Synthesis and antifungal activity of medicagenic acid saponins on plant pathogens: modification of the saccharide moiety and the 23α substitution

The study of structure-antifungal activity relationships of medicagenic acid saponins was widened to include synthetic glycosides of mannose, galactose, cellobiose, and lactose as well as a 23α-hydroxymethyl analog of medicagenic acid, namely, methyl 2β,3β-dihydroxy-23α-hydroxymethyl-Δ¹²-oleanene-28β-carboxylate, against Sclerothium rolfsii, Rhizoctonia solani, Trichoderma viride, Aspergillus niger, and Fusarium oxysporum. The native glucose-containing saponin was a more effective antifungal agent than the aforementioned saponins, except in the case of the cellobiose-containing derivative and F. oxysporum. A carboxyl substituent at the 23α position of the sapogenin brought about higher fungistatic activity than a methyl carboxylate which, in turn, was more effective than an hydroxymethyl group at the same position.