Radical strain-release photocatalysis for the synthesis of azetidines

The increasing popularity of four-member rings in drug discovery programs has vastly widen the research in synthetic chemistry, guiding the community to progress and reinvent old strategies for their crafting. Recently, the strain-release concept has been effectively used to build complex architectures. However, in contrast to the tens of described strategies accessing small carbocyclic derivatives, azetidine synthesis remains severely underdeveloped. Here, we report a mild visible-light-driven method to access densely functionalised azetidines by subjecting azabicyclo[1.1.0]butanes (ABBs) to radical strain-release (RSR) photocatalysis. We investigated the mechanism of this process using a combination of spectroscopic and optical techniques, and density functional theory (DFT) calculations. The chemistry is orchestrated by the activity of a novel organic photosensitiser, that governs the key energy-transfer process with diverse types of sulfonylimine precursors. The formed radical intermediates are intercepted by the ABB through an RSR process, leading access to azetidines in high chemical yields in a single operation. The power and generality of this photocatalytic method is demonstrated for various azetidine targets, including derivatives of Celecoxib and Naproxen.