Isomerism chiral centers

Assign configurations, using the sequence rule, to each chiral center of the stereo-isomeric isocitric acids and alloisocitric acids ... 

Radical addition to conjugated systems is an important part of chain propagation reactions. The rate constants for addition of cyclohexyl radical to conjugated amides have been measured, and shown to be faster than addition to styrene. In additions to RCH=C(CN)2 systems, where the R group has a chiral center, the Felkin-Ahn rule (p. 148) is followed and the reaction proceeds with high selectivity. Addition of some radicals, such as (McsSijaSi-, is reversible and this can lead to poor selectivity or isomerization. ... 

The mutually dependent elements of 25 are a pherochiral double bond and a chiral center of steroisomerism. Instead, the isomerism could be viewed as the result of a conceptual torsion of the partially occupied line between C(2) and... 

Isomers are molecules with the same composition (i. e. the same molecular formula), but with different chemical and physical properties. If isomers differ in the way in which their atoms are bonded in the molecule, they are described as structural isomers (cf citric acid and isocitric acid, D). Other forms of isomerism are based on different arrangements of the substituents of bonds (A, B) or on the presence of chiral centers in the molecule (C). 

Another type of isomerism arises when a molecule contains a chiral center or is chiral as a whole. Chirality (from the Greek cheir, hand) leads to the appearance of structures that behave like image and mirror-image and that cannot be superimposed ( mirror isomers). The most frequent cause of chiral behavior is the presence of an asymmetric C atom—i.e., an atom with four different substituents. Then there are two forms (enantiomers) with different configurations. Usually, the two enantiomers of a molecule are designated as L and D forms. Clear classification of the configuration is made possible by the R/S system (see chemistry textbooks). 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

Hydroformylation is an addition reaction of a hydrogen atom and a formyl group to an olefin to form two isomeric aldehyde products (Equation (1)). Both the aldehyde products are important chemicals //or///< /-aldehydes are industrially important because they are widely used for detergents and plasticizers the /i-o-aldehydes can be important intermediates for production of fine chemicals and drugs once the chiral center at the a-carbon to aldehyde is controlled. Progress in hydroformylation exactly traces that in the phosphine ligand chemistry and valuable aldehyde products have become available on an industrial scale. 

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms (pp. 17-19). The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers (Fig. 7-2). 

It was proved that in most cases all propagation and chain-transfer (chain-termination) steps take place without isomerization of the chiral center.326... 

The hetero-Diels-Alder reaction of activated butadienes with carbonyl compounds is a convenient method for the preparation of precursors of sugars. Up to three chiral centers are created simultaneously. The high-pressure [4 + 2]cycloaddition of l-methoxybuta-1,3-diene 32 to N-mono- and N,N-diprotected alaninals was investigated [42-45]. The Eu(fod)3-mediated reaction of 32 with alaninal 25 gave a mixture of four diastereoisomers, which was then subjected to acidic isomerization, leading to the thermodynamically more stable pair of adducts syn-33 and anti-34, with predominance of the latter isomer (Scheme 12). The N-monoprotected alaninals reacted with a moderate ryn-diastereoselectivity. This method was used in the synthesis of purpurosamines (see Sec. DI.C). 

In older literature optical isomerism of the type represented by d and l pairs was usually discussed in terms of "asymmetric carbon atoms" or "asymmetric centers." Now the terms chiral (pronounced ki-ral) molecules, chiral centers, and chirality (Greek "handedness") are preferred. 

When a molecule is chiral, then it will have two isomeric forms called enantiomers, each of which is the nonsuperimposable mirror image of the other. Enantiomers are distinct stereoisomers because they are compounds that have die same molecular formula and sequence of bonded elements but which differ in tile spatial arrangement of groups in the molecule. If a molecule is chiral, and thus has two enantiomers, it usually (but not always) contains at least one chiral center. In organic compounds a chiral center usually corresponds to an asymmetric tetrahedral carbon atom. 

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