Substituent Effects on
            <sup>13</sup>
            C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry

Duddeck, H.

doi:10.1002/9780470147252.ch4

Cited by 64 publications

(5 citation statements)

References 518 publications

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“…The γ ‐gauche effect arises from steric clashes at the α ‐stereocenters that make up the VDR or VSeSR system and produces a shift in the 13 C resonance of both allylic carbons as a result of steric crowding and increased electron density at these sites for a cis double bond (Figure ) . This enhanced electron density, when compared with a trans olefin, results in an upfield shift in the 13 C‐NMR for the carbons adjacent to the site of unsaturation . This rule can be extended to VDRs and VSeSRs because the amide bond has partial carbon–nitrogen double bond character .…”

Section: Resultsmentioning

confidence: 99%

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

2014

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Vicinal disulfide rings (VDRs) occur when a disulfide bond forms between adjacent cysteine residues in a protein and results in a rare eight-membered ring structure. This eight-membered ring has been found to exist in four major conformations in solution, divided between cis and trans conformers. Some selenoenzymes use a special type of VDR in which selenium replaces sulfur, generating a vicinal selenosulfide ring (VSeSR). Here we provide evidence that this substitution reduces ring strain, resulting in a strong preference for the trans conformation relative to cis in a VSeSR (cis:trans – 9:91). This was determined by using the “γ-gauche effect” which makes use of both 1H-NMR and two-dimensional (2D) NMR techniques for determining the amide bond conformeric ratio. The presence of selenium in a VSeSR also lowers the dihedral strain energy (DSE) of the selenosulfide bond relative to the disulfide bond of VDRs. While cis amide geometry decreases strain on the amide bond, it increases strain on the scissile disulfide bond of the VDR found in thioredoxin reductase from D. melanogaster (DmTR). We hypothesize that the cis conformation of the VDR is the catalytically competent conformer for thiol/disulfide exchange. This hypothesis was investigated by computing the DSE of VDR and VSeSR conformers, the structure of which was determined by 2D NMR spectroscopy and energy minimization. The computed values of the VDR from DmTR are 16.5 kJ/mol DSE and 14.3 kJ/mol for the C+ and T− conformers, respectively, supporting the hypothesis that the enzyme uses the C+ conformer for thiol/disulfide exchange.

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Section: Resultsmentioning

confidence: 99%

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

2014

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“…High-field shifts of 13 C NMR signals by atoms in the γ-position are extremely valuable and diagnostically the most useful of all substituent effects in 13 C NMR spectroscopy . In 13 C NMR spectra recorded for N -alkylketimines, Fraser et al observed very pronounced upfield shifts of −6 to −12 ppm for α-carbons syn to the N -alkyl groups in comparison with the same carbon when anti .…”

Section: Resultsmentioning

confidence: 99%

“…The values of Δ sa span a range of 8 ppm and show that the γ-effects of the groups at the imino nitrogens are deshielding in almost all cases provided that the configurations have been assigned correctly. However, if this condition is not met, the γ-effects would be normal and shielding as expected by analogy with imines, amidines, and other CN compounds. , In view of this disturbing situation and the recent revival of interest in syn − anti isomerizations of imines, we embarked on an NOE NMR study, which also allows unequivocal stereochemical assignment when only a single diastereomer is observable. We included the known iminoaziridines 1f , 1g , 4e , 4h , and 5 …”

Section: Resultsmentioning

confidence: 99%

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Synthesis, Configuration, and¹⁵N NMR Spectra of Iminoaziridines. Synthons Equivalent to Three Components of the Ugi Reaction

et al. 2007

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Monocyclic iminoaziridines and exo-endo diastereomers of spirocyclic iminoaziridines that are derived from norbornane are prepared in batches of up to 10 g to foster applications as building blocks in syntheses. N,N'-Disubstituted alpha-haloamidines, which are readily available in two steps from N-substituted alpha-halocarboxamides, are 1,3-dehydrohalogenated by strong bases such as alkali-metal hydrides or tert-butoxides to afford distillable oils or low-melting solids, which consist of slowly interconverting E-Z diastereomers of the title compounds. The scope and limitations are outlined for this reaction. The configurations E and Z that were assigned on the basis of homoallylic 1H-1H coupling and asymmetric solvent-induced shifts required that, in 13C NMR spectra, the observed gamma-effects of substitutents at the imino nitrogens were deshielding, contrary to the well-known shielding gamma-effects in all other types of C=N compounds. However, an NOE NMR study demonstrated unequivocally that the previous assignments are correct and hence the observed gamma-effects actually deshielding. The ranges of 15N NMR chemical shifts span more than 60 ppm. Neither the beta- nor the gamma-effects of substituents on both types of nitrogen follow a uniform increment pattern.

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“…It turned out that 13 C signals of carbons in γ-gauche position to a functional group are shifted upfield with respect to the parent compound, in which the functional group is replaced with a hydrogen atom (Figure ). This γ-gauche effect has been investigated for different functional groups and this phenomenon is comprehensively described, though a general explanation has not been given. During our investigations on the influence of sulfoxide groups on structure, stability, reactivity, and spectroscopic properties of the respective compounds, − we realized that the γ-gauche effect in sulfoxides is not well understood and that a convincing and consistent explanation is still missing.…”

Section: Introductionmentioning

confidence: 99%

Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides

Jung

Podlech

2018

J. Phys. Chem. A

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Reasons for the C NMR γ-gauche effect in sulfoxides, i.e., the distinct shielding of a carbon β to a gauche-oriented sulfoxide group were investigated. Several calculated and measuredC NMR data of open chain or thiane-derived sulfoxides revealed that an upfield shift is only observed for that γ-gauche position, in which the respective carbon is anti to the sulfoxide's sulfur lone pair. Carbons in γ-gauche position, which are synclinal to the lone pair, are not affected. The magnetic anisotropy of the S═O group was examined by generation of iso-chemical-shielding surfaces (ICSSs) and magnetically induced current maps. Stereoelectronic interactions were determined with natural bond orbital (NBO) and natural chemical shielding (NCS) analyses. The γ-gauche effect is best described by stereoelectronic interactions, especially those of the sulfur's lone pair with antibonding orbitals to a β-carbon in antiperiplanar orientation. An explanation based on steric interactions, which has frequently been referred to, is not suitable to describe the observed shielding effects. Furthermore, a description of the bonding situation in the S═O group is given. It can be understood as S-O triple bond, where the bond order is significantly reduced by antibonding contributions in some of the occupied molecular orbitals.

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Substituent Effects on ¹³ C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry

Cited by 64 publications

References 518 publications

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

Synthesis, Configuration, and¹⁵N NMR Spectra of Iminoaziridines. Synthons Equivalent to Three Components of the Ugi Reaction

Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides

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Substituent Effects on 13 C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry

Cited by 64 publications

References 518 publications

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

Conformational analysis of oxidized peptide fragments of the C-terminal redox center in thioredoxin reductases by NMR spectroscopy

Synthesis, Configuration, and15N NMR Spectra of Iminoaziridines. Synthons Equivalent to Three Components of the Ugi Reaction

Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides

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Substituent Effects on ¹³ C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry

Synthesis, Configuration, and¹⁵N NMR Spectra of Iminoaziridines. Synthons Equivalent to Three Components of the Ugi Reaction