Nuclear magnetic resonance label position

The direct attack of proton from the solvent on the intermediate dihydropyridine as well as the over-all mechanism of the reduction received support from the extent and position of deuterium labeling in the product from the reduction of l-methyl-4-phenyl-pyridinium iodide (7) with sodium borohydride in dimethylformamide and deuterium oxide. The l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (9) formed was shown by nuclear magnetic resonance (NMR) and mass spectral analysis to contain approximately one deuterium atom located at the 3-position.13,14 This is the result to be expected from the pathway shown in Eq. (3) if the electrophile were a deuteron. 

Godici, P. E., and Landsberger, F. R. (1975). Biochemistry 14, 3927. 1 3C Nuclear Magnetic Resonance Study of the Dynamic Structure of Lecithin-Cholesterol Membranes and the Position of Stearic Acid Spin-Labels. 

The elucidation of the biosynthetic pathway for the production of various metabolites has been extensively examined through the use of techniques that use isotopic labeling (stable isotopes and radioactive isotopes). Initially, radiolabeled precursors were introduced into plants and the resultant radioactive compounds were chemically degraded to identify the positions of the label. As the development of analytical instrumentation advanced, the isotopically labeled natural products were analyzed by mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy instead of chemical degradation. 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

The nuclear magnetic resonance spectrum gives very specific structural information that enables the position of labelled nuclei in a molecule to be defined. The technique has been especially important in studies of biosynthesis. Rapid... 

Figure 33 Biogenesis of some of the hexose-, pentose-, and cyclitol-derived components in ACAGAs from 6-IC I-D-giucose (according to Ref. 8). The labeling patterns in neomycin and vali-damycin measured by nuclear magnetic resonance (NMR) prove that all units are built up preferentially from C, (circled C atoms), C. or C, units (thick lines) rearranged by transketolase-and transaldolase-catalyzed reactions in passages through the pentosephosphate cycle. The paiiem in Cal products is hypothetical. The positions derived from the C6 of D-glucose are marked by a star.

In order to determine if a specific carbon was the critical one in the phototransposition reactions of these aromatic nitriles, 2,6-dideutero-4-methylbenzonitrile 6-d2 was examined. This substrate has all of the chemically distinct carbons labeled with a substituent so that their positions in the phototransposition can be monitored. As shown in Eq. (46.8), only the cyano-substituted carbon undergoes phototransposition, giving the specifically labeled products 7-dj and 8-d2 (as determined by C nuclear magnetic resonance [NMR] spectra of reaction mixtures). 

For the label of specificity, criterion (2), we must appeal to a more limited area of study, to spectroscopic and diffraction data. The most definitive data are, no doubt, those which indicate atom positions in the molecular aggregate. Thus, x-ray diffraction, neutron diffraction, and certain nuclear resonance studies of solids can provide more or less direct evidence that there are H atoms which occupy positions of close approach (hence bonding distance) to two other atoms. Electron diffraction spectra can yield the same information for gaseous species. More easily obtained, however, are IR and Raman spectra, which reveal specific involvement of H atoms by peculiarities in their vibrationeil degrees of freedom in the molecular aggregate. Finally, high resolution proton magnetic resonance studies provide a sensitive index of the electronic environment of the H atoms. 

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