Enantiomer recognizing pairs

The possibility of separating dilevalol from labetalol was considered as an option at the commencement of the dilevalol project. However, it quickly became clear, from work carried out in both Research and Development, that this approach might not be a viable option. Although the racemic pairs (RR + SS and RS + SR) were separable by crystallization, and although the optical resolution of the RR and SS enantiomers could be achieved through salt formation with a chiral acid, the direct yield of dilevalol was less than 20%. Nevertheless, it was recognized that if the recovery and recycling of the waste streams from the physical and optical resolutions could be carried out efficiently, considerable economies would be obtained (Scheme 1). 

You ve probably recognized that there s a simple mathematical relationship between the number of stereogenic centres and the number of stereoisomers a structure can have. Usually, a structure with n stereogenic centres can exist as 2 stereoisomers. These stereoisomers consist of 2 diastereoisomers, each of which has a pair of enantiomers. This is an oversimplification to be used cautiously because it works only if all diastereoisomers are chiral. We recommend that you find out how many diastereoisomers there are in every new molecule before considering enantiomers. 

A meso compound is represented by, for example, (l/ ,25), and the identical (15,2/ ) structure is not shown. In addition, only one enantiomer of a pair is shown. Where relevant, sawhorse projections are shown in eclipsed conformation, though it should be recognized that this is not the most stable. 

The biological response induced by a pair of enantiomers can differ in potency (quantitative difference) or in nature (qualitative difference). In the latter case, it is assumed that one enantiomer acts at one receptor site, whereas its antipode is recognized by other sites and possesses a different activity and toxicity profile. 

Il is useful to recognize at this point that comparing pictorial formulas is just one of several ways to determine the relationship between possible stereoisomcrie structures. If you have the time, making models is always useful, especially for visualization purposes. An even better way is to apply the rules of nomenclature to each siruclure and to determine the Rqy S conliguration of each stereocenter. Once you have become comfortable with this technique ns it applies to the different types of structural pictures, you may find that this is the qiiicke.sl method of all Once R-S assignments have been made to structures under consideration, their stereochemical relationship is obvious For two stereocenters, R.R and S.S are enantiomers of each other, R,S and S,R are enantiomers of each other, and any other combination is a pair of diastcreomers. 

One way (but not the only way) is to recognize that a pair of enantiomers is always possible for molecules that contain a single tetrahedral atom with four different groups attached to it. 

---