Stereogenicity and Stereoisomerism

The confusion of these terms ( prochiral , prostereogenic , and prostereoisomerism ) is so serious that most articles and textbooks other than those of biochemical fields tend to use enantiotopic , diastereotopic , and stereoheterotopic in place of these terms. However, such replacement conceals problematic situations to be settled, because the confusion reflects misleading situations concerned with chiral , stereogenic , and stereoisomerism , which are the cruxes of stereochemistry. 

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

Even a fourth variant is feasible, namely the submission of meso-compounds -these contain stereogenic centers but feature internal elements of symmetry and, hence, are achiral - to a desymmetrization, which in principle generates only one stereoisomer. The classical case is the hydrolysis of a diester to form a half-acid/es-ter as a single antipode. Generation of both optical isomers of synthetically useful 1,2-diol products in good yield and stereoisomeric purity applying this methodology has recently been demonstrated (see Fig. 2.11) [65]. 

The newly-developed dichotomy has advantages over the conventional dichotomy between enantiomers and diastereomers (the broken-Uned boxes in Fig. 10.11), because the latter causes the confusion due to the entangled criteria shown in Table 10.3. This means that a paradigm shift from the conventional terminology to the present terminology for stereoisomerism turns out to be inevitable (Fig. 10.11) (Fujita 2009e), where correspondence between terms for relationships and those for attributes are defined as follow enantiomeric — chiral f S -diastereomeric — f S -stereogenic and l 5-stereoisomeric — f 5-stereoisomerism. 

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. 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

The description of stereoselective reactions in which one new stereogenic unit is created, i.e., where a pair of enantio- or diastereomers can result, is straightforward. However, there are now numerous examples known of stereoselective reactions in which two or more stereogenic units are generated in the bond-forming step. Accordingly, more than two stereoisomers are formed. In principle, stating the ratio of the stereoisomeric products would suffice for the description of the outcome of such a reaction. However, mechanistic rationalization and prediction of the results are vastly simplified when subsets of the stereoisomers and their relative ratios are considered. Here the terms simple and induced diastereoselectivity play an important practical role. 

Most organic molecules incorporate at least one stereogenic element (centre, plane or axis) [26] and may, therefore, exist in more than one stereoisomeric form. These stereoisomers, as far as mechanistic argument is concerned, are distinct species, and the kinetics and... 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

In a substance with more than one stereogenic center, each center is independently specified as R or S. Streptimidone has two stereogenic centers and two double bonds. Only the internal double bond is capable of stereoisomerism. 

It has no stereogenic carbons and thus does not exist in stereoisomeric forms. 

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