Oxyphosphonium Enolate Equilibria in a (4+1)‐Cycloaddition Approach toward Quaternary C3‐Spirooxindole Assembly

An efficient and convergent (4+1)‐cycloaddition strategy toward the construction of spirooxindole benzofurans that involves the intermediacy of an isatin‐derived oxyphosphonium enolate is presented. Mechanistic investigations employing in situ NMR analysis of the reaction mixture revealed a correlation between phosphonium enolate structure and product distribution that was heavily influenced by the solvent and reaction temperature. An unexpected balancing act! The efficient phosphorus‐mediated one‐pot formal (4+1)‐cycloaddition between an in situ generated oxyphosphonium enolate and an ortho‐quinone methide provides spirooxindole benzofurans in appreciable yields and diastereoselectivity despite a delicate balance of intermediate equilibria.