Introdution to steroids

A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone and the anti-inflammatory drug dexamethasone.
The core of steroids is composed of twenty carbon atoms bonded together that take the form of four fused rings: three cyclohexane rings (designated as rings A, B and C in the figure to the right) and one cyclopentane ring (the D ring). The steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are special forms of steroids, with a hydroxyl group at position-3 and a skeleton derived from cholestane.^[1]
Hundreds of distinct steroids are found in plants, animals and fungi. All steroids are made in cells either from the sterols lanosterol (animals and fungi) or from cycloartenol (plants). Both lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene.^[2]

Structure

Steroids are a class of organic compounds with a chemical structure that contains the core of gonane or a skeleton derived therefrom. Usually, methyl groups are present at the carbons C-10 and C-13 – an alkyl side-chain at carbon C-17 may also be present.

The basic skeleton of a steroid, with standard stereo orientation. R is a side-chain at C-17.

Cholesterol. This steroid is the precursor to other steroids in the steroidogenesis.

Gonane is the simplest possible steroid and is composed of seventeen carbon atoms, bonded together to form four fused rings. The three cyclohexane rings (designated as rings A, B, and C in the figure below) form the skeleton of phenanthrene; ring D has a cyclopentane structure. Hence, together they are called cyclopentaphenanthrene.^[3]

Numbering of rings and of carbon atoms in gonane, the simplest possible steroid.

The structure of cholestane, one of the comparatively simpler steroids.

The more complex structure of cholic acid, a bile acid.

Commonly, steroids have a methyl group at the carbons C-10 and C-13 and an alkyl side chain at carbon C-17. Further, they vary by the configuration of the side chain, the number of additional methyl groups, and the functional groups attached to the rings. For example, sterols have a hydroxyl group attached at position C-3.
Some exemplary steroids with their structures:

The anabolic steroid testosterone, the principal male sex hormone.

Progesterone, a steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis.

Medrogestone, a synthetic drug with similar effects as progesterone.

An example of functional groups is the hydroxyl group at C-3 common to sterols.

β-Sitosterol, a phytosterol showing the hydroxyl group at C-3.