Configurations of enantiomers

No correlation exists between the configuration of enantiomers and the direction of optical rotation. 

Figure S. Comparison of configurations of enantiomers according to different rules of correspondence between proximal atoms (A to D). The rule marked H-H is the traditional one as defined by us (1), the one marked P-H is that of Prelog and Helmchen (S). The planes of reflection of 17 and 18 coincide with the molecular planes.

How to designate the configuration of enantiomers We have already seen that a (+) or (—) sign indicates the optical activity of an enantiomer. 

There are two systems to designate configuration of enantiomers the D and L system, and the (R) and (S) system (also known as the Cahn-Ingold-Prelog system). 

The fundamentals of structure and stereochemistry have been considered in previous chapters in some detail. There are, however, practical aspects of stereochemistry that have not yet been mentioned, particularly with regard to chiral compounds. How, for instance, can a racemic mixture be separated into its component enantiomers (resolution) what methods can be used to establish the configuration of enantiomers how can we tell if they are pure and how do we synthesize one of a pair of enantiomers preferentially (asymmetric synthesis) In this chapter, some answers to these questions will be described briefly. 

After CHEC-II(1996), more new stable dithiiranes 3 and dithiirane A-oxides have been characterized using X-ray single crystal structure analysis. This method has also been used to determine the absolute configuration of resolved enantiomers of 16, previously known and characterized by X-ray only as a racemic mixture <1997T12203>. (The assignment of the absolute configuration of enantiomers of 17 is discussed in Section 1.14.4.1.). 

Roussel, C., Rio, A. D., Pierrot-Sanders, J., Piras, P.,Vanthuyne, N. Chiral liquid chromatography contribution to the determination of the absolute configuration of enantiomers, J. Chromatogr. A, 2004, 1037,311-328. 

No obvious correlation exists between the (R) and (5) configurations of enantiomers and the direction [(+) or (—)] in which they rotate plane-polarized light. 

R 498 C. Roussel, A. Del Rio, J. Pierrot-Sanders, P. Piras and N. Vanthuyne, Chiral Liquid Chromatography Contribution to the Determination of the Absolute Configuration of Enantiomers , J.Chromatogr., A, 2004,1037,311... 

In this chapter the systems of nomenclature used to describe the absolute configuration of enantiomers and diastereomers as well as the possible modes of attack in enantioselective and diastereoselective reactions are explained. 

Taking advantage from the study of spectra-structure relationships, the analysis of CD and ORD spectra enable the investigation of the stereochemistry of chiral molecules and determine the absolute configuration of enantiomers [18]. Moreover, interchromophoric interactions such as exciton coupling can be evidenced, and the distance and orientation of the involved chromophore can be determined [19]. 

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