Overhauser effect spectroscopic

It is not only chemical shifts or couplings that can be used to define correlations in a 2D NMR spectrum information from through-space interactions as in the Nuclear Overhauser Effect (NOE, Section 4.11.4) can also be used. The NOES Y (Nuclear Overhauser Effect Spectroscop Y) experiment is one of the most useful in this context. It is a homonuclear technique that allows correlation of nuclei through space separated by less than 5 A. The occurrence of a cross peak therefore indicates that the corresponding two nuclei are close in... 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

Proton and 13C NMR spectral data of 33 derivatives of 64 have been tabulated and assigned <1996MRC409>. Several 3-oxo derivatives of 64 have been studied by proton and 13C NMR spectroscopy as part of a comprehensive investigation of their structural and spectroscopic properties <2003BCJ2361>. The regioselectivity of the formation of the six-membered ring in derivatives of 67 from 1,3-diketones has been established by proton NMR spectroscopy and nuclear Overhauser effects <1997CHE535>. 

Spectral methods, spectroscopic titration and NMR data (chemical shifts on binding, intermolecular Nuclear Overhauser Effects, relaxation and correlation... 

Subsequently, shallow water collections of Lyngbya majuscula from Puerto Rico and the Dry Tortugas yielded additional supplies of ATX as well as a new congener termed antillatoxin B (Figure 6.10) [149]. The structure of the new metabolite was determined largely by comparison with the spectroscopic data set for ATX, and stereochemistry deduced by Marfey s analysis for L-alanine while the i-N-methyl homophenylalanine was proposed based on nuclear Overhauser effect (nOe) and bioassay results. Substitution of i-N-methyl homo-phenylalanine, an intriguing amino acid of quite rare occurrence in natural products, for i-N-methyl valine... 

Ponolactone A and its glucoside are inhibitors of expansion and mitosis in plant cells. Ponolactone A was isolated from Podocarpus nakai and shown to have the structure (25) by a detailed study of its n.m.r. spectrum, utilizing the nuclear Overhauser effect. The C-14 epimer was prepared from the C-7 acetate of nagilactone. Inumakilactone B was isolated" from Podocarpus macrophyllus and shown by a combination of chemical and spectroscopic evidence to have the structure (26). Inumakilactone C was tentatively assigned structure (27) on the basis of spectroscopic evidence. 

Phytochemical studies on Narcissus obesus have resulted in the isolation of a new alkaloid named obesine (420) (see Fig. 24), accompanied by several known Amaryllidaceae alkaloids (74). The stereochemistry and structural determination of the alkaloid 420 have been carried out by spectroscopic analyses and by application of 2D NMR techniques. Although the H-6)8 proton (5 4.38) is masked by the H-3 proton (5 4.30-4.40), the H-6a proton (54.02) was assigned at higher field on the basis of the nuclear Overhauser effect (NOE) with H-12 endo (5 3.10) observed in the 2D NMR experiment. On the other hand, the a disposition of H-3 was confirmed by the NOE between H-3 (5 4.30-4.40) and H-12 exo (5 3.01). In the C-NMR spectrum of 420 a characteristic signal due to the C-11 carbon was observed at 82.7 (singlet) ppm. Also, a comparison of the H and NMR spectra of obesine (420) with those of the related alkaloid 3-epi-marconine (303) (50) was performed. 

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. 

Two minor alkaloids were also isolated from B. zeylanica timber. The structure of one of these, C1oN8N202, mp 223-224°C, was revised from 3,4-dihydroxy-2,2 -bipyridine (18) to 8-hydroxyquinoline-4-carbaldehyde oxime (2) based on synthetic, NMR and nuclear overhauser effect (NOE) difference NMR spectroscopic evidence (19). Natural occurrence of oximes, although rare in higher plants, is not without precedence, and the essential oil of Ruta montana L. has been reported to contain the bis-oxime of 3,4-hexanedione (20). The structure of the nonpolar minor alkaloid broussonetine, C22H16N2O4, mp 238-239°C, was elucidated as 3,4-bis(8-hydroxyquinolin-4-yl)-y-butyrolactone (4) (21). 

Older methods based on solubility changes upon complexation, or on partition coefficients between aqueous solutions and hydrophobic solvents, have been shown to lead to gross errors as compared to spectroscopic techniques (40) that are also less sensitive to the formation of emulsions, micelles, and so on. The traditional X-ray analysis of inclusion compounds is of limited significance for establishing complexation between lipophilic substrates and macrocyclic host, particularly in aqueous solution. The essential hydrophobic driving force for complexation, of course, is nonexistent in the crystal. The future development of NMR methods including shielding calculations and measurements of intermolecular nuclear Overhauser effects is expected to provide the most reliable information on intercavity inclusion complexes in solution as the basis for catalytic applications. 

The analytical situation becomes much more complicated with open-chain tetrapyrrole pigments, which occur as bile pigments in nature (Falk, 1989). Their conformation is by no means limited to planar or ruffled-planar, but all kinds of cw-tran -configured diastereomers are known and produce extremely differentiated and often complicated UV/vis spectroscopic shifts and extreme changes in solubility, caused by enormous changes in neighbor group interactions. The latter are best analyzed by NOE (nuclear Overhauser effects) (Sanders, 1996). 

Based on the empirical results/ X-ray-diffraction data/ and solution-phase NMR experiments/ a transition state model (6) has been advanced to explain the observed enantioselectivity. The presence of an ortho substituent in the A arylmaleimide reactant directs aluminum coordination to occur with the lone pair of electrons anti to the nitrogen atom. A 3,5-dimethylphenyl moiety present on the ethylenediamine framework blocks one face of the dienophile, resulting in approach of the diene from the backside. A considerable amount of spectroscopic evidence, most notably that obtained fromNOE (nuclear Overhauser effect) experiments, has been accumulated to support this model. A -arylmaleimide derivatives that lack an ortho substituent and other dienophiles (e.g., maleic anhydride) can coordinate to the aluminum catalyst in alternative modes such that the reactive olefin is far removed from the chiral environment of the ligand scaffold, thereby resulting in cycloaddition reactions that exhibit little or no enantioselectivity. 

A wide variety of chemical and spectroscopic techniques has been used to determine functionality in humic substances. Although nuclear magnetic resonance (NMR) spectroscopy has been used for a much shorter period of time than most other techniques for determining functional group concentrations, this technique has provided far more definitive information than all other methods combined. However, substantially more work must be done to obtain the quantitative data that are necessary for both structural elucidation and geochemical studies. In order to increase the accuracy of functional group concentration measurements, the effect of variations in nuclear Overhauser enhancement (NOE) and relaxation times must be evaluated. Preliminary results suggest that spectra of fractions isolated from humic substances should be better resolved and more readily interpreted than spectra of unfractionated samples. 

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