(3b,16a,21b,22a)-22-Acetoxy-16,24,28-trihydroxy-21-{[(2Z)-2-methyl-2-butenoyl]oxy}olean-12-en-3-yl b-D-glucopyranosyl-(1->2)-[b-D-glucopyranosyl-(1->4)]-b-D-glucopyranosiduronic acid