COSY, NMR spectroscopy

In particular, DP9 presented ambiguities associated with the proton assignments from which the 13C assignments were derived. Thus, it was necessary to use the COSY method to assign the proton absorptions first. Homonuclear COSY NMR spectroscopy allowed unambiguous assignment of proton chemical shifts in all cases. 

In individuals with a deficiency in methylacetoacetyl CoA thiolase (MACT), the conversion of 2-methylacetoacetyl-CoA into acetyl-CoA and propionyl-CoA is inhibited and an accumulation of abnormal catabolic products is observed in the urine. Williams etal. have successfully used both ID and 2D H NMR spectroscopy to investigate the urinary metabolites of two unrelated patients with this disorder. The urine spectra from both patients clearly showed the presence of both 2-methyl-3-hydroxybutyrate and tiglylglycine, which is characteristic for MACT deficiency due to the build-up of metabolites close to the position of enzyme deficiency. However, at 360 MHz, the H NMR spectra are quite crowded and difficult to interpret fully. This difficulty was overcome by the use of the 2D JRES H NMR experiment, which resolved all the proton-proton couplings into the second dimension, allowing much clearer spectral interpretation. In addition to the spin-echo and 2D H J-resolved NMR spectra, 2D H COSY NMR spectroscopy was used to determine the spin-spin coupling connectivities between the protons in the various urinary components. 

A most careful study was now carried out by NMR spectroscopy, not only by regular Ih 500 MHz and 13C but also by 1H COSY NMR spectroscopy [4,5). Individual compounds were isolated by preparative HPLC of the mixture in urushiol, particularly of kuro-urushi and all the isomers, including the cis-trans isomers were identified and characterized [Figure 4] [5). Particular care was taken to identify the water addition product [M]K" =371 daltons. It was found that water had indeed been added to the triene to form a C10 hydroxylated diene [Formula] [5]. This water addition is catalyzed by some additives such as ferric chloride which are used in the sugurome process to optimize the lacquer properties. The details of the process is a closely guarded secret and is slightly different from manufacturer to manufacturer. 

As Figure 9.10 shows, there are 10 possible tautomers of OxC, although several can be excluded for energetic reasons. DFT calculations (Figure 9.11) indicate that c(i) and d probably cannot exist. Despite this, OxC tautomerization could apparently be controlled using solvent polarity as indicated by the H-NMR spectra shown in Figure 9.12. Tautomers were identified by using Nuclear Overhauser Effect (NOE) and COSY NMR spectroscopy. In DMSO- g... 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Figure 1. Pulse sequences of some typical 2D-NMR experiments. COSY = correlation SpectroscopY, DQFCOSY = Double Quantum Filtered COSY, RELAY = RELAYed Magnetization Spectroscopy, and NOESY = Nuclear Overhauser Effect SpectroscopY.

The sine-bell, sine-bell squared, phase-shifted sine-bell, and phase-shifted sine-bell squared window functions are generally used in 2D NMR spectroscopy. Each of these has a different effect on the appearance of the peak shape. For all these functions, a certain price may have to be paid in terms of the signal-to-noise ratio, since they remove the dispersive components of the magnitude spectrum. This is illustrated in the following COSY spectra ... 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

Homonuclear 19F-19F experiments are the most commonly carried out, and they are also the most easily implemented on conventional NMR spectrometers. Among such experiments, 19F COSY correlation spectroscopy is probably the 2D 19F NMR technique most frequently encountered, mainly because of through space couplings that can make it otherwise difficult to infer definitive structural information from the presence or magnitude of observed correlations. It has been found to be particularly useful in the analysis of fluoropolymers. 

The relative configurations and predominant conformations of [l,2,3]oxathiazino[4,3- ]isoquinoline derivatives 177-182 were studied by means of H and 13C NMR spectroscopy with the applications of DNOE, 2D HSC, and 2D-COSY measurements. Their stereostructure includes an equilibrium between the conformers cis-1, trans and cis-2. In the /ra -structure, the B/C rings are trans-ane-WatcA with H-l lb and the N-5 lone electron pair trans-diaxial. In the cis structures, the hetero-rings are air-anellated in the cis-1 conformation C-l is in inside position, while in cis-2... 

Zheng et al. [1] postulated that the driving force for placing Zr and B on the same carbon might stem from interactions between the zirconium and oxygen or boron and chlorine atoms. However, an X-ray analysis of 22 revealed that there are no intra- or intermo-lecular interactions between any of these atoms [35]. Compound 22 was also unambiguously characterized by 1H-1H double quantum filtered COSY [36] and 13C-1H heteronuc-lear chemical shift correlation NMR spectroscopy [37,38]. Considerable differences in the chemical shifts of the diastereotopic Cp groups were found in both the XH and 13C NMR spectra. The NMR study unequivocally showed that the methine proton was at-... 

Fig. 8. Pulse schemes for 2H 2D NMR spectroscopy of weakly ordered, deuterated solutes (a) 2H Q-COSY, (b) 2H-13C HETCOR experiment with 13C decoupling, (c) 2H-2H COSY 2D experiment. (Reproduced by permission of American Chemical Society.)...

The most important two-dimensional NMR experiments for solving stmctural problems are COSY (Correlation SpectroscopY), NOESY (Nuclear Overhauser Enhancement SpectroscopY), HSC (Heteronuclear Shift Correlation) and TOCSY (Total Correlation SpectroscopY). Most modem high-held NMR spectrometers have the capability to routinely and automatically acquire COSY, NOESY, HSC and TOCSY spectra. 

NMR spectroscopy on nuclei in 2-aminopyrans, in many cases, fail to report shifts with a reference, or are assigned inaccurately (86H(24)935). Some examples of chemical shifts and references are given below the data for pyran 19 have been proven by the COSY technique (02X953). One paper (85MI2) was devoted to a comparative analysis of the spectra of pyrans 16, 17, as well as of those of a 2,6-dimethyl-4H-pyran analog. 

There are a large number of structural parameters for NMR of different nuclei and many examples of how they can be applied to the analysis of hydroxylamines, oximes and hydroxamic acids. Fortunately though, there are many very clear, meticulously written descriptions of INEPT, DEPT, INADEQUATE, COSY, NOESY and the like, in one- and two-dimensional NMR spectroscopy, that are cited in the references. Since their content is beyond the scope of the present chapter, a brief mention of some of the fundamental concepts that are essential for its understanding by the nonspecialist is in order. 

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. 

H and C NMR spectroscopies were systematically used to elucidate the structure of C-glycosylflavonoids the assignment of signals was based upon various experiments, HMBC, HMQC, COSY, etc., as shown in the following examples ... 

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