1H NMR spectra of 1-alkyl-1,2,3,4-tetrahydro-2-oxo-3-azocine-carboxylic acid derivatives and their analogues

¹H NMR spectra of 1-alkyl-1,2,3,4-tetrahydro-2-oxo-3-azocinecarboxylates and -carbonitriles indicated that the geminal protons at the 1-α-position were located in noneqnivalent magnetic environments. The difference in the chemical shifts amounted to as much as 0.73 ppm in the case of methyl 1-ethyl-1,2,3,4-tetrahydro-2-oxo-3-azocinecarboxylate at 26°, and the free energy of activation for the coalescence was estimated to be larger than 23 $\text{kcal}\text{mol}$. The same type of nonequivalence and coalescence phenomena were also observed even with dimethyl protons at the 1-γ-position of 1-isobutyl-1,2,3,4-tetrahydro-2-oxo-3-azocinecarboxylate (ΔG^≠_c = 19 $\text{kcal}\text{mol}$). The situation was hardly affected by the reduction of the CC double bonds. The nonequivalence was not observed, however, if the substituent at the 3-position was absent. Therefore, these novel ¹H NMR spectra of 1-alkyl protons in the title compounds were concluded to be due to strong coupling between the restricted rotation around N(1)—C(α) bond and inversion of the 2-oxoazocine ring which required high energy of activation.