Bi-imidazole nucleosides obtained by ring opening of etheno and substituted etheno derivatives of adenosine

Abstract

Etheno- and substituted ethenoadenosines 1a–1d are found to be converted into bi-imidazole nucleosides 2a–2d in basic aqueous solutions. The formyl- and oxalo-substituted etheno derivatives of adenosine (1b and 1c) are more labile than the other compounds. This difference can be explained by the electron-withdrawing effects of the formyl and oxalo groups that create a more pronounced partial positive charge on C-5 in the ethenoadenosine derivatives 1b and 1c. The structural characterisation of the bi-imidazoles is mainly carried out by ¹H and ¹³C NMR spectroscopy. In 2b and 2c some of the carbon resonance signals are strongly broadened, most likely due to a prototropic effect in the outer imidazole ring. Addition of trifluoroacetic acid to the NMR samples suppresses the effect and sharp signals could be observed for all carbons in the compounds. The structural characterisation of the bi-imidazoles is largely dependent on multiple-bond proton–carbon coupling constants (^>1J_C,H), and therefore the constants have been assigned and are reported.