Configuration Stereoisomers

The first two classes of selectivity distinguish between constitutional isomers the last one between stereoisomers (configurational isomers and, eventually, conformational isomers). 

STEREOISOMERS Configurational isomers (Same connectivity but different three dimensional arrangement of atoms)... 

According to the formula for the calculation of the theoretical number of stereoisomers (configurational isomers), x = 2W, 24 = 16 (where n = 4 = the number of stereogenic units) stereoisomers are possible for alitretinoin. Although the molecule contains five double bonds only those in the side chain can be considered, since because of ring strain the double bond in the six-membered ring can only have the Z configuration. 

Stereoisomers (configurational isomers) (same connectivity but different... 

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. 

The pair of isomers designated as and trans 2 butene have the same constitution both have an unbranched carbon chain with a double bond connecting C 2 and C 3 They differ from each other however m that the cis isomer has both of its methyl groups on the same side of the double bond but the methyl groups m the trans isomer are on oppo site sides of the double bond Recall from Section 3 11 that isomers that have the same constitution but differ m the arrangement of their atoms m space are classified as stereoisomers as 2 Butene and trans 2 butene are stereoisomers and the terms as and trans specify the configuration of the double bond... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Eleven chirality centers may seem like a lot but it is nowhere close to a world record It is a modest number when compared with the more than 100 chirality centers typ ical for most small proteins and the thousands of chirality centers present m nucleic acids A molecule that contains both chirality centers and double bonds has additional opportunities for stereoisomerism For example the configuration of the chirality center m 3 penten 2 ol may be either R or S and the double bond may be either E or Z There fore 3 penten 2 ol has four stereoisomers even though it has only one chirality center... 

Thiol (Section 15 13) Compound of the type RSH or ArSH Th reo (Section 7 11) Term applied to the relative configuration of two stereogenic centers within a molecule The threo stereoisomer has like substituents on opposite sides of a Fischer projection... 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

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. 

A study of the photolysis of A, B, and C has been reported. A gives both D and the cleavage product benzaldehyde. B gives only E. C gives benzaldehyde and the stereoisomer B. Discuss the ways in which the presence and configuration of the remote t-butyl group can control the product composition, and account for the formation of the observed products. 

How many alkenes have the molecular formula C5H10 Write their structures and give their lUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate. 

In Problem 4.25 you were asked to draw the preferred conformation of menthol on the basis of the information that menthol is the most stable stereoisomer of 2-isopropyl-5-methylcyclo-hexanol. We can now completely describe (—)-menthol structurally by noting that it has the R configuration at the hydroxyl-substituted carbon. 

Relative to each other, both hydroxyl groups are on the same side in Fischer projections of the erythrose enantiomers. The remaining two stereoisomers have hydroxyl groups on opposite sides in 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 configuration of the highest numbered chirality center. D-Threose and L-threose are enantiomers of each other ... 

Because the configuration at C-2 is lost on enolization, the enediol intermediate can revert either to D-glucose or to D-mannose. Two stereoisomers that have multiple chirality centers but differ in configuration at only one of them are refened to as... 

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