Organocatalytic Modified Guareschi–Thorpe Type Regioselective Synthesis: A Unified Direct Access to 5,6,7,8-Tetrahydroquinolines and Other Alicyclic[b]-Fused Pyridines

An unprecedented organocatalytic, regioselective, modified Guareschi-Thorpe type protocol toward the modular synthesis of 5,6,7,8-tetrahydroquinolines 22a-g and other alicyclic[ b]-fused pyridines 23-28 via the identification of Chitosan as a heterogeneous catalyst is reported. This novel strategy is operationally simple and showed a wide range of functional group tolerance and substrate compatibility. The proposed mechanistic pathway involves an imine-enamine cascade approach for the synthesis of structurally diverse alicyclic[ b]-fused pyridine heterocycles. The gram scale synthesis and identification of a new class of antifungal molecules 29-31 emphasize the practicality of this method.