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 (3Σg-) 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.