DOUBLY ALLYLIC HYDROPEROXIDE FORMED IN THE REACTION BETWEEN STEROL 5,7-DIENES AND SINGLET OXYGEN

Ergosterol and 7-dehydrocholesterol, common 5,7-conjugated diene stero ls, react with photochemically produced singlet oxygen very efficientl y to yield, in parallel pathways, the corresponding 5,8-endoperoxides and the 7 beta-hydroperoxy-5,8(9),22-trienol or -5,8(9)-dienol, respec tively. The hydroperoxides decompose in an acid-catalyzed reaction to generate hydrogen peroxide and the 5,7,9(11),22-tetraenol or 5,7,9(11) trienol, respectively, with 1:1 stochiometry. The molar ratio of endo peroxide to hydroperoxide was constant (16:5) with two different react ion solvents, two different photosensitizers, and at all time points b etween 5 min and 3 h from the start of irradiation. Ergosterol did not react with either hydrogen peroxide or superoxide ion under our react ion conditions. Inhibition studies with nitrogen, 2,5-dimethylfuran, b eta-carotene, and tert-butanol confirmed the involvement of singlet ox ygen in these reactions. The unstable hydroperoxide would be expected to have undesirable biological consequences if formed in vivo.