Helical stereogenic units

The author is well aware of the fact that many chemists still advocate use of the traditional RIS symbols also for at least some, if not all, compound types with helical stereogenic units, even if these are not in concordance with the proper helicity descriptors as in the case of axially chiral compounds where A M and A P. For the sake of consistency,... 

Finally, it could be asked why it is so important to properly identify stereogenic centers and prefer them to helical units The main practical reason is that most molecules, when realistically described, e.g., by a crystal structure, contain numerous helical units and it is exceedingly difficult, particularly with a computer program, to sort out those that are invariant to physical conditions (see Section 1.1.1.). In general, it is advisable to base specification of stereogenic units on constitutional or configurational distinctions of ligands, as these are normally less affected by external conditions than conformational properties. 

Stereogenic units of quite different types are responsible for the existence of enantiomers in the case of axially and planary chiral molecules whose helicities - P(lus) or M(inus) - can most easily be derived with the help of appropriate four-point figures inscribed in the respective structural formulae. 

Molecules with a single helical unit, such as in hydrogen peroxide, are rare. More commonly found are bundles of two to nine units with coincident central bonds (see Table 1). The combination of two, e.g., in cyclophanes, and four, e.g., in biaryls and allenes, helical units are particularly important. Such compounds have a stereogenic axis, as stereoisomers may be... 

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