NMR Techniques for the Structure Elucidation and Conformational Analysis of Natural Products

Abstract

For many years, we have been involved in the structure elucidation of biologically active natural products. As the availability of techniques for work in this area have changed, so has our ability to conduct detailed spectroscopic analysis on the isolated compounds. Several years ago it would not have been reasonable to determine the complete proton and carbon assignments for a complex, new natural product. The techniques currently available are such that it is now typically possible to prove all of the carbon and proton assignments, without relying on prior information or using chemical shift theory to assign carbon chemical shifts. Philosophically, this is an important distinction to make, for relatively few of the carbon-13 attributions in the literature could be regarded as proven unambiguously. Rather, reliance is usually placed either on model compounds or on additive or subtractive shifts based on the data for simple analogues. As a result, numerous proton and carbon-13 assignments must be either reconfirmed or revised, even in well-established natural products.