Triterpenes and Steroids
Generalities
Triterpenes—4,000 compounds built upon over 40 different skeletons—are C30 compounds arising from the cyclization of epoxy-3S-2,3-epoxy-2,3-dihydro-squalene, or, in rare cases, from squalene itself. Almost always hydroxylated at C-3 (because they arise from the opening of the epoxide), triterpenes present very high structural homogeneity: the major differences are in configuration, and are linked to the conformation adopted by the squalene epoxide (or by squalene) prior to cyclization; the cation resulting from this cyclization can subsequently undergo a series of 1,2 proton and methyl group shifts, which can be used to rationalize the occurrence of the different tetra- and pentacyclic skeleta characteristic of this group.
Structural homogeneity is also marked among the steroids: compounds as different, by their properties, as phytosterols, saponins, ecdysteroids, cardiac glycosides, or steroidal alkamines, all have the same basic skeleton.
3:
Tetracyclic triterpene* Steroid * Pentacyclic triterpene
* avti-o oorhonc pf tho 94-nncmnn are ni imhprprl 9d> p| ?42
TERPENOIDS
At first glance, it is fair to say that there are no fundamental differences between triterpenes and steroids, as these may be considered to be tetracyclic triterpenes having lost, at a minimum, three methyl groups (in fact it is the presence of the methyl groups at C-4 and C-14 which was initially used to distinguish steroids from triterpenes [Ourisson]). Only by considering the biosynthesis is it possible to separate the two groups: a compound like cycloartenol (C30) must be considered a 4,4-dimethylsterol (it is a sterol precursor), whereas euphol or the dammaranes (also with 30 carbon atoms) are tetracyclic triterpenes. Separating the two groups is not always easy: for example, where do the cucurbitacins belong? With sterols—which are derived from protostane—or, according to many authors, in the group of tetracyclic triterpenes? A 1991 publication covers the lanostanes in the chapter on triterpenoids and lanosterol in the chapter on phytosterols.
APPLICATIONS OF TRITERPENES AND STEROIDS
In view of their therapeutic and industrial applications, triterpenes and steroids constitute a group of secondary metabolites of the utmost importance.
• Consider the applications of cardiac glycosides, which no synthetic product
has completely replaced.
• Consider the applications of spirostane-type saponins, of sitosterol, and of stigmasterol, which are valuable starting materials for many approaches in biotechnology. They remain irreplaceable to fulfill the needs of the pharmaceutical industry for steroidal medications (contraceptive, anabolic, anti-inflammatory
agents).
• Consider the therapeutic applications of many saponin-containing drugs, used for the extraction of active compounds (aescin, glycyrrhizin), to obtain simple galenicals, or to prepare phytotherapeutic products.
• Consider the economic importance of licorice, a low-calorie sweetener, widely used in food technology.
• Consider the importance of saponins, in that their presence can substantially decrease the nutritional value of fodder (alfalfa), or impart to plants that are familiar in our every day environment a non-trivial toxicity.
• Consider their therapeutic potential in many varied fields: as cytostatics, insecticides, anti-inflammatory agents, products toxic to molluscs, analgesics, and
TRITERPENES AND STEROIDS: GENERALITIES
663
BIOSYNTHESIS OF TRITERPENES AND STEROIDS
Although steroids in animals, fungi, algae, and higher plants all arise from a common pathway which leads, from acetate, via mevalonate, to squalene epoxide, upon closer examination it appeal's that from that point, the biosynthetic pathways diverge considerably. The first sterol synthesized by animals and fungi is lanosterol; which is converted to cholesterol in most animals, and to ergosterol in fungi. In the case of the Eucaryotes capable of photosynthesis (Algae, Bryophytes, Pteridophytes, Spermatophytes), all of the sterols (phytosterols, cardenolides, spirostanes, solanidanes) arise from the stepwise demethylation of cycloartenol and from the opening of its 9(3,19-cyclopropane ring. In addition, these vegetable organisms are capable of cyclizing squalene epoxide in a conformation that leads specifically to the tetracyclic triterpenes of the laticiferous ducts of the Euphorbiaceae, as well as to the saponins with a pentacyclic triterpenoid aglycone, or to the modified triterpenes of the Rutales (quassinoids, meliacins, limonoids).
Initial cyclization
It is the opening of the epoxide which initiates the cyclization. In order for this cyclization to take place, the cyclization enzyme must stabilize the conformation of the polyisoprene in a manner that fulfills the stereoelectronic requirements of the cyclization. It is the initial conformation of the squalene epoxide which determines the orientation of the biosynthesis toward either steroids and cucurbitacins, or else triterpenes in the strict sense of the term.
Squalene-2,3-oxide
1. If the squalene epoxide is maintained in a chair-boat-chair conformation, die cyclization leads to a protostane cation, which is the immediate precursor of cycloartanes and cucurbitanes, by a series of 1,2-proton and methyl group shifts (these shifts are made possible by the frans-antiparallel arrangement of the protons
Ö64
FERPENOIDS
2. If the squalene epoxide is maintained in a chair-chair-chair conformation, the cyclization leads to a dammarane cation (see, for example, the aglycones of the saponins of ginseng), which can also undergo rearrangement:
- either by concerted migrations leading to tirucallol and euphol, the precursors of limonoids and quassinoids,
- or, in most cases, by the formation of an extra ring, leading to pentacyclic triterpenes (oleananes, ursanes, lupanes, friedelanes, taraxastanes),
- or, in rare cases, to form tetracyclic compounds with a six-membered D ring (baccharanes, shionanes).
3. A somewhat special case is that of triterpenes devoid of a hydroxyl group at C-3. They are generally derived from the direct cyclization of squalene: hopanes (characteristic of natural sediments), fernanes (the conformation of the precursor is of the chair-chair-chair-boat type).
4. In some cases, the cyclization is only partial (polypodatetraenes, malabaricanes), or on the contrary it is complete, and completely concerted (boehmerol, arborinol: chair-boat-chair-chair-boat), or even initiated from both ends of the precursor (onoceranes). Some unusual structures have also been described (C3i aldehydes from Iris), especially in the animal kingdom (siphonales from the Spongiae).
Shionone
Awv-Fernene
Boehmerol
Malabaricol
a-Polypodatetraene
Triterpenes resulting from an alternate mode of cyclisation
TRITERPENES AND STEROIDS: GENERALITIES
665
Fate of squalene: origin of triterpenes and steroids
Cucur bitanes
Cycloartanes stemjds
Pentacyclic triterpenes
667
Formation of the steroids
As described above, animals and fungi elaborate lanosterol—the rearrangement ends with the release of the proton at C-9—whereas plants make cycloartenol: the rearrangement ends with the formation of cyclopropane by a reaction which is probably catalyzed by an enzyme.
The conversion of a C30 skeleton to a C27 or smaller skeleton, in other words to a steroid, involves—at a minimum—a stepwise demethylation at C-4 and C-14; note also the opening of the cyclopropane ring and a migration of the double bond resulting from this opening. Both methyl groups at C-4 are lost through a series of oxidations (CH3 —> CH2OH —> CHO —> C02H) which ends with a decarboxylation. A preliminary oxidation of the hydroxyl group at C-3 leads to an cc-ketoacid, which facilitates the final decarboxylation. The methyl group at C-14 is eliminated by oxidation to formic acid.
etc.
Principle of the elimination of the methyl group at the 4-position
In animals, the side chain remains intact (cholestane), becomes truncated (C24 cholanes, C21 pregnanes), or even eliminated (Cjg androstanes, Cls estranes), but in plants, the side chain may be functionalized and cyclized (spiroketals, alkamines, ecdysteroids), shortened (pregnanes) and functionalized (cardenolides, conanines), or—this is frequent—it may possess one or two extra carbons in the form of a methyl (or methylene) group, or an ethyl (or ethylidene) group at C-24. This characteristic of C2g or C29 phytosterols from higher plants is also found in the algae (fucosterol), fungi (ergosterol), and marine organisms.
The introduction of extra carbon(s) into the side chain of the steroids results from transmethylations involving S-adenosylmethionine. The first transmethylation generally precedes the demethylations at C-4 and C-14, and the second one commonly takes place at; later time. Sterols with a 24-methylene or a 24-ethylidene group can isomerize to the corresponding 24-methyl- and 24-ethyl A2* sterols, which can yield, by a stereospecific reduction, the 24a-alkylsterols characteristic of plants, for example, sitosterol (ethyl group) or campesterol (methyl group).
The other possible fates of the steroid nucleus appeal' in the figure on p. 670, and will be summarized below.
668
TERPENOIDS
Examples of modifications of the C-17 side chain of phytosterols
Formation of triterpenes
The guiding principles which result in the elaboration of the chief triterpenoid skeletons appear in the figure on p.535. Although a fair number of the proposed sequences remain hypothetical, they are, however, supported by the fact that some of the proposed reactions can be achieved in vitro in acidic medium, under conditions that mimic biosynthesis. Furthermore, a certain number of experiments with labeled compounds—among others, with 13C double-labeled acetate—have been conducted: they demonstrate the validity of several of the proposed mechanisms.
The secondary modifications of triterpenes are rather limited: additional hydroxylations, dehydrogenations, functionalizations of the angular methyl groups, and lactonizations are the most common (see below: saponins). One exception lies in several Rutales families (Rutaceae, Meliaceae, Simaroubaceae, Cneoraceae) in which the initial tetracyclic skeleton can undergo profound modifications: oxidation, ring opening and closure, side chain elimination, and more (see p. 764 and 766).
BIBLIOGRAPHY
Brown, G.D. (1998). The Biosynthesis of Steroids and Triterpenoids, Nat. Prod. Rep., 15, 653-696.
Connolly, J.D. and Hill, R.A. (1996). Triterpenoids, Nat. Prod. Rep., 13, 151-169.
Mahato, S.B. and Sen, S. (1997). Advances in Triterpenoids Research, 1990-1994,
Phytochemistiy, 44, 1185-1236.
John Goad. L. (1991). Phytosterols, in "Methods in Plant Biochemistry, vol. 7, Terpenoids", OTiftrlv/nnH BV «m»n««*----i-> tt ••»«   ■ •
TRITERPENES AND STEROIDS; GENERALITIES
669
s
• JBr.
Fiedelin FRIEDELANES
Taraxasteroi TARAXASTANES
a-Amyrin URSANES (three 1,2 shifts)
Interconversions in the pentacyclic triterpene series (examples)_
Other examples of pentacyclic triterpenes ~
Multiflorenol
Bauerenol
TERPENOIDS
f
Spirostane (0), Spirosolane (N)
Cardanolide
Conanine
Chief basic steroidal skeleton found in higher plants
vi ;
Triterpenes and Steroids
1. Introduction..................................................................................................................672
2. Structure of Saponins...................................................................................................672
A. Structure of the Aglycones.............................................................................673
B. Structure of the Glycosides............................................................................676
3. Extraction, Characterization, and Quantitation............................................................677
4. Biological and Pharmacological Properties.................................................................681
5. Starting Materials for Steroid Hormone Semisynthesis..............................................684
A. Sapogenins.....................................................................................................684
B. Other Starting Materials.................................................................................687
C. Conversion of Stalling Materials to Steroids of Therapeutical Interest........688
6. Chief Saponin-containing Drugs.................................................................................688
A. Saponin-containing Drugs that are Chiefly Anti-inflammatory.....................688
Licorice.............................................................................................388
Common Horse Chestnut.................................................................694
B. Saponin-containing Drugs of Use in Phlebology and Proctology..................697
Butcher's Broom..............................................................................697
Fig wort.............................................................................................698
C. Saponin-containing Drugs Useful for Treating Cough..................................699
Senega Snakeroot.............................................................................699
Common Ivy.....................................................................................701
Primrose...........................................................................................702
D. Saponin-containing Drags of Use in Dermatology........................................703
Hydrocotyle......................................................................................703
Tepescohuite.....................................................................................704
M.,,.;™]^' ................................705