Structural Elucidation of Organic Compounds

The oxidative cleavage of C=C bond is a common type of reaction encountered in organic synthesis and has played a historical role in the structural elucidation of organic compounds. There are two main conventional methods to oxidatively cleave a C=C bond (1) via ozonol-ysis and (2) via oxidation with high-valent transition-metal oxidizing reagents. A more recent method developed is via the osmium oxide catalyzed periodate oxidative cleavage of alkenes. All these methods can occur under aqueous conditions. 

On the other hand, nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools for the structure elucidation of organic compounds. However, to solve the molecnlar strnctnre of a novel substance by NMR spectroscopy alone is often time-consnming (when compared to MS). Besides, the identification of components in a complex mixture usually requires the separation and/or isolation of the components of interest prior to NMR analysis. Therefore mnltiple preparatory chromatographic... 

Nuclear magnetic resonance spectroscopy (NMR) is one of the most powerful analytical methods for identification and structure elucidation of organic compounds. Since NMR spectra are recorded in solution, no phase transfer like in MS is necessary when coupled with LC techniques. Additionally, NMR is a non-destructive detection technique, allowing the analyte to be transferred for characterization using additional methods. As of today, LC—NMR coupling was used in a wide range of applications [65,66,67,68,69,70,71],... 

Directly recorded one-dimensional 13C NMR spectra are routinely obtained for structure elucidation of organic compounds except in instances where a very limited amount of sample precludes direct 13C detection. It is usually worthwhile to spend enough instrument time to obtain a 13C spectrum with good signal/noise ratio and with sufficient delay times between pulse repetitions to ensure that quaternary carbons with long Tx are clearly observed. 

The first part of this section (Section 20.2.1) will provide the reader with historical overview of NMR and with a brief description of the most typical experiments used in NMR for the structural elucidation of organic compounds. The second part of this section (Section 20.2.2) will focus mainly on the improvements carried out in the NMR as a hyphenated analytical technique for the elucidation of organic compounds and an understanding of the need to develop LC-NMR for the analysis of complex mixtures. 

Strategies for NMR spectroscopic structure elucidation of organic compounds have been reviewed extensively.34-37 In this section, we briefly describe a set of the most commonly useful 2D NMR spectra that is sufficient for most (though certainly not all) organic structure determination problems, and we comment on specific modifications of acquisition parameters that facilitate the analysis of mixtures. 

Roboz J. (1974) Combined GC-HRMS for identification and structure elucidation of organic compounds in natural product research. 6th lnt. Colloq. Chem. Coffee (Bogota, 4-9.6.1973) (ASIC, 1974), 81-8. 

As far as the area of knowledge Is concerned, AI software should be developed by first defining the knowledge domain of the particular problem addressed. Thus, the domain of the program DENDRAL is the structural elucidation of organic compounds, while that of the program MYCIN Is the medical diagnosis... 

The interpretation of spectroscopic data for the identification and structure elucidation of organic compounds is largely an empirical process and relies heavily on the use of previously accumulated reference data. Compilation of computer-readable spectroscopic data bases is nowadays feasible because most commercially available spectrometers have small built-in computers for the digital acquisition of measured spectroscopic data they are also equipped with a suitable mass storage device to store spectra or selected spectral data, or they provide the facility to transfer the recorded spectra to a more powerful external computer. If the computer-readable spectroscopic data are suitably organized, the analyst is provided with a very powerful tool for the identification of a compound, a group of compounds or a structure by means of suitable software, thereby avoiding the slow and tedious manual work otherwise involved [67,69]. 

Part Nuclear Magnetic Resonance Spectroscopy by W. Robien focuses on structure elucidation of organic compounds. Spectra similarity searches, spectrum prediction (from a given chemical structure), recognition of substructures and automatic isomer generation are the main topics they are still areas of scientific research in computer-assisted structure elucidation. 

This reaction had wide application in the structural elucidation of organic compounds before the modern spectroscopy. In addition, this reaction has been used to prepare 01-monoiodo and o -diiodoketones by using a less than theoretical amount of iodine. ... 

The idea of de novo structure elucidation is to find the correct structure without searching databases. A prominent starting point is the well known DENDRAL system [183], the development of which began already in the mid 1960 s. DENDRAL was developed for the automated structure elucidation of organic compounds by MS, after separation by gas chromatography (GC). 

K. Varmuza, P. Penchev, F. Stand, and W. Werther. Systematic structure elucidation of organic compounds by mass spectra classification./. Mol. Struct, 408/409 91-96,1997. 

K. Varmuza, W. Werther, F. Stand, A. Kerber, and R. Laue. Computer-assisted structure elucidation of organic compounds, based on mass spectra classification and exhaustive isomer generation. Software-Entwicklung in der Chemie, 10 303-314,1996. 

A series of studies have appeared that focus on relationships between molecular structure and NMR spectra. C NMR is a useful analytical tool for the structure elucidation of organic compounds due to the direct relationship between a carbon atom s local structural environment and its chemical shift value. However, because this relationship is not completely understood, NMR spectral interpretation can be complex. Spectral simulation techniques enable the construction of approximate spectra for structures whose... 

D-INADEQUATE spectroscopy can conveniently disclose the connectivity of each carbon atom embedded in a molecule and has found widespread use in structural elucidation of organic compounds. This technique is extremely powerful when the size of one-bond coupling (V) is much larger than that of long-range couplings... 

NMR spectroscopy is a very important tool for the identification or structure elucidation of organic compounds. However, among the 138 alkaloids described in the literature in the period, only 85 (61.2%) have their NMR data reported. The complete list of all NMR assignments for the alkaloids which have their data reported is presented in Table IV. 

M. Zippel, J. Mowitz, I. Koehler, and H. J. Opferkuch, Anal. Chim. Acta, 140,123 (1982). Spektren—A Computer System for the identification and Structure Elucidation of Organic Compounds. 

Hawkins, D.M., The problem of overfitting. J. Chem. Inf. Comput. Sci., 44, 1-12 (2004). Varmuza, K., Penchev, P., Stand, F. and Werther, W, Systematic structure elucidation of organic compounds by mass spectra calssification. J. Mol. Struct., 408/409, 91-96 (1997). Varmuza, K., Recognition of relationships between mass spectral data and chemical structures by multivariate data analysis. Anal. Sci., 17(SuppL), 1467-1470 (2001). 

Various spectrometric methods have been utilised in organic chemistry. Among them, nuclear magnetic resonance (NMR) spectrometry is the most useful for structural interpretation in various situations. Additionally, various computer program systems for structural elucidation of organic compounds have been developed. 

The system CHEMICS, developed for the automated structure elucidation of organic compounds, uses the connectivity stack to generate all possible chemical structures which are consistent with given structural information. The connectivity stack uses a notation for each substructure, e.g., C3 for a methyl and LC for chlorine, and with a set of rules the molecule is linearly coded in a list of substructures and a list of connections. The canonical code is defined as the labeling which offers the maximum linear representation of the upper triangle of the adjacency matrix. 

Structure generators have been developed basically for the structure elucidation of organic compounds. Therefore, various types of information about an unknown compound have been introduced to the structure generators to reduce the number of candidate structures. In particular, data on the probable and forbidden substructures are quite useful for the above purpose. The details can be obtained from Structure Determination by Computer-based Spectrum Interpretation. 

Pretsch E, Clerc JT, Seibl J and Simon W (1989) Tables of Spectral Data for Structure Elucidation of Organic Compounds, 2nd edn. Berlin Springer. 

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