Radical strain-release photocatalysis for the synthesis of azetidines

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.