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Theoretical and experimental studies on the molecular and electronic structures of cytisine and unsaturated keto-sparteines

Density functional theory calculations with the B3LYP hybrid functional have been performed to determine the equilibrium structures of cytisine, N-methylcytisine, multiflorine, 5, 6-didehydromultiflorine, anagyrine, thermopsine, 5, 6-didehydrolupanine, and aphyllidine. These molecules, except cytisine, multiflorine, and aphyllidine, exhibit a marked preference for one conformation with the ring C chair. Cytisine, which also adopts the ring C chair, is predicted to exhibit the endo/exo conformational equilibrium of the N– H group in the gas phase or solution. Multiflorine is instead found to exist as a mixture of C-boat and C-chair conformers in the gas phase or solution. The most stable (C-chair) conformer of aphyllidine shows a marked conformational flexibility, with the exo and endo wings of ring A being nearly isoenergetic. The electronic structures of these quinolizidine alkaloids have been studied by measuring and calculating significant features 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, compares favourably with experiment. Notably, the repercussion of stereoelectronic hyperconjugative effects on Dd(Heq/Hax) and D1J(CHeq/CHax) of the iN– CO– groups is correctly accounted for by the DFT results. Based on ab initio outer valence Green’ s function calculations, a reliable interpretation of the uppermost bands in the photoelectron spectra has been advanced. The theoretical results indicate a complex interaction of the n(N), n(O), p(N), p(CO), and p(CC) chromophores. The lowest-energy conformations of all compounds allow a consistent interpretation of the photoelectron spectra. The conformational equilibrium in the gas phase is partially revealed by the photoelectron spectrum of multiflorine but not by that of cytisine. The single-crystal X-ray structure was also determined for anagyrine hydrochloride hydrate.

Ab initio and DFT calculations; Structures; NMR chemical shifts and coupling constants; Photoelectron spectra

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