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

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 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.