Studies on the Constituents of Actinostemma lobatum MAXIM. IV. Structures of Lobatosides C, D and H, the Dicrotalic Acid Esters of Bayogenin Bisdesmosides Isolated from the Herb

Abstract

Eight bayogenin glycosides, lobatosides A-H, were isolated from the herb of Actinostemma lobatum MAXIM.(Cucurbitaceae). The isolation of lobatosides A-H and the structures of lobatosides A, C, D and H are described.Lobatoside A is 3-O-[α-L-arabinopyranosyl-(1→2)-β-D-glucopyranosyl]bayogenin (2β, 3β, 23-trihydroxyolean-12-en-28-oic acid). Lobatosides C, D and H are dicrotalic acid (3-hydroxy-3-methylglutaric acid) esters of the 28-[α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl] ester, 28-[β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl] ester and 28-[β-D-xylopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl] ester of lobatoside A, respectively. Dicrotalic acid is linked at one end to the C₄-hydroxyl group of the α-L-arabinopyranosyl 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"). Lobatoside H was proved to be identical with tubeimoside I isolated from the tuber of Bolbostemma paniculatum (MAXIM.) FRANQUET (Cucurbitaceae).