Dual Role of <i>C</i>,<i>N</i>-Cyclic Azomethine Imine as a Dipole and an Oxidant in the Synthesis of Pyrazolidine-Fused Tetrahydroquinolines and C3-Functionalized Quinolines

A selective synthesis of pyrazolidine-fused tetrahydroquinoline and C3-functionalized quinoline from <i>N</i>-silyl enamine and <i>C</i>,<i>N</i>-cyclic azomethine imine is presented, using [3 + 2] cycloaddition with varying reagent ratios. In this study, the <i>C</i>,<i>N</i>-cyclic azomethine imine serves dual roles as both dipole and oxidant, with the latter being unprecedented. The dual role of <i>C</i>,<i>N</i>-cyclic azomethine imine was confirmed through stepwise control reactions and deuterium-labeling studies.