Complete settlement of long-standing confusion on the term ‘prochirality’ in stereochemistry. Proposal of pro-RS-stereogenicity and integrated treatment with prochirality

Long-standing confusion on the term ‘prochirality’ in stereochemistry has been analyzed by a critical review on the definitions described in the IUPAC recommendations 1996 (Pure Appl. Chem., 1996, 68, 2193). Thereby, the confusion has been clarified to come from the fact that the scope and limitations of the terms ‘enantiotopic’, ‘diastereotopic’, and ‘stereoheterotopic’ are not so fully specified to discriminate between prochirality and prostereogenicity. Entangled situations due to the confusion have been avoided by abstracting the concept of pro-RS-stereogenicity from the conventional ‘prostereogenicity’ on the same line as the concept of RS-stereogenicity was separated from the conventional ‘stereogenicity’ (S. Fujita, J. Org. Chem., 2004, 69, 3158). This abstraction, which has been based on stereoisograms and RS-stereoisomeric groups, has provided us with a systematic approach for investigating the relationship between the prochirality and the pro-RS-stereogenicity. Thus, the prochirality and the pro-RS-stereogenicity can be discussed in terms of a common theoretical framework, that is, coset representations of RS-stereoisomeric groups, where the conventional terms on topicities are replaced by unambiguous terms on sphericities (homospheric, enantiospheric, and hemispheric) and RS-tropicities (RS-homotropic, RS-enantiotropic, and RS-hemitropic). Moreover, the pro-RS-stereogenicity defined in the present paper has been correlated to the capability of designating pro-R/pro-S descriptors without any ambiguity. Thereby, the long-standing confusion on the term ‘prochirality’ has been settled completely.