Acremolin, a stable natural product with an antiaromatic 1H-azirine moiety? A structural reorientation

Recently, acremolin (4), a novel modified base, was isolated from a marine-derived fungus and claimed to possess a structure with a 1H-azirine moiety. It is shown now that the reported NMR data are not compatible with this antiaromatic heterocycle, which should be an extremely unstable compound. An isomeric, substituted N2,3-ethenoguanine is presented as a plausible alternative structure of acremolin that is consistent with all spectroscopic data. Thus, 1H-azirines keep their classification as very short-lived intermediates.