Description
Nomenclature of Steroids
Steroids are made up of four fused rings, i.e., A, B, C, and D. These hydrocarbons are chemically cyclopentano perhydrophenanthrenes and have a 5-membered cyclopentane ring (D) and three phenanthrene rings. A
perhydrophenanthrene (rings A, B, and C) is the saturated derivative of phenanthrene.
The polycyclic hydrocarbon is termed 5α-cholestane. It is used for numbering
system of a steroid. It is represented with 5α since the hydrogen atom at position 5 is on the opposite side of the rings from the angular methyl groups at positions 18 and 19 on the β side of the molecule. The term cholestane is used for a steroid
with 27 carbons including a side chain of 8 carbons at position 17 on the β side.
Functional groups on the β side of the molecule are shown by solid lines , while
the groups on α side are shown by dotted lines. Side chains a t position 17 are
always β unless shown by dotted lines or in the steroid nomenclature (e.g., 17α).
Cyclohexane is drawn in a chair conformation. The 3-D conformational image of
5α-cholestane is shown below. Cyclohexane can undergo a flip in c onformation,
but structures of s teroids are rigid as they have at least one trans fused ring
system. These rings should be diequatorial to each other.
If the angular methyl groups at 18 and 19 positions are β and are perpendicular to
the plane of rings due to axial orientation (and not peripheral to the plane of rings
due to equatorial orientation ), the conformational orientation of the remaining
bonds of a steroid is allocated easily. For example, in 5α-cholestane the C-19
methyl group at posi tion 10 is always β and have an axial orientation; the two
bonds at position 1 should be either β-equatorial or α-axial as shown.
The orientation of remaining bonds on a steroid can be determined if the groups
on a cyclohexane ring are positioned on adjac ent carbon atoms
(vicinal, 1 2) of the ring (i.e., 1,2 to each other) and are trans to each other if their relationship is 1,2 -diaxial or 1,2 -diequatorial; while, they arecis if their relationship is 1,2-equatorial-axial.
The steroid chemists refer to the series of carbon -carbon bonds represented with heavy lines as the steroid backbone.
The cis or trans relationship of th e four rings is expressed in terms of the
backbone. The 5α-cholestane has a trans-anti-trans-anti-trans backbone
indicating that all rings are fused trans (diequatorial). It also indicates that the
bond equatorial to ring B at position 9 that forms part of ring C, is anti (i.e.,
trans) to the bond equatorial to ring B, at position 10 that forms part of ring A,
and so on.Many stereochemical possibilities exist for the steroid backbone. For example, 5β-cholestane has acis-anti-trans-anti-trans backbone.
The stereochemistry of rings affects the biological activity of a specified class of
drugs.
All the biologically active steroids comprise of cholestane-type backbone . In almost all the essential steroids, a double bond exists between positions 4 and 5 or 5 and 6. Therefore, there is no cis or trans relationship between rings A and B.The symbol indicates a C═C bond in a steroid. If the C═C is between 4 and 5 positions, the compound is a
4 -steroid. If the C═C is between 5 and 10
positions, the compound is a
5(10) steroid.
Cholesterol (cholest-5-en-3β-ol) is a
5 -steroid or a 5-sterol as it is an unsaturated alcohol.
No C═C are found in the bile acids that belong to the 5β series. For example,
cholic acid is an important cholesterol metabolite in human and animals, has 5β- cholestane backbone, and is named 3α,7α,12α-trihydroxy-5β-cholan-24-oic
Mostly, cardiac glycosides also belong to the 5β series. The other three steroid
hydrocarbons having 5α-cholestane configuration are shown below. Biologically active compounds are the members of 5α-pregnane, 5α-androstane, and 5α- estrane steroid cl asses. Pregnanes a re steroids having 21 carbon atoms, androstanes have 19 carbon atoms, and estranes have 18 carbon atoms with C-19 angular methyl group at C-10 replaced by hydrogen. Numbering is done same as in 5α-cholestane.Adrenocorticoids (adrenal cortex hormones) are pregnanes. For example , cortisone is a 17,21 -dihydroxypregn-4-ene-3,11,20-trione, and its acetate ester is
named as 17,21-dihydroxypregn-4-ene-3,11,20-trione-21-acetate.
