ELECTRON-TRANSFER OXIDATION OF ENOL DERIVATIVES OF 2,3-DIHYDROBENZOPYRAN-4-ONES

Dihydrobenzopyrones 1a-e and their enol acetates 3a-c have been submitted to oxidation under single electron transfer (SET) conditions, using three alternative ways of activation: chemical oxidation with cerium(IV) ammonium nitrate (CAN), photochemical oxidation using triphenylpyrylium tetrafluoroborate (TPT) as sensitizer or electrochemical oxidation. The most significant products obtained are diketones 4, hydroxyketones 5, rearranged benzopyrones 6, enones 9 and, in the case of enol acetate 3c, 2-methylchromone (10) and 1,2-diphenylethane (13). These results are rationalized according to three major pathways from the radical cations: i) formation of the alpha-carbonyl radicals I (through deprotonation of the enols 2(+.) or cleavage of the carbonyl-oxygen bond of their acetates 3(+.)), eventually followed by secondary oxidation to the carbenium ions II, ii) breaking of the bond linking C-2 with one of the substituents and iii) ring opening.