Gold‐Catalyzed Synthesis of Ortho‐Quinone Methide Analogues as Reactive Synthetic Precursors

Abstract 2‐Exomethylene indolin‐3‐ones are useful synthetic intermediates with a unique indole scaffold. However, their synthesis has not yet been reported owing to their reactivity and instability. In this study, a gold(I)‐catalyzed method for generating 2‐exomethylene indolin‐3‐one derivatives via the 5‐ exo ‐ dig ‐selective cyclization of 2‐(2‐amino)phenyl alkynyl ketones with an alkyne terminus was developed. The subsequent pseudo‐dimerization of the generated 2‐exomethylene indolin‐3‐ones and their [4+2] cyclization reactions with 1,1‐disubstituted alkenes afforded spiropseudoindoxyl and pyranoindole derivatives, respectively, in 21% to quantitative yields. Density functional theory calculations of the gold‐catalyzed cyclization provided insights into the 5‐ exo ‐ dig selectivity. Furthermore, the reactive 2‐exomethylene indolin‐3‐one was isolated for the first time based on the results from 1 H NMR spectroscopic monitoring. The isolated product was applied to various transformations. Our synthesis method promotes the development of bioactive agents and functional materials with complex cyclic structures.