Stereoisomers, chirality

When using Fischer projections for this purpose however be sure to remember what three dimensional objects they stand for One should not for example test for superim position of the two chiral stereoisomers by a procedure that involves moving any part of a Fischer projection out of the plane of the paper in any step... 

Many drugs are racemates, including 13-blockers, nonsteroidal anti-inflammatory agents, and anticholinergics (e.g benzetimide A). A racemate consists of a molecule and its corresponding mirror image which, like the left and right hand, cannot be superimposed. Such chiral ( handed ) pairs of molecules are referred to as enantiomers. Usually, chirality is due to a carbon atom (C) linked to four different substituents ( asymmetric center ). Enantiomerism is a special case of stereoisomerism. Non-chiral stereoisomers are called diaster-eomers (e.g., quinidine/quinine). 

A chiral stereoisomer is not superimposable on its mirror image. It does not possess a plane or center of symmetry. The nonsuperimposable mirror images are called enantiomers. A mixture of equal numbers of molecules of each enantiomer is a racemic form (racemate). The conversion of an enantiomer into a racemic form is called racemization. Resolution is the separation of a racemic form into individual enantiomers. Stereomers which are not mirror images are called diastereomers. 

PHARMACEUTICALS, CHIRAL. Stereoisomers are compounds which have the same molecular formula bnl differ in the arrangement of their atoms in space. Chiral compounds are compounds which... 

In contrast to the multiplicity of examples of diastereotopic face discriminations in combination with other enzyme specificity discussed later, there are relatively few alcohol dehydrogenase catalyzed reductions that involve diastereotopic face selectivity alone. However, whenever a carbonyl group is present in a single chiral stereoisomer, diastereotopically face-selective reductions are possible. The reductions of the L-homocysteine derivative (47) (48), and of the (-)-oxocineole (15,4/ )-(49) (50), are two... 

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. 

Optical activity is considered essential for protein chain folding and life would probably not be possible without dissymmetric molecules. Molecules lacking reflective symmetry (no plane of symmetry) are designated as dissymmetric. It should be noted that asymmetric molecules (possessing only a Cl axis element of symmetry) are a special class of dissymmetric molecules, but not all dissymmetric molecules will be asymmetric. Stereoisomers possess the same molecular bonding skeleton but differ in the absolute arrangement of the atoms in space. They are optically active and may be characterized as chiral. Stereoisomers that are related as object and nonsuperimposable mirror image are termed enantiomers. Those stereoisomers not so related are called diastereoisomers. 

Let us remember that according to the lUPAC report [138], a stereoselective polymerization is a process in which a polymer is formed from a RS mixture of chiral stereoisomer by preferential incorporation of one species into a growing polymer chain and a statistical copolymer of R and S antipodes is not obtained. The occurence of stereoselectivity can be experimentally proved when it is possible to separate the polymer into fractions having optical activity of opposite sign as it has been made e.g. for chiral poly-a-olefins by Pino s [139] group. Informations on stereoselectivity can be gained from structural and physical analysis, mainly by high field NMR. 

Solution 27 Structure J can be isolated as a chiral stereoisomer becanse of the large steric barrier to rotation about the bond connecting the rings. Biphenyl K has a plane of symmetry and is therefore achiral. The symmetry plane of K is shown here. Any chiral conformation of L can easily be converted to its enantiomer by rotation. It is only when a b and f e and rotation is restricted by bulky groups that chiral (optically active) stereoisomers can be isolated. 

Wu, Y.-T., Hayama, T., Baldrige, K.K. Linden, A. and Siegel, J.S. (2006) Synthesis of fluoranthenes and indenocorannulenes elucidation of chiral stereoisomers on the basis of static molecular hovi s. Journal of the American Chemical Society, 128(21), 6870-6884. 

Chiral stereoisomers are compounds with the same structure, which differ by spatial arrangements of atoms or substituents. Their molecules are mirror images (enantiomers) or do not have that property (diastereoisomers and epimers) [5]. Pairs of enantiomers and diastereoisomers of the compound with two-carbon chain with four different substituents R1-R4 and their relations are presented in Figure 10.2. 

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