Enantiomer nomenclature

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. 

The adduct with the re face bound to Rhodium, leading to the 5 enantiomer, is more stable than the Pro-R -coordinated adduct, and so would be the predominant diastereomer in solution. In our original papers on this work [71, 72], we labeled the more stable Pro-5 diastereomer MAJ and the Pro-/ diastereomer MIN in analogy to the nomenclature from Figure 2. For consistency with our later work, however, we use the designations PRO-R and PRO-5 in this chapter. 

There is another point of nomenclature that must be discussed, namely where a chiral center is involved. Taking the simple case of 2-butanol, CH3CH(OH)CH2CH3, we can explain the point as follows (Scheme 1). Two configurations are possible at the chiral center. In both the R and the S series, three conformations are possible. The +sc form in the R series and the -sc form in the S series are enantiomers and their free energies must be the same under achiral conditions. However, the - sc form in the R and that in the S series differ in free energies. Therefore, it is not sufficient to call a conformation -sc if a chiral center is involved. In this case we may have to call such conformations - sc(R) and — sc(S) to distinguish them. 

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.

In order to stress the distinction between actual molecules and molecules represented as models, the terms enantiomer/c and diastereower/c are only applied to the former, for the latter the correct terms are enantiomorphic and diastereomorphia5. The Morphic nomenclature is also applied when parts of a molecule, e.g.. isolated groups or fragments, are discussed6. 

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. 

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... 

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. 5.46 Cyclopropylamine conformations at the B3LYP/6-31G level. Structure 1 is a hilltop whose two imaginary frequencies indicate that it wants to undergo nitrogen pyramidalization and rotation about the C-N bond to form the transition states (nomenclature ts 1/2 connects 1 and 2, etc.) and, eventually, the minima. Each C, species has an enantiomer of the same energy...

Avnir et al. llbl have examined the classical definitions and terminology of chirality and subsequently determined that they are too restrictive to describe complex objects such as large random supermolecular structures and spiral diffusion-limited aggregates (DLAs). Architecturally, these structures resemble chiral (and fractal) dendrimers therefore, new insights into chiral concepts and nomenclature are introduced that have a direct bearing on the nature of dendritic macromolecular assemblies, for example, continuous chirality measure44 and virtual enantiomers. ... 

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