Skeletal Rearrangement of Pyridine Derivatives via a Light-Induced Ring Expansion

We report an efficient synthetic route to access 1,2-diazepines from readily available pyridines through dearomative ring expansion. By initially using previously established methods to obtain the pyridinium ylide intermediate on multigram scale, the diazepine was obtained through a 6π electrocyclic ring opening upon irradiation with 370 nm light. 1,2-Diazepines are among the least common nitrogen heterocycles present in FDA-approved drugs, likely due to the lack of synthetic pathways that enable access to these scaffolds rather than a lack of biological significance. While pyridine expansions are generally multistep procedures, our work has shown promise for a one-pot route to this expansion product. Taken together, this work provides access to synthetically difficult drug cores, while also introducing synthetic handles for further derivatization.