Structural and spectroscopic characterization of oxo-sparteines

• Published in: Chemical physics Vol. 301; no. 1; pp. 33 - 43
• Main Authors: Galasso, V., Asaro, F., Berti, F., Habuš, I., Kovač, B., De Risi, C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 31.05.2004
• Subjects: Ab initio and DFT calculations; NMR chemical shifts and coupling constants; Photoelectron spectra; Structures; Photoelectron spectra; Ab initio and DFT calculations; Structures; NMR chemical shifts and coupling constants
• ISSN: 0301-0104
• DOI: 10.1016/j.chemphys.2004.03.003

The equilibrium structures of the 11 possible oxo-derivatives of sparteine were investigated with the density functional theory, using the B3LYP functional. The conformational preferences of the seven amino-ketonic sparteines are consonant with those of the parent free base, whose lowest energy conformer consists of a chair–chair quinolizidine A/B- trans system and a boat–chair quinolizidine C/D- trans system. A similar situation also occurs for lactams 2-oxo- (lupanine), 15-oxo-, and 17-oxo-sparteine, whose ketonized ring has a half-chair/sofa, half-chair, and sofa shape, respectively. Unlike the other isomers, the most stable conformer of lactam aphylline (10-oxo-sparteine) adopts the chair–sofa A/B- transoid, chair–chair C/D- cisoid disposition of the two quinolizidine moieties. These theoretical predictions are consistent with the available X-ray experimental results. The electronic structure of the oxo-sparteines was examined by measuring and calculating relevant parameters of their NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, calculated by means of DFT formalisms, correlate well with observation. Notably, the effective manifestation of stereoelectronic hyperconjugative effects on Δ δ(H eq/H ax) and Δ 1 J(CH eq/CH ax) of the lactams is correctly accounted for by the DFT results. The ab initio outer valence Green's function calculations yielded a consistent description of the main features in the photoelectron spectra, i.e., location, splitting, and sequence of the n(N) and n(O) ionization energies, which also reflect the competing through-bond and through-space interactions within the sparteine framework.