Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds
Chapter 11: Using Combinations of 2D NMR Spectral Data for Ab Initio Structure Elucidation of Natural Products and Other Unknown Organic Compounds
Published:26 Sep 2018
Special Collection: 2018 ebook collectionSeries: New Developments in NMR
2018. "Using Combinations of 2D NMR Spectral Data for Ab Initio Structure Elucidation of Natural Products and Other Unknown Organic Compounds", Optimizing NMR Methods for Structure Elucidation: Characterizing Natural Products and Other Organic Compounds, Darcy C Burns, William F Reynolds
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As discussed in Chapters 8 and 9, different 2D and selective 1D NMR pulse sequences provide different information about molecular structure. However, the real advantage of these methods comes from using them in combination for complete structure elucidation. Chapter 11 uses Santonin as a model compound to illustrate this approach. We first describe a basic approach for structure elucidation, involving COSY, HSQC and HMBC spectra used in combination, which we have found often allows one to determine the skeletal structure of an unknown compound. Then, qualitative nuclear Overhauser enhancement (nOe) data are used in combination with vicinal 1H–1H coupling constants to deduce the relative stereochemistry of the molecule. While this basic approach is often sufficient to deduce the structures of natural products of similar complexity to Santonin, in other cases, additional information is needed to choose between alternative molecular structures. Chapter 11 covers ways in which the needed additional information can be obtained, using pulse sequences that were described in Chapters 8 and 9. For really difficult stereochemistry problems, two newer techniques are described, both involving partial alignment of the molecule in the magnetic field. This allows measurement of residual dipolar couplings or residual chemical shift anisotropy. Particularly when used in combination, measurements of these parameters can help to accurately define 3D structures of molecules.