Sterol structure, numbering system

Sterols (from the Greek (rr pe6s meaning solid) are complex solid alcohols found in the unsaponifiable lipid fraction from the tissues of most animals and many plants. The principal, but by no means the only sterol of the vertebrates, is cholesterol, the structural formula of which, together with the conventional numbering system, is shown... 

Plant hormone biosynthesis and metabolism are influenced and modified, too, as already mentioned, by a large number of typical secondary plant constituents of quite different structures (Eef. 5, 6). For instance, it has been shown that some oi-disubstituted acetic acid derivatives are active both as antiauxins and as inhibitors of the biosynthesis of cyclic terpenoids. Thus, some more simple model compounds synthesized by ourselves, for example substituted a-phenoxy-isobutyrio acids, are not only competitive auxin inhibitors but they are also able to inhibit both the gibberellin and sterol biosynthesis. The same is true for some well known plant growth retardants, such as CCO or AMO 1618, which are inhibitors of gibberellin biosynthesis both in plants and in Fus ium moniliforme, but at the same time they are inhibiting and modifying also the biosynthesis of steroids in the respective organisms as well as in animal in vitro systems (Eef. 29) ... 

Sterols with a cyclopropane on the side chain are more numerous and have been foimd in several orders of demosponges. Most of the known structures have a A5 tetracydic system and some stanols have been isolated. A few of the most significant examples are shown in Figure 19.26 in ascending order of the number of carbon atoms. 

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