Trans-N-quaternization

Different changes are observed when quaternization causes inversion of the quinolizidine ring from trans to cis, as is observed in the case of nupharidine (which is an N-oxide) (32) and deoxynupharidine methiodide (71) (41, 69). In such cases the tertiary carbon atoms in the 3 position (C-4, C-10) with respect to the new N+—O" or N+—C bonds exhibit a paramagnetic shift, and the secondary carbon atom (C-6) exhibits a diamagnetic shift in comparison with similar carbon atoms in the free base. 

The change of conformation of the N-substituted quinolizidine ring from trans to cis (as a result of quaternization of nitrogen) causes a change of stereochemistry of C-7 or C-7. In consequence, C-17 changes from a diaxial relationship in thiobinupharidine (16) to an axial-equatorial one or, in the case of cis-AB, cis-A B thiobinupharidine dimethiodide (41), to an equatorial-equatorial relationship. This causes a signal shift for C-17 downfield by 5 and 10 ppm, respectively (43). 

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