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

An unprecedented organocatalytic, regioselective, modified Guareschi–Thorpe type protocol toward the modular synthesis of 5,6,7,8-tetrahydroquinolines <b>22a</b>–<b>g</b> and other alicyclic­[<i>b</i>]-fused pyridines <b>23</b>–<b>28</b> 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[<i>b</i>]-fused pyridine heterocycles. The gram scale synthesis and identification of a new class of antifungal molecules <b>29</b>–<b>31</b> emphasize the practicality of this method.