INSIGHTS INTO THE RING-OPENING MECHANISM OF BENZENE METABOLISM; AN INVESTIGATION OF THE OXIDATION OF MODEL OXEPINS BY CYTOCHROME P450
Benzene is a known human carcinogen and environmental pollutant. The mechanism of its metabolism by cytochrome P450 is well understood. Initial oxidation of benzene produces benzene oxide/oxepin, valence tautomers in rapid equilibrium. Most metabolites of benzene are derived from benzene oxide and include phenol, catechols, quinones, and bioconjugates. A small number of ring-opened metabolites are derived from oxidation of oxepin. Muconaldehyde is one such metabolite; it is toxic and is known to react with glutathione and cross-link DNA. It is unclear how muconaldehyde is formed from oxepin. In the 1970’s Davies and Whitham postulated that oxepin is epoxidized by cytochrome P450 to form 2,8-dioxabicyclo[5.1.0]octa-3,5-diene (2,3-epoxyoxepin) that rapidly rearranges to form muconaldehyde. While Davies and Whitham were not able to observe the 2,3-epoxyoxepin intermediate, they used model compounds to generate ring opened products. Later, Greenberg et al demonstrated that dimethyldioxirane (DMDO) could epoxidize model oxepins at low temperature to form 2,3-epoxyoxepin intermediates; the intermediates were observed using low temperature NMR. Alternatively, Golding proposed that oxepin was oxidized by consecutive one electron oxidations with cerium ammonium nitrate (CAN) to form a radical cation intermediate that ring-opened to form corresponding dicarbonyl compounds. It is still unclear which mechanism is utilized by cytochrome P450 to form ring-opened metabolites. The present study involves the synthesis of model oxepins 2,7-dimethyloxepin and 4,5-benzoxepin that are used in synthetic and enzymatic oxidation studies. The incubations with 2.7-dimethyloxepin did not produce ring-opened metabolites, but rather dimethylphenols. This is likely an application of the Curtin-Hammett principle where the dimethyl-1,6-benzene oxide tautomer is energetically accessible and the activation barrier for formation of phenol derived products is lower than that to produce ring opened compounds. Oxidation of 4,5-benzoxepin with DMDO produced 1H-2-benzopyran-1-carboxaldehyde, while oxidation with CAN gave a novel dimeric compound. The enzymatic oxidation of 4,5-benzoxepin revealed that the major oxidation pathway utilized by P450 is epoxidation of oxepin to a 2,3-epoxyoxepin; the major product observed was 1H-2-benzopyran-1-carboxaldehyde. This is the first time a 2,3-epoxyoxepin derivative has been shown as an intermediate in enzymatic oxidation and gives insight into the ring opening mechanism of benzene metabolism. A small amount of the dimeric compound was also observed, indicating a minor oxidation pathway of consecutive one electron oxidations. Much work has been done to fully characterize and identify the novel dimeric compound.