Nuclear Overhauser enhancement positive

The difference between 2-CH2 and 6-CH2 is shown by the nuclear Overhauser enhancement (NOE) on the proton at Su = 6.67, if the methylene protons at 5 = 2.87 are irradiated. The assignment of the methylene C atoms can be read from the CH COSY segment. The C atoms which are in close proximity to one another at 5c = 113.3 and 113.8 belong to C-5 and C-7. Carbon atom C-5 is distinguished from C-7 by the pseudo-quartet splitting CJqh = 3.4 Hz to 1-H and A-H2) that involves the methylene group in the ortho position. 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

The latter, in contrast to nuclear Overhauser enhancement and exchange spectroscopy (NOESY), always feature positive NOEs (negative cross-peaks with respect to diagonal), eliminating known problems of NOEs vanishing or spin diffusion, depending on correlation time, when high field spectrometers are used for measurements of medium-size compounds. 

The cycloaddition of glycals (21a-d) and dibenzylazidocarboxylate gives compounds (22a-d) in yields greater than 70% (Scheme 5) <87JA285>. These [4 + 2] adducts are useful intermediates in the preparation of 2-amino-2-deoxy carbohydrates <89JA2295>. The cycloaddition is stereospecific and is controlled by the stereochemistry at the C-3 position stereochemistry has been assigned by nuclear Overhauser enhancement (NOE) studies. 

We described the basic aspects of NOESY in Section 10.1 as an introductory example of a 2D experiment. NOESY is very widely used in measuring macro-molecular conformation, as we see in Chapter 13. However, as shown in Fig. 8.4, the H— H nuclear Overhauser enhancement 17 varies from its value of +0.5 in small molecules to a limiting value of — 1 in large polymers with very long Tc, and at intermediate values of rc the NOE may vanish. An alternative is to use the NOE measured in the rotating frame, as this quantity is always positive. By analogy to NOESY, this technique has the acronym ROESY (rotating frame Overhauser enhancement spectroscopy),... 

For Rexco branched/cyclics, gating of the H-noise decoupling (so that it is applied only during signal acquisition) in order that nuclear Overhauser enhancement (n.O.e.) is suppressed causes the intensities of the methylene (29.85 p.p.m.) to decrease relative to that of the methyl (1. 15 p.p.m.). Presumably different relaxation mechanisms predominate in the two cases (j U) for the methylene group, relaxation is mainly dipolar, with positive Overhauser enhancement, whereas the methyl group may relax by spin-rotation with negative Overhauser enhancement. 

Alkyl-3-thiophenyl sulfolenes gave (Z. )-dienes (197) in a ratio of about 9 1 [20]. The geometry of the dienes was determined by nuclear Overhauser enhancement ( Oe) measurements (Scheme 6.64, Table 6.27, entries a-d). A trimethylsilyl group in the 3-position appeared to have a similar effect [106] (Table 6.27, entry e). 

The UV spectrum of kadsurin is almost superimposable on that of schizandrin (3.1b), a fact which, in addition to other spectral evidence, led to structural proposal 3.4a. The position of each functional group and the conformation of kadsurin and the related kadsurarin (3.5a) were elucidated by measurements of nuclear Overhauser enhancement of signal intensities (Table 5) 43). 

The numbering of atoms within the base and sugar moieties is according to the standard lUPAC/IUB rules [84S1]. The resonance positions of the imino protons incorporated in this survey are all based on one and/or two dimensional nuclear Overhauser enhancement spectroscopy. 

Figure 5.43. Gated decoupler method for recording 2D J-resolved C-NMR spectra. The upper portion shows the pulse sequence while the lower portion represents the effect of the pulse sequence on magnetization of a CH group decoupling during the preparation period provides nuclear Overhauser enhancement. The position of the two magnetization vectors in (d) will be dependent on 11 the sum of the two vectors which is detected will depend both on and the magnitude of the coupling constant, J. The signals are then said to be J-modulated. ...

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