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 alpha-L-rhamnopyranosyl-(1 -> 2)-alpha-L-arabinopyranoside moiety at the C-3 position is described. Betulinic acid 3 beta-O-alpha-L-rhamnopyranosyl-(1 -> 2)-[ -D-glucopyranosyl-(1 -> 4)]-alpha-L-arabinopyranoside isolated from Pulsatilla koreana and 28-O-beta-D-glucopyranosyl betulinic acid 3 beta-O-alpha-L-rhamnopyranosyl-(1 -> 2)-a-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 alpha-L-arabinose moiety are also reported. These results open the way to the synthesis of various lupane-type saponin derivatives as potentially bioactive compounds. (c) 2008 Elsevier Ltd. All rights reserved.