Divergent, Enantioselective Synthesis of Pyrroles, 3H-Pyrroles and Bicyclic Imidazolines by Ag- or P-Catalyzed [3+2] Cycloaddition of Allenoates with Activated Isocyanides

The divergent, stereoselective formal [3+2] cycloadditions of allenoates with activated isocyanides catalyzed by silver or phosphine-based catalysts were investigated. Silver catalysis is capable of delivering a range of 3H-pyrroles in high stereoselectivities. These enantioenriched heterocycles can either undergo sequential cyclisation with isocyanoacetates to deliver unprecedented bicyclic imidazolines with excellent yields and stereoselectivity or undergo unusual aromatization pathways leading to polysubstituted pyrroles. On the other hand, a simple mix-and-go procedure using an amino acid-derived phosphine as the catalyst produces pyrroles bearing a benzylic stereocenter with good enantioselectivity.