Features in the NMR spectra of the aglycones of Agave spp. saponins. HMBC method for aglycone identification (HMAI)

Introduction

The analysis and detection of steroidal saponins is mainly performed using chromatographic techniques coupled with mass spectrometry. However, nuclear magnetic resonance (NMR) spectroscopy is a potential tool that can be combined with these techniques to obtain unambiguous structural characterisation.

Objective

This work provides a review of the carbon-13 (¹³C)- and proton (¹H)-NMR spectroscopic data of aglycones from Agave saponins reported in the literature and also the development of an easy identification method for these natural products.

Methods

The database Scifinder was used for spectroscopic data collection in addition to data obtained from the Cadiz Allelopathy research group. The keywords used were Agave, spirostanic, furostanic, and saponin.

Results

The shielding variations produced by functional groups on the aglycone core and the structural features of the most representative aglycones from Agave species are described. The effects are additive for up to four long-range connectivities. A method for the identification of aglycones (HMAI) is proposed to classify aglycones from Agave spp. through the use of ¹H-NMR and heteronuclear multiple bond correlation (HMBC) experiments.

Conclusions

The HMBC spectrum is representative of the structural features of aglycones from Agave spp. The HMBC method for aglycone identification (HMAI) method allowed the identification of pure saponins or mixtures thereof and this method can be used in combination with chromatographic techniques coupled with mass spectrometry to provide a more thorough analysis of Agave samples that contain aglycones.