Oxidation of Acetylated Guanosine by 3,3-Disubstituted 1,2-Dioxetanes through Nucleophilic Attack on the Peroxide Bond: Model Studies on the Oxidative DNA Damage by Reactive Peroxides
摘要:
The reaction of the disubstituted 3-(methoxymethyl)-3-phenyl-1,2-dioxetane (1a) with the acetylated guanine nucleoside (2) in methanol affords 8-methoxyguanosine 5 as oxidation product, as well as guanine (6) and 1-methoxyribose 7 by deglycosylation (total yield ca. 30%). The dioxetane-derived reduction product constitutes the 1,2-diol 40, while the major dioxetane-derived product (85%) is omega-methoxyacetophenone (3a). A Grob-type fragmentation is made responsible for the exclusive formation of the dioxetane cleavage products in the reactions with the acetylated nucleosides 8-10 derived from adenine, cytosine, and thymine. Rather than redox chemistry, this guanosine oxidation, unprecedented for peroxides, is proposed to involve nucleophilic attack by the N-7 atom of the nucleosides on the peroxide bond of the dioxetane la electrophile to generate a zwitterionic intermediate. S(N)2 attack by methanol at the C-8 position of the guanine moiety in the zwitterionic intermediate leads to the 8-methoxyguanosine 5 and the diol 4a. Alternatively, heterolytic cleavage of the glycosidic bond affords the methoxylated ribose 7 (after methanol trapping) and the N-7-alkoxylated guanine. The latter, after protonation, subsequently undergoes Grob fragmentation into guanine (6) and the dioxetane decomposition products omega-methoxyacetophenone (3a) and formaldehyde. We propose that the present novel oxidation of guanosine is general for electrophilic peroxides and may constitute a prominent route of oxidative DNA damage. In contrast, the corresponding 3-(bromethyl)-3-phenyl- 1,2-dioxetane (1b) gave with the guanosine 2 an intractable, complex product mixture, for which presumably the bromo substituent is responsible on account of competitive alkylation chemistry. However, with the 2'-deoxythymidine 10, a novel acid-catalyzed ring-opening of the bromo-substituted dioxetane Ib to its beta-methoxy hydroperoxide 11b is observed, a reaction which does not take place for the methoxy-substituted dioxetane 1a. This unusual process for simple dioxetanes is rationalized in terms of stabilization of the intermediary benzylic cation by the adjacent beta-bromo substituent through neighboring group participation.
Reaction of 1,2-dioxetanes with heteroatom nucleophiles: adduct formation by nucleophilic attack at the peroxide bond
作者:Waldemar Adam、Markus Heil
DOI:10.1021/ja00040a017
日期:1992.7
The reactions of 3,3-disubstituted 1,2-dioxetanes 1 with numerous heteroatom nucleophiles, e.g., R 2 NH, R 3 N, RSH R 2 S, CN - , SCN - , Br - , Cl - , OH - , and O 2 .- , were investigated. Initial nucleophilic substitution at the sterically less hindered site of the dioxetane peroxide bond leads to addition, deoxygenation, and fragementation products. The observed S N 2 chemistry was substantiated
3,3-二取代的 1,2-二氧杂环丁烷 1 与许多杂原子亲核试剂的反应,例如 R 2 NH、R 3 N、RSH R 2 S、CN - 、SCN - 、Br - 、Cl - 、OH - 和O 2 .- ,进行了调查。在二氧杂环丁烷过氧化物键的空间位阻较小的位点处的初始亲核取代导致加成、脱氧和断裂产物。观察到的 SN 2 化学反应得到二氧杂环丁烷 1c 的证实,因为溴离子被邻近的醇盐离子位点置换得到环氧化物产物
1,3-Dioxolane Formation by Nucleophilic Attack of Diazoalkanes on the Peroxide Bond of 1,2-Dioxetanes
作者:Waldemar Adam、Alexander Treiber
DOI:10.1021/jo00083a027
日期:1994.2
The reaction of the 1,2-dioxetanes 1a-d with the diazoalkanes 2 alpha-eta was investigated. The two 3,3-disubstituted (3,3-dimethyl- and 3-(bromomethyl)-3-phenyl) dioxetanes (1a and 1b), trimethyldioxetane (1c), and tetramethyldioxetane (1d) gave with the various diazoalkanes 2 the corresponding 1,3-dioxolanes 3 (insertion products) and/or the dioxetane-derived ketones 4 (fragmentation). Nucleophilic attack by the negatively charged carbon pole of the diazoalkane on the sterically less hindered site of the dioxetane peroxide bond affords the 1,3-dioxolane 3 after cyclization with denitrogenation of the resulting O,N dipole. The O,C dipole, formed by the nucleophilic attack of the negatively charged nitrogen pole on the dioxetane, is proposed as precursor to the ketones 4 through Grob-type fragmentation with regeneration of the diazoalkane.