R, S system

Fischer projections and d-l notation have proved to be so helpful m representing carbohydrate stereochemistry that the chemical and biochemical literature is replete with their use To read that literature you need to be acquainted with these devices as well as the more modern Cahn-Ingold-Prelog R S system... 

The discoveries of optical activity and enantiomeric structures (see the box, page 97) made it important to develop suitable nomenclature for chiral molecules. Two systems are in common use today the so-called d,l system and the (R,S) system. 

The newer (R,S) system of nomenclature is superior to the older d,l system in one important way. The configuration of molecules with more than one... 

If a tetrahedral center in a molecule has two identical substituents, it is referred to as prochiral since, if either of the like substituents is converted to a different group, the tetrahedral center then becomes chiral. Consider glycerol the central carbon of glycerol is prochiral since replacing either of the —CH9OH groups would make the central carbon chiral. Nomenclature for prochiral centers is based on the (R,S) system (in Chapter 3). To name the otherwise identical substituents of a prochiral center, imagine... 

The compound is named as a substituted derivative of a deoxy sugar. The group replacing OH determines the configurational description. Any potential ambiguity should be dealt with by the alternative use of the R,S system to specify the modified stereocentre. 

If substitution at the terminal carbon atom of the carbohydrate chain creates a chiral centre, the stereochemistry is indicated by the R.S system. 

If there is a carbonyl group in the branch (or a terminal COOH or its equivalent), its position (assigned lowest number when stereochemistry is being considered) is used to define the configurational prefix (see examples 1 and 3 in Chart V). Use of the R,S system is generally preferred, as less open to misinterpretation. 

For the tetraric acids, the trivial name tartaric acid remains in use, with the stereochemistry given using the R,S system. Esters are referred to as tartrates (the second a is elided). 

Note. In the older literature, there is confusion about the use of D and L in the case of tartaric acids. It is therefore recommended to use the R,S system in this case. 

Note. In these compounds carbon atom number 1 has become chiral. When known, the stereochemistry at this new chiral centre is indicated using the R,S system ([13], Section E). 

R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland) devised the (R-S) system (Sequence rule) for designating the configuration of chiral carbon atoms. 

Unhappily, there is another, older convention for designating chiral molecules here the labels are D and L. The R S system is more systematic and more generally useful but old usages die hard. We shall encounter D and L when we talk about amino acids in chapter 10. 

Another type of isomerism arises when a molecule contains a chiral center or is chiral as a whole. Chirality (from the Greek cheir, hand) leads to the appearance of structures that behave like image and mirror-image and that cannot be superimposed ( mirror isomers). The most frequent cause of chiral behavior is the presence of an asymmetric C atom—i.e., an atom with four different substituents. Then there are two forms (enantiomers) with different configurations. Usually, the two enantiomers of a molecule are designated as L and D forms. Clear classification of the configuration is made possible by the R/S system (see chemistry textbooks). 

Figure 19-6 Designation of configuration about an asymmetric center by the R,S system. Substituent priority decreases in the order A, B, C, D.

The R,S system is quite general and has many advantages (and a few disadvantages) compared with the d,l notation for simple molecules. For diastereomers, it provides much clearer notations than meso, erythro,1 and threo1 that have been used for many years to designate the configurations of achiral and chiral diastereomers having two chiral carbon atoms ... 

A system that is easy to use and which is based on the sequence rules already described for the R,S system works as follows ... 

Write projection formulas for the following compounds and rename them by the R,S system ... 

A-2. Specify the configuration of each stereogenic carbon in the preceding problem, using the Cahn-Ingold-Prelog R-S system. 

There are several conventions to indicate the configuration of a stereocenter. The most precise is the IUPAC system that names the configuration around a stereocenter as R (for right handed rectus) or S (for left handed sinister). You have probably seen the R/S descriptors on drug information sheets. The details of the R/S system are beyond our purposes here, but any standard organic chemistry text will discuss how to assign these descriptors. 

Most amino acids, with the notable exception of glycine, are optically active. Their configurations are usually indicated by the D/L system (Figure 1.4) rather than the R/S system. Most naturally occuring amino acids have an L configuration but there are some important exceptions. For example, some bacteria also possess D-amino acids. This is important in the development of some antibacterial drugs. 

Thus, description of a simple change of fatty acid at the primary position required frequent changes of configuration prefixes. Also, the R/S system, like the older D/L one, did not account for the stereospecificity of the acylglycerol derivatives toward lipases (phospholipase A2 in particular). Finally, nonrandom distribution of fatty acids in natural or synthetic enantiomeric acylglycerols could not be systematically correlated by reference to either the R/S or D/L configuration. 

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