Gold- and Copper-Catalyzed [4+2] Benzannulations between Enynal or Enynone Units and 2(pi)-Systems

Gold-catalyzed formal [4+2] benzannulation between enynal or enynone units, including ortho-alkynyl(oxo)benzenes, and 2(pi)-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-Br?nsted acid system, decarbonylated aromatic compounds can be prepared in the reactions using alkynes as the 2(pi)-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. 1 Introduction 2 Gold-Catalyzed Reaction of Enynal or Enynone Units with Acetylenic Compounds 3 Copper-Catalyzed Reaction of Enynal Units with ­Acetylenic Compounds 4 Copper-Catalyzed Reaction of Enynal or Enynone Units with Alkenes 5 Gold-Catalyzed Reaction of Enynal Units with Carbonyl Compounds 6 Gold- and Copper-Catalyzed Intramolecular Benz­annulation and Cycloaddition 7 Gold-Catalyzed Intramolecular Benzannulation as a Synthetic Tool for Angucyclinone Antibiotics 8 Summary