Enantiomers notation

Fischer projections and d-l notation are commonly used to describe car bohydrate stereochemistry The standards are the enantiomers of glycer aldehyde... 

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive. 

The possible types of intermolecular associations between Rj and 5j are summarized in Table 1 using this notation. The first two interactions listed are related enantiomerically and are necessarily identical. The third interaction differs from the first two in that both enantiomers are involved. 

Serine hydroxymethyl transferase catalyzes the decarboxylation reaction of a-amino-a-methylmalonic acid to give (J )-a-aminopropionic acid with retention of configuration [1]. The reaction of methylmalonyl-CoA catalyzed by malonyl-coenzyme A decarboxylase also proceeds with perfect retention of configuration, but the notation of the absolute configuration is reversed in accordance with the CIP-priority rule [2]. Of course, water is a good proton source and, if it comes in contact with these reactants, the product of decarboxylation should be a one-to-one mixture of the two enantiomers. Thus, the stereoselectivity of the reaction indicates that the reaction environment is highly hydro-phobic, so that no free water molecule attacks the intermediate. Even if some water molecules are present in the active site of the enzyme, they are entirely under the control of the enzyme. If this type of reaction can be realized using synthetic substrates, a new method will be developed for the preparation of optically active carboxylic acids that have a chiral center at the a-position. 

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.

The notation ER or ERS to express the preferential conversion of R over S, has not gained widespread popularity. Any ambiguity arising from the use of the unsuperscripted notation is addressed in the accompanying text. With 0 < E < °° and ERS = 1 /Esr, it is common practice to express the enantiomeric ratio as / > 1, with separate indication of the preferred enantiomer. 

Figure 2.11 Some plausible confoimations of (2/f, 4Y )-l-hydroxy-2,4-dimethylhex-5-ene. How many different torsional isomers might one need to examine, and how would you go about generating them [Note that the notation 2/f, 45 implies that the relative stereochemical configuration at the 2 and 4 centers is R,S - by convention, when the absolute configuration is not known the first center is always assigned to be R. However, the absolute conformations that are drawn here are S,R so as to preserve correspondence with the published illustrations of Stahl and coworkers. Since NMR in an achiral solvent does not distinguish between enantiomers, one can work with either absolute configuration in this instance.]...

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). 

In all examples available, geometrically similar antipodes react preferentially in the case of mono- and di-substituted ethylenes respectively, the three exceptions in Table 5 being due to the fact that substrates with similar geometry have different notations (Fig. 5). No observable kinetic resolution is achieved in the platinum-catalyzed hydroformylation of 3-methyl-1-pentene whereas a slight enantiomer discrimination is observed in the case of 2,4-dimethyl-1-pentene. 

Model system for enantiomer control in accord with Ref. [264]. Notation is such pfiat jgnR) denotes the D enantiomer on the ground electronic state in vibrational state n, gtiL) l thte analogous state of the L enantiomer and em) is the th level of excited electronic state. 

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.

In Figure 3.5, regular projections of knots with crossing numbers less than seven are shown, together with their symbolic notations commonly used in knot theory. For each topologically chiral knot only one of the two topological enantiomers is shown. 

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