A complete 1D and 2D NMR studies of variously substituted 3-azabicyclo[3.3.1]nonan-9-ones

Intramolecular through-space NMR spectroscopic effect of steric compression on 1H NMR spectroscopy

The intramolecular through-space NMR spectroscopic effect of steric compression is related to intramolecular through-space van der Waals repulsion. The electron cloud of a proton can be pushed away by the electron cloud of a nearby proton or functional group. As the electron population of the sterically compressed proton is decreased (therefore deshielded), the chemical shift sharply moves downfield, which may result in ambiguity for the proton signal assignment. Also, the conformation of the local area of the sterically compressed proton can be altered by the steric repulsion, therefore, the coupling constant/coupling pattern of a sterically compressed proton could be influenced. This review summarizes and presents the impacts on the chemical shift and coupling constant by the 1H NMR spectroscopic effect of steric compression extracted from the reported examples from the 1950s to 2021.

Detailed 1 H and 13 C NMR structural assignments of four sesquiterpene lactones isolated from Eremanthus elaeagnus (Asteraceae)

A complete ¹ H and ¹³ C NMR analysis for a group of four sesquiterpene lactones isolated from Eremanthus elaeagnus (Asteraceae) is described in this work. ¹ H NMR, ¹³ C {¹ H} NMR, gCOSY, gHMQC, and gHMBC experiments were performed to provide sufficient structural information to allow an unequivocal assignment. All hydrogen coupling constants were measured, clarifying all hydrogen signal multiplicities.

Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction

This paper reports the synthesis of a series of piperidones 1-8 by the Mannich reaction and analysis of their structures and conformations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-ones, compounds 1 and 2, adopt a chair-boat conformation, while those in 2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9-ones, compounds 3-8, adopt a chair-chair conformation because of stereoelectronic effects. These stereoelectronic effects were analyzed by the (1) J C-H coupling constants, which were measured in the (13)C satellites of the (1)H NMR spectra obtained with the hetero-dqf pulse sequence. In the solid state, these stereoelectronic effects were investigated by measurement of X-ray diffraction data, the molecular geometry (torsional bond angles and bond distances), and inter- and intramolecular interactions, and by natural bond orbital analysis, which was performed using density functional theory at the ωB97XD/6311++G(d,p) level. We found that one of the main factors influencing the conformational stability of 3-8 is the interaction between the lone-pair electrons of nitrogen and the antibonding sigma orbital of C(7)-Heq (nN→σ*C-H(7)eq), a type of hyperconjugative interaction.

Crystal structure of 2,4-bis-(2-chloro-phen-yl)-7-tert-pent-yl-3-aza-bicyclo[3.3.1]nonan-9-one

The title compound, C₂₅H₂₉Cl₂NO, which is a chloro analog of 2,4-bis­(2-bromo­phen­yl)-7-(tert-pent­yl)-3-aza­bicyclo­[3.3.1]nonan-9-one [Park, Ramkumar & Parthiban (2012). Acta Cryst. E68, o2946], exists in a twin-chair conformation with an equatorial orientation of the 2-chloro­phenyl groups. The tert-pentyl group on the cyclo­hexa­none adopts an exocyclic equatorial position and is disordered between two orientations in a ratio 0.520 (8):0.480 (8). The crystal packing shows no directional contacts beyond van der Waals contacts.

2,4-Bis(2-eth-oxy-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

The asymmetric unit of the title compound, C24H29NO3, contains two independent mol-ecules, which each exibit a twin-chair conformation with an equatorial orientation of the ortho-eth-oxy-phenyl groups but different dihedral angles [41.3 (1) and 24.1 (1)°] between the benzene rings. In the crystal, pairs of weak C-H⋯O hydrogen bonds link the two different independent mol-ecules into dimers.

2,4-Bis(2-bromo-phen-yl)-7-tert-pentyl-3-aza-bicyclo-[3.3.1]nonan-9-one

The title compound, C₂₅H₂₉Br₂NO, is a tert-pentyl analog of 2,4-bis­(2-bromo­phen­yl)-3-aza­bicyclo­[3.3.1]nonan-9-one [Par­thiban et al. (2008 ▶). Acta Cryst. E64, o2385]. Similar to its analog, the title compound exists in a twin-chair conformation with an equatorial orientation of the 2-bromo­phenyl groups. The benzene rings are inclined to each other at a dihedral angle of 29.6 (3)°. The tert-pentyl group on the cyclo­hexa­none ring also adopts an exocyclic equatorial disposition.

2,4-Bis(4-eth-oxy-phen-yl)-1-methyl-3-aza-bicyclo-[3.3.1]nonan-9-one

In the title compound, C₂₅H₃₀NO₃, a crystallographic mirror plane bis­ects the mol­ecule. Although it is a positional isomer of 2,4-bis(4-eth­oxy­phen­yl)-7-methyl-3-aza­bicyclo­[3.3.1]non­an-9-one [C₂₅H₃₁NO₃, M_r = 393.51; Park et al. (2012c ▶). Acta Cryst. E68, o779–780], its mol­ecular weight is 392.50 due to the 50:50 ratio of the methyl group at bridgehead C atoms. However, the title compound exists in the same twin-chair conformation as its 7-methyl isomer. Also, the 4-eth­oxy­phenyl groups are equatorially oriented on the bicycle as in its isomer. In the title compound, the cyclo­hexanone ring deviates from an ideal chair (total puckering amplitude Q_T = 0.5390 Å) and the piperidone ring is closer to an ideal chair (Q_T = 0.6064 Å). These Q_T values are very similar to those of its isomer. Even though a center of symmetry passes through the 7-methyl analog, the benzene rings are oriented 26.11 (3)° with respect to each other, whereas the orientation is 53.10 (3)° for the title compound. The title compound exhibits inter­molecular N—H⋯O inter­actions [H⋯A = 2.25 (2) Å, versus 2.26 (2) Å for the analog].

2,4-Bis(4-eth-oxy-phen-yl)-3-aza-bicyclo-[3.3.1]nonan-9-one

The title compound, C(24)H(29)NO(3), exists in a twin-chair conformation with an equatorial orientation of the 4-eth-oxy-phenyl groups. The benzene rings are inclined to each other at an angle of 28.0 (1)°. In the crystal, weak C-H⋯O inter-actions link mol-ecules related by translation into chains along the b axis. The crystal packing exhibits π-π inter-actions between the benzene rings of neighbouring mol-ecules [centroid-centroid distance = 3.692 (3) Å].