Studies on the constituents of Actinostemma lobatum MAXIM. V. Structures of lobatosides B, E, F and G, the dicrotalic acid esters of bayogenin bisdesmosides isolated from the herb.

Abstract

The structures of lobatosides B, E, F and G, the dicrotalic acid esters of bayogenin bisdesmosides isolated from the harb of Actinostemma lobatum MAXIM. (Cucurbitaceae), were determined on the basis of chemical and spectral evidence.Lobatoside B is the dicrotalic acid (3-hydroxy-3-methylglutaric acid) ester of 3-O-[β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl]bayogenin 28-[α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl] ester. Dicrotalic acid is linked at one end to the C6-hydroxyl group of the terminal β-D-glucopyranosyl group in the C3-linked sugar moiety, and at the other end to the C4-hydroxyl group of the α-L-rhamnopyranosyl group in the ester-linked sugar moiety to form a macrocyclic structure (cyclic bisdesmoside).Lobatosides E, F and G are cyclic bisdesmosides of bayogenin similar to lobatoside B, but different in the numbers and species of the component sugars and positions of the ester linkages of the dicrotalic acid.