM addition to the seven‐membered ring of the azulenes to yield the Diels ‐ Alder products of type E . This competing reaction must occur on grounds of orbital‐symmetry conservation under SHOMO(azulene)/LUMO(ADM) control ( cf. Schemes 20–22 ). Several X‐ray diffraction analyses of the products were performed ( cf. Chapt. 4.1 ).