Azetidines are four-membered nitrogen-containing heterocycles that hold great promise in current medicinal chemistry due to their desirable pharmacokinetic effects. However, a lack of efficient synthetic methods to access functionalized azetidines has hampered their incorporation into pharmaceutical lead structures. As a [2+2] cycloaddition reaction between imines and alkenes, the aza Paternò-Büchi reaction arguably represents the most direct approach to functionalized azetidines. Hampered by competing reaction paths accessible upon photochemical excitation of the substrates, the current synthetic utility of these transformations is greatly restricted. We herein report the development of a visible light-enabled aza Paternò-Büchi reaction that surmounts existing limitations and represents a mild solution for the direct formation of functionalized azetidines from imine and alkene containing precursors.

This review discusses the current scope and limitations of the [2 + 2] photocycloaddition reaction between an imine and an alkene component, the aza Paternò–Büchi reaction, and highlights recent improvements within this area of research.

<div> <div> <p>Herein, we describe the application of Lewis acid-catalyzed carbonyl-olefin metathesis towards the synthesis of chiral, substituted tetrahydropyridines from commercially available amino acids as chiral pool reagents. This strategy relies on FeCl₃ as an inexpensive and environmentally benign catalyst and enables access to a variety of substituted tetrahydropyridines under mild reaction conditions. The reaction proceeds with complete stereoretention and is viable for a variety of natural and unnatural amino acids to provide the corresponding tetrahydropyridines in up to 99% yield.</p> </div> </div> <br>