Structural studies on 6‐methyl‐9‐carbamoyl‐tetrahydro‐4 H ‐pyrido[1,2‐ a ]pyrimidin‐4‐ones by 1 H, 13 C and 15 N NMR spectroscopy

Several 6‐methyl‐9‐carbamoyltetrahydro‐4 H ‐pyrido[1,2‐α]pyrimidin‐4‐ones have been prepared using phosgene iminium chloride. These compounds can exist in equilibrium as the cis (3A) imine ⇌ (3B) enamine ⇌ trans (3C) imine. 1 H, 13 C and 15 N NMR prove that the cis ‐ and trans ‐imine isomers are predominant in the equilibrium. 1 H NMR data reveal that the share of the 3B enamine form is negligible at measurable concentrations. The isomeric ratio 3A:3C is time dependent and can be monitored by measuring the CH 3 C‐6 and (CH 3 ) 2 N signals. The 13 C NMR data show that doublets in the range 42–45 ppm for C‐9 are only compatible with the imine forms 3A and 3C. The SCS values of the CH 3 C‐6 and OCN(CH 3 ) 2 groups were calculated and used for identification of the cis and trans isomers. 15 N NMR data show that the N‐1 chemical shift of the imine is approximately − 140 ppm for compound 3, whereas that of a fixed enamine is around − 267.8. This provides additional support for the predominance of the imine tautomers in the equilibrium 3A ⇌ 3B ⇌ 3C. 15 N data allow the stereoisomers 3A and 3C to be distinguished.