Gold-catalyzed benzannulation

Abstract

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 catalyst.