Chiral centers, more than one

Chiral centers, more than one, lljf Chiral stereomer, 69 Cholesterol, chirality in, 81 Cinnamaldehyde, 328 Cis-trans interconversion, 111 Cis-trans isomerism, in cyclic compounds, 163 Claisen condensation, 394 rearrangement, 439 Cleavage, oxidative, 117 Clemmensen reduction, 219, 311 Coenzyme A, 354 Collins reagent, 264 Collision frequency, 39 Configuration, 72 relative, 76 Conformation, 51 Conformational diastereomers, 78 enantiomers, 78 stereomers, 78... 

Noting the presence of one (but not more than one) chirality center is a simple rapid way to determine if a molecule is chiral For example C 2 is a chirality center m... 

One final very important point Everything we have said in this section concerns molecules that have one and only one chirality center molecules with more than one chirality center may or may not be chiral Molecules that have more than one chirality center will be discussed m Sections 7 10 through 7 13... 

We 11 continue with the three dimensional details of chemical reactions later m this chapter First though we need to develop some additional stereochemical principles con cernmg structures with more than one chirality center... 

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

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

Let s look at one more example of a compound with more than one chirality center, the tartaric acid used by Pasteur. The four stereoisomers can be drawn as follows ... 

Some molecules have more than one chirality center. Enantiomers have opposite configuration at all chirality centers, whereas diastereomers have the same configuration in at least one center but opposite configurations at the others. Epimers are diastereomers that differ in configuration at only one chirality center. A compound with n chirality centers can have a maximum of 2n stereoisomers. 

Carbohydrates with more than one chirality center are shown in. Fischer projections by stacking the centers on top of one another. By convention, the carbony] carbon is always placed either at or near the top. Glucose, for... 

A molecule such as CHQBrl, which exists in two different forms that are not superim-posable mirror images, is said to be chiral. The two different forms are referred to as enantiomers, or optical isomers. Any molecule in which four different groups are bonded to carbon will be chiral the carbon atom serves as a chiral center. Molecules may contain more than one chiral center, in which case there can be more than two enantiomers. 

Molecules With More Than One Chiral (Stereogenic) Center... 

Fig. 1. Introduction of one or more than one stereogenic elements (center, axis, plane or helix) leads to different types of chiral dendrimers...

Molecules have more than one stereogenic (chiral) center diastereomers... 

Two compounds are diastereomers when they contain more than one chiral center. If the number of dissymmetric centers is given by N, then the number of possible diastereomers is given by 2N. Of these 2 v diastereomers, each will be characterized by its mirror image, so that the number of enantiomers is given by 2NI2. Whereas the physical properties of enantiomers in an achiral environment are necessarily identical, the physical properties (including solubility) of diastereomers are normally different. The differences arise since there is no structural requirement that the crystal lattices of different diastereomers be the same. For instance, the solubility of an (SS )-diastereomer could differ substantially from that of the (/ S)-diastereomer. However, it should be remembered that the solubility of the (SS)-diastereomer must be exactly identical to that of the (I 7 )-diastereomer, since these compounds are enantiomers of each other. At the same time, the solubilities of the (SI )-diastereomer and the (I S)-diastereomer must also be identical. 

If a molecule contains more than one chiral center, there are other forms of stereoisomerism. As mentioned in Section 1.1, nonsuperimposable mirror images are called enantiomers. However, substances with the same chemical constitution may not be mirror images and may instead differ from one another... 

To obtain VRCI for molecules containing more than one chiral center, the root mean square of VRCI for all the chiral centers is taken. 

Since the commercial introduction of the P-CAC in 1999, several industrial applications have been shown to be transferable to the system. Moreover, users in the biopharmaceutical and foodstuff industry have seen their productivity increasing dramatically as a result of using the P-CAC technology. Furthermore, a P-CAC has been shown capable of continuously separating stereoisomers when using chiral stationary phases even when there is more than one chiral center in the desired molecule. Below some of the applications are described in more details. Others are proprietary and hence cannot be disclosed. 

Recently, CE has been developed for the analysis of drug substances. It is not employed with great frequency for quality control (owing to inherent sensitivity issues), but on occasion, CE procedures can be employed when HPLC procedures have failed to adequately measure the impurities. CE is particularly useful for the separation of closely related compounds such as the diastereomers and enantiomers of compounds with more than one chiral center. Figure 1.8 illustrates the separation of the diastereomers quinine (QN) and quinidine (QD) and some impurities using CE [17]. 

Stereoisomers with more than one chiral center and which are not mirror images of each other hence, stereoisomers that are not enantiomers of each other. For example, L-threonine and D-threonine are an enantiomeric pair whereas L-threonine and D-allothreonine are a diastereomeric pair (as is L-threonine and L-allothreo-nine). Diastereomers will have similar physical, chemical, and spectral properties but those properties will not be identical. If n is the number of chiral centers, then the maximum number of stereoisomers will be equal to 2. However, the actual number for a given set of isomers may be less than 2 due to the presence of meso forms. See Enantiomer Epimer Meso Form... 

A stereoisomer having more than one chiral center and having an internal plane of symmetry. Hence, meso compounds do not exhibit optical activity. 

Some important organic compounds have more than one chiral center. Multiple chiral centers indicate the presence of multiple stereoisomers. 

Asymmetric synthesis has achieved a position as one of the most important areas of modem organic chemistry. Dnring the past 20 years the number of publications in this area has been vast. On the pallet of organic reactions that are used in asymmetric synthesis, cycloadditions possess a prominent position, since they are some of the most efficient methods for creating new chiral centers with control of stereochemistry (1-4). The ability to introduce more than one new chiral center in a single step with control of both relative and absolute stereochemistry makes cycloaddition reactions highly attractive key reactions for stereoselective synthesis. 

Moving two-dimensional enantiomorphs out of the plane into three-dimensional space allows them to become congruent, i.e., identical. However, some two-dimensional aspects remain. When the two faces of a particular figure are examined, one perceives that the faces, and the corresponding half-spaces10, of the three-dimensional space, are enantiomorphic, and their chirality sense can be specified by Re/Si descriptors. Furthermore, for figures with more than one stereogenic center the Ikjul and ZjE descriptions are preserved in three-dimensional space. 

The classical method of EPC synthesis is the preparation of the chiral compound in racemic form and subsequent separation of the enantiomers ( optical resolution ). If the compound contains more than one stereogenic center, it is first prepared as a diastereomerically pure racemate and then submitted to optical resolution. 

MOLECULES WITH MORE THAN ONE CHIRAL CENTER... 

Epimers differ in configuration about a single chiral center in molecules with more than one chiral center. Anomers are epimers in which the chiral site was formerly a carbonyl C. 

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