RS-Stereodescriptors Determined by RS-Stereogenicity and Their Chirality-Faithfulness

Abstract

The foundation of the Cahn-Ingold-Prelog (CIP) system has been redefined by employing the concept of RS-stereogenicity derived from stereoisograms for analyzing stereoisomerism, where R-and S-stereodescriptors are pairwise given to two molecules in an RS-diastereomeric relationship. The conventional definitions of RS-stereodescriptors which are based on the original term "chirality" (along with enantiomeric and diastereomeric relationships) and on the revised term "stereogenicity'" are altogether abandoned. Thereby, the present foundation is shown to be distinct from and to be more consistent than the conventional one which assigns R-and S-stereodescriptors to two molecules in an enantiomeric relationship as well as in a diastereomeric relationship. In order to harmonize the present foundation with the conventional one and to leave conventional results unchanged as far as possible, RS-stereodescriptors given originally to a pair of RS-diastereomers are considered to be translated into RS-stereodescriptors for a pair of enantiomers, where the concept of chirality-faithfulness of priority sequences has been proposed to testify whether the translation process of RS-stereodescriptors is faithful or not. As a result, the specification of configurations is rationalized on a more succinct basis than the conventional foundation which emphasizes the dichotomy between enantiomers and diastereomers. A paradigm shift from enantiomers to RS-stereoisomers has been pointed out so that over-simplified features of the conventional dichotomy between enantiomers and diastereomers have been discussed by means of stereoisograms.

Stereoisograms of trigonal bipyramidal compounds: I. Chirality and RS-stereogenicity free from the conventional “chirality” and “stereogenicity”

“…Although the pairing of A and C is satisfied in each pair (i.e., chirality-faithful [12]), the configuration index for one of the pair is different from the other of the pair, i.e., TBPY-5-15 vs. TBPY-5-16. If we strictly take account of chirality faithfulness [12], we should tentatively change the configuration index TBPY-5-16 into TBPY-5-15 for 11 and 12, where the CIP priority a > b > c > d > p (5) is tentatively adopted in place of the CIP priority a > b > c > d > p (5) > p (6).…”

“…Because the pair of RSdiastereomers coincides with the pair of enantiomers 6 and 6, the C/A-descriptors are interpreted to be reassigned to the latter pair of enantiomers 6 and 6 in terms of chirality faithfulness [12]. The above Type I case is chirality-faithful because the CIP priority a > b > c remains unchanged on the action of reflections.…”

“…When the CIP priority is presumed to be a > b > c > p > p, the trigonal bipyramidal compounds with [Mabcpp] (Type V, Fig. 6) are differentiated by the following C/A-descriptors: Because 8 and 9 are achiral, they are not chirality-faithful [12]. Hence, the uppercase letters C and A are changed into lowercase letters c and a.…”

“…If we strictly take account of chirality faithfulness [12], we should tentatively change the configuration index TBPY-5-16 into TBPY-5-15 for 11 and 12, where the CIP priority a > b > c > d > p (5) is tentatively adopted in place of the CIP priority a > b > c > d > p (5) > p (6).…”

“…Then, the R/Sdescriptors based on the RS-stereogenicity are linked to the chirality through chirality faithfulness [12].…”