Classification of stereoisomers. Flowcharts without and with the intermediate concept of RS-stereoisomers for mediating between enantiomers and stereoisomers

Abstract

The conventional flowchart for the classification of isomers into constitutional isomers and stereoisomers and for the further classification of stereoisomers into enantiomers and diastereomers is concluded to be misleading from a viewpoint of equivalence relationships and equivalence classes. A new flowchart based on equivalence relationships and equivalence classes is proposed, where a set of isomers a set of stereoisomers, and a pair of enantiomers are recognized as equivalence classes under the respective equivalence relationships, that is, an isomeric relationship, a stereoisomeric relationship, and an enantiomeric relationship. A constitutionally-anisomeric relationship (a revisal for a constitutionally-isomeric relationship) and a diastereomeric relationship are redefined as non-equivalence relationships. After the introduction of stereoisograms, a more comprehensive flowchart is proposed, where an RS-stereoisomeric relationship is incorporated as an intermediate concept for mediating between a stereoisomeric relationship and an enantiomeric relationship. The comprehensive flowchart detects RS-stereoisomers, which are classified into five types in accord with type-I to type-V stereoisograms. The comprehensive flowchart is applied to tetrahedral derivatives, allene derivatives, ethylene derivatives, square planar complexes, and oxirane derivatives. The results are summarized in the form of isomer-classification diagrams, which are correlated to correlation diagrams of stereoisograms. The different ways taken by van’t Hoff (‘asymmetry’, ‘stereogenicity’) and Le Bel (‘dissymmetry’, ‘chirality’) at the beginning of stereochemistry are now integrated by the comprehensive flowchart.