Correlated spectroscopy structure elucidation

TOCSY Two-dimensional total correlation spectroscopy To elucidate structure of organic molecules To establish proton coupling network and molecular connectivity... 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

Structure elucidation of three related derivatives of ring system 2 ( temozolomide 9a, mitozolomide 9b, and the related acid derivative 9c) has been carried out by 13C and 15N NMR spectroscopy <1995J(P1)249> (Scheme 2). The 1SN NMR chemical shifts measured in dimethyl sulfoxide (DMSO) solutions are listed in Table 1. For compound 9a, Lowdin charges of the nitrogen atoms have also been calculated and found to have a linear relationship with the experimentally determined chemical shifts of these atoms. The NMR data of 9a have been correlated with those of a series of heterocycles of related structure by the same team <2002MRC300>. 

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. 

The structures of the compounds were elucidated by a combination of NMR techniques (lH-, 13C-, and 13C-DEPT NMR) and chemical transformation, enzymatic degradation, and as well as mass spectrometry, which gives information on the saccharide sequence. A more recent approach consists of an extensive use of high-resolution 2D NMR techniques, such as homonuclear and heteronuclear correlated spectroscopy (DQF-COSY, HOHAHA, HSQC, HMBC) and NOE spectroscopy (NOESY, ROESY), which now play the most important role in the structural elucidation of intact glycosides. These techniques are very sensitive and non destructive and allow easy recovery of the intact compounds for subsequent biological testing. 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

The structure of a compound separated from the antifungal constituents of the Chinese liverwort Marchantia polymorpha L. (Marchantiaceae) was identified by H and 13C NMR spectroscopy and established to be 13,13 -0-isopropylidene-riccardin 17 <2006MI34>. The acetonide structure of 17 emerged from the 13C NMR spectra and HMBC correlations. For the structural elucidation of a new . 

All the spectroscopic approaches applied for structural characterization of mixtures derive from methods originally developed for screening libraries for their biological activities. They include diffusion-ordered spectroscopy [15-18], relaxation-edited spectroscopy [19], isotope-filtered affinity NMR [20] and SAR-by-NMR [21]. These applications will be discussed in the last part of this chapter. As usually most of the components show very similar molecular weight, their spectroscopic parameters, such as relaxation rates or selfdiffusion coefficients, are not very different and application of these methodologies for chemical characterization is not straightforward. An exception is diffusion-edited spectroscopy, which can be a feasible way to analyze the structure of compounds within a mixture without the need of prior separation. This was the case for the analysis of a mixture of five esters (propyl acetate, butyl acetate, ethyl butyrate, isopropyl butyrate and butyl levulinate) [18]. By the combined use of diffusion-edited NMR and 2-D NMR methods such as Total Correlation Spectroscopy (TOCSY), it was possible to elucidate the structure of the components of this mixture. This strategy was called diffusion encoded spectroscopy DECODES. Another example of combination between diffusion-edited spectroscopy and traditional 2-D NMR experiment is the DOSY-NOESY experiment [22]. The use of these experiments have proven to be useful in the identification of compounds from small split and mix synthetic pools. 

All available methods of diffraction, microscopy, and spectroscopy are used for structure elucidation in present-day materials chemistry.1,2 For detailed structure determination, even powders suffice for the most part because of the advances in diffraction profile analysis. These advances in structural tools enable more meaningful correlations of structure with properties and phenomena. Catalysis is becoming more of a science partly because of our ability to unravel the structures and surfaces of catalysts. Phase transitions of all varieties5 are being investigated more and more by chemists. 

The H, 13C and 7Li study of polycyclic anions includes ID and 2D methods that enable an unequivocal structure elucidation of the molecules. Through-bond and through-space interactions as well as correlation spectroscopy allow the study of various aspects of lithium-reduced polycyclic hydrocarbons described in the following sections. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

Parallel acquisition NMR spectroscopy (PANSY) has emerged as a method to provide great time savings for NMR data acquisition and structure elucidation.95 In contrast to the concept of time-sharing evolution, PANSY experiments use a separate receiver for each nucleus. As a proof of concept, Kupce and co-workers demonstrated the potential of PANSY by simultaneously acquiring correlation and correlation spectra, both... 

Prior to the advent of 2D methods, selective spin decoupling was used extensively in both proton NMR and in heteronuclear (especially 13C) NMR to ascertain which sets of nuclei contribute to observed spin coupling. Such information is critical to assignment of resonances and to the elucidation of the structure of an unknown molecule. 2D methods now largely supply this information much more efficiently, by correlations that depend on the existence of spin coupling. The homonuclear version of one such experiment is called COSY (correlation spectroscopy), and the heteronuclear version is known by several acronyms, most commonly HETCOR (lieferonuclear correlation). 

The value of COSY stems from its dependence on the presence of spin coupling between the nuclei involved in the correlation. As we have seen, such coupling for protons is usually limited to three or four chemical bonds, hence provides some specificity that is helpful for structure elucidation. On the other hand, useful complementary information can be obtained from longer range interactions among a set of coupled nuclei. The standard method for obtaining such information is described by two acronyms—TOCSY (for total correlated spectroscopy, which best describes the aim of the experiment) and HOHAHA (for fiomonu-clear Hartmann-Hahn, which better describes the mechanisms employed). 

A structure can be deduced from a set of substructures. The latter is derived from multispectra. The structure elucidation involves extracting the set of substructures from multispectra and then assembling them. Let us consider two-dimensional Correlated SpectroscopY (COSY) as shown in Fig. 2. A cross-peak indicates that two protons in atoms have a spin coupling, which implies that they are separated by two or three bonds... 

TOCSY (Total Correlation Spectroscopy) is another important homonuclear 2D correlation experiment where correlations arise due to the presence of homonuclear scalar coupling.In the standard COSY experiment, crosspeaks appear for spins in which the scalar coupling occurs over typically two to four bonds. In the TOCSY experiment crosspeaks can appear for spins separated by many more bonds as long as they are part of a contiguous network of coupled spins. The correlations are effected by the application of a series of low-power rf pulses termed the spin-lock. The duration of the spin-lock period determines the extent to which the correlations are propagated through the spin system. The TOCSY experiment is a useful complement to the COSY methods for the elucidation of complex structures. 

Heteronuclear correlation spectroscopy is not restricted to the spin pair H- C. H-X correlations are also used for structure elucidation, e.g. for... 

Of course, chemical shift data and, even more, tin-tin coupling satellite patterns are essential data for the structure elucidation of the condensation complexes of salicylaldoxime and di-/t-butyltin oxide however they prove their effectiveness mainly at confirmatory level when the structure is known from another information source. This was illustrated by the extensive use of X-ray data to confirm the solution structure of the major species M to be that of the crystal state structure of compound 1. Two-dimensional H-" Sn HMQC spectroscopy mainly strengthens the evidence and enables preliminary correlations between V( n, H) couplings and geometrical parameters to be established. 

Two dimensional sequences are commonly used in solution state NMR spectroscopy to elucidate connectivity of atoms within molecular structures. Sequences are available for heteronuclear shift correlation, homonuclear correlation (COSY and INADEQUATE) and other sequences for longer-range connectivities. In general the basis of these... 

To summarize modern NMR spectroscopy. Chapter 8 carries out the total structural proof of a single complex natural product. This chapter illustrates the tactics and strategies of structure elucidation, from one-dimensional assignments to two-dimensional spectral correlations, culminating in stereochemical analysis based on Overhauser effects. 

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