Unusually large 13C NMR chemical shift differences between neutral and protonated glycocyamidines.: New insights on previously reported chemical shift assignments and chemical properties

Unusually large C-13 chemical shift differences are observed between neutral and protonated glycocyamidine (2-aminoimidazolinone) derivatives. Upon-protonation, the guanidine (N-C=N) and carbonyl (C=O) carbons undergo an upfield shift of approximately 12 and 17 ppm, respectively; relative to the neutral species. For neutral glycocyamidine derivatives, the C-13 chemical shifts for the carbonyl carbon reside above 190 ppm. In the protonated form, the carbonyl resonances are in the mid to lower 170s. These values are indicative of the protonation state of the glycocyamidine moiety. The present study serves as a useful reference in assisting the characterization of future glycocyamidine-based natural products and derivatives.