Two Basic Strategies

Structurally, steroidal saponins have two quite distinct parts: a steroid part and a sugar part. The chemical synthesis of steroids was one of the hottest fields during 1950s to 1970s after the discovery of steroidal hormones and contraceptives [7]. Almost all the famous synthetic chemists of that period did something related to steroids. And then a big steroid industry was established. Some steroids of the steroidal saponins are important industrial materials (e.g., diosgenin, tigogenin, and hecogenin, etc.); others, which can be readily prepared from them and their industrial degradation products, include pregnenolone, dehydroisoandrosterone, and progesterone (Scheme 1). It is also practical to isolate considerable amounts of some steroids from the corresponding plants after removal of the "glycoforms.''

The chemical synthesis of oligosaccharides has been a major focus since the 1980s [8] as a result of our increased understanding of the importance of these sugars in many life processes [3]. Although with low efficiency and sometimes with tremendous difficulties, almost all the sugar linkages known in nature have been synthesized.

Since the chemical syntheses of oligosaccharides and steroids have been studied intensively, the key to synthesizing a saponin turns out to be the construction of the glycosidic bond between the oligosaccharide and the steroid. Accordingly, there are two basic strategies, outlined in Scheme 2: one is to fabricate an oligosaccharide donor and then attach it to the aglycone (strategy 1); the other is to begin by con-

Pregnenolone Dehydroisoandrosterone Scheme 1 Some important industrial steroids.
Scheme 2 Two basic strategies for the synthesis of saponins.

necting the first monosaccharide to the aglycone, then manipulate the protecting groups of the sugar moiety, and extend the sugar chain sequentially (strategy 2). Strategy 1 looks straightforward; however, it is risky, considering the unavailability of generally applicable glycosylation protocols [8]. In employing strategy 2, the formation of the glycosidic bond between the sugar and the steroid can be sought to be stereospecific and in high yields at the monosaccharide level, but the subsequent protecting group manipulation becomes lengthy and complicated. From another point of view, a series of saponins with identical oligosacchride chains but different steroids can be readily prepared by means of strategy 1, by connecting the oligosac-charide donor to various steroids; if strategy 2 is used, a series of saponins with the same steroid but different oligosaccharide chains similar in pattern can be readily prepared by employing a number of monosaccharide donors in each glycosylation steps. The systematic synthesis of both series of saponins would facilitate the evaluation of the respective biological roles of the sugar part and the steroid part of saponins.

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