Dual role of C,N-cyclic azomethine imine as a dipole and an oxidant in the synthesis of pyrazolidine-fused tetrahydro-quinolines and C3-Funtionalized quinolines

초록

C,N-cyclic azomethine imines have been used as dipoles in various kinds of cycloaddition reactions It’s because they are easy to synthesize various derivatives which have biological activities. In general, C,N-cyclic azomethine imines react with electron-deficient dipolarophiles via normal-electron-demand (NED) pathways based on FMO theory. However, we explored a new reactivity mode by employing electron-rich, in-situ generated endocyclic -silyl enamine intermediates as dipolarophiles. Interestingly, the reaction pathway strongly depends on the stoichiometry. When the equivalent of N-silyl enamines was higher than the equivalent of C,N-cyclic azomethine imines, pyrazolidine derivatives could be synthesized through [3+2] cycloaddition. In contrast, when C,N-cyclic azomethine imines were used in excess, they played a dual role?not only as dipoles but also as internal oxidants. We confirmed that the N-silyl enamines and C,N-cyclic azomethine imines proceeded with [3+2] cycloaddition first, and then, the excess amount of C,N-cyclic azomethine imines worked as oxidants to rearomatize the cyclic intermediate to synthesize C3-functionalized quinolines. This dual role of azomethine imines, particularly their unprecedented function as oxidants, was supported by control experiments and deuterium labeling studies.