Stereoisomerism stereogenic centers

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. 

Steam distillation, TEARS assay, 667 l-Stearoyl-2-arachidonoyl-GPC, 737, 738 Stereogenic centers chiral alkenes, 884-5, 1144 dihydrogen tiioxide, 131 Stereoisomerism... 

Figure 1.11 Stereoisomers a carbon atom bonded to fonr different substituents in a chiral carbon or a stereogenic center. Such molecules cannot be superimposed upon their mirror image. A receptor will recognize one stereoisomer but not another. Such stereoisomers are designated as either R or S. Stereoisomerism may also occur around double bonds, producing cis or trans orientations of the substituents on either face of the double bond.

When a stereoisomer has more than one stereogenic center (e.g., n), the number of theoretically possible enantiomers can be derived from the 2" formula. The four types of stereoisomerism are shown in Figure 1. 

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. 

Figure 1.55. C6o-chlorin dyad obtained as mixture of four diastereoisomers that result from configurational (the asterisk marks the stereogenic center in question) and conformational dia-stereoisomerism (resulting from hindered rotation around the bond connecting the chlorin unit to the pyrrolidine ring).

Starting from achiral materials, two stereoisomeric phosphonylated dihydroxy pyrrolidines (275) and (276), containing four stereogenic centers, have been synthesized enantioselectively, employing a combination of enzymatic and transition-metal-mediated methods. Both compounds contain features of the transition state of the enzyme-catalysed fucosyl transfer reaction and represent building blocks of potential inhibitors against this class of enzymes. The synthesis of new sugar-derived phosphonic acids e.g. (277) from protected... 

S)-Metolachlor is the active ingredient of the herbicides Dual and Bicep, which are widely applied in the United States and were developed by the Swiss company Ciba-Geigy (now Novartis). The steric hindrance near the aryl-N bond restricts rotation about that bond. This restricted rotation results in a type of stereoisomerism called atropisomerism, resulting in aR and a,S configurations about the chiral aryl-N bond axis.113 There is also a stereogenic center (position /) on a carbon next to the N-atom in the amino side chain. All four possible stereoisomers of metolachlor are shown in Figure 9-3. 

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]. 

Previous examples in this section (especially equations 90 and 91, and 95-99) have made it clear that the use of substituted vinyl groups in oxy-Cope rearrangements of vinylbicycloalkenes leads to products with at least four stereogenic centers with essentially complete stereocontrol. If the vinyl moiety is not only cyclic but contains one or more chiral centers, then a large number of stereoisomeric products could result. In the search for control over the stereochemical outcome, Paquette et al. have investigated a series of chiral cyclopentene organometallic reagents (Scheme 17). ... 

The situation is the same when the two stereogenic centers are present in a ring. There are four stereoisomeric l-bromo-2-chlorocyclopropanes a pair of enantiomers in which the halogens are trans and a pair in which they are cis. The cis compounds are diastereomers of the trans. 

Only three, not four, stereoisomeric 2,3-butanediols are possible. These three are shown in Figure 7.11. The (27 ,37 ) and (25, 35) forms are enantiomers of each other and have equal and opposite optical rotations. A third combination of stereogenic centers, (27 ,35), however, gives an achiral structure that is superposable on its (25,37 ) mirror image. Because it is achiral, this third stereoisomer is optically inactive. We call achiral molecules that have stereogenic centers meso forms. The meso form in Figure 7.11 is known as mei o-2,3-butanediol. 

Since there are four stereogenic centers and no possibility of meso forms, there are 2", or 16, stereoisomeric hexoses. All 16 are known, having been isolated either as natural products or as the products of chemical synthesis. 

A molecule that contains both stereogenic centers and double bonds has additional opportunities for stereoisomerism. For example, the configuration of the stereogenic center in 3-penten-2-ol may be either R or S, and the double bond may be either E or Z. There are therefore four stereoisomers of 3-penten-2-ol even though it has only one stereogenic center. 

Onc we grasp the idea of stereoisomerism in molecules with two or more stereogenic centers, we can explore further details of addition reactions of alkenes. 

PROBLEM 7.19 There are two other stereoisomeric tartaric acids. Write their Fischer projections, and specify the configuration at their stereogenic centers. 

Glyceraldehyde can be considered to be the simplest chiral carbohydrate. It is an aldotriose and, since it contains one stereogenic center, exists in two stereoisomeric forms the d and l enantiomers. Moving up the scale in complexity, next come the aldotetroses. Examination of their structures illustrates the application of the Fischer system to compounds that contain more than one stereogenic center. 

Hydrogenation of racemic a-monosubstituted P-keto esters normally results in four possible stereoisomeric hydroxy esters. Under appropriate conditions, however, a single stereoisomer with two contiguous stereogenic centers can be ob-... 

The sulfur atom is a stereogenic center in both the sulfurane and the thiiranium ion, and this may influence the stereochemistry of the reactions of stereoisomeric aikenes. Thiiranium ions can be prepared in various ways, and several have been characterized, such as the examples below. 

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