Stereoisomers structure

Stereoisomers Structural isomers having an identical chemical constitution but exhibiting differences in the spatial arrangement of their atoms are called stereoisomers [7], One case of stereoisomerism, denoted asymmetric chirality, comprises molecules that are mirror images of each other. Such pairs of molecules are called enantiomers. Figure 1.2.3 illustrates the two chiral molecules of 1-bromo-1-chloroethane. The line in the middle represents a symmetry plane. Note that it is... 

Amphetamine and methamphetamine occur as structural isomers and stereoisomers. Structural isomers are compounds with the same empirical formula but a different atomic arrangement, e.g., methamphetamine and phentermine. Stereoisomers differ in the three-dimensional arrangement of the atoms attached to at least one asymmetric carbon and are nonsuperimposable mirror images. Therefore, amphetamine and methamphetamine occur as both d- and L-isomeric forms. The two isomers together form a racemic mixture. The D-amphetamine form has significant stimulant activity, and possesses approximately three to four times the central activity of the L-form. It is also important to note that the d- and L-enantiomers may have not only different pharmacological activity but also varying pharmacokinetic characteristics. 

Enantiomers are stereoisomers structured like rnirror-images (see Fig. 1). A main problem related to producing a pure single enantiomer is that selective synthesis is often not feasible or too expensive. In contrast, conventional synthetic procedures are less expensive but non-selective (they deliver the racemic 1 1 mixture). Since usually only one enantiomer has the desired physiological effect (the other might be ineffective or harmful), such mixtures need to be separated for example, by kinetic resolution, crystallisation, or chromatography. However, the yield achievable by this approach is inherently limited to 50% only. 

If two conformational i.somers arc cooled to a temperature low enough to prevent their interconversion by bond rotation or other motion, then they can be described us stereoisomers structures with the same connectivity but with different atomic arrangements in three dimensions. For example, the conformers in (d) are nonsuperimposable mirror images (enantiomers) if bond rotation is not allowed to occur (make a model ). Those in (g) are not enantiomers, but they are still stereoisomers. The temperatures required to "freeze out" confor-inaiional interconversion are very low on the order of -200 C for substituted ethanes and -I00 C for cyclohexanes. Conformers of these types are occasionally referred to as iiuoivonvertihle stereoisomers,... 

Since structures 1 and 2 differ only in the arrangement of their atoms in space, they represent stereoisomers. Structures 1 and 2 are also mirror images of each other thus 1 and 2 represent a pair of enantiomers. Structures 3 and 4 correspond to another pair of enantiomers. Structures 1-4 are all different, so there are, in total, four stereoisomers of 2,3-dibromopentane. 

Sanggenons E (175) 147), [ajp —86°, C45H44O12, and P (178) 147), [ajo +215°, both amorphous powders are C-14 stereoisomers. Structure assignments were based on spectral evidence. 

The complex mixture of regioisomers and stereoisomers structurally related to the endoperoxides and thromboxanes (prostaglandin-like compounds) produced from arachidonic acid and esterified to phospholipids and cholesterol are classified as isoprostanes. These compounds are used as markers of in vivo oxidative injury in biological systems and considered reliable because they are specific, stable and not affected by the lipid content of the diet (Chapter 13. E.4). However, these secondary oxidation products are produced in relatively small concentrations because they are derived from arachidonic acid, which is a minor polyunsaturated lipid compared to linoleic acid in biological samples. Very sensitive gas chromatography-mass spectrometric (GC-MS) and liquid chromatography-MS techniques are required to analyse isoprostanes in biological samples. 

Butene, CH3CH=CHCH3, is a simple example of an alkene that exists as a stereoisomer. Structurally, 2-butene contains a planar C=C unit with two methyl groups attached to it. Rotation around the rigid C=C unit is impossible (the 7i-bond ensures that it is locked into position), so the methyl groups... 

Similar drawings are provided for diastereomer 45 (45A and 45B), and another line drawing is provided for 46 (46A and 46B), which is the mirror image of 45. Careful inspection of 45 and 46 reveals something different from previous stereoisomers Structure 45 is superimposable on 46, which means that these two structures are the same (they represent one molecule, 45 = 46). Make a model of both 45A and 46A. Pick up 45A, rotate it by 180°, and lay it on top of 46A. They are a perfect fit all atoms match up. Note thcU the atoms will match only in an eclipsed rotamer. Examine Fischer projection 45B to see that both bromine atoms are on the same side of the molecule, as are both hydrogens. The up and the down groups are both methyl. 

The presence of double bonds and the possibility of an oxygen atom at different positions in the molecule account for the occurrence of isomers. There are structural isomers and frequently stereoisomers. Structural isomers as well as cis-trans isomers are usually separated in gas chromatography with standard columns, the method of choice for the analysis of essential oils. However, the separation of enantiomers needs specific chiral columns based on cyclodextrins. Therefore, in most publications where these columns were not used, there is no differentiation between the enantiomers. 

For each case in Figure 6.14, we have stereoisomers—structures with the same connectivities but differing arrangements of the atoms in space. They are not enantiomers, so they must be diastereomers. The novelty lies in the fact that these stereoisomers interconvert by a translation or reorientation of one component relative to the other. In some ways these structures resemble conformers or atropisomers, which involve stereoisomers that interconvert by rotation about a bond. For the supramolecular stereoisomers, however, interconversion involves rotation or translation of an entire molecular unit, rather than rotation around a bond. Note that for none of the situations of Figure 6.14 do we have topological stereoisomers. In each case we can interconvert stereoisomers without breaking and reforming bonds. 

Two or more compounds that have the same molecular formula but different molecular structures are called isomers. The two main classes of isomers are structural isomers and stereoisomers. Structural isomers differ in the order in which atoms are bonded to each other. For example, hexane, 2-methylpentane, and 3-methylpentane are structural isomers because they all have the molecular formula CgHj, but they have different carbon chains. Structural isomers have different physical and chemical properties. 

The presence of stereoisomers in polymers is indicated in IR spectra by the appearance of new absorption frequencies, shifting of absorption frequencies, and band broadening. Methods for measuring the stereoisomeric composition of a polymer chain depend on knowledge of the relationship between these spectral properties and the stereoisomer structure. 

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