Synthesis and structure-activity relationship of oleanolic mono- or di-glycosides against Magnaporthe oryzae

Saponins are naturally-occurring units with broad diversity and are usually recognized as phytoanticipins. In order to develop new saponin chemical entities with high activity against Magnaporthe oryzae, we selected oleanolic acid (OA), which has wide natural distribution and rich content in plants. We used the ability of OA to act as an aglycone for glycosylation to obtain information on the structure-activity relationship (SAR) for rational molecular pesticide design. Oleanolic mono- or di-glycosides were synthesized at either the C₃-hydroxy and/or C₂₈-carboxyl position, using trichloroacetimidate or glycosyl bromide donors, respectively. Structures were confirmed by [¹H]-,[¹³C]-NMR. Furthermore, the activity of the synthesized glycosides against M. oryzae was assessed in vitro, based on the mycelium growth rate. The twenty five oleanolic mono- or di-glycosides comprised fourteen saponins with 3-monosaccharide residue 1a-1n, six saponins with 28-monosaccharide residue 2a-2f, and five saponins with 3, 28-monosaccharide residue 3a-3e; all showed different activities against M. oryzae according to their different structures. We concluded that the optimal oleanolic mono- and di-glycoside structure for activity against M. oryzae is a C₃ connection of a hexose such as mannose, galactose, or glucose, in combination with a C₂₈ connection to a small group such as allyl or a C₃ connection to a pentose accompanied by a larger group such as another pentose or heptenyl at C₂₈.