Catalyst-Controlled Switchable (5 + 4)/(3 + 4) Cycloadditions for the Divergent Synthesis of Pyrazole-Fused Seven- and Nine-Membered Heterocycles

Developing diversity-oriented synthetic approaches for medium-sized rings is of great interest, and the divergent synthesis of medium-sized rings with different ring sizes poses a challenging task. In this study, we present a catalyst-controlled switchable (5 + 4)/(3 + 4) cycloaddition strategy for the divergent synthesis of seven- and nine-membered heterocycles. Utilizing two different 4-aminopyridine Lewis base catalysts, pyrazole-fused benzazepines or benzoxazonines can be efficiently assembled from the same substrates. These products exhibit good stability without interconversion under the reaction conditions or elevated temperatures. The origin of the catalyst-controlled chemoselectivity was elucidated by means of computational studies involving DFT calculations, distortion/interaction analysis, and noncovalent interaction analysis.