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

Abstract

Abstract2‐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 1H 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.