Synthesis of two natural betulinic acid saponins containing α-l-rhamnopyranosyl-(1→2)-α-l-arabinopyranose and their analogues

A concise synthesis of naturally occurring betulinic acid saponins bearing an α-l-rhamnopyranosyl-(1→2)-α-l-arabinopyranoside moiety at the C-3 position is described. Betulinic acid 3β-O-α-l-rhamnopyranosyl-(1→2)-[β-d-glucopyranosyl-(1→4)]-α-l-arabinopyranoside isolated from Pulsatilla koreana and 28-O-β-d-glucopyranosyl betulinic acid 3β-O-α-l-rhamnopyranosyl-(1→2)-α-l-arabinopyranoside isolated from Schefflera rotundifolia were easily synthesized for the first time using a stepwise glycosidation approach. The overall syntheses involved eight linear steps starting from allyl betulinate and commercially available l-arabinose, l-rhamnose and d-glucose. The syntheses of betulin and betulinic acid O-glycoside analogues containing an α-l-arabinose moiety are also reported. These results open the way to the synthesis of various lupane-type saponin derivatives as potentially bioactive compounds.