Photochemical Cyclization of a-Amino Esters to Access 3-Azetidinones

A light-driven cyclization of readily available a-amino esters to 3-azetidinones has been developed. This method leverages chromophore activation with the acyl imidazole to generate the triplet diradical species under mild conditions without the need for photosensitizers or transition metals. A selective hydrogen atom transfer event, followed by intramolecular Norrish-Yang radical coupling occurs to yield the N-heterocycle, with facile elimination of the imidazole group to access the 3-azetidinone. Computational calculations reveal the role of the protecting group in favoring the Norrish-Yang cyclization pathway.