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- May 15, 2018 -

The basic structure of steroids, gonane (cyclopentanoperhydrophenanthrene), has 17 carbons which are arranged as three six-member carbon rings to which a five-member carbon ring is attached (Fig. 1). Each carbon has one or two hydrogens attached. The gonane structure can be represented without showing the carbons and hydrogens, as shown in Figure 1.

Figure 1. Chemical structure of gonane, the basic steroid nucleus.

Steroids can be divided into different groups of parent compounds, based on the number of carbons that they contain (Fig. 2). In addition to gonanes, which consist of 17 carbons, estranes consist of 18 carbons (C18 steroids) and include estrogens. Androstanes have 19 carbons (C19 steroids) and include androgens. Pregnanes contain 21 carbons (C21 steroids) and include progesterone and corticosteroids. This chapter focuses primarily on C18, C19, and C21 steroids. Cholanes have 24 carbons and include bile acids, and cholestanes have 27 carbons and include cholesterol as well as cholesterol-like compounds. The compounds in this group are also referred to as sterols.

Figure 2. Classification of steroids, based on the number of carbons in the molecule.

In each group of parent steroids, compounds differ in their characteristics because of the presence of different functional groups on the molecules. Common functional groups include the ketone group, hydroxyl group, and double bond, as shown in the chemical structure of the cortisol molecule in Figure 3. Other functional groups include the carboxyl and aldehyde groups, which are present in the molecules of bile acids and aldosterone, respectively (see Fig. 3). An important characteristic of the C18 steroids is the presence of an aromatic ring that is found in estrogens (e.g., estradiol) (see Fig. 3).

Figure 3. Functional groups present in chemical structures of steroids.