Tetracyclic and Pentacyclic Triterpenes

For many different vegetable oils there is a characteristic qualitative and quantitative distribution of the tetracyclic and pentacyclic triterpenes, and this is thus utilized as a fingerprint for their identification. These compounds are present in either the free or the esterified form. 

Compounds such as (69) (from gum mastic, Pistacia len-tiscus, Anacardiaceae) and (70) (Fig. 23.24) may be regarded as products of trapping of a cationic intermediate of the cyclization of squalene 2,3-epoxide (2) to tetracyclic and pentacyclic triterpenes (Harrison, 1988). 

B Formation of Tetracyclic and Pentacyclic Triterpenes from Squalene Epoxide. In most microorganisms and in plants and animals the cyclic triterpenes are derived from squalene-2,3-epoxide, which is formed from squalene by a monooxygenase (C 2.6.5). The action of different, squalene epoxide cyclases results in the formation of different types of cyclic triterpenes. 

An impressive cationic domino polycyclization has been developed by Corey and coworkers in their short and efficient enantioselective total synthesis of aegicer-adienol (1-150), a naturally occurring pentacyclic nor-triterpene belonging to the 3-amyrin family [42]. Thus, the treatment of the enantiopure monocyclic epoxy tet-raene 1-147 with catalytic amounts of methylaluminum dichloride induces a ca-tion-JT-tricyclation by initial opening of the epoxide to form the tetracyclic ketone 1-148 in 52 % yield, and its C-14 epimer 1-149 in 23 % yield, after silylation and chromatographic separation (Scheme 1.37). Further transformations led to aegicer-adienol (1-150) and its epimer 1-151. 

The msyority of natural triterpenes are pentacyclic compounds. The next largest group are the tetracyclic triterpenes. There is also a smaller number of triterpenes with various other cyclic structures. The only important acyclic triterpene is squalene (and its 2,3-oxide as a metabolic intermediate). Most triterpenes are alcohols (3-OH). They are found free and as glycosides (saponins) (Karrer, 1958 Robinson, 1975) or esters. Free triterpenes are often components of resins, latex, or cuticle. Saponins are powerful surface-active agents and can cause lysis of red blood cells. Some of them have been... 

Terpene synthesis in nature is a complex process involving successive electrophilic additions followed by a variety of skeletal rearrangements, including those of the Wagner-Meerwein variety. These reactions are typically catalyzed by enzymes and are responsible for the wide array of structural diversity in these compounds, including 6-6-6-5 tetracycles, 6-6-6-6-5 pentacycles, 6-6-6-6-6 pentacycles, and the less abundant acyclic, monocyclic, bicyclic, tricyclic, and hexacyclic triterpenoids. Each of the more than 100 triterpene skeletons identified in nature are formed through the involvement of several multifunctional triterpene synthases. 

---