Magnetic Resonance in Chemistry

Friebel, Stefan; Harris, Robin K.; Kenwright, Alan M.

doi: 10.1002/(SICI)1097-458X(199705)35:5<290::AID-OMR85>3.0.CO;2-Zpmid: N/A

The magnetization of the mobile domain for a lamellar two‐region system following the application of the Goldman–Shen pulse sequence was simulated under various conditions in order to evaluate the effectiveness of several suggested techniques for obtaining spin diffusion data separate from the effects of spin–lattice relaxation. It is shown that in general none of the methods are completely effective, but the conditions of applicability are discussed and simulated examples showing deviations from ideality are illustrated. © 1997 by John Wiley & Sons, Ltd.

Bringmann, Gerhard; Koppler, Dagmar; Scheutzow, Dieter; Porzel, Andrea

doi: 10.1002/(SICI)1097-458X(199705)35:5<297::AID-OMR88>3.0.CO;2-Jpmid: N/A

One‐ and two‐dimensional NOE and ROE experiments were applied to investigate the relative configuration at the stereogenic biaryl axes of naphthylisoquinoline alkaloids, natural biaryl compounds mainly occurring as stable atropisomers. The influence of ortho substitution at the biaryl axes on the dynamic behavior of the atropo‐diastereomers and the determined NOE effects was investigated. As an example, the axial configuration of ancistrobrevine A was elucidated by the method. © 1997 by John Wiley & Sons, Ltd.

Shekar, S. C.; Hamilton, J. A.

doi: 10.1002/(SICI)1097-458X(199705)35:5<302::AID-OMR89>3.0.CO;2-Apmid: N/A

Interleaved sideband families that arise from distinct isotropic chemical shifts and the associated chemical shift anisotropy (CSA) tensors in magic angle spinning NMR are conventionally disentangled using two‐dimensional schemes. However, for the case of just a pair of distinct chemical sites, a scheme is suggested that involves a pair of one‐dimensional experiments whose sum and difference spectra separate the interleaved sideband families. This method was applied to phospholipid bilayers. The 31P NMR sidebands of phosphatidylcholine and dioleoylphosphatidylethanolamine are separated and CSA values for each phospholipid were calculated. © 1997 by John Wiley & Sons, Ltd.

Szántay, Jr, Csaba; Csepregi, Zsuzsanna; Aranyosi, Péter; Rusznák, István; Tőke, László; Víg, András

doi: 10.1002/(SICI)1097-458X(199705)35:5<306::AID-OMR93>3.0.CO;2-Npmid: N/A

High‐field 1H and 13C NMR methods (e.g. 1H{1H} and 13C{1H} NOE spectroscopy) were used to examine the tautomeric characteristics, particularly with regard to the question of azo–hydrazone tautomerism, of two azo dyes. Full 1H and 13C NMR signal assignments are given for both molecules. The NMR results indicate that in DMSO solution the tautomeric equilibria are shifted predominantly toward the hydrazone from II in both compounds. The presence of a fast II=III interconversion could also be plausibly assumed. © 1997 by John Wiley & Sons, Ltd.

Zhuo, Jin‐Cong

doi: 10.1002/(SICI)1097-458X(199705)35:5<311::AID-OMR94>3.0.CO;2-Mpmid: N/A

17O, 13C and 1H NMR spectra for para‐ and meta‐substituted 4‐arylaminopent‐3‐en‐2‐ones (acyclic enaminones, 1 and 2) and 3‐arylaminocyclohex‐2‐en‐1‐ones (cyclic enaminones, 3 and 4) are reported. The 17O, 13C and 1H shift values of these enaminones correlate well with σm0 and σp‐ constants in the correlations for meta and para derivatives, and with pKa values of the corresponding anilines. Dual substituent parameters analyses were also performed. Correlations of 17O and 13C chemical shifts of the carbonyl groups with those of the corresponding N‐acylanilines indicate that the enaminone moiety as a whole has electronic properties similar to those of the RCONH group. The 17O shift values of the carbonyl O atoms of enaminones correlate well with their 1H and 13C data. Shieldings of 33–45 ppm for O atoms are observed for 1 and 2 compared with 3 and 4, respectively. This is attributed to intramolecular hydrogen bonding. © 1997 by John Wiley & Sons, Ltd.

Ozawa, Tomonaga; Isoda, Yuichi; Watanabe, Hiroshi; Yuzuri, Tomoaki; Suezawa, Hiroko; Sakakibara, Kazuhisa; Hirota, Minoru

doi: 10.1002/(SICI)1097-458X(199705)35:5<323::AID-OMR98>3.0.CO;2-5pmid: N/A

Rotational barriers about the C—N bonds of several N‐acyl‐N‐methyl‐α‐amino acids and their esters R3CO—N(R4)—CR1R2—COOR5, and also N‐Boc‐protected dipeptides, were determined and the steric effects caused by the substituents R1–R5 are discussed by comparing them with the results of MM3 calculations on these amides. Bulky substituents on both the acyl group and the nitrogen atom were shown to have lower ΔG‡. In the series of N‐acylglycines with variable R3, the 15N chemical shifts were correlated with ΔG‡. © 1997 by John Wiley & Sons, Ltd.

Riddell, Frank G.; Rogerson, Martin

doi: 10.1002/(SICI)1097-458X(199705)35:5<333::AID-OMR83>3.0.CO;2-Ipmid: N/A

cis‐2‐tert‐Butyl‐5‐(tert‐butylsulphonyl)‐1,3‐dioxane displays a double tert‐butyl coalescence at low temperatures in its 13C CP/MAS NMR spectrum associated with slowing down of the rotation of both tert‐butyl groups. The overlap of the resonances precluded the measurement of rates of rotation from lineshape analyses. However, 13C T1ρ measurements above coalescence allowed rates of rotation and activation parameters for the rotations of both groups to be derived. The C‐tert‐butyl group has ΔH‡=26.3±3.6 kJ mol‐1 and ΔS‡‐13±14 J K‐1 mol‐1. The S‐tert‐butyl group has ΔH‡=31.8±2.2 kJ mol‐1 and ΔS‡‐9 J K‐1 mol‐1. © 1997 by John Wiley & Sons, Ltd.

Moloney, Gerard P.; Iskander, Magdy N.; Craik, David J.

doi: 10.1002/(SICI)1097-458X(199705)35:5<337::AID-OMR86>3.0.CO;2-5pmid: N/A

Condensation of (–)‐norephedrine with excess formaldehyde under mild conditions leads to formation of the 2:1 condensation product N,N′‐methylenebis(4‐methyl‐5‐phenyl)oxazolidine compared with the reaction with 1 mol of formaldehyde, which leads to 4‐methyl‐5‐phenyloxazolidine. 1H and 13C NMR spectroscopy was used to monitor the stability of this compound and its decomposition products. The 2:1 condensation product is found to be stable in CDC13 but breaks down rapidly in CD3OD to yield a 50:50 mixture of 4‐methyl‐5‐phenyloxazolidine and 3‐hydroxymethyl‐4‐methyl‐5‐phenyloxazolidine. Upon addition of D2O to this equimolar mixture, the latter compound decomposes to norephedrine and formaldehyde, whereas the former compound is stable. © 1997 by John Wiley & Sons, Ltd.

PROVERA, S.; MARCHIORO, C.; DAVAILLI, S.; CASE, N.

doi: 10.1002/(SICI)1097-458X(199705)35:5<342::AID-OMR81>3.0.CO;2-Mpmid: N/A

1H and 13C NMR spectral data for several pyrido[2,3‐b]pyrazines and pyrido[2,3‐b]pyrazine‐N‐oxides are reported. The chemical shift assignments were based on 1D‐NOE, gradient HSQC and gradient HMQC experiments for some model compounds. © 1997 by John Wiley & Sons, Ltd.