Strategy 1 Synthesis of Maidong Saponin C

Maidong, one of the most commonly used Chinese herbs, is held to nourish ''yin'' (tonic effect; "yin" is one of the basic philosophical concepts in traditional Chinese medicine). Saponins are believed to be the active principles in Maidong plants, which are found in several species, including some in the Ophiopogon and Liriope genera. Thus far more than 60 steroidal saponins have been isolated from these species [2c]. Yu et al. isolated Maidong saponin C (16) from the tuber of Liriope muscari [12]. Pharmacological studies showed that 16 possessed strong anti-inflammatory activity [13].

According to strategy 1, Maidong saponin C (16) was broken down into tri-saccharide donor 17 and steroid 18 (Scheme 7). Again, a trichloroacetimidate donor (17) was used; without a neighboring participation group, an SN2 reaction is expected to achieve the formation of a ^-glycosidic bond [8b]. Steroid 18 was conceivably derived from diosgenin by introducing a 1^-OH. The 3-OH of 18 was believed to be more active than the 1/8-OH, and therefore protected as a TBS ether.

Steroid 18 was prepared from diosgenin in 6 steps and 14% yield (Scheme 8). The configuration of the 1a-OH (22) could not be inverted by using SN2-type reactions, probably owing to the stereohindrance of this position. Therefore an oxidation-reduction sequence was used to epimerize the 1a-OH (22 ^ 18). The trans-

Maidong Saponin C (16)

D-Galactose + D-GIucose + D-Xylose Diosgenin

Scheme 7 Retrosynthesis of Maidong saponin C (16).

formation of 20 to diol 21 under Li/NH3 reduction was found to be capricious; the A6 product or other products sometimes dominated.

Schmidt glycosylation [8b] was used twice to prepare the trisaccharide donor 17 (Scheme 9). Attempts to use xylopyranosyl imidate donor 28 in the regioselective glycosylation of diol 26, which has an equatorial 3-OH and an axial 4-OH as diol 11, were not successful. The deprotection of the anomeric allyl group of trisaccharide 29 was found to be problematic. When reactions were catalyzed by palladium spe-

Scheme 8 Preparation of aglycone 18. (a) DDQ, 69%. (b) H2O2, 68%. (c) Li/NH3, 53%. (d) TBSCl, 90%. (e) PDC, 92%. (f) NaBH4, 70% (for 18).

AcO AcO

AcO AcO

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