Catalyst-free formal [3 + 2] cycloaddition of stabilized N,N-cyclic azomethine imines to 3-nitrobenzofurans and 3-nitro-4H-chromenes: access to heteroannulated pyrazolo[1,2-a]pyrazoles.

We have studied the [3 + 2]-cycloaddition of various N,N-cyclic azomethine imines to 3-nitrobenzofurans. This process is a rare example of their dearomatization. We have also extended this process to the related 3-nitro-4H-chromenes as dipolarophiles. Both reactions provide access to benzofuro- and chromeno-condensed pyrazolo[1,2-a]pyrazoles with 100% atom economy in a diastereoselective manner under mild eco-friendly conditions. Finally, on the basis of DFT calculations, the mechanistic insights into the mentioned [3 + 2]-cycloadditions and explanations of the experimentally determined limitations of the method are given. Hirshfeld atomic charge values of push-pull heterocycles were suggested as a criterion for a priori assessment of the possibility of their dipolar cycloaddition with N,N-cyclic azomethine imines.