Stereogenic centers Stereoisomers, also

Stereogenic center (Section 5.3) A site in a molecule at which the interchange of two groups forms a stereoisomer. A carbon bonded to four different groups is a tetrahedral stereogenic center. A tetrahedral stereogenic center is also called a chirality center, a chiral center, or an asymmetric center. 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

The essence of asymmetric synthesis is producing a new stereogenic center in such a manner that the product consists of stereoisomers in unequal amount. In most cases, this can be achieved by the formation of a new sp3 stereocenter. There is also another type of asymmetric reaction in which the employed substrates contain either a stereogenic unit or a pro-stereogenic unit apart from the functional group, and asymmetric synthesis occurs even though the nature of the reaction is not directly related to the newly formed sp3 stereocenter. The Wittig reaction is invoked for the asymmetric synthesis of such molecules.47... 

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 R was a stereogenic center, of the four possible stereoisomers only the two tnmv-3-halo-isomers were obtained. A modification of the so-obtained [3-lactams [216] has been also reported consisting in a dehalogenation procedure giving rise to 3-unsubstituted [i-lactams, (Scheme 96). 

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. 

More elaborate molecules can also have a plane of symmetry. For example, there are only three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral but the third is achiral. In the achiral stereoisomer, the substituents are located with respect to each other in such a way as to generate a plane of symmetry. Compounds that contain two or more stereogenic centers but have a plane of symmetry are called meso forms. Because they are achiral, they do not rotate plane polarized light. Note that the Fischer projection structure of meio-tartaric acid reveals the plane of symmetry. 

Hauske et al. subjected ester 23 with the chirality center also outside of the peri-cyclic framework to the Ireland-Claisen rearrangement and observed a very strong influence of the stereogenic center. 24 was obtained as a single stereoisomer in good yield [19]. 

What is it about their structures that leads to chirality in 2-chlorobutane but not in 2-chloropropane Notice that, in 2-chlorobutane, carbon atom 2, the one marked with an asterisk, has four different groups attached to it (Cl, H, CH3, and CH2CH3). A carbon atom with four different groups attached to it is called a stereogenic carbon atom. This type of carbon is also called a stereogenic center because it gives rise to stereoisomers. 

Despite the lUPAC recommendation to use the term "chirality center," the terms stereocenter and stereogenic center are more commonly used. However, these terms have a broader definition a stereocenter, or stereogenic center, is defined as a location at which the interchange of two substituents will generate a stereoisomer. This definition includes chirality centers, but it also includes c/s and trans double bonds, which are not chirality centers. 

Where they exist, we also identify enantiomeric pairs. A molecule with n stereogenic centers has 2 stereoisomers (except when special symmetry conditions are present). When two stereoisomers are mirror images of each other they form an enantiomer pair. Each stereoisomer has only one enantiomer. Any other pairing of stereoisomers is considered to be a diastereomer pair. Thus, when there are... 

As we found when we considered acyclic compounds containing two stereogenic centers, the number of stereoisomers of cyclic compounds depends on whether or not the centers are equivalent. This also true for cychc compounds. First, we U examine the isomeric cis- and 1-bromo-2-chlorocyclobutanes (Figure 8.14a). These compounds are diastereomers. The compounds in Figure 8.14 are arranged to demonstrate the mirror image relationship of the two tram enantiomers, whose configurations are IR,2R and 15,25. There are also two enantiomeric cis isomers. 

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