Stereoisomers chiral molecules

The minor images of bromochlorofluoromethane have the sane constitution. That is, the atoms are connected in the sane order. But they differ in the anangement of then-atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable minor image are classified as enantiomers. The word enantiomer describes a paiticulai- relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, Is that person a cousin Fuithennore, just as an object has one, and only one, minor image, a chiral molecule can have one, and only one, enantiomer. 

Consider the stereochemical relationships between these flexible stereoisomers. A flexible molecule is chiral only if each of its conformers is chiral and if no two conformers are mirror images. Which, if any, of the stereoisomers are chiral Rexible chiral molecules are enantiomers only if each of their conformers are mirror-images. Which, if any, of the stereoisomers are enantiomers and which are diastereomers ... 

The FDA (and similar regulatory agencies, as reviewed by Daniels et al., 1997) has become increasingly concerned with the safety of stereoisomeric or chiral drugs. Stereoisomers are molecules that are identical to one another in terms of atomic... 

In living organisms, chiral molecules are usually present in only one of their chiral forms. For example, the amino acids in proteins occur only as their l isomers glucose occurs only as its D isomer. (The conventions for naming stereoisomers of the amino acids are described in Chapter 3 those for sugars, in Chapter 7 the RS system, described above, is the most useful for some biomolecules.) In contrast, when a compound with an asymmetric carbon atom is chemically synthesized in the laboratory, the reaction usually pro-... 

Stereoisomers Structural isomers having an identical chemical constitution but exhibiting differences in the spatial arrangement of their atoms are called stereoisomers [7], One case of stereoisomerism, denoted asymmetric chirality, comprises molecules that are mirror images of each other. Such pairs of molecules are called enantiomers. Figure 1.2.3 illustrates the two chiral molecules of 1-bromo-1-chloroethane. The line in the middle represents a symmetry plane. Note that it is... 

The single most important physical property that differentiates enantiomers is their ability to rotate the plane of plane polarized light. This property is called optical activity and is displayed only by chiral molecules. Thus, stereoisomers which are also chiral are known as optical isomers. Chiral molecules that rotate polarized light in a clockwise fashion are termed dextrorotatory (d) while those that rotate the beam counterclockwise are levorotatory (/). Enantiomers have optical rotations of die same magnitude but of different signs (d or /). 

These rules and applications are summarized completely in most introductory organic texts. It is important to be able to assign R,S configurations to stereogenic centers in molecules and to construct chiral molecules given the R or S configuration. In this way it is very easy to determine the stereochemical relationships between stereoisomers. 

Enzymes often prove to be the catalyst of choice for numerous transformations, and their prowess is particularly noteworthy for the synthesis of chiral molecules. The ability of biocatalysts to impart chirality through conversion of prochiral molecules or by transformation of only one stereoisomer of a racemic mixture stems from the inherent chirality of enzymes. As noted in the introduction to this book (Chapter 1), the chiral drug market is increasing, partly as a result of the need to produce single enantiomers as advocated by the U.S. Food and Drag Administration.1 The ability to extend the patent life of a drug through a racemic switch also plays a role in this increase. An example of a racemic switch is Astra Zeneca s Esomeprazole, a proton pump inhibitor (see Chapter 31).2... 

Molecules such as 2-chlorobutane are termed chiral. Chiral molecules exist as either of two stereoisomeric structures. These stereoisomers, a pair of nonsuperimposable mirror images, are called enantiomers. 

Problem 5.21 Answer True or False to each of the following statements and explain your choice, (a) There are two broad classes of stereoisomers. (/ ) Achiral molecules cannot possess chiral centers, (c) A reaction catalyzed by an enzyme always gives an optically active product, (d) Racemization of an enantiomer must result in the breaking of at least one bond to the chiral center, (e) An attempted resolution can distinguish a racemate from a meso compound. M... 

The enantiomers of a chiral compound have identical physical and chemical properties. Accordingly, abiotic processes such as air-water exchange, sorption, and abiotic transformation are generally identical for both enantiomers. However, biochemical processes may differ among stereoisomers because they can interact differentially with other chiral molecules such as enzymes and biological receptors. Thus, enantiomers may have different biological and toxicological effects. 

Of the nine stereoisomers of inositol, the scyllo-isomer has no axial hydroxyl, the myo-isomer has one, the epi-, chiro-, and neo-isomers have two, and the alio-, cis-, and mneo-isomers have three hydroxyl groups (Figure 1). Of these, six isomers scyllo-, myo-, epi-, neo-, cis-, and mr/co-i somers) have one or more planes of symmetry in the molecule (meso compounds) and are therefore not chiral. D-chiro- and L-e/u>o-i somers do not have a plane of symmetry and are chiral molecules moreover they are enantiomers of each other. The alio-isomer is unique - the conformational isomer of (10) (Figure 2) is (11) which is also its enantiomer Since interconversion between conformational isomers is rapid, a//o-inositol exists as a 50/50 mixture of the two enantiomers at room temperature. Therefore, although alio-inositol is chiral, the compound is optically inactive at room temperature because it is a racemic mixture a chiral reagent, such as an enzyme, would be expected to preferentially react with one enantiomer and not the other. 

The earlier, almost universally accepted belief that if one enantiomer of a chiral molecule demonstrates pharmacological activity, the other enantiomer will be pharmacologically inert, is not valid. It must be anticipated that all stereoisomers of an organic molecule will exhibit pharmacological effects, frequently widely different and unpredictable. Many examples of qualitative and quantitative differences in metabolism of enantiomers are documented (57). 

The spatial relationships that exist for the human body also exist at the molecular level because the molecules of nature exist as three-dimensional symmetrical and asymmetrical figures. One of the most common asymmetric molecules is a tetravalent carbon atom with four different ligands attached to it. The spatial arrangement of the atoms in this molecule is shown in Fig. 2, The carbon atom depicted in Fig. 2 is the asymmetric center of the molecule, and the molecule is a chiral stereoisomer. If the molecule and its mirror image are nonsuperimposable, the relationship between the two molecules is enantiomeric, and the two stereoisomers are enantiomers. Carbon is not the only atom that can act as an asymmetric center. Phosphorus, sulfur, and nitrogen are among some of the other atoms that can form chiral molecules. 

A clockwise rotation of a beam of plane-polarized light by a stereoisomer, usually used to denote a specific enantiomer of a chiral molecule, that is d- or (-1-)-ephedrine... 

One of the most important advantages of biocatalysts is their stereospedficity. An example of a stereospedfic biotransformation is given in Figure 2.3. In 1992 the Food and Drug Administration (FDA) in the United States addressed the issue of whether it mattered, for drug approval purposes, that a preparation of a synthetic chiral molecule (one or more as)rmmetric centers) contains not one but two compounds (racemates) -the two mirror-image stereoisomers (individual enantiomers). 

Interconvertibility of stereoisomers is of great prac, cal signiffcance because it limits their isolahi/ity. Hard-to-interconvert stereoisomers can be separated (with special methods, of course, for resolution of enantiomers) and studied individually among other things, their optical activity can be measured. Easy-to-interconvert isomers cannot be separated, and single isolated isomers cannot be studied optical activity cannot be observed, since any chiral molecules are present only as non-resolvable racemic modifications. 

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