Chair to Boat Interconversion and Face to Face Interactions in Isomeric Aryl-Substituted Perhydrocyclopentaquinolizines

The structure and the conformation of the two isomeric 3,5-di(4-methoxyphenyl)perhydrocyclopenta[ij]quinolizines 1 and 2 have been determined by a combination of NOE experiments, analysis of vicinal J
coupling constants, and DFT computations. The two aryl rings were found to exhibit a face to face
disposition, and variable-temperature NMR spectra allowed the determination of the corresponding rotation
barriers, as well as chair to boat and nitrogen inversion processes of the quinolizine rings. The structure
and the conformation of the two corresponding ammonium salts 1-H+ and 2-H+ were also obtained in
solution by the same techniques:  in addition, their solid-state structures were determined by X-ray
diffraction.