The amide protonation of (-)-N-benzoylcytisine in its perchlorate salts

Abstract

The (13)C NMR spectrum of (-)-N-benzoylcytisine perchlorate does not show a double set of signals typical of amide compounds, although this effect has been observed for the other diamine derivatives of cytisine. This observation means that in solution there must be the state of equilibrium between two forms of the cation with the protonated amide groups. DFT calculations have indeed indicated two preferred tautomeric forms with protonated oxygen atoms of amide groups. In the solid state however, according to X-ray analysis of perchlorate and perchlorate hydrate of N-benzoylcytisine the oxygen atom of the amide group in the six-membered ring A is preferred protonation site as compared with the oxygen in benzoic moiety. (-)-N-benzoylcytisine salt is the first compound from among the known derivatives of quinolizidine alkaloids that are not N-oxides, in which in solid state only the oxygen atom at cyclic amide is protonated instead of nitrogen atom or oxygen in benzoic moiety.

Keywords: Computational chemistry; Crystal structure; N-benzoylcytisine; NMR; Quinolizidine alkaloids.