The reactions of thymine and thymidine with ozone

The ozonolysis of thymine and thymidine has been investigated by a product study complemented by kinetic studies using spectrophotometry, conductometry and stopped-flow with optical and conductometric detection. Material balance has been obtained. Ozonolysis of thymine (k = 3.4 x 10(4) dm(3) mol(-1) s(-1)) leads to the formation of the acidic (pK(a) = 4) hydroperoxide 1-hydroperoxymethylene-3-(2-oxopropanoyl)urea 5 (similar to34%), neutral hydroperoxides (possibly mainly 1-hydroperoxyhydroxymethyl-3-(2-oxopropanoyl)urea 6, total similar to41%) and H2O2 (25%, with corresponding formation of 1-formyl-5-hydroxy-5-methylhydantoin 11). The organic hydroperoxides decay (similar to1.1 x 10(-3) s(-1) at 20 degreesC, 1.3 x 10(-4) s(-1) at 3 degreesC) releasing formic acid (formation of 5-hydroperoxy-5-methylhydantoin 18) and also to some extent H2O2 (and 11). After 100 min, the formic acid yield is 75%. Upon treatment at high pH, it increases to 100%. Reduction of the organic hydroperoxides with bis(2-hydroxyethyl)sulfide (k = 50 dm(3) mol(-1) s(-1)) leads to 11 whose subsequent treatment with base yields 5-hydroxy-5-methylhydantoin 13 in 100% yield. It is suggested that the Criegee ozonide formed upon reaction with ozone at the C(5)-C(6) double bond opens heterolytically in two directions with subsequent opening of the C(5)-C( 6) bond. In the preferred route (75%), the positive charge resides at C(6). Deprotonation at N(1) gives rise to 5, while its reaction with water yields 6. Loss of formic acid yields 5-hydroperoxy-5- methylhydantoin 18. Reduction of 5 and 6 with the sulfide yields 11. In the minor route (25%), the positive charge remains at C(5) followed by a reaction with water. The resulting alpha-hydroxy hydroperoxide rapidly loses H2O2 (formation of 11). In basic solution, singlet dioxygen is formed (8%). The concomitant product, 5,6-dihydroxy-5,6-dihydrothymine has been detected. In the ozonolysis of thymidine, the rapid formation of conductance (k = 0.55 s(-1)) is due to the release of acetic acid (18%). In this reaction a short-lived hydroperoxide is destroyed. As a consequence of this, 25 s after ozonolysis the total hydroperoxide yield is only 78% (including 8% H2O2). The products corresponding to acetic acid are suggested to be CO2 and N-(2-deoxy-beta-D-erythropentofuranosyl)formylurea 22. A number of organic hydroperoxides have been detected by HPLC by post-column derivatisation with iodide. An acidic hydroperoxide such as 5 in the case of thymine is not among the products. Upon sulfide reduction, the organic hydroperoxides yield mainly (43-50%) N-1-(2-deoxy-beta-D-erythropentofuranosyl)-5-hydroxy-5-methylhydantoin 23. The reasons for some striking differences in the ozonolyses of thymine and thymidine are discussed.