Constructing Four-Membered Heterocycles by Cycloisomerization

Four-membered heterocycles are highly sought after in modern drug discovery as they provide beneficial properties to the target molecules. Despite tremendous efforts by the synthetic research community, there is a need for a simple and new method to incorporate these motifs into the design molecules. Herein, we reveal a cycloisomerization strategy for the construction of oxetane and azetidine rings via metal hydride hydrogen atom transfer/radical polar crossover, which is challenging both enthalpically and entropically. This method is suitable for synthesizing polysubstituted four-membered heterocycles. This mild and functional-group tolerant reaction has a broad substrate scope, including the spiro structure, which is an important motif in drug discovery research. Various four-membered heterocyclic building blocks can be synthesized by product derivatization. We also discuss the reaction mechanism, focusing on the four-membered ring formation, by deuterium experiments and DFT studies.