Fortschritte der Chemie organischer Naturstoffe / Progress in the Chemistry of Organic Natural Products (eBook)

Contents 6
List of Contributors 8
Chlorophyll Catabolites 9
1. Introduction 10
2. Chlorophyll Catabolites from Vascular Plants 14
3. Chlorophyll Catabolites from the Green Alga Chlorella protothecoides 38
4. Chlorophyll Catabolites from Marine Organisms 40
5. Conclusions and Outlook 42
References 45
Steroidal Saponins 53
1. Introduction 53
2. Isolation 54
3. Structure Elucidation 57
4. Biological Activity 70
5. Biosynthesis of Steroidal Glycosides 77
6. Report of New Steroidal Saponins ( 1998– Mid- 2006) 78
7. Conclusion 134
Author Index 151
Subject Index 161

Steroidal Saponins (p. 45-47)

N. P. Sahu, S. Banerjee, N. B. Mondal, and D. Mandal

Indian Institute of Chemical Biology, 4 Raja S C Mullick Road, Kolkata 700 032, India

1. Introduction

The medicinal activities of plants are generally due to the secondary metabolites (1) which often occur as glycosides of steroids, terpenoids, phenols, etc. Saponins are a group of naturally occurring plant glycosides, characterized by their strong foam-forming properties in aqueous solution. The cardiac glycosides also possess this property but are classified separately because of their specific biological activity. Unlike the cardiac glycosides, saponins generally do not affect the heart. These are classified as steroid or triterpenoid saponins depending on the nature of the aglycone. Steroidal glycosides are naturally occurring sugar conjugates of C27 steroidal compounds. The aglycone of a steroid saponin is usually a spirostanol or a furostanol. The glycone parts of these compounds are mostly oligosaccharides, arranged either in a linear or branched fashion, attached to hydroxyl groups through an acetal linkage (2, 3). Another class of saponins, the basic steroid saponins, contain nitrogen analogues of steroid sapogenins as aglycones.

Steroidal glycosides have drawn much attention in the last few decades not only as economically important raw materials for the pharmaceutical industry used in the production of various steroidal hormones (4–7) but also as biologically active compounds (8–13) and as ingredients for cosmetics (14). General reviews dealing with steroid saponins have been published earlier by Tschesche and Wulff (15), Elks (16, 17) and Takeda (18). Following our previous review of steroid saponins (19) which covered the literature up to 1980 a number of reviews dealing with specific aspects of spirostanes, furostanes and their glycosides have appeared (20–32) covering the literature up to early 1998. The present review is a compilation of steroid saponins isolated during the period 1998 to mid 2006 together with their biological activities. It also includes a summary of the latest developments in purification processes and structure elucidation techniques (mainly NMR and mass spectrometry).

2. Isolation

The methods for isolation of steroid saponins are similar to those of triterpenoid saponins. Since glycosides, as a class, are particularly prone to enzymatic or microbial degradation, processing of plant material needs to be started soon after collection to avoid delays. Air-dried powdered plant material is defatted and then extracted, either with cold or hot methanol or ethanol or with 50% aqueous ethanol or methanol at ambient temperature. Usually the extract is concentrated at reduced pressure, macerated with water, and partitioned successively using ethyl acetate and n-BuOH. Most of the saponin constituents are found in the n-BuOH soluble fraction. However, highly polar glycosides may be found in the aqueous layer.

Steroidal saponins are usually highly polar compounds occurring as complex mixtures, and their separation into individual components is a formidable task. The traditional purification and separation process for steroidal saponins consists of repetitive chromatography on silica gel columns using chloroform-methanol and/or chloroform-methanol-water as eluent, followed by fractional crystallization, preparative TLC, etc.