2D-NMR analysis

The structure elucidation of the kinamycins was a formidable challenge, and the information presented below draws from the work of several research groups over a period of more than 20 years. As will be shown, the originally proposed structure of the kinamycins contained a cyanamide rather than a diazo function. Subsequent synthetic and biosynthetic studies led to replacement of the cyanamide with a diazo function. The structural elucidation was challenging, in part, because of the high degree of unsaturation of the kinamycins, which limits the utility of H and 2D NMR analysis. In addition, because these structures were unprecedented, there were no clear benchmarks for comparison at the time. The pathway from isolation to determination of the correct structure is described below. 

Balas, L. and Vercauteren, J., Extensive high-resolution reverse 2D NMR analysis for the structural elucidation of procyanidin oligomers. Magn. Reson. Chem. 32, 386, 1994. 

Es-Safi, N.E. et ah. Structure of a new xanthylium salt derivative. Tetrahedron Lett. 40, 5869,1999. Es-Safi, N.E. et ah, 2D NMR analysis for unambiguous structural elucidation of phenolic compounds formed through reaction between (-l-)-catechin and glyoxylic acid. Magn. Reson. Chem. 40, 693, 2002. 

D NMR analysis of daphnicyclidin F (61), C23H27NO5, and chemical correlation of daphnicyclidin D with 61, indicated that 61 is the 2 -hydroxy form of daphnicyclidin D [49]. 

A total structure including partial stereochemistry has been proposed for MTX on the basis of extensive spectroscopic analysis (Mnrata et al. 1992, 1993, 1994). The 2D NMR analysis of MTX led to the structnral elncidation of some partial stmctnres snch as the C1-C36 moiety. However, overlap and poor resolntion of H and NMR signals hampered the elncidation of the H- H cotmectivity,... 

The structures of these compounds (shown in Fig. 10) were elucidated unambiguously by spectroscopic methods including ID and 2D NMR analysis, and also by comparing experimental results with hterature data [52]. 

Another intriguing sugar-based metabolite from a cyanobacterium is cyclodextrin 33, isolated from a Hawaiian Tolypothrix byssoidea strain. A range of related metabolites were produced these proved inseparable and were analyzed as a mixture. However, acetylation of the mixture yielded peracetate derivatives that could be separated by HPLC and analysis of both the purified peracetate and the unmodified mixture was used in the elucidation of the major component 33. MS and extensive NMR analysis led to the structure shown. 2D NMR analysis was crucial in the determination of the individual sugar units, while their sequence was solved by difference nuclear overhauser effect (NOE) experiments. The configurations of all sugar units were presumed to be D on the basis of comparison with related cyclodextrins. The Tolypothrix cyclodextrins were found to block the activity of some of the other toxic metabolites produced by this cyanobacterium. " ... 

HRFAB-MS and 13C-NMR spectrum. The most notable differences with compound (94), was the absence of the C-6 -C-7 double bond and an additional methoxy group, that was located at C-6 , according to a HMBC experiment. As in the case of the preceding compounds, the structure was confirmed by 2D NMR analysis and CD determined the absolute configuration. With those data, the authors determined the structure of (96) as 6 p-methoxy-dihydro-scutionin aB, Fig. (52). 

From the root bark of Zizyphus mauritiana, a new cyclopeptide alkaloid mauritine J (54), together with known alkaloids, was isolated. Its structure was established by homo- and heteronuclear 2D-NMR analysis and compared with that of amphibine E (55) (51). From Zizyphus oenoplea, some cyclopeptide alkaloids were isolated (52-54). 

In cases in which just a select coupling or correlation is needed from a spectrum to complete an assignment or stmcture determination, a complete 2D analysis may not be required. There are one-dimensional analogues of 2D NMR experiments which can provide the specific information in a more time-efficient manner than performing a fuU 2D NMR analysis. Examples of ID analogs for NOESY and HETCOR experiments are described that use field gradients to select the resonance of interest... 

Treatment of pristinamycin IIa with meta-chloroperbenzoic acid afforded a compound to which the structure (79) was initially assigned, resulting from epoxidation of the more substituted double bond (12,13-C). This material did not display chemical properties characteristic of an epoxide as the assumed epoxide moiety remaining after treatment with nucleophilic reagents. Michael-type addition products on the dehydroproline ring were observed after treatment with thiols or amines (see Sect. 5.4.5). 2D-NMR analysis of the product from reaction of pristinamycin IIa with mCPBA showed that a transannular oxidative cyclization had taken place leading to formation of (80). The reaction can be considered to involve initial epoxidation of the 12,13-double bond followed by an intramolecular nucleophilic attack by the 37-hydroxy of the enol ether (Scheme 19). A similar transannular oxidative cyclization reaction has been reported for the reaction of l,5-dimethylcyclooct-4-en-l-ol with meta-chloroperbenzoic acid [125]. 

TROSY (transverse relaxation-optimized spectroscopy) and CRIPT (cross-correlated relaxation-induced polarization transfer) or CRINEPT (cross-correlated relaxation-enhanced polarization transfer) for the two-dimensional (2D) NMR analysis of N-. H-labeled homo-oligomeric macromolecules with masses ranging from 110-800 kDa. Practical applications of these methods are, for instance analyses of intermolecuiar interactions in supramolecular complexes or conformational changes of a single macromolecule upon interactions with other molecules. 

IR spectrum of symbioimine (35) showed absorption bands for hydroxyl (3450 cm ), iminium (1690 cm ), and sulfate (1240, 1140, 1050 cm ) groups. The NMR signal at 188.0 (C-5) implied the presence of an iminium functionality in this water-soluble amphoteric compound. Its structure, which consists of a characteristic 6,6,6-tricyclic iminium ring possessing an aryl sulfate moiety, was deduced by 2D-NMR analysis (Fig. 6). 

Fig, 6 Planar and stereo structure of symbioimine (35) based on 2D-NMR analysis... 

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