Antioxidant properties of ethoxyquin and some of its oxidation products

Abstract

Ethoxyquin (1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline (I)), ethoxyquin nitroxide (II), 2,6-dihydro-2,2,4-trimetbyl-6-quinolone (III) and l,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline (IV) were prepared and purified in order to study their antioxidant properties in fish meal and/or oil. I (also commercally available) and III were equally effective in fish meal, but III and IV were better in oil than I and II. In AIBN initiated methyl linoleate autoxidation, the order of efficiency in terms of the stoichiometric factor (n), using BHT as a standard, was IV > I > BHT > II. On the other hand, III gave very little inhibition in this system. In fish meal and oil, I was converted to III and to a 1,8'-dimer (V). The same products were formed in tert-butoxy oxidation of I. The nitroxide (II) was also formed from I but this was only seen by ESR in various organic solvents. It is likely to play some role in the antioxidant mechanism of I. The 1,8'-dimer was isolated and found to have no antioxidant properties in oil. Another reaction product of 1,2,4-dimethyl-6-ethoxyquinoline was also prepared. It was found to show prooxidant effects in fish oil.