作者:Roman Flyunt、Jacob A. Theruvathu、Achim Leitzke、Clemens von Sonntag
DOI:10.1039/b204067k
日期:2002.8.27
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 (pKa
= 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.
胸腺嘧啶和胸腺嘧啶脱氧核苷的臭氧化反应已通过产物研究和利用分光光度法、电导法和带有光学和电导检测的停流技术的动力学研究得到补充。得到了物料平衡。胸腺嘧啶(k= 3.4 × 104 dm3 mol−1 s−1)的臭氧化反应导致形成酸性(pKa= 4)氢过氧化物1-羟过氧亚甲基-3-(2-氧丙酰基)脲5(约34%)、中性氢过氧化物(可能主要是1-羟过氧羟甲基-3-(2-氧丙酰基)脲6,总计约41%)和H2O2(25%,相应地形成1-甲酰基-5-羟基-5-甲基乙内酰脲11)。有机氢过氧化物衰减(20 °C下约为1.1 × 10−3 s−1,3 °C下为1.3 × 10−4 s−1),释放甲酸(形成5-羟过氧-5-甲基乙内酰脲18),也在一定程度上释放H2O2(和11)。100分钟后,甲酸产率为75%。在碱性高pH条件下处理后,产率增加到100%。用二(2-羟乙基)硫醚(k= 50 dm3 mol−1 s−1)还原有机氢过氧化物,产生11,其后续在碱性条件下的处理以100%的产率得到5-羟基-5-甲基乙内酰脲13。提出的机理是,在C(5)-C(6)双键与臭氧反应形成的Criegee臭氧化物在两个方向上异裂开环,随后C(5)-C(6)键断裂。在优先的路径(75%)中,正电荷位于C(6)。在N(1)处去质子化产生5,而其与水的反应产生6。失去甲酸形成5-羟过氧-5-甲基乙内酰脲18。用硫醚还原5和6产生11。在次要路径(25%)中,正电荷保持在C(5),然后与水反应。产生的α-羟基氢过氧化物迅速失去H2O2(形成11)。在碱性溶液中,形成单线态双氧(8%)。同时产物5,6-二羟基-5,6-二氢胸腺嘧啶已被检测到。在胸腺嘧啶脱氧核苷的臭氧化反应中,快速形成的电导(k= 0.55 s−1)是由于释放乙酸(18%)。在这个反应中,一种短寿命的氢过氧化物被破坏。因此,臭氧化反应25秒后,总氢过氧化物产率仅为约78%(包括8% H2O2)。与乙酸对应的产物被认为是CO2和N-(2-脱氧-β-D-赤藓糖戊糖基)甲酰脲22。通过碘化物的柱后衍生化,通过HPLC检测到多种有机氢过氧化物。像胸腺嘧啶中的5这样的酸性氢过氧化物不在产物之列。用硫醚还原时,有机氢过氧化物主要(43-50%)产生的为N1-(2-脱氧-β-D-赤藓糖戊糖基)-5-羟基-5-甲基乙内酰脲23。讨论了胸腺嘧啶和胸腺嘧啶脱氧核苷臭氧化反应中一些显著差异的原因。