Isolation and structure elucidation of gymnemic acids, antisweet principles of Gymnema sylvestre.

The structure of gymnemagenin (3 beta,16 beta,21 beta,22 alpha,23,28-hexahydroxy-olean-12-ene), the sapogenin of the antisweet principles of Gymnema sylvestre, was established by X-ray analysis of the 3 beta,23;21 beta,22 alpha-di-O-isopropylidene derivative. On the basis of this result, the structure of deacylgymnemic acid was elucidated as the 3-O-beta-glucuronide from the carbon-13 nuclear magnetic resonance spectra. Five antisweet principles, gymnemic acid-III, -IV, -V, -VIII, and -IX, were isolated in pure states from the hot water extract of leaves of Gymnema sylvestre. Of these, three (GA-III, -IV, and -V) were known, while two (GA-VIII and -IX) were new compounds. The structures of GA-VIII and -IX were elucidated as 3'-O-beta-D-arabino-2-hexulopyranosyl gymnemic acid-III and -IV, respectively.