Synthesis of glycyrrhetic acid diglycosides and their cytoprotective activities against CCl4-induced hepatic injury in vitro

Glycyrrhetic acid diglycosides 16, 24, 25, 42 and 46, with respectively beta-D-glucuronopyranosyl-(1-->3)-beta-D-glucopyranose, -(1-->6)-alpha-D-glucopyranose, -(1-->6)-beta-D-glucopyranose, -(1-->6)-beta-D-galactopyranose, and beta-D-galacturonopyranosyl-(-->2)-beta-D-glucopyranose as sugar components at the O-3 positions on the aglycons, were synthesized. In vitro cytoprotective activities, against CCl4-induced hepatic injury, of the synthetic diglycosides, methyl beta-D-glucuronopyranosyl-(1-->4)-alpha-D-glucopyranosyl-D-glycyrrhetinate 33 and methyl esters 15 and 23 (the precursors of 16 and 24 respectively) were compared with those of glycyrrhizin 1 and beta-D-glucuronopyranosyl-(1-->2)-beta-D-glucopyranosyl-glycyrrhetic acid 2. Of the glycosides 16, 24, and 25, with beta-D-glucuronopyranosyl-glucopyranose as the sugar component, 16 and 24 were as cytoprotective as 1 and 2, whereas 25 showed no remarkable activity. From stereomodels of the glycosides these differences in activity were inferred to be due to the stereochemistries of the terminal beta-D-glucuronopyranoses in the molecules. Glycoside 46, in which the terminal beta-D-glucuronopyranose of 2 was replaced by beta-D-galacturonopyranose, was as potent as 2. Further, it was confirmed that a free COOH group on the E ring of aglycon was essential for the activity.