1,2‐Dioxetanes and α‐Peroxylactones

Visible Light-Induced and Singlet Oxygen-Mediated Photochemical Conversion of 4-Hydroxy-α-benzopyrones to 2-Hydroxy-3-oxo-2,3-dihydrobenzofuran-2-carboxamides/carboxylates Using Rose Bengal as a Photosensitizer

“…The in situ generation of singlet oxygen from its triplet state under the reaction conditions received experimental confirmation from the fact that no product was detected when two representative reactions (Table , 0% yields for 3a and 5a ) were performed in the presence of DABCO, which serves as an effective quencher for singlet oxygen . This 1 O 2 then undergoes a rapid [2 + 2]-cycloaddition across the enolic double bond of the tautomer 1′ of hydroxycoumarin ( 1 ) to form dioxetane intermediate 6 (non-isolable), which in turn generates a radical species 7 as results of homolytic cleavages of both the bonds “ a ” and “ b ” . In the next step, formation of hydroperoxide intermediate 8 (non-isolable) is anticipated due to participation of 7 in the radical recombination process, both intramolecularly (leading to the five-membered furan ring) and intermolecularly with the alkoxy/aminoalkyl and hydrogen radicals generated in situ from alcohols ( 2 )/amines ( 3 ) under the reaction conditions.…”