R and S absolute configuration

R) and (S) absolute configurations are referred to the tertiary C atoms of the principal (drain. 

Designation of optical isomers can be by the symbols D and L, which are used to indicate the relationship between configurations based on d (+)-glycer-aldehyde as an arbitrary standard. If such relationship is unknown, the symbols (-I-) and (-) are used to indicate the sign of rotation of plane polarized light (i.e., dextrorotatory and levorotatory). In 1956, Cahn et al. [1] presented a new system, the (R) and (S) absolute configurations of compounds. 

It is convenient to classify these polymers on the basis of the tacticity of the main chain only thus, polymers obtained from one antipode, in which all the asymmetric carbon atoms of the lateral chains have the same absolute configuration, will be considered as homopolymers. On the contrary, polymers made up by macromolecules in the side chains of which asymmetric carbon atoms having R or S absolute configuration alternate more or less regularly, can be considered as copolymers of the two antipodes of the monomer having respectively R and S absolute structure. 

Identify what appears to be the compound s central molecular scaffold and estimate how flexible or inflexible that system may be in terms of adopting different shapes or movements in 3D space locate any asymmetric centers and if stereochemical information is displayed in the structure then discern the R or S absolute configuration nomenclature for each of these sites. 

A new category of stereoisomers is possible when a molecule contains more than one stereogenic center. In terms of absolute configuration, every stereo-genic center is treated as an independent unit and assigned the appropriate R or S absolute configuration. However, the presence of more than one stereogenic center poses new problems. 

In determination of the absolute configuration of a-chiral primary amines, BINOL derivatives were used as chiral derivatizing agent.10 In this procedure, the chiral substrate was derivatized with R and S enantiomers of the 2,-methoxy-l,l -binaphthalene-8-carbaldehyde and the XH spectra of both diastereomers were compared. Comparison of the chemical shift differences of the diastereomers has allowed determination of the absolute configuration of the chiral substrate [5]. 

Applying it to glucose, whose absolute configuration has been determined, the molecule is represented as follows with the symbols R and S specified at various asymmetric carbon atoms. 

The use of a reference axial system, whether right- or left-handed, is completely analogous to the Cahn, Ingold, and Prelog convention (75-77) regarding the specification of the absolute configuration of chiral molecules R and S as depicted in Scheme 11 for molecules with large (L), medium (M), and small... 

The 36d-LAH complex was applied to the reduction of ketone oximes and their O-tetrahydropyranyl and O-methyl derivatives to optically active amines (69). Results for a variety of phenyl alkyl and dialkyl ketones are shown in Table 4. The predominant amines formed all were of the S absolute configuration with optical purities up to 56%. The oxime hydroxy group presumably reacts with the less hindered H2 in the 36d-LAH complex (cf. Scheme 6) to form an oxime complex (45), which probably undergoes infermolecular hydride transfert of H2 from a second molecule of the 36d-LAH complex (Scheme 8). Asymmetric reduction with the ethanol-modified 36d-LAH reagent gave amines of R con-... 

Figure 13.5. Examples of n-o reactions (R and S denote absolute configurations)...

Some generic structures of /3-amino acids are shown in Fig. 6.40. Since, in /3-amino acids, two C-atoms separate the amino and carboxylate groups, there are two possible locations for attachment of a single side chain (i.e., /32 and /33), or even two or more side chains (e.g., /32,3 and /32,2,3, respectively). In a /3-peptide, these symbols can be used as prefixes, e.g., the /33/32-dipeptide in Fig. 6.40 becomes /33-HAla- /32-HVal for R=Me and R = i-Pr. The stereodescriptors (R) and (S) should be used to specify the absolute configuration at the stereogenic centers. The same rules apply to y-amino acids and y-peptides. 

The first example of asymmetric induction in transfer of chirality from the chiral sulfur atom to the prochiral carbon atom was described by Goldberg and Sahli in 1965 (197). It concerns the pyrolysis of the optically active p-tolyl tra s-4-methylcyclohexyl sulfoxides 258. It was found that on pyrolysis at 200 to 250°C, optically active sulfoxides (R)-258 and (5)-258 yield optically active 4-methylcyclohexenes-l 259, with the absolute R and S configurations, respectively, at the newly formed chiral carbon atoms (Scheme 25). The optical purities of the 4-methylcyclohexenes-l that were formed depended largely on the temperature of pyrolysis. Thus, the values of 42 and 70% optical purity were noted for 259 at 250° and 200°C, respectively. The formation of the cycloolefins 259, whose absolute configurations are the same as those of the starting optically active sulfoxides 258, indicates that the pyrolysis reaction proceeds... 

Reaction of 299 with benzaldehyde was found to give an equimolar mixture of diastereomeric j3-hydroxysulfoxides (314). Addition of 299 to a-tetralone 300 was more satisfactory, since the corresponding diastereomeric/3-hydroxysulfoxides 301 were formed in a 1.8 1 ratio. Their subsequent desulfuration with Raney nickel yielded levorota-tory 1-hydroxy-1-methyl-1,2,3,4-tetrahydronaphthalene 302 of unknown absolute configuration and optical purity. Similarly, addition of 299 to cyclohexene oxide leads to the formation of diastereomeric /3-hydroxysulfoxides 303 in a 2 1 ratio which, after separation, may be desulfurized to give (R,R)- and (S,S)- trans-2-methylcyclo-hexanols 304, respectively. Analysis of NMR spectra of the... 

Since the enantiomers of the carboxylic acid 42-H can easily be separated via its diastereomeric salts (Scheme 10) [59], many of the other bicyclopropylidene derivatives can also be obtained in enantiomerically pure form by transformations of the acids (R)- and (S)-42-H. The absolute configuration of (i )-42-H was proved by an X-ray crystal structure analysis of its (f )-a-phenylethylamide [59]. 

The absolute configurations of amino acids 198a (the racemic counterpart was designated 186c) and 198i on the basis of X-ray crystal structure analysis data of the a-azido esters 197a-Me and 197i-Bn were (R) and (S,S,S), respectively. 

T.C. Rosen, S. Yoshida, R. Frdhiich, K.L. Kirk, G. Haufe, Fiuorinated phenyicyciopropyiamines. 2. Effects of aromatic ring substitution and of absolute configuration on inhibition of microbial tyramine oxidase, J. Med. Chem. 47 (2004) 5860-5871. 

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