Spiranes, axial chirality

Axial Chirality. For a system with four groups arranged out of the plane in pairs about an axis, the system is asymmetric when the groups on each side of the axis are different. Such a system is referred to as an axial chiral system. This structure can be considered a variant of central chirality. Some axial chiral molecules are allenes, alkylidene cyclohexanes, spiranes, and biaryls (along with their respective isomorphs). For example, compound 7a (binaphthol), which belongs to the class of biaryl-type axial chiral compounds, is extensively used in asymmetric synthesis. Examples of axial chiral compounds are given in Figure 1-5. 

Among the several molecular assemblies able to display axial chirality, well known examples include allenes (XXXI), spiranes (XXXII), and biphenyls (XXXIII). 

Axial chirality (e.g., spiranes with cyclic skeleton)... 

Axial chirality the atoms are placed in two perpendicular planes that are nonsymmetrical and cannot rotate freely (allenes, cumulenes, spiranes, and substituted ortho-biphenyls). 

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. 

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis), irons-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and 5 descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to references 8 (see p. 43) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

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