Triterpenes, structural chemistry

Triterpenes, structural chemistry, 136 Tropopause, emissions model, 605 Troposphere ozone analysis, 605 trifluoromethyl peroxynitrate, 743 Tryptophan... 

Triterpenes are C30 compounds, produced from two molecules of famesyl pyrophosphate (FPP) condensed head-to-head. Representative structures are shown in Fig. 20. The compounds shown represent only a few of the many triterpenes found in plants. They are chosen to suggest the great variety of structures and to illustrate specifically some of the compounds that have played, or are playing, key roles in studies of triterpene chemistry and biochemistry. For a detailed survey of triterpene structures the reader is referred to Connolly and Overton (1972) and Ourisson et al. (1964), as well as Devon and Scott (1972). Analytical techniques for triterpenoids, and the distribution of triterpenoids in plants, have been reviewed by Rastogi and co-workers (Kulsh-reshtha et al., 1972 Pant and Rastogi, 1979). Chandler and Hooper (1979) have recently reviewed the literature on fiiedelin and associated triterpenes. 

A comprehensive rationale for structural and stereochemical outcome of squalene cyclization in terms of conformation dictated by the cyclase has been build up by the Zurich school. It provides a convenient basis for discussing various triterpene structures (263, 323). These conformations are described in terms of section-wise folding of the squalene chain into a chair (C), or boat (B) conformation or a part remaining unfolded (U). The following discussion of triterpenoids relevant to wood chemistry is based on these considerations. 

These structures were revised in the literature, "O. Shirota, H. Morita, K. Takeya and H. Itokawa, Revised structures of cangorosins, triterpene dimers from Maytenus ilicifolia, J. Nat. Prod. 60 (2), 111-115 (1997)". The related literatures were as follows, a) Shirota, O., Morita, H., Takeya, K. and Itokawa, H., Structures of Xuxuarines, Stereoisomeric Triterpene Dimers from Maytenus chuchuhuasca. Tetrahedron 51 (4), 1107-1120 (1995), b) Shirota, O., Morita, H., Takeya, K. and Itokawa, H., Novel Stereoisomeric Triterpene Dimers, Xuxuarines Aa and Ap, from Maytenus chuchuhuasca. Chemistry Letters 10I-I02 (1995), c) Shirota, O., Morita, H., Takeya, K. and Itokawa, H., Five New Triterpene Dimers from Mayterms chuchuhuasca, J. Nat. Prod. 60 (11), 1100-1104 (1997) and d) Shirota, O., Morita, H., Takeya, K. and Itokawa, H., New Geometric and Stereoisomeric Triterpene Dimers from Maytenus chuchuhuasca, Chem. Pharm. Bull. 46 (1), 102-106 (1998). 

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl diphosphate produce a rich variety of structural types in the sesquiterpene series. We will have more to say about the chemistry of higher terpenes, especially the triterpenes, later in this chapter. For the moment, however, let s return to smaller molecules to complete the picture of how isoprenoid compounds arise from acetate. 

Presently, triterpenoids constitute the third largest family of terpenoids with over 1500 compounds (excluding nor derivatives) embracing some 40 skeletal types. Because they are biogenetically and structurally related to the biologically important steroids, triterpenoids have attracted considerable attention and, together with steroids, have provided an experimental basis for the development of principles of conformation analysis. The chemistry of triterpenoids has been reviewed several times (38, 89, 151, 176, 245, 304, 309, 351). A special review covers synthetic efforts in this area (9). Figure 8.1.27 lists triterpene skeletal types of frequent occurrence in nature or those relevant to our discussion. 

Ames, T. R., J. L. Beton, A. Bowers, T, G. Halsall, and E. R. H. Jones The Chemistry of the Triterpenes and Related Compounds. Part XXIII. The Structure of Taraxasterol, T -Taraxasterol (Heterolupeol), and Lupenol-I. J. Chem. Soc. 1954, 1905, and references cited therein. 

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