The increasing popularity of four-member rings in drug discovery has prompted the synthetic chemistry community to advance and reinvent old strategies to craft these structures. Recently, the strain-release concept has been used to build complex architectures. However, although there are many strategies for accessing small carbocyclic derivatives, the synthesis of azetidines remains underdeveloped. Here we report a photocatalytic radical strategy for accessing densely functionalized azetidines from azabicyclo[1.1.0]butanes. The protocol operates with an organic photosensitizer, which finely controls the key energy-transfer process with distinct types of sulfonyl imines. The radical intermediates are intercepted by the azabicyclo[1.1.0]butanes via a radical strain-release process, providing access to difunctionalized azetidines in a single step. This radical process is revealed by a combination of spectroscopic and optical techniques and density functional theory calculations. The power and generality of this method is illustrated with the synthesis of various azetidine targets, including derivatives of celecoxib and naproxen.

A perspective on the literature dealing with the organocatalytic asymmetric preparation of axially chiral N-heterocycles is provided. A particular focus is devoted to rationalize the synthetic strategies employed in each case. Moreover, specific classes of organocatalysts are shown to stand out as privileged motives for the stereoselective preparation of such synthetically challenging molecular architectures. Finally, an overview of the main trends in the field is given.1 Introduction2 Five-Membered Rings2.1 Arylation2.2 Dynamic Kinetic Resolution2.3 Ring Construction2.4 Central-to-Axial Chirality Conversion and Chirality Transfer2.5 Desymmetrization3 Six-Membered Rings3.1 Desymmetrization3.2 (Dynamic) Kinetic Resolution3.3 Ring Construction3.4 Central-to-Axial Chirality Conversion4 Conclusion