Toward aflatoxin B2: an unexpected additive effect in a Dِtz benzannulation reaction

The addition of tolane to an intermolecular Dötz reaction has a profound effect upon the product distribution of the reaction such that cyclopentannulation becomes a major pathway rather than benzannulation. In addition the incorporation of an electron-rich acetylene into the cyclopentannulated product proceeds in a contra-steric fashion, a set of results which has been rationalised in terms of the reversible formation of intermediate π-complexes at the branch point of the benzannulation sequence.