R,S sequence rules

Apparent opposite chiral recognitions with 19 and 20 are attributed only to the (R,S) sequence rule by Cahn et al. (66AG(E)385). 

We may distinguish between enantiotopic or diastereotopic faces in trigonal moieties (> C=0 and > C=C <). The faces of prochiral moieties can be differentiated as the Re or Si face by applying R) or S) sequence rules to the three substituents in the plane as viewed from each face. " ... 

Absolute Stereochemistry Absolute stereochemical assignment of each stereocenter (R vs S) Cahn-Ingold-Prelog Convention (sequence rules)... 

Chemical Properties. Because of its chiral center, malic acid is optically active. In 1896, when tartaric acid was first reduced to malic acid, the levorotatory enantiomer, S(—), was confirmed as having the spatial configuration (1) (5,6). The other enantiomer (2) has the R configuration. A detailed discussion of configuration assignment by the sequence rule or the R and S system is available (7). 

R,S convention (Section 9.5) A method for defining the absolute configuration at chirality centers using the Cahn-IngoId-Prelog sequence rules. 

The compound is named as a disubstituted deoxy sugar. Configuration is determined by regarding the substituent having priority according to the Sequence Rule ([13], Section E), as equivalent to OH. Any potential ambiguity should be dealt with by the alternative use of the R,S system to specify the modified stereocentre. 

R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland) devised the (R-S) system (Sequence rule) for designating the configuration of chiral carbon atoms. 

The R, S, convention, proposed by R.S. Cahn, C ingold and V Prelog in 1956 is also based on a three dimensional model rather than on the projection formula and has been adopted by the IUPAC. It has also been called the Sequence rule is quite straightforward and involves the following simple steps. 

For compounds 7.42, the preferred substrate always had the 3D geometry shown here, which corresponded to an (R)- or -absolute configuration, depending on priorities assigned by the sequence rule, e.g., the (S,S)-enan-tiomer for X = CH2. The situation is not so clear-cut with compounds 7.43, with which contradictory results were obtained. 

Sequence rule 5. i.e., R> S, covers pseudoasymmetric" units, requiring descriptors r and s. Note that the descriptors R and 5 refer to isolated ligands, i.e., the r,s descriptors are not... 

If however the Q-tetrahedron (ABYZ) were treated in the usual way for specifying the chirality (by viewing towards the comer with lowest priority and determining the (R) or (S) sequence of the three comers left according to the sequence rule 2)) this might reverse several assignments (as have been deduced so far) and would result in much confusion. 

Cahn, R. S. An introduction to the sequence rule. A system for the specification of absolute configuration. J. chem. Education 41, 116... 

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