Figure 1. 13C NMR peak assignments for 3a. 3 and 3', the former is preferred as the compound gives another exchangeable one-proton singlet at 6 5.35 characteristic of H-CY of an enamino ket0ne. I Moreover, the tetrahydropyrazine 3f should disproportionate to the corresponding pyrazine and piperazine. 8 The structure 3 is further corroborated by 13C NMR spectra (peak as-signmentg for 3a is shown in Figure 1). The most significant peak is ...
