Chirality stereochemical descriptors

Fig. 4-2. Planar-chiral and central-chiral stereochemical descriptors for 2-methyl-ferrocenecar-boxylic acid.

As mentioned in the introduction, the first empirical correlation between the absolute configuration of dopants and the handedness of induced cholesterics was proposed in 1975.20 The first attempt to find a general correlation was a few years later Krabbe et al.58 related the sense of the cholesteric to a stereochemical descriptor of the dopant based on the effective volume of the substituents and listed many compounds following this rule. However, exceptions were described at that time,59 and, furthermore, this approach neglects the role of the structure of the nematic solvent in determining the sense of the cholesteric. It is well known that chiral compounds may induce cholesterics of opposite handedness in different nematics.60,61... 

Figure 5 Stereochemical descriptors for (a) chiral tetrahedral compounds, and (b) square-planar complexes...

The chiral identity of a molecule is included in the nomenclature of inorganic compounds, and today s comprehensive system is based upon suggestions made in 1990 in IUPAC s Recommendations on Nomenclature of Inorganic Chemistry [84], and ACS s Inorganic Chemical Nomenclature [90]. The basis for the usage of stereochemical descriptors was laid by Brown [91,92], from which three types of chiral descriptor conventions were developed (i) Steering-wheel-convention [93], (ii) Skew-lines convention [94] and (iii) Oriented-skew-lines convention [95]. 

Fig. 12. The four diastereoisomeric 1 1 adducts which can be formed on reaction of the bisdiene 37 with the syn-bisdienophile 38. The stereoelectronic control in operation during the reaction is such that only the syn/endo-H isomer 40 has been detected [21, 38, 110, 112]. The remote stereochemical descriptors, syn and anti, refer to the relative configurations of the endoxide bridges across each newly-formed cyclohexene ring. The close stereochemical descriptors, exo-H and endo-H, refer to the relative configuration adopted by the hydrogen atoms at the ring junctions associated with the newly created chiral centers...

This procedure is elegant and useful, but conventional in that, like most enantioselective syntheses, it involves an achiral starting material desymmetrised by a chiral reagent. However, as shown in Scheme 3, the methodology was extended in a remarkable and unusual direction through the application of similar chemistry to racemic starting materials [7]. To allow a generalised discussion of this and subsequent procedures, it is helpful to introduce special stereochemical descriptors for use with 1,3-diols and their de-... 

R - and S - are relative stereochemical descriptors. Thus, (R, R ) indicates two centres of like chirality (either both R- or both S-) and (R, S ) indicates two centres of unlike chirality. (RS) and (SR) are used to denote racemates (see RS-). 

CAS now registers and names substances with partially defined stereochemistry. Previously, partial stereochemistry was generally ignored. The presence of unknown chiral centers is indicated by the addition of the term [partial]- to the end of the normal stereochemical descriptor. When the reference ring or chain has incompletely defined chiral atoms/bonds, the format cites the stereochemistry using R and S terms with their nomenclature locants for all known centers. If this method is used to describe a substance for which only relative stereochemistry is known, rel is added to the stereochemical descriptor. Any stereochemical descriptor marked as rel always cites the first centre as R-. 

M-), (P-) Stereochemical descriptors (M = minus, P = plus) introduced to describe the chirality of helical molecules. Extension of the CIP system to planar chirality gave an alternative description aR/aS for helical molecules such as helicenes, aR invariably = (M) and aS = (P), but for compounds showing planar chirality the reverse, with pR = (P) and pS = (M). Best avoided. See Section 7.3.2. 

Absolute configuration The arrangement in space of the ligands of a stereogenic unit, which may be specified by a stereochemical descriptor such as R or S, D or L, P or M. See also chirality sense, chirality element, stereogenic element... 

The simplified version of the structure graph presented in this paper as Fig. (8) indicates only those structures, which are on the shortest path from the educts to the products. (In the graph the stereochemical descriptors R, S, Z and E show the configuration of the eventual centers of chirality at C-l, C-3, C-4, C-8 and C-9, and/or the conformation of the eventual C=C bond at C-3, C-8 and C-9, respectively. Indication of the configuration of a center of chirality precedes that of the conformation of... 

A. If the side chain of the amino acid is a methyl group, the compound is referred to as alanine. What is the Cahn-Ingold-Prelog stereochemical descriptor for the chiral center in alanine ... 

The basic problem of this suggestion was that the chirality descriptors obtained by the central procedure are the opposite of those derived from the planar approach. Schlogl consequently changed his stereochemical nomenclature [5,9] and. 

Attempts to produce descriptors similar to cis and iruns for stereochemically more complicated coordination entities have failed to achieve generality, and labels such as fac and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chiraiity symbols. 

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