One-Pot, Highly Regioselective 1,3-Dipole Cycloaddition Promoted by Montmorillonite for the Synthesis of Spiro[indole-pyrrolizine], Spiro[indole-indolizine], and Spiro[indole-pyrrolidine] gem-Bisphosphonates

Various spiro[indole-pyrrolizine], spiro[indole-indolizine], and spiro[indole-pyrrolidine] gem-bisphosphonates were prepared by multicomponent reactions between isatins, tetraethyl vinylidenebis(phosphonate), and amino acids in the presence of montmorillonite. The one-pot reactions proceeded by 1,3-dipole cycloadditions of azomethine ylides formed in a decarboxylative manner. The proposed mechanism is in line with experimental data that confirmed that the azomethine ylide is formed in a decarboxylative manner; this provides new insight into the underlying mechanisms of such cycloadditions. The method has many notable features, such as a broad substrate scope, high efficiency, and high regioselectivity.