Nuclear Overhauser effect NOE difference

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>. 

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>. 

Several physical methods have been employed to ascertain the existence and nature of ICs infrared (IR) absorption spectroscopy nuclear magnetic resonance (NMR) spectroscopy,14 including JH nuclear Overhauser effect (NOE) difference spectroscopy, H 2-D rotating-frame Overhauser effect spectroscopy (2-D ROESY),15 and solid-state 13C cross-polarization/magic angle spinning (CP/MAS) spectroscopy 16 induced circular dichroism (ICD) absorption spectroscopy 17 powder and singlecrystal X-ray diffraction 18 and fast atom bombardment mass spectrometry (FAB MS). 

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). 

Heteronuclear Multiple Quantum Correlation) and HMBC (Heteronuclear Multiple Bond Correlation). Application of nuclear Overhauser effect (nOe) difference spectroscopy and nuclear Overhauser effect spectroscopy (NOESY) complete the analysis, giving atomic spatial relationships. Sensitivity problems can be alleviated using Homo Hartmann-Hahn spectroscopy (HOHAHA or TOCSY, Total Correlation Spectroscopy). For weak nOes a rotating frame experiment, i.e. ROESY (Rotating frame Overhauser Effect Spectroscopy) is useful, and may be the best experimental method to sequence chains of sugars [5]. 

We will briefly consider in this section various aspects of homonuclear spin-de-coupling experiments and nuclear Overhauser effect (NOE) difference spectra. Obviously any detailed treatment is far beyond the size limitations of this chapter. Moving next to ID NMR techniques, we wiU briefly consider the utilization of selective spin-population transfer (SPT) and experiments which rely on these principles such as INEPT and DEPT, off-resonance proton decoupling techniques, decoupler gating experiments, and finally spin—lattice or Tj relaxation techniques. 

A handy method for solving these types of problems is nuclear Overhauser effect (NOE) difference spectroscopy. This technique is based on the same phenomenon that gives rise to the nuclear Overhauser effect (Section 4.5), except that it uses homonuclear, rather than a heteronuclear, decoupling. In the discussion of the nuclear Overhauser effect, attention was focused on the case in which a hydrogen atom was directly bonded to a atom, and the hydrogen nucleus was saturated by a broadband signal. In fact, however, for two nuclei to interact via the nuclear Overhauser effect. 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

A conformational comparative study has been performed on a penem, a carbapenem, and a 1/3-methylcarbapenem, bearing the same C-2 and C-6 side chains, using both NMR and theoretical tools. The corresponding calculations have been performed at the 3-21G level using the ab initio MO method, while 111 NMR measurements and nuclear Overhauser effect (NOE) enhancements were carried out in DzO solution. It arose from this study that there are conformational differences in the side chains of these three compounds in the physiological environment in particular, the conformation of the C-6 side chain in the penem appears to be different from that in the carbapenem <1998BMC367>. 

The compound 35 possesses exo- and air-stereochemistry of the methyl and the benzoate groups based on the nuclear Overhauser effect (NOE) data. Based on the C-N distance (r), the Woodward parameter (h) and the sum at the N-atom (SN) value from X-ray data, N-l is in a pyramidal environment. N-3 is on maximum resonance with the Jt-framework of the adjacent C=0 group. This is reflected in a shorter C(2)-N(3) bond (1.375(2) A, compared to the C(2)-N(l) bond distance [1.438(2)A]. JZN values are as follows N-3 = 359.99°, and ]N-1 =311.74° and, = 0,601 A. The X-ray crystallographic data support some of the conclusions derived from AMI calculations, viz. N-l and N-3 atoms are in chemically distinct environments and form C-N bonds of different strengths. This would have implications on the... 

The nuclear Overhauser effect (NOE) is a consequence of the modulation of the dipole-dipole interactions (through space) between different nuclei and is correlated with the inverse sixth power of the internuclear distance. Experimentally, the NOE is the fractional change in intensity of one resonance when another resonance is irradiated in a double-irradiation experiment. The NOE phenomenon is intimately related to spin relaxation. The NOE varies as a function of the product of the Larmor frequency, co0, and the rotational correlation time, tc. In small molecules, tc is short relative to uo"1. In this extreme motional narrowing situation, the frequency... 

---