Medicinal foodstuffs. IX. The inhibitors of glucose absorption from the leaves of Gymnema sylvestre R. BR. (Asclepiadaceae): structures of gymnemosides a and b.

Although the glycosidic fraction from the dried leaves of Gymnema sylvestre R. BR., gymnemic acid, was reported to be effective for diabetes, it showed little inhibitory activity on the increase of serum glucose level in oral glucose-loaded rats. From the glycosidic fraction, six triterpene glycosides, gymnemosides a, b, c, d, e, and f, were isolated together with nine known triterpene glycosides. The structures of gymnemosides a and b were determined on the basis of chemical and physicochemical evidence as 21-O-tigloyl-22-O-acetylgymnemagenin 3-O-beta-D-glucopyranosiduronic acid and 16-O-acetyl-21-O-tigloylgymnemagenin 3-O-beta-D-glucopyranosiduronic acid, respectively. In addition, an acetyl group linked to the 16- or 22-hydroxyl group in gymnemosides a and b was found to migrate easily to the primary 28-hydroxyl group, while the acyl migration from the 28-position was rarely observed. The inhibitory activity of each triterpene glycoside from gymnemic acid was examined to determine its impact on the increase of serum glucose level in oral glucose-loaded rats. Gymnemoside b and gymnemic acids III, V, and VII were found to exhibit a little inhibitory activity against glucose absorption, but the principal constituents, gymnemic acid I and gymnemasaponin V, lacked this activity.