Artefact formation during acid hydrolysis of saponins from Medicago spp.

Artefact compounds obtained during acid hydrolysis of saponins from Medicago spp. (Fabaceae), have
been monitored and evaluated by GC-FID. Their identification has been performed by GC-MS and 1H and
13C NMR. Saponins with different substituents on the triterpenic pentacyclic aglycones were considered,
and their hydrolysis products were detected and quantified during 10 h of time course reaction. From
soyasapogenol B glycoside the well known soyasapogenols B, C, D and F were obtained together with a
previously undescribed sapogenol artefact identified as 3b,22b,24-trihydroxyolean-18(19)-en and named
soyasapogenol H. From a zanhic acid saponin two major artefact compounds identified as 2b,3b,16atrihydroxyolean-
13(18)-en-23,28-dioic acid and 2b,3b,16a-trihydroxyolean-28,13b-olide-23-oic acid
were obtained, together with some zanhic acid. Other compounds, detected in very small amount in the
reaction mixture, were also tentatively identified based on their GC-MS and UV spectra. The other most
characteristic saponins in Medicago spp., hederagenin, bayogenin and medicagenic acid glycosides, under
acidic condition of hydrolysis, released instead the correspondent aglycones and generated a negligible
amount of artefacts. Nature of artefacts and mechanism of their formation, involving a stable tertiary
carbocation, is here proposed and discussed for the first time.