Structural and spectroscopic characterization of oxo-sparteines

Abstract

Abstract 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 Δδ(Heq/Hax) and Δ1J(CHeq/CHax) 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.