Chemical Studies on the Oriental Plant Drugs. XXIX. Saponins and Sapogenins of Ginseng : Further Study on the Chemical Properties of the Side Chain of Dammarane Type Triterpenes

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The chemical properties of the side chain of dammarane type triterpenes are discussed. Dehydrochlorination of the chloro compounds (II and IV) derived from ginsenosides R_b1, R_b2 and R_c or their prosapogenins by the conc. HCl hydrolysis afforded the compounds having the side chain with isopropylidene (I or III) and isopropenyl double bond (VII or V). The ratio of the occurrence of the two types of double bond systems depends on the dehydrochlorination reagents used. On treating with BF₃ etherate in ether, 20 (R)-protopanaxadiol (III) gave isodehydro-protopanaxadiol (VIII), which was also obtained as an artifact from the crude hydrolysate of Ginseng saponins. On treating with NBS betulafolienetriol (VI) yielded a bromo compound, C₃₀H₅₁O₃Br, whose stereochemical structure was finally established to be XVI by X-ray crystallographic analysis. A similar compound (XVIII) was obtained from 20 (R)-protopanaxadiol (III) with NBS. The mechanism of the bromination reaction is elucidated.