Ring closing and photooxidation in nitrogen analogues of 3-hydroxyflavone

An epoxide intermediate in the Algar–Flynn–Oyamada (AFO) synthesis of flavones is reaffirmed through the use of quinolone analogues to 3-hydroxyflavone (3HF). Stepwise synthesis of analogues 3-hydroxy-2-phenyl-1,4-dihydro-4-quinolone 11 and 3-hydroxy-1-methyl-2-phenyl-1,4-dihydro-4-quinolone 12, via chalcone formation, epoxidation, ring closing and final oxidation, has been accomplished. The intermediary of an epoxide is further supported by blocking cyclization with methoxy substitution at the 2′-position (1A). Absorption/emission spectroscopy of 11 and 12 shows large red shifts, as seen in 3HF, indicative of an excited state intramolecular proton transfer mechanism. Nitrogen analogues demonstrate photooxidative stability similar to that of 3HF.