15N NMR studies of guanidines. II—the fused‐in guanidine unit of some oxoheterocycles: A combined 15N NMR, 13C NMR and IR study

Abstract 15N NMR spectroscopy was used to extend previous studies, by 13C NMR and IR spectroscopy, of fused oxoheterocycles which contain the guanidine unit. These compounds fall into three clearly definable structural types, through‐conjugated, ring‐conjugated or imide‐like, any or all of which may result from the cyclization reactions used for their synthesis. It is shown that 15N NMR and IR criteria are of equal value in distinguishing these, whereas 13C NMR is in general unhelpful. A qualitative correlation exists between v(CN) for the guanidine unit, and the summed 15N NMR chemical shifts for the two π‐donor nitrogen atoms. The present observations should help to establish spectroscopic criteria for structural elucidation.