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Palmer, Robert B.; Andro, Tina M.; Natale, Nicholas R.; Andersen, Niels H.
doi: 10.1002/(SICI)1097-458X(199607)34:7<495::AID-OMR899>3.0.CO;2-5pmid: N/A
A series of 14 4‐(3′,5′‐disubstituted isoxazolyl)‐1,4‐dihydropyridine calcium channel antagonists were examined using variable‐temperature proton nuclear magnetic resonance spectroscopy and nuclear Overhauser effect (NOE) experiments. Two of these compounds, the 1,4‐dihydro‐2,6‐dimethyl‐4‐[5′‐methyl‐3′‐(4″‐fluorophenyl)isoxazol‐4′‐yl]‐3,5‐pyridinedicarboxylic acid dimethyl ester (3a) and 1,4‐dihydro‐2,6‐dimethyl‐4‐[5′‐methyl‐3′‐(4″‐bromophenyl)isoxazol‐4′‐yl]‐3,5‐pyridinedicarboxylic acid dimethyl ester (5a), were synthesized to assist in the clarification of ambiguities discovered in the low‐temperature spectra of 1,4‐dihydro‐2,6‐dimethyl‐4‐(5′‐methyl‐3′‐phenylisoxazol‐4′yl)‐3,5‐pyridinedicarboxylic acid diethyl ester (2b). The solid‐state structure of 3a is also reported. The solution‐state rotameric preferences of the 14 compounds are reported and compared with those calculated at the AM1 level. C‐4—C‐4′ bond rotation barriers were also calculated at the AM1 level for nine of the systems examined. Several species failed to display temperature‐dependent signals associated with hindered rotation owing to highly biased rotameric equilibria at the temperatures required to freeze out the rotation. The seven experimental rotation barriers (ΔG≠ from ⩽26 to 40.4 kJ mol−1) reported are 1–6.8 kJ mol−1 higher than ΔH≠ calculated at the AM1 level.
Caldwell, Gary W.; Diane Gauthier, A.; Mills, John E.
doi: 10.1002/(SICI)1097-458X(199607)34:7<505::AID-OMR913>3.0.CO;2-Ypmid: N/A
The 1H (400 MHz) and 13C (100 MHz) NMR peak assignments of thebaine derivatives [free base (1), hydrochloride (2 and 3), methiodide (4) and N‐oxides (5 and 6)], codeine N‐oxide (7) and 14β‐hydroxycodeinone N‐oxide (8) were performed using various one‐ and two‐dimensional techniques. The configurations of the quaternary nitrogen‐substituted alkaloids were determined. The reaction of thebaine, codeine and 14β‐hydroxycodeinone with H2O2 produced thebaine N‐oxide with the major product (6; Rf 0.62) having an N‐CH3 axial configuration and both codeine N‐oxide (7; Rf 0.40) and 14β‐hydroxycodeinone N‐oxide (8; Rf 0.73) with the major product having an N‐CH3 equatorial configuration.
Li, Zheng; Li, Du; Owen, Noel L.; Len, Zongkang; Cao, Zhengzhong; Yi, Yijun
doi: 10.1002/(SICI)1097-458X(199607)34:7<512::AID-OMR912>3.0.CO;2-8pmid: N/A
The structure of a new chromone glycoside isolated from smilax glabra, named smiglanin [3,5,7‐trihydroxychromone 3‐O‐α‐L‐rhamnopyranoside] (C15H16O9), was determined using NMR techniques. The carbon skeleton of the molecule was established mainly by the 2D INADEQUATE method coupled with a computerized spectral analysis. Complete 1H and 13C resonance assignments were made based on the INADEQUATE and other 1D and 2D NMR techniques, including long‐range HETCOR and indirect detection HMBC. The vicinal proton–proton coupling constants, together with proton–proton distance constraints derived from the NOESY experiment, were incorporated as molecular modeling parameters to give the energetically most favorable conformation.
Katritzky, Alan R.; Ghiviriga, Ion; Leeming, Peter; Soti, Ferenc
doi: 10.1002/(SICI)1097-458X(199607)34:7<518::AID-OMR914>3.0.CO;2-Lpmid: N/A
Deuterium‐induced chemical shift differences on the 13C (DIS) are reported for a series of Schiff bases of salicylaldehydes, 1‐(phenyliminomethyl)naphthalen‐2‐ol and 4‐chloro‐1,7‐phenanthrolin‐10‐ol. Provided that corrections for the hydrogen donor atom and diamagnetic effects of the surrounding groups are made, plots of logarithms of two bond DIS vs. the chemical shift of the exchangeable proton are linear for compounds in which tautomeric equilibria are absent or strongly displaced in favor of one form. Molecules with comparably populated tautomers in fast exchange fall off this correlation. By this method, anils of salicylaldehydes were found to exist as hydroxy forms with localized hydrogen bonds while 1‐(phenyliminomethyl)naphthalen‐2‐ol and 4‐chloro‐1,7‐phenanthrolin‐10‐ol were found to be tautomeric mixtures dominated by the hydroxy forms. DIS also revealed that 4‐chlor‐1,7‐phenanthrolin‐10‐ol exists in solution as an aggregate of two molecules.
doi: 10.1002/(SICI)1097-458X(199607)34:7<527::AID-OMR915>3.0.CO;2-Kpmid: N/A
17O chemical shift substitution parameters obtained previously from the investigation of the conformationally rigid 1,6‐anhydro‐β‐D‐hexopyranoses were applied to five α,β‐D‐hexopyranoses: allose, glucose, mannose, gulose and galactose. The 17O chemical shifts predicted for the endocyclic ether oxygens, and also for the anomeric hydroxyl oxygens, were in good agreement with the experimental data. For the anomeric carbons and hydroxyl oxygens the 13C and 17O chemical shift differences between α‐ and β‐D‐hexopyranoses showed a linear relationship.
doi: 10.1002/(SICI)1097-458X(199607)34:7<532::AID-OMR910>3.0.CO;2-Cpmid: N/A
For many heteronuclei, inverse (1H detected) heteronuclear Overhauser effect spectroscopy (HOESY) should be superior in theory over heteronucleus detected HOESY. In practice, strong t1 noise artifacts completely cancel the benefits of inverse HOESY. However, pulsed field gradient (PFG)‐enhanced inverse HOESY yields 2D spectra without any t1 noise, consisting only of genuine cross peaks. The applicability of the method is demonstrated for thymolphthalein monophosphate hydrate using 31P,1H PFG‐HOESY and for n‐butyllithium using 7Li,1H PFG‐HOESY. In addition to the absence of t1 noise, the merit of PFG‐inverse HOESY is improved digital resolution in the usually crowded 1H domain.
Burlot, L.; Cherton, J.‐C.; Convert, O.; Larpent, C.; Dennetiere, B.; Dardoize, F.
doi: 10.1002/(SICI)1097-458X(199607)34:7<538::AID-OMR920>3.0.CO;2-Upmid: N/A
Under mild basic conditions, Chaetoglobosin M leads to the formation of two new bridged chaetoglobosins with preservation of the epoxide function. The structures of these new toxins are proposed on the basis of high‐resolution MS and NMR data and supported by molecular modelling.
doi: 10.1002/(SICI)1097-458X(199607)34:7<545::AID-OMR916>3.0.CO;2-Lpmid: N/A
A comparison of longitudinal and transverse frame magnetization transfer methods is provided using modified Bloch equations for a two‐spin system representative of a methylene group exchanging between two chemically distinct sites. This simple formalism incorporates chemical shift, auto‐ and cross‐relaxation terms, chemical exchange first‐order rate constants and r.f. field terms for ROESY experiments. Overhauser and exchange magnetization transfer is emulated at three limits of molecular reorientation time (τc). The calculated build up curves suggest a parasitic effect of ROE on chemical exchange magnetization transfer. In addition, for molecules harboring spin systems exchanging between sites such that the exchange rate approximates the transverse cross‐relaxation rate, and reorienting in solution at rates slower than that defined by the CAMELSPIN limit (ω0τc≈1.118), the calculations suggest observation of exchange or Overhauser cross peaks may be compromised. Finally, the effect of off‐resonance spin locking r.f. power level is explored upon exchange or Overhauser build‐up curve profiles. The results suggest off‐resonance approaches that at low r.f. spin locking power may adversely affect ROE or exchange build‐up rates and may compromise either exchange rate determination or internuclear distance estimation.
Stott, Katherine; Keeler, James
doi: 10.1002/(SICI)1097-458X(199607)34:7<554::AID-OMR919>3.0.CO;2-Epmid: N/A
A new experiment, HETGOESY, is described in which 13C–1H NOE enhancements are detected directly on 1H. The experiment uses pulsed field gradients for coherence selection, thus completely avoiding the problems associated with difference spectroscopy. It is shown, both theoretically and experimentally, that the new experiment has sensitivity advantages over the conventional 13C{1H} steady‐state NOE difference experiment. The new experiment also has the advantage that it uses selective excitation in the generally better resolved decoupled 13C spectrum rather than the 1H spectrum.
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