Abstract
1H–15N HMBC spectra of norditerpenoid alkaloids and their synthetic azabicyclic analogues were obtained to investigate the impacts of the through-space effect of steric compression, protonation, and formation of intramolecular hydrogen bonding on the 15N NMR spectroscopy of these natural products and their piperidine-containing analogues. A rare 15N NMR effect of steric compression is demonstrated in half-cage A/E-rings of norditerpenoid alkaloid free bases and their synthetic azabicyclic analogues, in which the distribution of the lone pair of electrons of the tertiary amine N-atom is sterically restricted by bridged cycloalkanes, e.g., cyclopentane, cyclohexane, and cycloheptane rings. This results in significant changes in the 15N chemical shift, typically by at least ∼10 ppm. The lone pair of electrons of the N-atom in the piperidine ring are sterically compressed whether the bridged cyclohexane ring adopts a chair or boat conformation. The 15N chemical shifts of 1α-OMe norditerpenoid alkaloid free bases significantly increase (ΔδN ≥ 15.6 ppm) on alkaloid protonation and thence the formation of an intramolecular hydrogen bond between N+-H and 1α-OMe. The intramolecular hydrogen bonds between the N-atom and 1α-OH of 1α-OH norditerpenoid alkaloid free bases, karacoline, condelphine, and neoline stabilize their A-rings, adopting an unusual twisted-boat conformation, and they also significantly increase δN of the tertiary amine N-atom.
Highlights
For nonisotopically enriched samples, the sensitivity of 15N NMR is limited by the low natural abundance of 15N nuclei (0.36%, I = 1/2) and its intrinsic low receptivity
A rare, possibly the only case that has demonstrated the through-space 1H NMR effect of steric compression caused by a secondary amine deshielding a proton that is close to the N
We are investigating the effects of such a steric compression in the synthetic azabicycles (5−12) and their related norditerpenoid alkaloids (13−21)9,10 and how such steric interactions impact the tertiary amine using 1H−15N HMBC spectroscopy to report on the environment of the Natom
Summary
Compared with the 1H signals in a sample, the signal-to-noise ratio will be of the order of 104 smaller. This essentially precludes direct measurement without recourse to extended acquisition times or extremely concentrated samples. These limitations can be overcome through judicious use of inverse-detected 15N NMR experiments, e.g., 1H−15N heteronuclear multiple-bond correlation (1H−15N HMBC) spectroscopy.. We are investigating the effects of such a steric compression in the synthetic azabicycles (5−12) and their related norditerpenoid alkaloids (13−21) and how such steric interactions impact the tertiary amine using 1H−15N HMBC spectroscopy to report on the environment of the Natom Atom in space has been reported for imino[14]annulene (4, Figure 1). We are investigating the effects of such a steric compression in the synthetic azabicycles (5−12) and their related norditerpenoid alkaloids (13−21) and how such steric interactions impact the tertiary amine using 1H−15N HMBC spectroscopy to report on the environment of the Natom
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