4,5-Dihydro-4,5-dihydroxyimidazoles as products of the reduction of 2-nitroimidazoles. HPLC assay and demonstration of equilibrium transfer of glyoxal to guanine

An HPLC method has been employed to study the electrochemical reduction (mercury cathode at −800 mV with respect to calomel electrode) of the 2-nitroimidazole benznidazole (N′-benzyl-(2′-nitro-1′-imidazoyl)acetamide). The principal product of this reduction is the cyclic guanidinium ion 3c (protonated N′-benzyl-(2′-amino-4′,5′-dihydro-4′,5′-dihydroxy-1-imidazoyl)acetamide), which forms in a linear fashion as the nitroimidazole is reduced and accounts for 75% of the product upon completion of the reduction. To perform the HPLC analysis quantitatively an authentic sample of this product (isolated as cis-trans isomers) was prepared as the sulfate salt through the reaction of N-benzyl-2-guanidinoacetamide sulfate with aqueous glyoxal. The two isomers of 3c arise through the nonreductive decomposition of the 2-hydroxylaminoimidazole, which is the product of a four-electron reduction of the nitroimidazole. Analysis of high field 1H NMR spectra also showed that the two isomers of 3c were the principal products following electrochemical reduction, neutral aqueous zinc reduction, and radiation chemical reduction. Previous investigations using NMR of the reductions of misonidazole (3-methoxy-1-(2′-nitro-1′-imidazoyl)-2-propanol) and 1-methyl-2-nitroimidazole have shown that the corresponding dihydroimidazoles 3a and 3b are the major products. The agreement of these various NMR and HPLC results suggests that the formation of dihydroimidazoles 3 is a general phenomenon for model reductions of 2-nitroimidazoles in neutral aqueous solution. Previous workers have shown that 2-nitroimidazole reduction mixtures, when treated with guanine derivatives, form the adduct 4 derived from the guanine and glyoxal. This work demonstrates that this adduct is also formed when authentic samples of 3a, 3b, and 3c are reacted with 2′-deoxyguanosine. A quantitative HPLC analysis, however, demonstrates that the reaction does not proceed to completion, and in fact the equilibrium for formation of 4 is unfavorable. This suggests that guanines are not useful derivatizing agents for the quantitative assay of "glyoxal-like" products formed in chemical or biological reductions of 2-nitroimidazoles. Keywords: nitroimidazole, reduction of nitroimidazoles, glyoxal from nitroimidazoles.