R-/S- nomenclature

The currently used (R,S)-nomenclature of asymmetric C-atoms and similar tetracoordinate configurations is based upon the sequential order of the ligands derived from the CIP rules. If one orients an asymmetric C-atom in such a manner that the fourth ligand points backwards, the indices of the first three ligands of an (I )-configuration increase in a clockwise pattern, and counterclockwise for an (S)-configuration. 

Which will generally correspond to the RJt isomer when Rl and RM are simple alkyl groups and Rl takes precedence over RM in the R,S nomenclature. 

Now, you may have noticed that the hydroxyl group in methylecgonine is oriented differently from that in tropine. In methylecgonine it is easy to define the position of the hydroxyl, since this is a chiral centre and we can use the R/S nomenclature. An alternative stereoisomer of tropine exists, and this is called pseudotropine. How can we define the configuration for the hydroxyl when the plane of symmetry of the molecule goes through this centre and means this centre is not chiral but can exist in two different arrangements ... 

Schemes 3 and 4 show the topological arrangements for trien in both octahderal and square planar (trans) geometries. These stereochemical arrangements are available for any of the linear tetramine ligands (50)—(56), but the R,S nomenclature must be assigned for each individual polyamine. The R,S nomenclature shown in Scheme 3, applicable to trien and 3,2,3-tet, must be systematically reversed for 2,3,2-tet.

A succinct but complete discussion of R,S nomenclature is found in J. A. March, Advanced Organic Chemistry, Reactions, Mechanism, and Structure, 4th ed., Wiley, New York, 1992, pp. 109-111. 

According to the most recent IUPAC nomenclature, the R/S nomenclature may also be used with trigonal pyramidal TPY-3 structures. For TPY-3 structures, the vacant coordination site may be considered to be occupied by a lone pair of electrons or a phantom atom and given lowest priority. In Figure 5.4, we illustrate the use of this nomenclature for ethylmethyphenyltelluronium ion.18... 

Deduce the absolute configuration of L-cysteine according to the R/S nomenclature. 

Which group in each pair is assigned the higher priority in R,S nomenclature ... 

Step [1] Assign priorities to the two substituents on each end of the C=C by using the priority rules for R,S nomenclature (Section 5.6). 

A word of caution should be added with respect to Chemical Abstracts, as far as chirality assignments of homoannular substituted ferrocene derivatives are concerned. Until the 8th collective index, only (-f) and (—) are found as chirality indicators. For quite a long time, no descriptors were given at all, only the remark stereoisomer , followed by the registry number, which does not allow identification of a compound easily. This fact is in sharp contrast to the claims of Chemical Abstracts Service authors that they would consequently use Schlogl s central descriptors [20, 21]. Since volume 114, the (R, S ) nomenclature for ferrocene derivatives begins to appear, but its application is not very consequent, at least at the time where the book was written, and it is advisible to examine the orginal article rather than trust Chemical Abstract s descriptors. 

The/ /S and pro-R/S nomenclature systems are described in Eliel, E. L., Wilen, S. H., Stereochemistry of Organic Compounds, Wiley, New York, 1994, as well as in most organic chemistry textbooks. 

It just so happens that (,V)-laclic acid is the same as (+)-lactic acid (see Section 3.3.2.1). However, the + or — sign of the optical rotation is not related to the R,S nomenclature (i.e. a (+)-enantiomer could be either an U-enantiomer or an 5-enantiomer). 

The R,S nomenclature can be assigned to Fischer projections by drawing the substituent of lowest priority in a vertical position (i.e. at the top). Although Fischer projections can be rotated in the plane of the paper by 180°, changing the position of substituents by 90° requires double exchanges . 

The R-S nomenclature, which is an absolute naming scheme, is shown in Figure 9.6. It is commonly used by organic chemists, but is rarely used by biochemists since it is difficult to apply to molecules, such as tetroses, pentoses, hexoses, etc., which may have more than one chiral carbon. The predominant monosaccharides found in nature have the D configuration. 

Nomenclature of chemical compounds. Most undergraduate organic textbooks provide a more detailed account of die nomenclature used for organic molecules, including the R,S nomenclature for chiral centers. 

A second element of chirality is the configuration of the tertiary carbon atom of the growing polymer chain nearest to the metal atom. In fact, a new stereogenic center is formed in the growing chain at every propene insertion (Scheme 4). The standard Cahn—Ingold—Prelog R, S nomenclature jjg used here. 

Although the R, S nomenclature used to characterize the chirality at the metal atom for the Cs symmetric ligands can be used also for the C symmetric ligands of this section, in this case we preferred the more mnemonic and explanatory notation according to which, the relative disposition of the ligands that presents the coordinated monomer in the more (less) crowded region, is referred to as inward (outward) propene coordination . 

One speaks of a violation of the symmetry (here parity violation). It should be noted that the modern R, S nomenclature is used in the figure, whereby the R-amino acids normally correspond to the D-amino acids in the old nomenclature and the 5-amino acids correspond to the L-amino acids. We shall use both nomenclatures here, because the D, L terminology is widely used in biochemistry. In physics, one tends to use the R, L nomenclature which simply stands for Right/Left. If one takes into account the chemical equilibrium (3.1) at room temperature, the small enthalpy of reaction is reflected in the equilibrium constant ... 

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