CMR. Chemical shift

The CMR chemical shifts of the symmetrically substituted pavine alkaloid argemonine are as indicated below. The A -methyl signal at 640.6 is shielded... 

The CMR chemical shifts for cularine are indicated in the structure below. ... 

The following CMR chemical shift assignments have been made for emetine... 

Coupling of the more activated enamine 335, obtained in two steps from tabersonine, with 21-cyano-20-epipandoline (331) in the presence of AgBF4 in THF solution gave the dimer 334 in 20% yield. Its structure was based on spectral data, particularly CMR. Diagnostic were the chemical shifts for C-2 at 104.1 ppm (versus 103.4 ppm in ervafoline) and the upfield shift of C-6 at 35.3 ppm (ascribed to the -y effect). 

The data flux of a two-dimensional carbon-proton shift correlation is similiar to that described in Fig. 2.50(a) for a. /-resolved 2D CMR experiment, with one difference Instead of carbon-proton couplings JCH, proton chemical shifts (iH are stored in the evolution time tl. Fourier transformation in the (2 domain thus yields a series of NMR spectra with carbon-13 signals modulated by the attached proton Larmor frequencies. A second Fourier transformation in the domain generates the dH, Sc matrix of a two-dimensional carbon-proton correlation. 

As the range of chemical shifts is an order of magnitude larger for 1 C than for H, CMR is better suited for investigating stereoisomers. This has been demonstrated by 1- C NMR measurements on 2,4-dichloropentane and 2,3-dichlorobutane, which are models... 

Physical phenomena other than rates and equilibrium constants can be correlated by Hammett-type relationships. For example, as Figure 2.4 shows, in 13C nuclear magnetic resonance spectroscopy (called Cmr) the chemical shift of the cationic carbon in 17 is correlated by Brown s cr + values.21 And the C=0... 

Since the late 1950s PMR spectroscopy has contributed immensely to many areas of the chemistry of alkaloids (7). With the advent of Fourier transform spectrometers CMR has rapidly approached the level of PMR in its application to problems of structural elucidation and stereochemistry. In the case of the alkaloids many classes of the isoquinoline family have been studied. These alkaloids are of particular interest not only because of their widespread occurrence in nature but also because of their pharmacological activity (2-5). Wenkert et al. (6) were the first to review progress in this area. More recently, Shamma and Hindenlang (7) have made an extensive compilation of chemical shift data on amines and alkaloids that includes many... 

The chemical shifts reported here were obtained from spectra recorded using CDC13 as the solvent unless otherwise stated and are in ppm downfield from TMS. For those not familiar with CMR and the associated experimental procedures references (8-13) should be consulted. 

A close examination of the chemical shifts of the aliphatic carbon atoms of the diastereomeric pairs, 81 and 82, 83 and 84, and 85 and 86, has shown that the diastereomers may be differentiated by CMR spectroscopy. The change in stereochemistry at C-T between em/iro-/i-hydrastine (81) and f/ireo-a-hydrastine (82) caused deshielding in 82 at C-3, C-4, and the A-methyl group by +2.3, +2.5 and +0.2 ppm, respectively, whereas C-T was shielded by —0.9 ppm. Chemical shift differences of similar magnitude were observed for the pairs, 83 and 84, and 85 and 86 except that in these systems C-T underwent a slightly larger shielding. 

The development of Fourier transform NMR spectroscopy made carbon NMR (13C NMR or CMR) possible, and high-field superconducting spectrometers allowed it to become nearly as convenient as proton NMR ( H NMR). Carbon NMR determines the magnetic environments of the carbon atoms themselves. Carbonyl carbon atoms, alkyne carbon atoms, and aromatic carbon atoms all have characteristic chemical shifts in the 13C NMR spectrum. 

The remaining chemical shifts of gelsemine are deduced by comparison with simple models and, where ambiguities remain, by comparison with the CMR spectra of a dihydro and tetrahydro derivative taking advantage of the environmental dissimilarity of some of their carbon centers. Thus saturation of the vinyl group would be expected to affect... 

Confirmation of the structure of hazuntinine (49) came from the CMR spectrum, which showed close similarity with the spectrum of vincadifformine (2) except for C-14 and C-15, and C-10 and C-ll (20). The former two carbons at 51.8 and 57.0 ppm were clearly attached to oxygen from their chemical shift and are therefore the site of the epoxide linkage. The chemical shift differences of C-10 (+23.0ppm) and C-ll (+22 ppm) in comparison with vincadifformine (2) demonstrate the ortho relationship of the two aromatic methoxyl groups. 

There is little doubt that C-NMR ( CMR ) will become of great importance in the near future as instrumentation becomes more widely available. The structural implications are considerable the chemical shifts of cover over 200 ppm and fall into very characteristic ranges. Further, by using the off-resonance de-... 

The PMR spectra of several reduced proaporphines have been recorded both in deuteriochloroform and in pyridine-dg. PMR chemical shifts for 8,9-dihydroglaziovine in deuteriochloroform are shown below. The CMR spectra of fifteen reduced proaporphines have been recorded. 

The CMR spectral assignments shown below have been made for j8-hydra-stine, a-hydrastine, corlumine, and adlumine. Chemical shifts at C-3 and C-4 were found to be diagnostic of the relative stereochemistry at C-1 and C-9. ... 

The superscript italic letters a, c,... on the cmr numbers denote interchangeable chemical shifts. 

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