Gold‐Catalyzed Benzannulations

Abstract Gold catalysts possess not only sigma‐Lewis acidic property but also pi‐electrophilic (i.e., soft and carbophilic) nature which in turn activates alkynes for nucleophilic addition. By the use of this characteristic property of gold catalysts, construction of polyaromatic framework, which is commonly present in natural products or optoelectronic materials, is easily accessed by benzannulation strategy. For example, benzannulation of ortho‐alkynylbenzaldehydes with alkynes occurs in the presence of gold catalysts to produce naphthyl ketones. The reaction proceeds through a benzopyrylium intermediate, which is formed via pi‐activation of the ortho‐alkynyl group by gold catalysts followed by attack of oxygen of the carbonyl group to the electron deficient alkyne, and subsequent [4+2] cycloaddition between the resulting intermediate and external alkynes. Not only the intermolecular benzannulation but also intramolecular benzannulation takes place easily. This chapter describes almost all recent results on benzannulation using gold and related catalysts along with our findings on this topic. Together with fundamental methodologies of benzannulation using gold catalysts, application of those methods to the synthesis of natural products and heterocycles is mentioned