NOE difference experiments

NOE-difFerence spectroscopy is particularly valuable for distinguishing stereoisomers, for it relies solely on intemuclear distances, and thus avoids any problems of ambiguity or absence associated with couplings. With smallish molecules, it is best carried out in the above 1D maimer, because 2 s are necessary for tire transmission of the NOE. The transmission process becomes more efficient with large molecules and is almost optimal for proteins. However, problems can occur with molecules of intemiediate size [3f]. A 2D version of the NOE-difference experiment exists, called NOESY. 

It is important that constant temperature be maintained throughout the nOe difference experiment. If the instrument is fitted with a constant-temperature device, then it is advisable to adjust it to a few degrees above room temperature so that it maintains a constant temperature accurately. 

Avoid having moving metal objects near the magnet when carrying out nOe difference experiments, to prevent random variations in frequency. A small line-broadening ( 2 Hz) can also be applied to the spectra before or after subtraction, to reduce subtraction artifacts. 

Some extensive nOe difference experiments were performed to established the relative configuration of several polysubstituted perhydropyrrolo[2,l- ]thiazoles <1999JME2977, 2004TL4139>. The other recorded spectra for this type of compounds were used only to confirm the structures. There are no systematic studies. 

There are two types of NOE experiments that can be performed. These are referred to as the steady-state NOE and the transient NOE. The steady-state NOE experiment is exemplified by the classic NOE difference experiment [15]. Steady-state NOE experiments allow one to quantitate relative atomic distances. However, there are many issues that can complicate their measurement, and a qualitative interpretation is more reliable [16]. Spectral artifacts can be observed from imperfect subtraction of spectra. In addition, this experiment is extremely susceptible to inhomogeneity issues and temperature fluctuations. 

The stereoselectivity of the jco-cyclization was extremely high and only the (Z)-isomers were formed as confirmed by nuclear Overhauser effect (NOE) difference experiments <1997SC367>. 

Fig. 9.7. ID NOE difference experiments on met-aquo myoglobin. (A) Reference spectrum (a) and difference spectrum (b) observed upon saturation of peak g. (B) Intensity of negative/and d signals in (b) as a function of the irradiation frequency. Signal/is maximal when the irradiation frequency is on g, signal d shows a steady increase with increasing frequency [39].

The through-space distances involved in H H proximity determinations are quite small, and the effect decreases as the inverse of the sixth power of the distance, through space, between the protons. The usual observable enhancement is less than 20%. To increase the sensitivity, we use the NOE difference experiment, in which a conventional H spectrum is computer sub-... 

This technique differs from the old NOE difference experiment we looked at in Chapter 5, where we selectively saturate one resonance (make Mz = 0) over a long period of time (the mixing time) whereas other spins are perturbed and reach a steady-state level of... 

The great potential of the H, Li NOE for structural research was recognized in 1986 independently by two groups. Avent et al. [154] reported ID NOE difference experiments for hydrido[tris(trimethylsilyl)methyl]-metalates, while Bauer et al. [155] introduced the H, Li HOESY experiment. The pulse sequence for these types of experiment, (ix). 

A complete NMR characterization was performed to elucidate the structure. For comparison, the same set of experiments was collected for the parent drug substance. Initial NMR characterization data were consistent with a proposed structure, but reevaluation was performed when a new reaction pathway was proposed. A second structure was proposed that was also consistent with the NMR data. Additional NMR experiments were performed to differentiate between the two structures. These included nOe difference experiments to probe spatial relationships within the molecule. 

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