N-15 NMR-STUDIES OF GUANIDINES .2. THE FUSED-IN GUANIDINE UNIT OF SOME OXOHETEROCYCLES - A COMBINED N-15 NMR, C-13 NMR AND IR STUDY

N-15 NMR spectroscopy was used to extend previous studies, by C-13 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 N-15 NMR and IR criteria are of equal value in distinguishing these, whereas C-13 NMR is in general unhelpful. A qualitative correlation exists between nu(C=N) for the guanidine unit, and the summed N-15 NMR chemical shifts for the two pi-donor nitrogen atoms. The present observations should help to establish spectroscopic criteria for structural elucidation.