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.