Racemization, definition

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

In order to illustrate an example of process design for the manufacture of enantiopure drug substances on an industrial SMB system, consider manufacturing 10 ton/ year of an enantiopure drug. The racemic drug by definition is a 50 50 mixture of each enantiomer (products A and B). The goal is to process enantiopure drug substances in order to obtain 99 % purity for both the extract and the raffinate. 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

Within this approach, clathrates (for a definition see Chapter 1 in Vol. 140 of this series) and related lattice-type aggregates may be considered as multi-supramolecular systems where guest molecules are included in a crystal matrix. They allow a great many applications which have been specified in Vol. 140, first of all the separation of enantiomers by enantioselective recognition and inclusion of racemic guest molecules. 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

The formal view. The formal view is much simpler. The racemic catalysts have a twofold axis and therefore C2-symmetry. Both sites of the catalysts will therefore preferentially co-ordinate to the same face (be it re or si) of propene. Both sites will show the same enantiospecificity the twofold axis converts one site in the other one. Subsequently, insertion will lead to the same enantiomer. According to the definition of Natta, this means that isotactic polymer will be formed. If the chain would move from one site to the other without insertion of a next molecule of propene, it will continue making the same absolute configuration at the branched carbon atom. Hence, no mistake occurs when this happens. 

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

Now let us look at triad segments of our poly(vinyl chloride). The first one had meso adjacent units and is isotactic by definition of the meso, racemic argument but the adjacent chlorides are not on one side of the plane. 

These definitions are clarified by considering a portion of a polymer chain such as XVII. Chain segment XVII has a total of 9 repeating units but only 8 dyads and 7 triads. There are 6 meso dyads and 2 racemic dyads (m) —, (r) —, There are 4 isotactic, 2 heterotactic, and 1 syndiotactic triads mm) = (mr) — j, (rr) — A. 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

In a series of agonists and antagonists (for definitions, see section 2.4), the eudismic affinity quotient can also he defined as a measure of stereoselectivity. Because of widespread misconceptions, the distomer of a racemate is often considered inactive and of no consequence to pharmacological activity, an idea reinforced by the fact that resolution (i.e., separation) of racemates is economically disadvantageous. In the 1980s, Ariens and his associates (Ariens et al., 1983 Ariens, 1984, 1986) published a series of influential books and papers that showed the fallacy of this concept and pointed out the necessity of using pure enantiomers in therapy and research thankfully, this message has now been learned. 

---