Structural and spectroscopic characterization of oxo-sparteines

The equilibrium structures of the 11 possible oxo-derivatives of sparteine were investigated by DFT, 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 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.