Flavone–Nitrogen Heterocycle Conjugate Formation by 1,3-Dipolar Cycloadditions



Azomethine ylides generated in situ from the reaction of N-[2-, 3- and 4-(chromon-2-yl)phenyl]glycine and paraformaldehyde can be trapped with dipolarophiles in 1,3-dipolar cycloaddition reactions to yield flavone–nitrogen heterocycle dyads. These azomethine ylides proved to be reactive with electron-poor dipolarophiles, affording the expected adducts. The use of microwave irradiation as an alternative source of heating has significant advantages and allows a reduction of both reaction time and temperature. The use of benzaldehydes as dipolarophiles afforded flavone–oxazolidine dyads, and the results indicate that electron-withdrawing groups in the para position of the benzaldehyde increases the dipolarophile reactivity. This work was also extended to the use of meso-tetrakis(pentafluorophenyl)porphyrin as a dipolarophile, allowing new flavone–dihydroporphyrin conjugates to be prepared.