R-S notation

The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chiral ity axis Such compounds are so infrequently encountered however we will not cover the rules for specifying their stereochemistry in this text... 

The Z)-statistic concept can be readily extended to the two-factor case, where (A, B,) are corresponding ranks, i.e. if A and B are the ordinal form of observation sets (the A7B notation is used instead of Lehman s (R.S) notation in order to avoid confusion between similar symbols). If rank positions are tied, they are replaced by their mid-rank, yielding modified distributions A and B. By a straightforward extension of Eq.(6), the modified Z)-statistic is computed as [17]... 

Figure 2.10 Hydrolysis of a racemic secondary ester or transesterification of the corresponding secondary alcohol with CALB as catalyst both yield the same enantiomer as product. The product of hydrolysis is the (R)-alcohol while the product of transesterification is the (R)-ester. The R,S-notation in this case is done on the assumption that Rj has higher priority than R2. This is not necessarily in the same order as large , small in model considerations.

Moscowitz, A., Mislow, K. J. Am. Chem. Soc. 84, 4605 (1962). Although they follwed the axial instead of planar chirality convention in their R,S-specification for chiral (rans-cyclooctene, both procedures give the same R,S-notation in this case. 

The enantiomers shown are related as a right-hand and left-hand screw, respectively. Chiral allenes are examples of a small group of molecules that are chiral, but don t have a chirality center. What they do have is a chirality axis, which in the case of 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chirality axis. Such compounds are so infrequently encountered, however, we will not cover the rules for specifying their stereochemistry in this text. 

For example, (7) is trans-2-methylcyclohexanol in addition the chirality of C-l and C-2 should be designated by the R/S) notation, and the correct systematic name is therefore (lR,2R)-2-methylcyclohexanol (7a), since the mirror image molecule, (lS,2S)-2-methylcyclohexanol (7b) is also the trans isomer. The cis isomer is also chiral, and diastereoisomeric with the trans isomer the two enantiomers would be (lS,2R)-2-methylcyclohexanol (24a) and ( R,2S)-2-methylcyclohexanol (24b). 

R/S notation the four different atoms or groups attached to a central atom are ranked a,b,c,d, by molar mass. The lowest (d) is directed away from the viewer and the sequence of a-b-c produces clockwise (R) or counter-clockwise (S) configuration. 

Figure 7.2 The structure of the faster reacting enantiomer in lipase-catalyzed esterification in kinetic resolution of racemic secondary alcohols or hydrolysis of the corresponding esters. Small and large refer to the relative size of the groups and not to the R/S notation.

The R-/S- notation is valid only for the the absolute configuration of center having single bonds only. In the case of a double bond, the traditional cis/trans nomenclature system is not sufficiently accurate and the E-IZ- is currently preferred. 

An example of a twofold simple axis of symmetry (C2 axis) can be seen in planar (S,S)-1, 2-dimethylcyclopentane (1) (Figure 2), which can be brought to an indistinguishable position by a 180° rotation around this axis. As the R-S notation attached to the two asymmetric carbon atoms indicates, these centers have the same chirality in general a C axis may have n identical asymmetric (/IS) units disposed 2n/n apart around this axis. 

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