On the structure and spectroscopic properties of sparteine and its diastereoisomers

The equilibrium structures of the lupine alkaloid sparteine, its diastereoisomers (alpha-sparteine and beta-sparteine), and two related tricyclic diamines were investigated with the density functional theory ansatz B3LYP and HF-MP2 method. With the exception of beta-sparteine, these free molecules are structurally simple, since one conformer predominates both in the gas phase and in solution. The mono-protonated forms of all these molecules are characterized by intramolecular asymmetric hydrogen bonds. The di-protonated species have the same structural preferences as the parent free bases. All the theoretical structural predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron spectra were measured and assigned by means of ab initio outer valence Green's function calculations. The overwhelming predominance of the lowest-energy conformer of all compounds is confirmed by various spectroscopic results. However, the structure of beta-sparteine is very special. The C-13 NMR spectra in solution at room temperature are indicative of an effective C-2 molecular symmetry. From the analysis of the NMR spectra from +20 to -85degreesC it is inferred that the most stable C-1 conformer of beta-sparteine is almost exclusively present in solution. At room temperature, it rapidly interconverts between two enantiomeric forms to produce the NMR spectrum of a time-averaged C-2 conformer. (C) 2003 Elsevier B.V. All rights reserved.