An umpolung strategy for chemoselective metal-free [4 + 2] annulation of azlactones: access to tetrahydro-β-carbolin 1,3-diketone frameworks

Abstract

A series of tetrahydro-β-carbolin 1,3-diketone frameworks can be synthesized under mild conditions utilizing azlactones and indole-2-ylamides. This highly efficient [4 + 2] annulation further broadens the application of azlactones derived from amino acids via the umpolung strategy in organic synthesis, featuring good compatibility. Furthermore, control experiments indicate that the addition of the oxidant DDQ could complete the homocoupling of azlactones without a metal mediator. Moreover, DFT computational calculations were performed to further elucidate the chemoselectivity of the reaction and a plausible mechanism is proposed to explain the reaction process.