Gold- and copper-catalyzed [4+2] benzannulations between enynal or enynone units and 2π-systems

Gold-catalyzed formal [4+2] benzannulation between enynal or enynone units, including ortho-alkynyl(oxo)benzenes, and 2π-systems, such as alkynes and carbonyl compounds (enol forms), produces aromatic ketones in good to high yields. By simply changing the catalyst from gold to copper-Brønsted acid system, decarbonylated aromatic compounds can be prepared in the reactions using alkynes as the 2π-system. An efficient synthetic approach to angucyclinone antibiotics, (+)-ochromycinone and (+)-rubiginone B₂, is developed by applying the present benzannulation to an intramolecular version. The reaction proceeds most probably through the formation of the benzo[c]pyrylium ate complex via the intramolecular nucleophilic addition of the carbonyl oxygen atom to the alkyne part of enynal or enynone units, which is induced by carbophilic gold and copper catalysts.