Pro-R/Pro-S-Descriptors Specified by RS-Diastereotopic Relationships, Not by Stereoheterotopic Relationships

Abstract

The pro-R/pro-S system has been redefined by using the concept of pro-RS-stereogenicity, where two ligands in an RS-diastereotopic relationship are pairwise characterized by pro-R-and proS -descriptors on the basis of stereoisograms. As for a practical criterion for determining RS-diastereotopic relationships, a symmetry criterion has been developed as a new matter by extending the concept of stereoisograms to testify RS-diastereotopic relationships. The conventional definitions of pro-R/pro-S-descriptors which are based on the original term "prochirality" (along with enantiotopic and diastereotopic relationships) and on the revised term "prostereogenicity" (along with stereoheterotopic relationships) are altogether abandoned. The revised concept of prochirality has a purely geometric meaning, which stems from the term enantiotopic relationship or equivalently enantiospheric equivalence class (S. Fujita, Symmetry and Combinatorial Enumeration in Chemistry, Springer-Verlag, Berlin-Heidelberg (1991)). The two concepts for describing intramolecular environments, i.e., the pro-RS-stereogenicity and the prochirality, are clarified to correspond to the concepts of RS-stereogenicity and chirality, which have been formulated on the basis of stereoisograms in order to bring about a harmonized viewpoint to stereoisomeric and geometric intermolecular features of stereochemistry (S. Fujita,

Systematic Enumeration and Symmetries of Cubane Derivatives

“…The labels "C" and "A" assigned originally to a pair of RS-diastereomers are diverted to specify a pair of enantiomers by considering the concept of chirality faithfulness. [100,128] An RS-diastereomeric relationship stems from RS-stereogenicity, which is inherent in type-I, type-III, and type-V stereoisograms. See the double-headed arrows in the horizontal directions of Figure 32 The priority sequence is presumed to be A 1 > B 2 > H 3 , where each encircled number indicates the priority number.…”

“…Hence, Figure 38 shows a chirality-faithful case. [100,128] C/A Descriptors Assigned to Cubane Derivatives of Type III Let us examine a quadruplet of RS-stereoisomers shown in the type-III stereoisogram of Figure 35. A pair of C/A descriptors is assigned to a pair of RS-diastereomeric cubane derivatives 111 and 112, as shown in the upper part of Figure 39, linked with a brace.…”

“…Strictly speaking, however, the configuration number 14342444 of 111 is different from 14243444 of 111. Thus, Figure 39 shows a chirality-faithful case, [100,128] in which pairwise enantiomers, however, have different configuration numbers. The assignment of C/A descriptors to a pair of RS-diastereomeric cubane derivatives 19 and 20 is shown in Figure 40, where they are contained in the type-V stereoisogram of Figure 37.…”

Type-itemized enumeration of quadruplets of RS-stereoisomers: II—cycle indices with chirality fittingness modulated by type-V quadruplets

“…In Part I, the method of type-itemized enumeration reported in [22] has been substantially simplified, where cycle indices with chirality fittingness (CI-CFs) have been modulated by type-IV quadruplets after the extension of Fujita's proligand method to meet Fujita's stereoisogram approch [30]. As a continuation of Part I, the present paper as Part II deals with another simplified method using CI-CFs modulated by type-V quadruplets.…”

“…Type-itemized enumerations of quadruplets have been conducted according to three kinds of relationships contained in stereoisograms [21,22]. More recently, symmetry-itemized enumerations of quadruplets have been conducted by extending Fujita's USCI approach [10], where tetrahedral derivatives [23,24], allene derivatives [25,26], and three-membered heterocycles [27][28][29] are combinatorially enumerated after the preparation of necessary data (e.g., mark tables, inverse mark tables, subductions of coset representations).…”