Extended pseudoasymmetry and geometric prochirality clarifying the scope of the concepts of holantimers and stereoisograms

Pseudoasymmetry is discussed in an extended fashion by means of the stereoisogram approach (Fujita, S. J. Org. Chem.2004, 69, 3158–3165; (Fujita, S. Tetrahedron2004, 60, 11629–11638), where an adamantane skeleton and an 8-azabicyclo[3.2.1]octane skeleton are used as polycyclic skeletons for constructing stereoisograms of five types (Types I–V). The term pseudoasymmetry is characterized as the concurrent participation of achirality and RS-stereogenicity in terms of a stereoisogram of Type V, while the term asymmetry is characterized as the concurrent participation of chirality and RS-stereogenicity in terms of a stereoisogram of Type I. Prochirality as a purely geometric concept is also discussed from the viewpoint of the stereoisogram approach. The stereoisogram approach has brought about a paradigm shift from enantiomers to RS-stereoisomers as equivalence classes, so that the categorization of compounds by the stereoisogram approach is shown to be conceptually different from the conventional categorization described in the Nobel Prize lecture of Prelog (1975). Thereby, the stereoisogram approach is concluded to serve as an integrated methodology for organic and inorganic stereochemistry.

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