Research on 1-AZA two-ring systems. 21. Basicities, stereochemistry, and 1H and 13C NMR spectra of pyrrolizidine and its homologs in aqueous and aqueous acetonitrile solutions. Prediction of the pKa value of trans-fused pyrrolizidine

Cis-trans-conversion of the bicycle in pyrrolizidines: The effect of the preferred conformations on the properties of the bases and the thermodynamics of conformational transformation of the ring fusion type. (Review)

“…As known [34], the latter has the lowest basicity, and the energy of interaction between its molecules and the stationary phase will consequently be smaller than in the case of the isomers 18 and 19. According to data in [17,33], the differences in the dispersion and orientational interactions and also in the energy of formation of the hydrogen bond between the sorbate and sorbent molecules act in one direction during the chromatography of the investigated compounds 11, 18, and 19 on triethanolamine, leading to more rapid passage of the highly strained pyrrolizidine 11 through the column.…”

“…Identification of the signals for the protons at the α-C atoms in such spectra was difficult, but it was clear that their lower limit, where the signal of the H-8 proton in cis-fused pyrrolizidines is usually located, is shifted upfield [34]. This fact was interpreted as reflecting the displacement of the conformational equilibrium toward the trans-fused forms 10A and 11A [34]. The spectra recorded on instruments working at 250 MHz and above make it possible to interpret them more fully and to determine the position of the signal for the H-8 proton.…”

“…In aqueous solution the unstrained and weakly strained pyrrolizidines 1, 8, 9, 18, 19, and 22 have comparatively high basicity (pK a 10.88-11.94) [32,34,[44][45][46] and are apparently the strongest bases among saturated amines with tertiary nitrogen atoms [17,46]. This feature is due to the geometry of the cis-fused pyrrolizidines, which have the unshared electron pair of the nitrogen atom sterically accessible for hydration [7,17,45,46].…”

“…This feature is due to the geometry of the cis-fused pyrrolizidines, which have the unshared electron pair of the nitrogen atom sterically accessible for hydration [7,17,45,46]. As far as the basicity of the highly strained pyrrolizidines with preferred trans-fusion of the rings is concerned, it has only been measured for compounds 10 and 11 (in aqueous acetonitrile for the first and in water and aqueous acetonitrile for the second) [34]. In both solvents the latter is almost 1.5 times weaker as a base than its isomer 18 [34].…”

“…The 1 H NMR spectra of the highly strained pyrrolizidines 10 and 11, obtained on instruments working at 60 and 90 MHz, differ greatly in form from the spectra of the corresponding unstrained isomers 22 and 19, recorded on the same instruments under analogous conditions [28,29,34]. Identification of the signals for the protons at the α-C atoms in such spectra was difficult, but it was clear that their lower limit, where the signal of the H-8 proton in cis-fused pyrrolizidines is usually located, is shifted upfield [34].…”