Solvent-free Povarov reaction for synthesizing ferrocenyl quinolines: antioxidant abilities deriving from ferrocene moiety.

Twenty-two 2-phenyl-4-ferrocenylquinolines are synthesized by Povarov three-component-reaction (3CR) among the substituted anilines, benzaldehydes, and ferrocenylacetylene with Ce(OTf)3 being catalyst in the absence of solvents. The antioxidative effects of the obtained quinolines are estimated by quenching 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) cationic radical (ABTS(+·)), 2,2'-diphenyl-1-picrylhydrazyl (DPPH), and galvinoxyl radicals, and by inhibiting Cu(2+)/glutathione (GSH)-, hydroxyl radical (·OH)-, and 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH)-induced oxidations of DNA. It is found that the ferrocenyl group instead of hydroxyl group generates the antioxidative effect for quinoline to quench radicals and to protect DNA against radical-induced oxidations. The antioxidative effect generated by ferrocenyl group can be further increased by the electron-donating moieties such as furan, -N(CH3)2, -OCH3, and ferrocenyl group, while the electron-withdrawing groups such as -NO2 and -Cl are not beneficial for quinolines to be antioxidants. The ferrocenyl group in quinoline exhibits higher antioxidant activity than hydroxyl group in Trolox.