Photochemical Carbene-Transfer Reaction of Vinyldiazo Compounds: Synthesis of 1-Pyrrolines via Methylenation/[2+3] Annulation

Herein, we report a new class of vinyldiazo compounds called bromo-diazoenals that effectively suppress the inherent 2π-electrocyclization typically observed in vinyldiazo compounds under photochemical conditions. Upon irradiation with visible light, these Br-diazoenal compounds generate singlet enalcarbenes, which provide direct access to the carbene center, allowing methylene-transfer reactions with vinyl azides. The subsequent [2+3] annulation furnishes the bromo-enal-functionalized 1-pyrrolines bearing a quaternary stereocenter. Given that the 1-pyrroline scaffold is a privileged structural motif in various biologically active natural products, this methodology offers a valuable and sustainable approach to these important heterocycles.