Nuclear Overhauser effect reaction

The observed polarization is primarily associated with the former parahydrogen protons. However, other protons may also experience a drastic signal enhancement due to nuclear spin polarization transferred to these nuclei via the nuclear Overhauser effect (NOE) or similar processes, both in the final reaction products as well as in their precursor intermediates. 

Benzoylation of D-g/ycero-D-gw/o-heptono-1,4-lactone with an excess of benzoyl chloride and pyridine afforded the hept-2-enono-1,4-lactone as the main product (198). The di- and triunsaturated compounds were isolated in very low yield from the mother liquors (199). Higher yields of the di- and triunsaturated derivatives 153 and 154 were obtained when the /5-elimination reaction was performed with triethylamine on the previously synthesized per-O-benzoyl D-g/ycero-D-gw/o-heptono-1,4-lactone. Employing 10% triethylamine in chloroform, the lactone 153 was obtained as an E, Z dias-tereomeric mixture in 9 11 ratio as determined by H n.m.r. When 20% triethylamine was used, the furanone 154 was obtained in 59% yield (200). Its structure was assigned, on the basis of H and 13C n.m.r. spectra, as 3 -benzoyloxy - (5Z)-[(Z)-3 - benzoyloxy - 2 - propenyliden] -2(5 H)- furanone. The stereochemistry of the exocyclic double bonds was established (201) by nuclear Overhauser effect spectroscopy (NOESY). 

Although one diastereomer 10 was largely favored, the product was obtained as a mixture of diastereomers, and the previously unreported minor diastereomer 11 was also characterized. The stereochemistry of the products was established by nuclear Overhauser effect (NOE) studies. A plausible mechanism assumes the intermediacy of an acetal, and its reaction with 2-methoxypropene generated from 2,2-dimethoxypropane [20]. In order to test this mechanism, the dimethyl acetal of salicylaldehyde was synthesized and reacted independently with both 2,2-dimethoxypropane and 2-methoxypropene. Indeed, both reactions gave the same products as those from the reaction of salicylaldehyde with 2,2-dimethoxypropane (Scheme 4). The condensation of salicylaldehyde and 2,2-dimethoxypropane was also carried out in CD3CN and reaction progress was followed by H NMR spectroscopy. This experiment also confirmed the formation of the acetal from salicylaldehyde (8 5.52, singlet, C//(OMe)2). 

Nuclear Overhauser effect (NOE) difference measurements were used to assign structure 79 for the product of reaction of diphenylnitrile imine with 5-ethylsulfonyl-2-methyl(27/)pyridazinone. Thus in the H NMR spectrum the ot/, o-protons of the arylhydrazino moiety (which were identified by two-dimensional heteronuclear multiple quantum correlation (2-D HMQC) spectroscopy) were shown in differential NOE (DNOE) experiment to be significantly enhanced on irradiation of pyridazine hydrogen H-7, proving their steric proximity <2000JST13>. 

Tetracoordinate l,2A4-oxaselenetane 47 is synthesized by a ring-closure reaction of /3-hydroxylalkyl selenide in 54% yield <1997CC1671>. Similarly, oxaselenetanes 48, 35, and 36 are obtained in 44%, 56%, and 65% yields, respectively (Scheme 14) <1997CC1671, 1998PS501>. These compounds were found to be air stable, colorless plates at room temperature, and the relative stereochemistry between the 3- and 4-positions of 35 and 36 has been determined by differential nuclear Overhauser effect (NOE) experiments. 

Giles and coworkers have recently reported the use of TiCU in CH2CI2 at -78°C for the stereoselective isomerization of naphthyl- and phenyl-dioxolanes to yield pyrans that are important in aphid pigments. 13 Other Lewis acids besides TiCl4 such as BF3, SnCU, Ti(IsoPr)2Cl2 and Ti(IsoPr)3Cl are not effective in such reactions. NMR and nuclear Overhauser effect spectroscopies were used to establish stereochemistry. Some diol side product (13 %) was also formed. The major effort in this research was to clearly identify products of the isomerization. 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

One interesting facet of the germacrane-type sesquiterpenoids is the conformation of the ten-membered ring. This aspect has previously been discussed in terms of transannular electronic effects (anomalous u.v. spectra) and transannular chemical reactions (Cope rearrangement and cyclisations to eudesmane and/or guaiane types). Recently, the power of two spectroscopic techniques has been brought to bear on this problem. The first of these is the use of the Nuclear Overhauser Effect (NOE) and the second is the A"-ray analysis of a suitable derivative. 

The formation of cyclic structures and polymerization of aryl cyanates was followed from the changes in signal intensities of the reaction products. All intermediate spectra consist of two well separated signals arising from cyanate and triazine groups, respectively. Since a chromium acetylacetonate is added in order to eliminate the Nuclear Overhauser Effect and reduce the spin-lattice... 

Nozaki-Hiyama-Kishi couplings, 253 Nozaki-Hiyama-Kishi reaction, 297, 312 nuclear Overhauser effect, 386 nucleophilic aromatic substitution, 422... 

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