Line of stereoisomerism

A third type of configurational interdependence exists if two elements are so interrelated that a change in the configuration of one automatically alters that of the other. This characterization applies to the two centers of 1,4-cyclohexanediol of the type Cg+g hi (5,51). Consequently only two isomers exist and a single pair of descriptors suffices for their distinction. We can remove the mutual dependence of the two elements by waiving the requirement that a line of stereoisomerism be occupied by bonds. The H and OH ligands have different distributions in the isomers about the line between C(l) and C(4), and the usual terms cis and trans express this relationship. Undoubtedly this is the most convenient description and the only one now available, but should we go further and say that the proper element of stereoisomerism in this case is this achiral line of torsion, and that its further factorization into two graphochiral centers is unwarranted ... 

Even this brief list may suffice to show that it would be a formidable task to develop a system of factorization free of avoidable redundancies, and that such a system would not be satisfactory even if it avoids arbitrary choices. It would require a rule disqualifying certain centers or lines of stereoisomerism on the basis of their relationships to other potential elements in the same molecule. Such definitions would not be self-contained. Moreover, the products of factorization that would take the place of those dropped cannot be limited to points or lines that are merely differently defined. There would have to be a virtually open-ended proliferation of new elements. This highly undesirable feature would not be offset by a major benefit of the revised system such as a correlation between the numbers of elements and of stereoisomers, because a complete elimination of all redundancies does not seem possible. We conclude that the system of choice is the one based on the principle that the elements of stereoisomerism allow no further factoring. Accordingly we think it best to retain the definitions given in Sects. IV and VI and their strictures that all centers and lines be occupied by atoms or bonds. 

Cahn et al. (4) thought it beneficial to have two methods available for specifying the sense of chirality of the biaryls, as examples of either axial chirality or conformational chirality. As we have merged both into a single class—the line of stereoisomerism—any justification for two modes of description has vanished. Except for the modifications envisaged earlier, the procedures of Section 4 of the Sequence Rule (4) seem well suited to describe the isomerism of the line. Thus the terms MIP would become the general descriptors of the chiral line of torsion. 

The prochirality concept is not necessarily an expression of a precursor-product relationship because there exist stereoselective reactions at pro-chiral elements that do not generate elements of chirality. An illustration of this is the reversible enzymatic dehydration of citric to cu-aconitic acid. In this process two prochiral centers of citric acid disappear and we obtain an achiral line of stereoisomerism that physically coincides with a prochiral plane of prostereoisomerism. 

In the third chapter, Hans Hirschmann and Kenneth R. Hanson provide a detailed analysis of the principles of stereochemical classification or factorization. In contrast to the system earlier proposed by Cahn, Ingold, and Prelog (and recently extended and modified by Prelog and Helmchen) featuring centers, axes, and planes of chirality, Hirschmann and Hanson here present an alternative scheme not limited to chiral structures. This scheme for the factorization of stereoisomerism uses as principal elements the center and line of stereoisomerism. Numerous examples are given. 

These investigations have followed three main lines, (1) alterations in the amino-alcohol nucleus, (2) variation in the alkyl or acyl side-chains, (3) influence of stereoisomerism. Tropine and ecgonine, the basic components of atropine and cocaine, lend themselves to such investigations, but scopine, the amino-alcohol of hyoscine is so labile that systematic modification of this alkaloid has not yet been possible. 

Figure 3. Some major types of centers (X) and lines (X—X) of stereoisomerism. Different degrees of ligand diversity are possible for most of these types.

The elements of stereoisomerism considered thus far consist of a point occupied by an atom and a line wholly occupied by one or more bonds. The ligands are joined directly to these elements. This emphasis on bonding relationships appears to be proper, as the distinction between constitutional and steric isomerism similarly depends on established patterns of connectedness. From this point of view it seems less than satisfactory if direct connectedness between specific atoms is assumed, when chemical theory envisions no such localized bond. This situation prevails in the description of ir complexes such as the metallocenes. Initially (44a), the 1,2- (22) or 1,3-heteroligated ferrocenes were considered to... 

The operation that interconverts stereoisomeric planes can be viewed as a ir rotation of the plane that breaks the bond(s) between the plane and the extraplanar atom(s) and then restores it (or them) in a sterically different way. The axes of this rotation pass through the center of the unsaturated system. As they are infinite in number (46) they cover the plane and in a sense define it. Although several of these axes coincide with bonds, the operation differs profoundly from that characteristic of a line of torsion. Rotation of the plane causes a change in bonding, whereas torsion about a line alters the torsional angles between the ligands that are attached to the terminal atoms of the bond that defines the line. 

An element of prostereoisomerism is a partial structure that can be converted into an element of stereoisomerism not otherwise present, by considering one of a pair of homomorphic groups to be different from the other. The groups involved in this operation are necessarily heterotopic. Depending on the character of the element of stereoisomerism thus produced, one can divide the elements of prostereoisomerism into centers, lines, and planes and subdivide them, as appropriate, into those that are (fully) prochiral, only prographochiral, only... 

The claimed lack of ambiguity does not preclude a physical overlap between elements. If one chooses to regard as stereoisomers the three nonsuperposable staggered forms resulting from the torsion of a carbon-carbon bond with six different ligands, we have three separate elements of stereoisomerism, two centers and one line. This is the minimum number required to account for the existence of the 12 stereoisomers. 

A line of research that has aroused much interest in recent years is the study of head-to-head, tail-to-tail polymers (96-98). Their direct synthesis has little likelihood of being successffil as head-to-tail sequences usually predominate in vinyl polymerization. One possibility for their preparation is through the chemical modification of suitable preformed polymers. In the case of the head-to-head, tail-to-tail polypropylene, different stereoisomeric forms have been isolated, depending on the method of preparation. In the general scheme, the precursor is an unsaturated polymer obtained by polymerization of the disubsti-tuted butadiene (2,3-dimethylbutadiene or 2,4-hexadiene) then, by chemical or catalytic reduction, this polymer is converted into the desired polypropylene, whose stmcture can then be examined by NMR spectra. Head-to-head, tail-to-... 

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... 

Fig. 15.45. Intramolecular 1,3-dipolar additions of stereoisomeric nitrile oxides to form stereoisomeric isoxazo-lines.

A general and precise description of stereoisomerism in polymers is suggested on the basis of the repetition theory which describes the distinct patterns along a line that can be obtained from a three-dimensional motif. The probability models for describing the" stero-sequence length in various possible cases of interest in stereoregular polymers are discussed. It is shown that for describing the stereosequence structure, the simplest probability model must involve a Markov chain with four probability parameters. 

Trimedlure may exist in eight different isomeric forms, depending on which of the four dotted lines of Figure 3 the chlorine atom resides upon, and whether the methyl and ester groups are cis or trans. Our preparations of trimedlure are not pure compounds but mixtures of isomers. Trimedlure has been separated into two solid isomers and a liquid fraction. Tests at our Hawaii laboratory indicated that one of the solids and the liquid portion of trimedlure are attractive, but the other solid is not attractive 91). Apparently, stereoisomerism can play an important role in insect attraction. 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

H.-G. Schmarx, A. Mosandl and K. Grob, Stereoisomeric flavour compounds. XXXVIII dkect chir ospecific analysis of y-lactones using on-line coupled EC-GC with a chkal separation column , Chromatographia 29 125-130 (1990). 

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