Abstract
Monoalkylated acylguanidines are important functional groups in many biologically active compounds and additionally applied in coordination chemistry. Yet a straightforward assignment of the individual NH chemical shifts and the acylguanidine conformations is still missing. Therefore, in this study, NMR spectroscopic approaches for the chemical and especially the conformational assignment of protonated monoalkylated acylguanidines are presented. While NOESY and ^3^J~H, H~ scalar couplings cannot be applied successfully for the assignment of acylguanidines, ^4^J~H, H~ scalar couplings in ^1^H,^1^H COSY spectra allow for an unambiguous chemical shift and conformational assignment. It is shown that these ^4^J~H, H~ longโrange couplings between individual acylguanidinium NH resonances are observed solely across allโtrans (w) pathways. Already one cis orientation in the magnetisation transfer pathway leads to signal intensities below the actual detection limit and significantly lower than crossโpeaks from ^2^J~NH, NH~ couplings or chemical exchange. However, it should be noted that also in the case of conformational exchange being fast on the NMR time scale, averaged crossโpeaks from allโtrans ^4^J~H, H~ scalar couplings are detected, which may lead at first glance to an incomplete or even wrong conformational analysis. Copyright ยฉ 2010 John Wiley & Sons, Ltd.