Enantiomers and Diastereoisomers

The molecule 1 has two stereogenic centres and has a different set of substituents at each centre accordingly, there are four stereoisomers of 1. Four stereoisomers also result if in 1 one replaces, say, an NH2 by an 

A molecule with n stereogenic centres can have a maximum of 2/7 stereoisomers. 

so that one substituent, i.e. F, is now common to both stereogenic centres. 

Diastereoisomers have different physical properties and react at different rates they are Stereoisomers that are not enantiomers. 

A particular stereoisomer of 1 with two stereogenic centres can have only one enantiomer, but more than one diastereoisomer. 

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The importance of the three stereogenic centers became evident as two of the stereoisomers [(2S,3S,7S) and (2S,3R,7R)] were recognized early as sex pheromones and that other enantiomers and diastereoisomers were often found to be inhibitory to the attractive response. Recently, the sawfly pheromone field has undergone a major advance with the recognition that several sawfly species synthesize and utilize sex pheromones of different structural types than the 3,7-dimethylpentadecan-2-ols. Shorter and longer chain lengths (undecan-2-ols 2 and tridecan-2-ols 3) and an additional methyl group in position 9 or 11 (4 and 5) characterize these new pheromone discoveries (Fig. 1). With an ad-... 

Whatman PX-5/25 ODS 25 cm x 4.6 mm Pirkle type 1A column Hexane-propan-2-ol-acetonitrile (170 21 15), [1 mL/min] Not given Separation of enantiomers and diastereoisomers of the drug and its metabolites. [98]... 

To initiate the equilibrium, triethylamine was added and the reaction followed frequently by H-NMR. Under these conditions, equilibrium was reached within 3 hours (Fig. 6.15). Following this, the full hbrary (DCL-F) including aldehyde (49) was constructed. This library initially behaved similarly to DCL-G, with several nitroaldoladducts forming competitively. A total of 20 individual (3-nitroalcohol adducts is possible including enantiomers and diastereoisomers. However, within 1 hour, a clear amplification of a single diastereomeric pair, supposedly iminolactone (56), became clearly visible. The amphfication effect then gradually continued until all... 

One of the problems attaching to the HPLC separation of peptides is the analysis of stereoisomers (enantiomers and diastereoisomers), that is, of peptides that differ only in the configuration of their amino acid residues. 

Fig. 9 Top-view on enantiomers and diastereoisomers of adsorbed glycine and alanine. The amino nitrogen and the two oxygen atoms form bonds to the surface and create a chiral footprint configuration. The surface-induced absolute configuration, as indexed with a superscripted i, is specified using CIP-rules and by giving an atom or group closer to the surface a higher priority...

A fundamental subclassification is that of stereoisomers, which can be divided into enantiomers and diastereoisomers. Either two stereoisomers are related to each other as object and nonsuperimposable mirror image, or they are not. In the former case, they share an enantiomeric relationship. This implies that the molecules are dissymmetric (chiral), and chirality is the necessary and sufficient condition for the existence of enantiomers. An example of an enantiomeric relationship is illustrated in diagram III which shows the (R)- and (S)-enantiomers (see Section 4.b) of... 

Stereoisomers which are not enantiomers are diastereoisomers. While a given molecule may have one and only one enantiomer, it can have several diastereoisomers. However, two stereoisomers cannot at the same time be enantiomers and diastereoisomers of each other. Enantiomeric and diastereoisomeric relationships are thus mutually exclusive. Diagram IV shows the ( )- and (Z)-diastereoisomers (also... 

Diastereoisomers When a molecule contains several asymmetrical atoms of carbon (see chiral), two types of isomers, enantiomers and diastereoisomers, exist. The former are mirror images. The latter contain some carbon atoms which have an identical structure, others which are enantiomeric. For example, if we have a molecule with two asymmetrical carbons, the first carbon atom can have... 

In drug discovery nowadays, it is common practice to profile all enantiomers and diastereoisomers of drug candidates, especially for synthetic drugs, if a mixture of isomers is being considered for drug development or if the compounds are prone to isomerization. 

For a technical short chain chlorinated paraffin (SCCP) mixture containing 60% chlorine by weight, the theoretical number of congeners (defined as constitutional isomers and homologues) is 4,200 [11, 16]. It should be noted that the complexity would actually be an order of magnitude greater than that indicated in Table 1 because chlorine substitution at a secondary carbon atom usually produces a chiral carbon atom so that enantiomers and diastereoisomers would be generated. 

The absolute configurations at phosphorus of resolved organophosphorus nerve agent enantiomers and diastereoisomers have not been firmly established Some probable assignments have been made on the basis of an X-ray analysis and some chemical correlations (Scheme 10). The X-ray analysis has shown that the dextrorotatory enantiomer of... 

A few typical examples of stereoisomers viz., enantiomers and diastereoisomers are illustrated... 

IVaditionally, isomerism has been classified as dther constitutional or steric, with the latter class being subdivided into enantiomers and diastereoisomers. The flowchart, Figure 1, illustrates this traditional subdivision. 

Scheme 7.—Formation of Branched and Straight-chain Aldoses and Ketoses (Enantiomers and Diastereoisomers Not Shown). ...

This chapter buiids on the concepts introduced in Chapter 14 make sure you understand aii the terms used to describe stereochemistry that are defined there, in particuiar make sure you are absoiuteiy ciear on the meanings of chiral, achiral, enantiomer, and diastereoisomer, aiong with what the designators R, S, +,... 

The or-analogue 159 has been made by a Pd-catalysed reaction involving an allylic phosphate as reactant, whilst others have also described routes to 159, its hydrogenated analogue, and the enantiomer and diastereoisomers of this, using enzymic desymmetrization to introduce chirality the products were evaluated for their ability to inhibit tumour necrosis factor-a. A chemo-enzymatic approach has also been used to make the cyclopropane-fused analogue 160 and... 

Table 4 HPLC methods for separation of all drugs, their enantiomers and diastereoisomers. 

Hereinafter, the possibilities of enantiomer and diastereoisomer separations based on crystallization are shown by known examples. 

Most terpenes contain one or more chiral centers. Of several terpenes, the optically inactive form and the 1- and d-form occur in different plants. The enantiomers and diastereoisomers differ regularly in their odor characteristics. For example, menthol (XIV in Table 5.33) in the 1-form... 

Generally, in liquid crystals, it is only phases that contain optically active materials that exhibit chiral liquid crystal modifications. Thus at least one substance in the liquid crystal system must be a stereoisomer that contains at least one asymmetrically substituted atom, and which is present in a greater concentration than its opposite enantiomer. It is the configurational isomers in the system that give rise to chiral properties. Included in configurational isomers are two distinct classes of stereoisomers enantiomers and diastereoisomers [2]. Enantiomers are two molecules that are related to one another as object and nonsuperimposable mirror image, as shown in the upper part of Fig. 1. Diastereoisomers usually contain more than one asymmetric atom, and pairs of diastereoisomers do not share a superim-posable mirror image, as shown in the lower part of Fig. 1 [1, 2]. 

Isomers that differ from each other only in the way atoms are oriented in space. There are two types of stereoisomers enantiomers and diastereoisomers. 

The comprehensive chiral HPLC method development approach designed for brivanib alaninate enantiomers and diastereoisomers separation included [150] the following ... 

Crystallization methods have been greatly improved in their rationale and efficiency, as a consequence of a better knowledge of the properties on which the separation processes are based, such as identification of racemate types, phase diagrams of enantiomer, and diastereoisomers systems. 

Stereoisomers have the same constitution, with all the same atoms joined together by the same type of bonds but a different shape. The existence of stereoisomers depends on the theory of organic structure and the fact that chemical bonds have directional properties. Stereoisomers can be further divided into two mutually exclusive groups enantiomers and diastereoisomers (sometimes elided as diastereomers). Enantiomers are distinguishable molecules related as object and mirror image. Diastereoisomers are all other stereoisomers. 

What stereoisomers would you expect for the following compounds Label double bonds or Z and chiral centers R or S. Distinguish enantiomers and diastereoisomers. Which isomers would you expect to be optically active ... 

Armstrong, D.W., He, F.-Y. and Han, S.M., Planar chromatographic separation of enantiomers and diastereoisomers with cyclodextrin mobile phase additives, /. Chromatogr., 448 345, 1988. 

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