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

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