R and S nomenclature

In most steroids the B-C and C-D rings are fused, usually in a trans manner. The lower side of the steroid is denoted a, the upper side of the steroid is denoted (3, usually drawn as projected helow the plane of the paper, which is shown as broken lines, and above the plane of the paper, which is drawn as solid lines. Thus, substituents attached to the steroid are also characterized as a and (3. Cholesterol has eight chiral centres, therefore 256 stereoisomers are theoretically possible, but only one exists in nature Stereogenic centres in steroid side chains are denoted preferentially with the R and S nomenclature. 

R) and (S) Nomenclature of Asymmetric Carbon Atoms 181 5-4 Optical Activity 185... 

Does this mean that (/ >butan-2-ol is the dextrorotatory isomer because it is named (.R), and (S)-butan-2-ol is levorotatory because it is named (5) Not at all The rotation of a compound, (+) or (-), is something that we measure in the polarimeter, depending on how the molecule interacts with light. The (R) and (S) nomenclature is our own artificial way of describing how the atoms are arranged in space. 

The alternative names for these two isomers in terms of R and S nomenclature refers to their relative positions within the molecule, but the explanation of this system is beyond the scope of this text and it does not add further relevance or explanations to the principles outlined. (See optical activity in the Glossary.)... 

The absolute configuration at a chiral tetracoordinate center can be described using the D and l nomenclatures (but there are drawbacks, see [25]), or with the R and S nomenclature to be summarized below. 

The R and S nomenclature was first presented in 1951 by Cahn and Ingold [26], and then consolidated and extended by Cahn, Ingold and Prelog [27,28]. The essential part of this nomenclature (also called the CIP nomenclature) of chiral centers is the sequence rule, i.e., a set of arbitrary but consistent rules which allow a hierarchical assignment of the substituents (a > b > c > d). 

The R and S nomenclature has also been extended to the chiral plane (see for example [44]). Thus, structure XXXV has the (S) absolute configuration. Some confusion between chiral axes and chiral planes has led to the proposal of a new definition of the chiral plane and of a new procedure for specifying planar chirality [45]. 

Identify asymmetric carbon atoms, and name them using the R) and (S) nomenclature. 

The R and S nomenclature system is very useful in naming biological products such as a-tocopherol. Natural vitamin E is (2R,4 i ,8 R)-a-tocopherol (37), but commercial vitamin capsules contain the acetates of both the natural vitamin and of (2S,4 R,8 R)-a-tocopherol. Both acetates are hydrolyzed to the phenol in the intestine before absorption into the body, but the acetate of natural vitamin E is hydrolyzed more rapidly. ... 

The R and S nomenclature system can also be used for structures with an axis or plane of chirality if we apply the additional rule that substituents on the end of an axis or on the surface of a plane nearer the observer are arbitrarily given higher priority than those further away. Some authors augment the R and S labels in order to specify that the designation applies to axial or planar chirality, so the terms Ra and Sa (or aR and aS) for axial chirality and Rp and Sp (or pjR and pS) for planar chirality also appear in the literature. ... 

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