Benzene is employed in large quantities in the chemical industry and is an ubiquitous contaminant in the environment. There is strong epidemiological evidence that benzene exposure induces hematopoietic malignancies, especially acute myeloid leukemia, in humans, but the chemical mechanisms remain obscure. E,E-Muconaldehyde is one of the products of metabolic oxidation of benzene. This paper explores the proposition that E,E-muconaldehyde is capable of forming Gua-Gua cross-links. If formed in DNA, the replication and repair of such cross-links might introduce structural defects that could be the origin of the carcinogenicity. We have investigated the reaction of E,E-muconaldehyde with dGuo and found that the reaction yields two pairs of interconverting diastereomers of a novel heptacyclic bis-adduct having a spiro ring system linking the two Gua residues. The structures of the four diastereomers have been established by NMR spectroscopy and their absolute configurations by comparison of CD spectra with those of model compounds having known configurations. The final two steps in the formation of the bis-nucleoside (5-ring → 6-ring → 7-ring) have significant reversibility, which is the basis for the observed epimerization. The 6-ring precursor was trapped from the equilibrating mixture by reduction with NaBH4. The anti relationship of the two Gua residues in the heptacyclic bis-adduct precludes it from being formed in B DNA, but the 6-ring precursor could readily be accommodated as an interchain or intrachain cross-link. It should be possible to form similar cross-links of dCyt, dAdo, the ε-amino group of lysine, the imidazole NH of histidine, and N termini of peptides with the dGuo-muconaldehyde monoadduct.

Vinyl chloride induces hepatic angiosarcomas, which are otherwise rare malignancies. The biochemical basis involves the formation of the epoxide, which reacts with DNA to give ∼98% of the 7-(2-oxoethyl) adduct (4) of dGuo plus small amounts of the etheno derivatives of dGuo, dCyd, and dAdo. The carcinogenicity is generally ascribed to the etheno adducts, not 4, because 4 has been shown to disappear from cells rapidly and to have negligible mutagenicity, which argues against its biological importance, whereas etheno adducts are both persistent and mutagenic. It has also been shown that apurinic sites derived from 4 are unlikely to be crucial lesions. A confounding factor with regard to the etheno hypothesis is that etheno adducts arise in unexposed cells by reactions of various lipid peroxidation products. The present study explores the possibility that a major contributor to the carcinogenicity of vinyl chloride may be formamidopyrimidine (FAPy) 12, N-[2-amino-6-[(2-deoxy-β-d-erythro-pentofuranosyl)amino]-3,4-dihydro-4-oxo-5-pyrimidinyl]-N-(2-oxoethyl)-formamide, which can arise by ring opening of 4, although its formation has not been observed until the present study. N7 adduct 4 undergoes deglycosylation to give 7-(2-oxoethyl)-Gua (13) in acid and imidazolium ring-opening to 12 in base. At pH 7.4, both processes occur with the formation of 12 representing ∼10% of the product mixture. FAPy 12 spontaneously cyclizes to 22, which upon mild acid treatment yields the deglycosylation product 2-amino-3,4,7,8-tetrahydro-7-hydroxy-4-oxopteridine-5(6H)-carbaldehyde (14). The structure of 14 has been established by NMR and mass spectroscopy and by independent synthesis. Reaction of the epoxide of crotonaldehyde with dGuo failed to give either 13 or 14, indicating that both compounds are unique products of the reactions of dGuo with the epoxides of vinyl monomers. Although FAPy 12 was found to be unstable, carbinolamine 22 arising from cyclization of 12 may be an important contributor to the carcinogenicity of vinyl chloride.