Chiral: axis centers

The enantiomers shown are related as a right hand and left hand screw respectively Chiral allenes are examples of a small group of molecules that are chiral but don t have a chirality center What they do have is a chirality axis, which m the case of 2 3 pen tadiene is a line passing through the three carbons of the allene unit (carbons 2 3 and 4)... 

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

The use of diazodicarboxylates has been recently explored in Cinchona alkaloid catalyzed asymmetric reactions. Jprgensen [50] reported the synthesis of non-biaryl atropisomers via dihydroquinine (DHQ) catalyzed asymmetric Friedel-Crafts ami-nation. Atropisomers are compounds where the chirality is attributed to restricted rotation along a chiral axis rather than stereogenic centers. They are useful key moieties in chiral ligands but syntheses of these substrates are tedious. 

In addition to this nonequivalence, the imprecise definition of the chirality axis and difficulties in dealing with stereogenic centers in polycyclic structures led to confusion in the case of certain adamantanes, cyclohexanes and spiranes (see Section 1.1.5.3.3.). These problems were solved in the late 1970s and it was recommended1 that the PjM rather than the obsolete aR/aS description be used. This use is recommended as it leads to a considerable simplification of static Stereochemistry. 

Table 7. Slcreomodels with Stereogenic Centers That Were Described by a Chirality Axis"...

Spiranes have caused many problems. The stereomodel 3, of general type 3a, is a good example of where the obvious necessity of specifying stereogenic centers was circumvented by the chirality axis despite too low a symmetry of the skeleton (C2v). 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

The stereochemical course of a-alkylation of both L-isoleucine and D-allo-isoleucine derivatives 61 and 62 is controlled predominantly by the chiral axis in the enolate intermediate, whereas the adjacent chiral center C(3) has little effect. 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

The descriptor a stands for axis or axial. Molecules that exhibit atropisomerism are chiral because of the presence of a chiral axis rather than a chirality center. For a discussion of chiral axes and atropisomerism, see E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, Wiley-Interscience New York, 1994, Chap. 14. 

Chirality element, element of chirality A stereogenic axis, center, or plane that is reflection variant. See also stereogenic element. 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

I.4.C. Chiral Molecules without a Stereogenic Center (Molecules Containing a Chiral Axis)... 

A few classes of organic molecules have a chiral axis although they do not have a stereogenic center.34 The mirror image of such a molecule is not superimposable so it is possible to have enantiomers without the... 

A series of 5-aiyluracils 308 were optimized as a potent GnRH antagonist. They presented a fixed chirality center and a chirality axis. The occurrence of two stable diastereomers depends on the barrier to rotation of the exchanging atropisomers. 

Molecules without chirality centers can be chiral. Biphenyls that are substituted can exhibit an axis of chirality. When A B, and X Y, the two conformations are nonsuperimposable mirror images of each other that is, they are enantiomers. The bond connecting the two rings lies along a chirality axis. 

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