C-13 NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY OF ATISINE AND VEATCHINE-TYPE C-20-DITERPENOID ALKALOIDS FROM ACONITUM AND GARRYA SPECIES

13C NMR spectra of the atisine- and veatchine-type alkaloids, as well as certain of their derivatives, have been obtained by the Fourier transform technique at 25.03 MHz. With the help of single-frequency off-resonance proton decoupling techniques, additivity relationships, and the effects induced by certain structural changes, self-consistent assignments of nearly all the resonances have been made. The 13C NMR spectra are also analyzed to identify skeletal features of the atisine and veatchine-type alkaloids of use in the structure determination of new C20-diterpenoid alkaloids. On the basis of the 13C NMR analysis of atisine and veatchine as well as a temperature-dependence study of atisine, the existence of C-20 epimers in these alkaloids is demonstrated. A 13C NMR study of the behavior of the oxazolidine ring of atisine in non-ionic and ionic solvents indicates that the C-20 epimers of atisine do not exist in an equilibrium mixture in solution and are not interconvertible via a zwitterion as reported earlier. © 1978.