Chapter 13: Nuclear Magnetic Resonance in Saponin Structure Elucidation

Saponins are natural glycosides occurring primarily in plants. These natural compounds are present in many traditional medicines, attracting researchers to investigate their chemistry and pharmacological/biological activity. The isolation and structure elucidation of saponins can be formidable tasks in view of their structural diversity, amphiphilic character, and the fact that they occur in complex mixtures. Traditionally, structure elucidation of these compounds has been done by using extensive hydrolytic or enzymatic degradation and chemical derivatization together with GCMS and ¹H and ¹³C 1D nuclear magnetic resonance (NMR) methods in order to identify the aglycone, the number and type of monosaccharide residues present, and their mutual connections. Nowadays, the application of modern NMR techniques to the structure elucidation of saponins provides rich structural information, as well as detailed pictures of their stereochemistry. Oleanane triterpene and steroidal saponins, along with steroidal alkaloids (also included here under the broad saponin designation), illustrate the structural discussions throughout the present text. Rather than providing extensive coverage of the saponin structural elucidation examples that are scattered in the literature, this chapter presents parallel objective examples on how to get structural information by using specific NMR techniques and the ever-growing proton and carbon chemical shifts database that is now available in the literature.