Synthesis of Betavulgaroside III, a Representative Triterpene seco-Glycoside

Triterpene seco-glycosides constitute a small family of the plant saponins which feature a terminal seco-saccharide appendage deriving supposedly from oxidative scission of a monosaccharide unit. Herein, we have developed synthetic approaches for the first time to the access to these molecules. Betavulgaroside III (1), a representative congener occurring in Beta vulgaris and Achyranthes fauriei, is successfully synthesized in a total of 31 steps with l-arabinose, d-glucose, and oleanolic acid as starting materials. The longest linear sequence requires 23 steps and in an overall 0.9% yield (from d-glucose). The synthesis features oxidative elaboration of the seco-saccharide unit prior to assembly of the triterpene 3,28-bisglycoside. This tactic has been proven superior, in the attempts to the synthesis of the more easily accessible 2′′-epi-betavulgaroside III (2), to that employing oxidative cleavage of the terminal saccharide unit at an advanced triterpene 3,28-bisglycoside scaffold.