Spiranes, optically active

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. 

The possibility of optical activity in ring-compounds was recognised by van t Hoff. Baeyer introduced the name spiran (from Latin spira, twisted) for compounds in which two rings at right-angles are joined by a carbon atom (I). Optical activity may arise, as in the compound II ... 

The property of chirality is determined by molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. Examples include certain allenes, spiranes, alkylidenecyclo-alkanes, and biaryls as well as other specific examples. Some specific molecules that have been isolated in optically active form are given in Scheme 2.2. The configuration of these molecules is established by subrules in the Cahn-Ingold-Prelog convention. We will not describe these here. Discussion of these rules can be found in Ref. 1. 

Another interesting group of compounds where chirality often occurs is spir-anes (Figure 2.17). It is sufficient to substitute any two hydrogen atoms in both spirane rings with any substituents (they can be identical) to make this molecule optically active. 

FIGURE 2.17 Examples of spiranes and an example of optically active disubstituted spirane. 

Compounds containing nitrogen atom that are capable of forming four bonds and having different substituents may be optically active. In such compounds, there is a certain analogy to a chiral tetrahedral carbon atom. This type of compounds includes ammonium salts, amine oxides, and spiranes where nitrogen atom is a central one (Figure 2.26). 

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