Enantiomers nomenclature systems

The description of ci-amino acids as D or L is a holdover from an older nomenclature system. In this system (5)-alanine is called L-alanine. The enantiomer would be D- or ( )-serine. The l (laevo, turned to the left D = dextro, turned to the right) designation refers to the ct-carbon in the essential amino acids. In alanine, there is a single a-carbon that is asymmetric. When two asymmetric centers are present as in L-threonine, the stereochemistry of both carbons must be considered. The common form of L-threonine is the 25,3R stereoisomer. 

Over the years, several nomenclature systems have been developed to characterize the relationship between enantiomers. The system based on optical activity and the classification of enantiomers as dextrorotatory [d or (+)] or levorotatory [1 or (—)] already has been described. However, this system of nomenclature is of limited applicability because the sign of rotation, (+) or (—), does not predict the absolute configuration or the relative spatial arrangement of atoms in the enantiomers. In an attempt to designate the precise configurations about carbon centers of asymmetry, the Cahn-Ingold-Prelog RjS system have been developed and adopted as the most commonly used nomenclature system for isomers. 

In 1891 Emil Fischer devised a nomenclature system that would allow scientists to distinguish between enantiomers. Fischer knew that there are two enantiomers of glyceraldehyde that rotated plane-polarized light in opposite directions. He did not have the sophisticated tools needed to make an absolute connection between the structure and the direction of rotation of plane-polarized light. He simply decided that the (-L) enantiomer would be the one with the hydroxyl group of the chiral carbon on the right ... 

One enantiomer is distinguished from the other enantiomer in a pair of enantiomers by placing a prefix before the name of the compound. There are two nomenclature systems for this purpose. The most general system is the lUPAC R-S system where the prefixes R- and S- are used to distinguish between the enantiomers. We will not consider the R-S system since it is not universally used by biological scientists. The latter more often use an older nomenclature system which uses the prefixes d- and l-. 

Ans. The d-l nomenclature system applies only for the case of a stereocenter attached to two different carbon substituents R and R, a hydrogen, and a hetero atom substituent X (e.g., OH or NH,)- The molecule is drawn so that the R and R substituents are vertical while the H and hetero atom substituents are horizontal. R, the carbon substituent which has the most highly oxidized carbon atom (the carbon with the least number of hydrogens) attached to the stereocenter, is placed up. The enantiomer with the X substituent on the right-hand side is the D-enantiomer while that with the X substituent on the left-hand side is the L-enantiomer ... 

This chapter will introduce the concepts of asymmetry and chirality as they apply to stereoisomers. Enantiomers are nonsuperimposable mirror images that are different compounds, identifiable only by differences in the physical property known as specific rotation. Enantiomers arise when a molecule has one or more atoms (including carbon) with different substituents (from different substituents for carbon). Such atoms are known as stereogenic (chiral) atoms (most of the examples in this book will deal with a stereogenic carbon atom). With more than one stereogenic center, another type of stereoisomer results known as a diastere-omer. All of these are types are stereoisomers, and a nomenclature system is in place to correlate the structure with what is known as absolute configuration. 

Figure 8.3 Examples of different biological effects of enantiomers. S and R refer to a particular system of nomenclature used to describe chiral carbon, (see Appendix A8.1)...

Thus far, we have discussed the nomenclature of different types of chiral systems as well as techniques for determining enantiomer composition. Currently,... 

This chapter has provided a general introduction to stereochemistry, the nomenclature for chiral systems, the determination of enantiomer composition and the determination of absolute configuration. As the focus of this volume is asymmetric synthesis, the coming chapters provide details of the asymmetric syntheses of different chiral molecules. 

Amino acids exist in nature as only one of these enantiomers. Except when the R group is CH2SH, the stereogenic center on the a carbon has the S configuration. An older system of nomenclature names the naturally occurring enantiomer of an amino acid as the L isomer, and its unnatural enantiomer the D isomer. 

The second is based on the geometry of the molecule and makes use of the skew-lines convention it is usually applied only to octahedral complexes. The two enantiomers are identified by the symbols A and A in this system. The C/A nomenclature is not required for those chelate complexes where the skew-lines convention is completely unambiguous (see Sections IR-9.3.4.11 to 9.3.4.14). 

If we name these two enaatiomers using only the lUPAC system of nomenclature that we have learned so far, both enantiomers will have the same name 2-butanol (or sec-butyl alcohol) (Section 4.3F). This is undesirable because each compound must have its own distinct name. Moreover, the name that is given a compound should allow a chemist who is familiar with the rules of nomenclature to write the structure of the compound from its name alone. Given the name 2-butanol, a chemist could write either structure I or structure II. 

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