Stereoisomers definition

Since the stereochemistry of the triene system of LTB4 had not been determined prior to synthesis, a number of stereoisomers of LTB4 were prepared for purposes of definitive comparison of physical properties and bioactivity with biologically produced LTB4. The various stereoisomers of LTB4 were much less active biologically than LTB4 itself. 

In this paper, asymmetric carbon atoms are considered only in paraffins and substituted paraffins, and the following definition will be retained (cf. Sec. 36(b)) A carbon atom is called asymmetric if the four bonded radicals arc pairwise structurally different. (Thus, it is not sufficient to require that the four radicals are not stereoisomers in order to declare a carbon atom asymmetric. One could envisage other, possibly useful, definitions.)... 

However, if both maleic and fumaric acid gave the dl pair or a mixture in which the dl pair predominated, the reaction would be stereoselective but not stereospecific. If more or less equal amounts of dl and meso forms were produced in each case, the reaction would be nonstereoselective. A consequence of these definitions is that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific, but at most stereoselective. For example, addition of bromine to methylacetylene could (and does) result in preferential formation of trans-1,2-dibromopropene, but this can be only a stereoselective, not a stereospecific reaction. 

The lUPAC 1974 Recommendations, Section E, Fundamental Stereochemistry, give definitions for most of the terms used in this chapter, as well as rules for naming the various kinds of stereoisomers. They can be found in Pure Appl. Chem., 1976, 45,... 

A definitive feature of this highly stereoselective new route to substituted tetrahydrofurans is that both syn and anti allylic diol stereoisomers typically afford identical tetrahydrofuran products. Thus, there is no need for stereoselective... 

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

The traditional limitation of the stereoisomer concept to isomeric compounds with different configurations stems from the idea that configurations are not interconvertible under the observations conditions. ) A definition of stereoisomers on this basis is questionable as its application is then dependent upon observation conditions. It also fails to account for the fact that many mobile interconversions of configurations (e.g. configurations involving tricoordinate Nitrogen or pentacoordinate Phosphorus) are known, as well as thermally stable conformations (e.g. the atropisomers). 

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. 

On the other hand, selective, usually applied to a synthesis, means that of all the possible isomers only one isomer is obtained. However, if the reaction product was/is a mixture of isomers one could speak then of the "degree of selectivity". Since usually one of the isomers will be the predominant isomer, we may say that the reaction (or the synthesis) is selective with respect to this particular isomer. As in the case of "specificity", we may refer to "regioselectivity" or to "stereoselectivity" (either diastereoselectivity or enantioselectivity) and may say, for instance, that a synthesis is 80% diastereoselective. According to the most updated terminology "diastereomers" are all the "stereoisomers" that are not "enantiomers", so geometrical isomers are also included in such a definition. 

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

By definition, it is necessary to recognize geometric isomers, isotopomers and stereoisomers as distinct species. Moreover, there is the pragmatic issue that regio-selectivity, isotopic labeling and stereo-chemical investigations are three very important avenues of mechanistic enquiry. 

These methods were developed to avoid the definition and parameterization of numerous fragments or atomic types and to allow the prediction of lipophilicity for stereoisomers or conformers. Only a selection of methods are highlighted here, since an excellent extensive review was recently published [34]. 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

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