Structural analysis of complex saponins of Balanites aegyptiaca by 800 MHz 1H NMR spectroscopy

The main saponin (1) present in the mesocarp of Balanites aegyptiaca fruit is a mixture of 22R and 22S epimers of 26-(O-beta-D-glucopyranosyl)-3-beta-[4-O-(beta-D-glucopyranosyl)-2-O-(alpha-L-rhamnopyranosyl)-beta-D-glucopyranosyloxy]-22,26-dihydroxyfurost-5-ene. This structure differs from a previously reported saponin isolated from this source by the site of attachment of the rhamnosyl residue, and presumably represents a structural revision of the latter. The main saponin (2) present in the kernel is a xylopyranosyl derivative of 1. The use of high-field NMR enabled the practically complete assignment of 1H and 13C chemical shifts of these complex saponins, existing as a mixture of C-22 epimers. Moreover, the work represents a new approach to structural elucidation of saponins: direct preparative-scale HPLC-RID of crude extracts followed by high-field NMR investigations supported by ESI-MSn.