The reactions of thymine and thymidine with ozone

Abstract

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 × 10⁴ dm³ mol⁻¹ s⁻¹) leads to the formation of the acidic (pK_a = 4) hydroperoxide 1-hydroperoxymethylene-3-(2-oxopropanoyl)urea 5 (∼34%), neutral hydroperoxides (possibly mainly 1-hydroperoxyhydroxymethyl-3-(2-oxopropanoyl)urea 6, total ∼41%) and H₂O₂ (25%, with corresponding formation of 1-formyl-5-hydroxy-5-methylhydantoin 11). The organic hydroperoxides decay (∼1.1 × 10⁻³ s⁻¹ at 20 °C, 1.3 × 10⁻⁴ s⁻¹ at 3 °C) 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 dm³ 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 α-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-β-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%) N₁-(2-deoxy-β-D-erythropentofuranosyl)-5-hydroxy-5-methylhydantoin 23. The reasons for some striking differences in the ozonolyses of thymine and thymidine are discussed.