Stereoisograms for three-membered heterocycles: III. R/S-stereodescriptors for characterizing the RS-stereogenic aspect of absolute configuration

摘要

The stereoisogram approach presumes the presence of three pairs of attributes, i.e., chirality/achirality, RS-stereogenicity/RS-astereogenicity, and sclerality/asclerality, in contrast to the modern stereochemistry which presumes the presence of a single pair of chirality/achirality. From this viewpoint of the stereoisogram approach, the scope and limitations of R/S-stereodescriptors of the Cahn-Ingold-Prelog (CIP) system are discussed, where oxirane derivatives are used as probes. In particular, correlation diagrams of stereoisograms are used to comprehend the global and local symmetries of oxirane derivatives. Thereby, R/S-stereodescriptors of the CIP system are found to specify local symmetries at each positions of an oxirane derivative, so that the global symmetry of the oxirane derivative requires the other types of descriptors proposed in Part II of this series (-descriptors and Z/E-descriptors). A correlation diagram of each position clarifies that R/S-stereodescriptors characterize the RS-stereogenic aspect of absolute configuration in the specification of local symmetries, but by no means the chiral aspect of absolute configuration. In other words, a pair of R/S-stereodescriptors is assigned to a pair of RS-diastereomers, not to a pair of enantiomers. This fact stems from the misleading foundation of the modern stereochemistry that it presumes a pair of chirality/achirality as a single pair of attributes. The pair of R/S-stereodescriptors originally assigned to a pair of RS-diastereomers should be interpreted subsidiarily to characterize the corresponding pair of enantiomers, where the concept of chirality faithfulness is necessary. Chirality-unfaithful cases are discussed by means of stereoisograms. Exceptional cases in the practices of the modern stereochemistry, e.g., pseudoasymmetry and 'geometric enantiomers', are discussed in a rational fashion according to the stereoisogram approach.