Chirotopic atoms

The ease of glycerol, with chirotopic diastereotopic hydrogens in the absence of a stereo-genic unit, requires special treatment. The first descriptor is derived as above, but the second is that of the constitutionally nearest chirotopic atom. 

Chirotopic The property of any atom, and, by extension, any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in a chiral environment [83]. Achirotopic is the property of any atom or point that does not reside in a chiral environment (see also [84]). Chirotopic atoms located in chiral molecules are enantiotopic by external comparison between enantiomers. Chirotopic atoms located in achiral molecules are enantiotopic by internal and therefore also by external comparison. All enantiotopic atoms are chirotopic [83]. 

Finally, in Chapter 18 we present an alternative, universal stereochemical classification of chemical transformations based on (a) overall loss, (b) no loss/gain, and (c) overall gain of chirotopic atoms we label these chirotopoprocesses as chirotopolysis, chirotopomutation and chirotopogenesis, respectively. Further subclassification is carried out using the dual criteria of rotativity (expected optical activity) and stereoselectivity (preferential formation of one stereoisomer over another). We also introduce and define the novel concepts of chiroselectivity and chirospecificity. Finally, the merits of the classification of chirotopoprocesses are discussed, and the stereotopoprocesses and chirotopoprocesses are correlated in relation to the stereotopic molecular faces. 

Table 15.1. Classification of Astereogenic/Stereogenic, Achirotopic/Chirotopic Atoms...

The eight examples in Figure 15.1 illustrate the four types of astereogenic/stereogenic, achirotopic/chirotopic atoms - o, o, s, and s. It turns out that (a) all four types of atoms - o, o, s and s - are present in achiral molecules, and (b) only o and s are found in chiral molecules. 

Every atom in a molecule is either chirotopic (types o and s ) or achirotopic (types o and s). The degree of chirotopicity of a given molecule, Cm, is equal to the sum of the degrees of chirotopicity of all chirotopic atoms of types o and s ... 

In Chapter 18, we develop an alternative classification on the basis of a net change of the number of chirotopic atoms Ac. The latter classification is based on the overall loss, no... 

Chirotopicity is a local geometry that produces chirality. Not only are the atoms in a chiral environment said to be chirotopic, but the spaces around atoms in a chiral environment are also considered chirotopic. Atoms (or spaces) in an achiral environment are said to be achirotopic. 

The terms enantiotopic and diastereotopic describe the relationship between a pair of atoms or groups in a molecule. Sometimes it is also useful to describe the local environment of a single atom, group, or location in a molecule (even if it does not coincide with an atomic center) as chiral or not. A chirotopic atom or point in a molecule is one that resides in a chiral environment, whereas an achirotopic atom or point does not. All atoms and all points associated with a chiral molecule are chirotopic. In achiral molecules, achirotopic points are those that remain unchanged (are invariant) upon execution of an S that is a. symmetry operation of the molecule. For most situations, this means that the point either lies on a mirror plane or is coincident with the center of inversion of the molecule. Importantly, there will generally be chirotopic points even in achiral molecules. 

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