Comparison of the Efficiencies of the Fused Heterocyclic Compounds, 9H-Xanthene-2,7-diols, and Related Chain-Breaking Phenolic Antioxidants

The antioxidant activities of 9H-xanthene-2,7-diols that are structurally related to tocopherols and 2,2′-alkylidenedi(hydroquinones) were measured by the inhibition of the thermally initiated autoxidation of tetralin at 60 °C using an oxygen-absorption method. The stoichiometric factors were calculated as n = 3.3—5.5 for 9H-xanthene-2,7-diols and 0.5—0.9 for 2,2′-alkylidenedi(hydroquinones) that lack the fused six-membered heterocyclic ring. Comparison of the alkyl substituent on the bridged methylene carbon showed that 9-methyl-9H-xanthene-2,7-diols increased in n value compared with 9-ethyl-9H-xanthene-2,7-diols. For the 9-methyl-9H-xanthene-2,7-diols, alkyl groups ortho to the phenolic hydroxyl group decreased the Rinh value: two methyl groups by 10-fold, one methyl group by 1.9-fold, and one t-butyl group by 1.4-fold, compared to that for the 9-methyl-9H-xanthene-2,7-diol. On the other hand, 2,2′-alkylidenedi(hydroquinones) were poorer antioxidants than the structurally comparable 9H-xanthene-2,7-dio...