Configurational isomers Stereoisomers

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

Stereoisomers, on the other hand, are compounds with the same molecular formula, and the same sequence of covalently bonded atoms, but with a different spatial orientation. Two major classes of stereoisomers are recognized, conformational isomers and configurational isomers. 

Now for a rather unexpected twist. We have seen that if there are n chiral centres there should be 2" configurational isomers, and we have considered each of these for n = 2 (e.g. ephedrine, pseudoephedrine). It transpires that if the groups around chiral centres are the same, then the number of stereoisomers is less than 2". Thus, when n = 2, there are only three stereoisomers, not four. As one of the simplest examples, let us consider in detail tartaric acid, a component of grape juice and many other fruits. This fits the requirement, since each of the two chiral centres has the same substituents. 

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

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

The four possible stereoisomers of the diastereomeric mixture of ( )-2-(lH-imidazol-4-yl)-cyclopropylamine (17) have not been separated yet (except very recently both tram isomers (Table 4) [25]), but as a result of this molecular modelling study it was observed that a congruent pharmacophoric conformation can only be adopted by the two tram configured isomers [24]. The conformationally restricted pyrrolidine derivatives immepyr (21) and SCH 50971 (22) support this pharmacophore... 

From a structural point of view a number of different isomers could be present. In principal the three different hydroxyl groups in the ligand are able to interact with the central metal ion thus giving rise to three different constitutional isomers. The stereochemistry of the different configurational isomers of gadobutrol is best described in view of the stereochemistry of the parent compound Gd-DOTA. In this complex chirality results from the restriction of free rotation around the bonds of the ligand caused by the inclusion of the central metal ion. Thus four stereoisomers are generated upon complexation of Gd(III) by DOTA. 

Stereoisomers have the same order of atom attachments but different arrangements of the atoms in space. Cis-trans isomerism is one kind of stereoisomerism. For example, two substituents on a cycloalkane can be on either the same (c/s) or opposite (trans) sides of the mean ring plane. Stereoisomers can be divided into two groups, conformational isomers (interconvertible by bond rotation) and configurational isomers (not interconvertible by bond rotation). Cis-trans isomers belong to the latter class. 

All pairs have the same bond patterns and are stereoisomers. Since they are not interconvertible by a-bond rotations, they are configurational isomers, conformational isomers conformational isomers... 

Trans-1,2-dibromoethene and cis-1,2-dibromoethene are configurational isomers (they can only be interconverted by breaking and remaking a bond, in this case the pi-bond of the alkene), they are both achiral (their mirror images are identical to themselves), and they are diastereomers (although they are stereoisomers, they are not enantiomers). 

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. 

Because the double bond has an E configuration, the total number of configurational isomers which are theoretically possible is 24 = 16 (four chirality centres). However, as a result of constitutional symmetry twelve of the theoretically possible isomers are in fact six pairs of identical compounds. Therefore only ten stereoisomers are possible lRyVRy2RyTR lRyVRy2Sy2 S ... 

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

Other interesting examples of positional isomer separation involving NPC are (a) the separation of dihydrodipyridopyridopyrazines, a new family of antitumor agents, on a silica Nucleosil 50 A-lOpm column [43] and (b) the separation of celecoxib isomers by Chiralpak AD column [44], NPC was also employed successfully for the resolution of (a) four configurational isomers of a steroidal calyx pyrrole [45], (b) regio- and stereoisomers of eicosanoids [46], (c) retinal and retinol isomers [47], and (d) several ElZ isomers pairs of vitamin A [48],... 

A particular three-dimensional arrangement is called a configuration. Thus, stereoisomers differ in configuration. The cis and trans isomers in Section 4.13B and the biomolecules starch and cellulose in Section 5.1 are two examples of stereoisomers. 

In Secs. 4.20 and 5.6, we learned that stereoisomers can be classified not only as to whether or not they are mirror images, but also—and quite independently of the other classification—as to how they are interconverted. Altogether, we have (a) configurational isomers, interconverted by inversion (turning-inside-out) at a chiral center (b) geometric isomers, interconverted—in principle—by rotation about a double bond and (c) conformational isomers, interconverted by rotations about single bonds. 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

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