Steroids are an important class of compounds occurring in all animals and plants, which have many important functions from regulating growth (anabolic steroids) and sex drive (all sex hormones are steroids) to acting as a self-defence mechanism in plants, frogs, and even sea cucumbers. A steroid is defined by its structure: all steroids contain a basic carbon framework consisting of four fused rings—three cyclohexane rings and one cyclopentane ring—labelled and joined together as shown in the margin. Just as in the decalin system, each ring junction could be cis or trans, but it turns out that all steroids have all trans-junctions except where rings A and B join, which is sometimes cis. Examples are cholestanol (all trans) and coprostanol (A and B fused cis).

Because steroids (even those with a cis A–B ring junction) are essentially substituted trans-decalins, they can’t ring-flip. This means, for example, that the hydroxyl group in cholestanol is held equatorial on ring A while the hydroxyl group in coprostanol is held axial on ring A. The steroid skeleton really is remarkably stable—samples of sediment 1.5 × 10⁹ years old have been found to contain steroids still with the same ring-junction stereochemistry.

مواضيع ذات صلة

Distinguishing acid derivatives by carbon NMR is difficult

E1cB eliminations in context

Anion-stabilizing groups allow another mechanism—E1cB

The regioselectivity of E2 eliminations

E2 elimination from vinyl halides: how to make alkynes

E2 eliminations from cyclohexanes

E2 eliminations can be stereospecific

E2 eliminations have anti-periplanar transition states

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms