Stereoisomers diastereomers

Diastereomers (Section 9.6) Non-mirror-image stereoisomers diastereomers have the same configuration at one or more chirality centers but differ at other chirality centers. 

Similar schemes can be developed easily for analogous reactions of acceptor-substituted polyenes. For example, a triene with an acceptor group in 1-position can form six regioi-someric products of Michael addition and electrophilic capture, and each of these exists as E/Z stereoisomers, diastereomers and/or enantiomers. Thus, reactions of this type are only useful if both the regio- and stereoselectivity can be controlled fortunately, only one isomeric Michael adduct is formed in many cases. This is true in particular for polyunsaturated Michael acceptors which bear at least one triple bond besides one or more double bonds. An additional feature of the latter substrate type is that nucleophilic additions can... 

The phenyl substituent can be introduced either cis or trans to the tert-butyl group. The two alcohols are therefore stereoisomers (diastereomers). 

An immediate consequence of Pasteur s law is that the relationship between enantiomers is established by symmetry alone and does not require any knowledge of molecular bonding connectedness (constitution). This is in contrast to diastereomers, the other class of stereoisomers Diastereomers are not related by symmetry, and their relationship can be defined only by first specifying that their constitutions are the same—otherwise, there would be nothing to distinguish them from constitutional isomers. Thus enantiomers, which have identical scalar properties and differ only in pseudoscalar properties, have more in common with homomers than with diastereomers, while diastereomers, which differ in all scalar properties, have more in common with constitutional isomers than with enantiomers.51, 52 It therefore makes more sense, in an isomer classification scheme, to give priority to isometry rather than to constitution.52 In such a scheme there is no need for the concept stereoisomer the concept retains its usefulness only because it normally proves convenient, in chemical reaction schemes, to combine enantiomers and stereoisomers in a common class. 

If a molecule has two or more asymmetric centres then there are various possible structures that are possible and we use terms like stereoisomers, diastereomers and enantiomers to discuss them. To illustrate the relative meaning of these terms, let us look at the... 

Bromine addition to alkenes is an example of a stereospecific reaction. A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other. In this case the starting materials, in separate reactions, are the E and Z stereoisomers of 2-butene. The chiral dibromides from (Z)-2-butene are stereoisomers (diastereomers) of the meso dibromide formed from (7 )-2-butene. 

Diastereomers (Section 5.6) Non-mlrror-image stereoisomers diastereomers have the same cortfiguration at one or more chl-rahty centers hut differ at other chlrahty centers. 

If compounds have the same topology (constitution) but different topography (geometry), they are called stereoisomers. The configuration expresses the different positions of atoms around stereocenters, stereoaxes, and stereoplanes in 3D space, e.g., chiral structures (enantiomers, diastereomers, atropisomers, helicenes, etc.), or cisftrans (Z/E) configuration. If it is possible to interconvert stereoisomers by a rotation around a C-C single bond, they are called conformers. 

Diastereomers= stereoisomers which are not rnirror images usually have different physical properties... 

Stereoisomer I is not a mirror image of III or IV so it is not an enantiomer of either one Stereoisomers that are not related as an object and its mirror image are called diastereomers diastereomers are stereoisomers that are not enantiomers Thus stereoisomer I is a diastereomer of III and a diastereomer of IV Similarly II is a diaste reomer of III and IV... 

Because diastereomers are not mirror images of each other they can have quite different physical and chemical properties For example the (2R 3R) stereoisomer of 3 ammo 2 butanol is a liquid but the (2R 3S) diastereomer is a crystalline solid... 

Steroids are another class of natural products with multiple chirality centers One such compound is cholic acid which can be obtained from bile Its structural formula IS given m Figure 7 12 Cholic acid has 11 chirality centers and so a total (including cholic acid) of 2" or 2048 stereoisomers have this constitution Of these 2048 stereoiso mers how many are diastereomers of cholic acid s Remember Diastereomers are stereoisomers that are not enantiomers and any object can have only one mirror image Therefore of the 2048 stereoisomers one is cholic acid one is its enantiomer and the other 2046 are diastereomers of cholic acid Only a small fraction of these compounds are known and (+) cholic acid is the only one ever isolated from natural sources... 

The CIS and trans isomers of 4 methylcyclohexanol are stereoisomers but they are not related as an ob ject and its mirror image they are diastereomers... 

Stereoisomers that are not enantiomers are classified as diastereomers Each enantiomer of erythro 3 bromo 2 butanol is a diastereomer of each enantiomer of threo 3 bromo 2 butanol... 

If they are stereoisomers are they enantiomers or diastereomers" (Molecular models may prove useful in this problem )... 

Relative to each other both hydroxyl groups are on the same side m Fischer pro jections of the erythrose enantiomers The remaining two stereoisomers have hydroxyl groups on opposite sides m their Fischer projections They are diastereomers of d and L erythrose and are called d and l threose The d and l prefixes again specify the con figuration of the highest numbered chirality center d Threose and l threose are enan tiomers of each other... 

Diastereomers (Section 7 10) Stereoisomers that are not enan tiomers—stereoisomers that are not mirror images of one another... 

Stereochemistry (Chapter 7) Chemistry in three dimensions the relationship of physical and chemical properties to the spatial arrangement of the atoms in a molecule Stereoelectron ic effect (Section 5 16) An electronic effect that depends on the spatial arrangement between the or bitals of the electron donor and acceptor Stereoisomers (Section 3 11) Isomers with the same constitu tion but that differ in respect to the arrangement of their atoms in space Stereoisomers may be either enantiomers or diastereomers... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

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