Nomenclature of enantiomers

Another point connected to chirality is the nomenclature of enantiomers. In the beginning, the optical isomers were distinguished with (+) and (—) signs or d (dextro) and / (levo), indicating the direction in which the enantiomers rotate a plane of polarized light. In this nomenclature, (+) or d stands for a rotation to the right (clockwise), whereas (—) or l indicates a rotation to the left (counterclockwise). The main drawback of such an assignment is that one cannot derive the number of chirality centers from it. Rather, it is necessary to apply the R/S... 

There is one property in which enantiomers do differ and that is the direction in which they rotate the plane of plane-polarised light. This phenomenon of optical activity provides the basis for the nomenclature of enantiomers. Thus (1), which rotates the plane of plane-polarised light in a clockwise direction ([aJo = +14.6° (c = 1, 5M HCl)),tll is denoted (+)-alanine while the enantiomer (2) which has an equal and opposite rotation under the same conditions ([a]o = -14.6°) is denoted (-)-alanine. Since the rotations due to the individual enantiomers in a mixture are additive, to a first approximation, the net measured rotation may be used as a guide to the enantiomeric composition. 

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. 

Figure 3.8 The two enantiomers of a-aminoacids. Here we follow the classic nomenclature of l- and D-aminoacids for indicating the two chiral forms. In terms of the S, R nomenclature, L-aminoacids correspond to the S absolute configuration -except for cystein, which is R.

Just as it is convenient to distinguish enantiomers and diastereomers by nomenclature symbols (R, S, E, Z, etc.) it is desirable to provide names for stereoheterotopic ligands or faces. The basic nomenclature to this end has been provided by Hanson 4,6) and is closely related to the nomenclature of stereoisomers. 

Fig. 11.19. Still-Gennari olefination of a racemic a-chi-ral aldehyde with an enan-tiomerically pure phosphonate as kinetic resolution I—Loss of the unreactive enantiomer ent-B of the aldehyde (R stands for the phenylmenthyl group in the Horner-Wadsworth-Emmons products the naming of the products in this figure is in agreement with the nomenclature of Figures 11.17 and 11.18).

The 191 problems in this book cover most of the area of stereochemistry, including nomenclature, stereogenic elements (centers, axes, planes) and their descriptors, symmetry, inorganic stereochemistry, determination of enantiomer excess, conformation of acyclic and cyclic compounds, and more. The answers, in addition to providing solutions to the problems, frequently include additional explanations of the underlying principles. The problems are ordered more or less in order of increasing difficulty. (I had a hard time with some of the problems toward the end myself )... 

For nomenclatural classification, a pentacoordinated phosphorus compound, P(L],L2,L3,L4,L5), with five different ligands, Lx. . . L5, attached to a skeleton of five positions identified by the indices s =1... 5, and of trigonal bipyramidal symmetry, D3h, can exist as 5 /6=20 isomers, or 10 pairs of enantiomers. To represent these isomers one defines a Reference Molecule ME in which the indices of all the ligands and the indices of all the skeletal positions coincide. Those indices will not coincide in the isomers of ME. The skeletal indices are assigned to the Reference Molecule according to some definitions,65 for example, as shown in ME. The ligand indices are assigned to the Reference Molecule... 

As mentioned before, most toxaphene components are chiral. Polychlorinated bornanes contain a plane of symmetry. The structures of enantiomers are generated by reflection of the substituents at C2, C3, and C9 with those at C6, C5, and C8, respectively (see Fig. 4). Consequently, enantiomers of chlorinated bornanes have a different nomenclature which causes additional troubles with naming [71,83]. 

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. 

Sir Christopher Ingold (1893-1970) was bom in Ilford, England. In addition to determining the mechanism of the 5 2 reaction, he was a member of the group that developed nomenclature for enantiomers. (Seep. 188.) He also participated in developing the theory of resonance. 

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-. 

We use the shorthand prefixes r and m to designate the racemic mixture of enantiomers and the meso compound, respectively. The currently accepted nomenclatures are 3R54i 5-dimethylhexane for the racemate and 3/ 45-dimethylhexane for the meso stereoisomer. 

Configurationally chiral, optically active polymers having stereogerric centers in the side chain or main chain can be obtained by enantiomer-selective pol5nnerization (lUPAC nomenclature asymmetric enantiomer-differentiating polymerization). In enantiomer-selective polymerization, one antipode of a racemic chiral monomer is preferentially polymerized to afibrd an optically active polymer. In this process, kinetic resolution of the racemic monomer takes place. The first clear polymerization of this typ>e was reported for propylene oxide. 

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