Stereoisomers classification

Since the presence of a plane of symmetry in a molecule ensures that it will be achiral, one a q)ro h to classification of stereoisomers as chiral or achiral is to examine the molecule for symmetry elements. There are other elements of symmetry in addition to planes of symmetry that ensure that a molecule will be superimposable on its mirror image. The trans,cis,cis and tmns,trans,cis stereoisomers of l,3-dibromo-rranj-2,4-dimethylcyclobutaijte are illustrative. This molecule does not possess a plane of symmetry, but the mirror images are superimposable, as illustrated below. This molecule possesses a center of symmetry. A center of symmetry is a point from which any line drawn through the molecule encouniters an identical environment in either direction fiom the center of ixnimetry. 

As noted previously, the classification of stereoisomers preferred by contemporary organic chemists is the enantiomer-diastereomer dichotomy17 and this may be quite conveniently applied to coordination compounds. Thus, complexes (9a) and (9b) are enantiomers, but (9a) and (9c), and (9b) and (9c), are diastereomers. Older terminology might have led to the description of A and B as optical antipodes and to (A+B) and C as geometrical isomers. 

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. 

The olefinic substrates that are cis-trans isomers are by modern stereochemical nomenclature more generally termed diastereomers. That is, they are stereoisomers that are not enantiomers. The fact that they contain no asymmetric carbons is irrelevant to this classification. 

The structure of the sex pheromone for the Fucus species, fucoserratene (11), was elucidated in 1973.16 The positions and geometries of alkenes were revealed by comparison of the gas chromatographic behavior with those of the isomeric conjugated 1,3,5- and 2,4,6-octatrienes. To date, a series of hydrocarbons and epoxides 1-11 and their stereoisomers have been identified within the pheromone bouquets of more than 100 different species of brown algae.17-23 Identification of these compounds was based on a combination of gas chromatography-mass spectrometry (GC-MS) analysis and by comparison with authentic synthetic compounds. These sex pheromones were all lipophilic, volatile compounds that consisted of C8 or Cn linear or monocyclic hydrocarbons or their epoxides. The monocyclic compounds have a cyclopropane, cyclopentene, or cyclo-heptadiene structure. Interestingly, the relationships between the chemical structures of pheromones and the taxonomical classifications of algae are unclear (Table 1). 

Fig. 2 is offered as a scheme allowing immediate comparison of the conventional and isometry-based classifications. These lead by distinct dichotomic pathways to the same four classes, namely homomers, enantiomers, diastereoisomers and constitutional isomers. Note however that the isometry-based classification has the disadvantage of not explicating stereoisomers as a class of isomers. 

The geometry-based classification of stereoisomers, as discussed above, discriminates two mutually exclusive categories, namely enantiomers amd diastereoisomers. 

The configuration-conformation classification of stereoisomers lacks a well defined borderline. In the continuum of energy values, intermediate cases exist which are difficult to classify. In the author s opinion (see also [19]), the boundary between configuration and conformation should be viewed as a broad energy range encompassing the value of 80 kJ/mol (ca. 20 kcal/mol), which is the limit of fair stability under ambient conditions. 

The classification of stereoisomers according to the two independent criteria of symmetry and energy is presented graphically in Fig. 3. Representing all cases of... 

Fig. 3. Summary of the classification of stereoisomers (reproduced from [19], with permission from Marcel Dekker Inc., New York).

Inherently chiral derivatives can be also obtained from calix[4]arenes if three different units are incorporated in the order ABAC or if only two different phenolic units are present, provided these derivatives are fixed in conformations having no symmetry plane and center. Figure 13 gives a survey of the possibilities. For such a classification, one should keep in mind that hydroxy groups (or methoxy groups but ethoxy groups are on the borderline) can pass the annulus. Their orientation may be necessary in a description of the actual conformation of such compounds. It must not be indicated, however, if different stable stereoisomers are to be... 

A pair of geometric isomers are, then, diastereomers. Where do they fit into the other classification scheme, the one based on how the stereoisomers are interconverted... 

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. 

Figure 7 Structure of murisolin (43) and group classifications of its stereoisomers on the basis of simple model compounds 44.

This classification is useful for molecules which do not have stereoisomers. This point is important in chemistry. The asymptotic hamiltonians of normal molecules are invariant to parity. For stereoisomers, the molecule assumes under inversion a configuration in space which cannot be made to coincide with the original configuration by rotation. For these type of molecules, we will talk of a symmetry-broken molecular hamiltonian. These right and left hand modifications exist as real molecules that can interconvert into each other via transition structures having appropriate symmetry. From the present standpoint, there exists different electronic wave functions for the R- and L-molecules. Thus, each subset cannot be used to expand wave functions of the other. 

Finally, in Chapter 18 we present an alternative, universal stereochemical classification of chemical transformations based on (a) overall loss, (b) no loss/gain, and (c) overall gain of chirotopic atoms we label these chirotopoprocesses as chirotopolysis, chirotopomutation and chirotopogenesis, respectively. Further subclassification is carried out using the dual criteria of rotativity (expected optical activity) and stereoselectivity (preferential formation of one stereoisomer over another). We also introduce and define the novel concepts of chiroselectivity and chirospecificity. Finally, the merits of the classification of chirotopoprocesses are discussed, and the stereotopoprocesses and chirotopoprocesses are correlated in relation to the stereotopic molecular faces. 

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