Hydroacridines XX [1]. Stereochemical factors influencing the C-13 NMR protonation shifts of six-membered saturated azaheterocyclic tertiary amines

The influence of stereochemical factors upon C-13 NMR shifts induced by N-p rotonation of saturated azaheterocyclic tertiary amines was investigated us ing (4a alpha,8a beta,9a beta, 10a alpha)-tetradecahydro-10-methylacridine, (4a alpha,8a beta,9a beta, 10a alpha)-10-ethyl-terradecahydroacridine, (4a alpha,8a alpha, 9a beta, 10a alpha)-tetradecahydro-10-methylacridine, and (4a alpha,8a alpha,9a beta, 10a alpha)-10-ethyl-tetradecahydroacridine as m odel compounds. The magnitudes of the protonation shifts depend on the foll owing stereochemical factors: (i) whether protonation occurs through an axi al or an equatorial nitrogen electron lone-pair, (ii) the relative steric o rientation (gamma-gauche or gamma-anti) of the nitrogen and carbon atoms in gamma position, and (iii) the geometry of the entire carbon skeleton. Simi lar stereochemical dependences were found for the protonation shifts on N-m ethyl-trans-decahydroquinoline, analyzed on the basis of chemical shift dat a from literature. The observed protonation shifts can be well rationalized in terms of the LEFS (linear electric field shift) theory.