Steroid Nomenclature and Structure

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Chapter: Medicinal Chemistry : Steroids

Steroids consist of four fused rings (A, B, C, and D). Chemically, these hydrocarbons are cyclopentano per hydro phenenthrenes.


STEROID NOMENCLATURE AND STRUCTURE

Steroids consist of four fused rings (A, B, C, and D). Chemically, these hydrocarbons are cyclopentano per hydro phenenthrenes. They contain a five-membered cyclopentane (D) ring and the three rings of phenanthrene. A perhydro phenanthrene (ring A, B, and C) is the saturated derivative of phenanthrene.


The polycyclic hydrocarbon, known as 5α-cholestane, is used to illustrate the numbering system for a steroid.

·The ring juncture or backbone carbons are shown in the structure of 5α-cholestane with a heavy dark line.

·Solid lines denote groups above the plane of the nucleus (β-configuration) and dotted or broken lines denote groups below the plane (α-configuration). If the configuration of substituent is unknown, its bond to the nucleus is drawn as a wavy line.

·The configuration of the H at C-5 is always indicated in the name.

·Circles were sometimes used to indicate α-hydrogens and dark dots to indicate β-hydrogens.

·Compounds with 5α-cholestane belong to allo-series, while compounds derived from 5 β-cholestane belong to the normal series.

·If the double bond is not between sequentially numbered carbons, in such cases, both carbons are indicated in the same.

·When a methyl group is missing from the side chain, this is indicated by the prefix ‘nor’ with the number of carbon atom, which has disappeared.


The symbol Δ is often used to designate a C = C bond in a steroid. If C = C is in between carbons 5 and 4, the compound is referred to as a Δ4 steriod, and if the C = C bond is between positions 5 and 10, the compound is designated as Δ5(10) steroid. Example, Estra-1,3,5(10) triene-3,17b-diol.


Since 17 β-estradiol contains 18 carbon atoms, it is considered as a derivative of estrane, a basic nucleus.


Stereochemistry: The absolute stereochemistry of the molecule and any substituent is shown with solid (β) and dashed (α) bonds; a (axial) bond is perpendicular to the plane of the molecule while equatorial bond (e) is horizontal to the plane of the molecule.


The aliphatic side chain at position is always assumed to be of β-configuration.

The term cis and trans are used occasionally to indicate the backbone stereochemistry between rings. For example, 5 α steroids are A/B trans and 5 β-steroids are A/B cis. The terms syn and anti are used analogously to trans and cis.

Conformations: There are six asymmetric carbon atoms in the nucleus 5, 8, 9, 10, 13, and 14. Therefore, there are 26 = 64 optically active forms possible. Cholestane, androstane, and pregnane can exist in two conformations, that is, chair form and boat form.


Chair confirmation is more stable than boat confirmation due to less angle strain, and hence, all cyclohexane rings in the steriod nucleus exist in the chair confirmation.

 

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