Light-Activated BF₃·Et₂O‑Promoted Generation of Singlet Oxygen and Cascade Reaction of Unsaturated Amides

Herein, BF₃·Et₂O-promoted O-insertion/spirocyclization/fluorination/ring-expansion of unsaturated amides to synthesis of spiro[benzo[b]-[1,4]dioxepine-3,5′-oxazole] skeletons in the presence of natural light and O₂ (³Σ_g^–) was reported. Air was the oxygen source of the ¹O₂-generation and O-insertion reaction under metal-free and mild conditions. BF₃·Et₂O played multiple roles, such as Lewis acid, activating reagent, and fluorine source in the reported cascade. A mechanism involving ¹O₂ generation/activation of double bond/O-insertion/spirocyclization/fluorination/ring expansion was supposed.