Chemical Studies on the Oriental Plant Drugs. XXVII. The Acid Catalyzed Reactions and the Absolute Configuration at C(20) of Dammarane Type Triterpenes

On acid treatments, betulafolianetriol (VII) having the same configuration at C(20) as dammarenediol II (VI) gave an equilibrated mixture of VII and its C(20) epimer (VIII). The derivatives of betulafolienetriol (IV) and VII having a hydroxyl or methoxyl group at C(12) also showed the configurational equilibration at C(20) hydroxyl under the same condition, whereas those without hydroxyl or methoxyl at C(12) showed no or negligible epimerization at C(20). The acid catalyzed pyran ring formation from the side chain of betulafolienetriol (IV) gave 3, 20-epi-panaxadiol (XII) and 3-epi-panaxadiol (XI) by the retention and the inversion of its C(20)-configuration, respectively. It reveals that C(20)-configuration of panaxadiol (I) and protopanaxadiol (III) (epimeric at C(3) and C(20) of IV) are same as that of dammarenediol I (V). Since the chirality at C(20) of panaxadiol (I) has already been established as R, dammarenediol I type triterpenes have (R)-configuration at C(20), and dammarenediol II type compounds S. This conclusion has also been supported by the IR-absorptions of C(20) keto and C(20) hydroxyl of 12-keto-3-acetates (XVII and XVIII) of betulafolianetriol (VII) and dihydroprotopanaxadiol (X). The mechanism of epimerization at C(20) on the acid treatments of dammaran-20-olcompounds having O-function at C(12) was discussed.