Abstract
This review outlines methods to investigate the structure of natural products with emphasis on intramolecular hydrogen bonding, tautomerism and ionic structures using NMR techniques. The focus is on 1H chemical shifts, isotope effects on chemical shifts and diffusion ordered spectroscopy. In addition, density functional theory calculations are performed to support NMR results. The review demonstrates how hydrogen bonding may lead to specific structures and how chemical equilibria, as well as tautomeric equilibria and ionic structures, can be detected. All these features are important for biological activity and a prerequisite for correct docking experiments and future use as drugs.
Highlights
This review has discussed how rather simple NMR measurements can help in determining hydrogen bonds, the presence of tautomerism or the occurrence of ionic or zwitter ionic natural compounds
Hydrogen bonding may be important in defining the structure
Hydrogen bonding may be important in defining the structure of molecules, in the sense that side-chains are fixed and the hydrogen bond energies can be quite high
Summary
An excellent overview dealing with this trend is given by Newman and Cragg [1]. Several reviews exist, covering natural products and drugs in some way [3,4,5,6,7,8]. Examples are presented to illustrate the theme of the paper. The present review will not deal with assignment of NMR spectra of natural products based on 2D NMR techniques, as this topic has been covered recently [9,10] and has been described in a very illustrative way [11,12].
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