ORGANIC STRUCTURE CHARACTERIZATION BY NATURAL-ABUNDANCE N-15 NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY - RAUWOLFIA ALKALOIDS AND MODEL COMPOUNDS

15N chemical shifts of yohimbine, reserpine, and several structurally related alkaloids and model compounds as well as those of several trifluoroacetate salts have been obtained at the natural-abundance level. Both the nature of the quinolizidine ring fusion at N-5 and substituents in a γ-gauche conformation markedly affect 15N resonance positions. The latter factor induces shieldings of magnitudes (7-12 ppm) comparable to those observed in 13C NMR. Where γ effects are absent, a cis-fused quinolizidine nitrogen is shielded by 13-15 ppm compared with a trans-fused one. Protonation deshields nitrogens in both series, but the displacement is larger for the cis-fused case and serves to characterize this geometry. Hyperconjugation between the nitrogen lone pair and adjacent antibonding C-H orbitals is tentatively proposed to rationalize the shift difference between the cis and trans cases. The structure of sparteine is confirmed as existing in the all-trans configuration. Nitrogen resonance positions are solvent sensitive in a predictable manner. © 1979, American Chemical Society. All rights reserved.