Determination of the Configuration and Conformation of α-, β-, and Isotripiperideine Carbon-13 Nuclear Magnetic Resonance Spectroscopy

The constitution, configuration, and conformation of the three isomeric tripiperideines (α-, β-, and iso-) have been established by ¹³C NMR spectroscopy. α-Tripiperideine (1) exhibits a five-line spectrum at room temperature which changes to a 15-line spectrum at low temperatures. This is due to a slowing down of the equilibration between three asymmetric topomers of conformation B, which, at room temperature, average to apparent C₃ symmetry. The conformation F of β-tripiperideine (2) is established by comparison of the observed ¹³C chemical shifts with calculated ones using the approach of empirical increments. The same procedure enables one to prove the dominant configuration and conformation I of isotripiperideine (3). By comparison of the most stable conformations of 1,2, and 3 it was possible to estimate the energetic limits of the syn-axial lone pair interaction (generalized anomeric effect) between two nitrogen atoms.