Efficient Synthesis of Polysubstituted Pyrroles Based on [3+2] Cycloaddition Strategy Utilizing [1,2]-Phospha-Brook Rearrangement under Brønsted Base Catalysis.

An efficient method for the synthesis of polysubstituted pyrroles was established based on the [3+2] cycloaddition strategy utilizing the [1,2]-phospha-Brook rearrangement under Brønsted base catalysis. The less-explored approach of the [3+2] cycloaddition, that is, the reaction of a C3 subunit with imines, was successfully achieved by making use of newly designed C3 subunits containing the requisite umpolung. The two-step formal [3+2] cycloaddition involves the catalytic generation of an α-oxygenated propargyl anion through the [1,2]-phospha-Brook rearrangement followed by γ-addition to the imine under Brønsted base catalysis and the subsequent intramolecular cyclization mediated by Au catalyst or a halogenation reagent to afford polysubstituted pyrroles having a variety of substituents in a positional selective manner. The pyrroles thus synthesized were amenable to further transformations, such as palladium-catalyzed cross-coupling reactions. The operationally very simple method with readily available substrates provides new access to a diverse array of well-organized polysubstituted pyrroles.