Group-theoretical discussion on the E/Z-nomenclature for ethylene derivatives. Discrimination between RS-stereoisomeric groups and stereoisomeric groups

The hierarchy of point groups, RS-stereoisomeric groups, stereoisomeric groups, and isoskeletal groups is discussed to comprehend the chirality, RS-stereogenicity, stereogenicity, and isoskeletal isomerism for ethylene derivatives. The RS-stereoisomeric groups for ethylene derivatives have been clarified not to coincide with their stereoisomeric groups, so that diastereomers (E/Z-isomers) are not identical with RS-diastereomers. To discuss the relationship among RS-diastereomers, m-diastereomers, and isoskeletal isomers, we have proposed the concepts of extended stereoisograms and extended stereoisogram sets, where the term "m-diastereomers" is coined to show its difference from the traditional term "diastereomer". Thereby, ethylene derivatives are classified into Types II-II/II-II/II-II, IV-IV/IV-IV/IV-IV, etc. on the basis of relevant stereoisograms (Types I to V). The stereoisomerism of ethylenes has been concluded to be treated in terms of m-diastereomers characterized by the E/Z-nomenclature but not to be treated in terms of RS-diastereomers characterized by the RS-nomenclaure.