S and R configuration

Figure 2-69. The two enantiomers of lactic acid assignment of R and S configurations to the enantiomers of lactic acid after ranking the four ligands attached to the chiral center according to the Cl P rules (OH > COjH > Me > H).

The January 1994 issue of the Journal of Chemical Edu cation contains an article that describes how to use your hands to assign R and S configurations... 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Sila-procyclidine, (cyclohexyl)phenyl[2-pyrrolidin-l-yl]silanol, may be prepared by hydrolysis of the corresponding methoxysilane (220) and is interesting in that it can form two types of hydrogen-bonded structure depending on whether it is enantiomerically pure or a racemate. In the racemate, the compound forms centrosymmetric dimers of (R)- and (S)-configuration molecules with an 0---N distance of 1.791 A. In the pure (R)-compound, however, the molecules are linked into infinite chains via intermolecular 0-H---N hydrogen bonds (0---N distance 2.792 A) (221), again similar to those in (2-morpholinoethyl)diphenylsilanol shown in Fig. 3. 

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... 

We compared the TE and toxicity of KL-1-14 synthesized in the R- and S-configuration [with 0.6 mol% Choi as helper lipid (see above)]. The transfection efficiencies for both lipids were statistically similar. Thus, for further experiments, KL-1-14 was synthesized as racemat. 

Spectrophotometric assays can be used for the estimation of the enantiosel-ectivity of enzymatic reactions. Reetz and coworkers tested 48 mutants of a lipase produced by epPCR on a standard 96-well microtiter plate by incubating them in parallel with the pure R- and S-configured enantiomers of the substrate (R/S-4-nitrophenol esters) [10]. The proceeding of the enzyme catalyzed cleavage of the ester substrate was followed by UV absorption at 410 nm. Both reaction rates are then compared to estimate the enantiomeric excess (ee-value). They tested 1000 mutants in a first run, selecting 12 of them for development of a second generation. In this way they were able to increase the enantiomeric excess from 2% for the first mutants to 88% after four rounds of evolutive optimization. 

Problem 5.18 For the stereoisomers of 3-iodo-2-butanol, (a) assign R and S configurations to C and C. (b) Indicate which are (i) enantiomers and (ii) diastereomers. (c) Will rotation about the C—C bond alter the configurations ... 

A second chiral center generated in a chiral compound may not have an equal chance for R and S configurations a 50 50 mixture of diastereomers is not usually obtained. 

We include this information in the name of a molecule, very much like we did for R and S configurations. For example, if the double bond is between carbons numbered 5 and 6 on a parent chain, then we would add the term (5E) or (5Z) at the beginning of the name. 

As established by NMR spectroscopy and circular dichroism, the dia-stereomerically pure chloro complexes can be epimerized in the presence of donor solvents to afford mixtures of (R)- and (S)-configurational isomers. The isomer ratio depends on solvent, R, and lanthanide ion. Importantly, selective epimerization allowed enrichment in either antipode with diastereomerically pure complexes obtained in a single recrystallization step. This was a crucial prerequisite for the preparation of molecular structure of [Me2SiCp" ( + )-neomenthylCp ] Lu(/2-Cl)2Li(Et20)2 has been determined by single-crystal X-ray analysis (Fig. 4) [42]. 

It is necessary to distinguish between two different H-bridge exchange processes. Complexes with cyclic, symmetrical diene ligands like 37g, 37o, 38g, 38o. 39g, and 39o show an exchange between endo-H-5 and endo-H-6 or endo-H-7, respectively. By this process, the R and S configurations are interconverted via an achiral 16-electron transition state. The relative positions of the monodentate ligands and the diene unit are unaffected by this... 

Comparison of the 4Ci configuration with the chair form given in the question leads to the conclusion that the compounds are anomers and hence diastereomers. The anomeric centres have the R and S configuration, respectively. 

In summary, a-hydrazino acids and natural and unnatural a-amino acids could be obtained in both the R and S configurations by this practical method. 

Unfortunately, the carbolithiation reaction of lithium species with 6-/V, /V-dimethylamino fulvene and subsequent transmetallation reaction leads to titano-cenes with stereoisomers, as an equimolar mixture of the R and S form of the substituted lithium cyclopentadienide is formed. Subsequently, this equimolar mixture of R and S configured substituted lithium cyclopentadienide is used for the transmetallation reaction and therefore, a mixture of the 25% of RJt, 25% of the S,S and 50% of the R.S form of the chiral titanocene dichloride is obtained. 

The lithiated bis-lactim ether 22 reacts with acetophenone and acetone with >95% de at C-3 to give the (3R)-aldol-type adducts 26. With acetone, only one diastereomer 26a can be detected both in the H-nmr- and l3C-nmr-spectrum, with acetophenone a 1 1-mixture of the two diastereomers 26 b with (R)- and (S)-configuration at C-3. They show both in the 1 H-nmr-spectram the expected high-field shift of the C-6-H, indicating the folded conformation 2717). 

In the preceding section, we saw there are four stereoisomers (two pairs of enantiomers) of 2-bromo-3-chlorobutane. These four isomers are simply all the permutations of (R) and (S) configurations at the two asymmetric carbon atoms, C2 and C3 ... 

Of examinations with commercially available chiral monoalkylamines and ethanolamines, complexation of chiral acerand 53 (RRRR and SSSS having four chiral carbons of R and S configurations, respectively) and norpseudoephedrine 63 (having two chiral carbons of. -configuration) in chloroform is shown as a representative example in Fig. 16. The (SSSS)-53 63 complex reveals a higher intensity of the absorption maximum near 550 nm compared to that of (RRRR)-53 63. This means that norpseudoephedrine is more easily accommodated in the cavity of (SSSS)-53 than that of (RRRR)-53, in accord with CPK molecular model examinations. 

The R S ratio in the racemic crystal of pip-1 before irradiation is 50 50, since the 1 -cyanoethyl groups of the A and B molecules have R and S configurations. After the irradiation, the A molecule has the R configuration, while the B molecule became the disordered structure with R S ratio of 20 30. This indicates that the R S ratio in a crystal changed from 50 50 to 70 30. This is why the specific rotatory power of the chloroform solution containing the irradiated pip-1 crystal showed +30°. 

Both of the racemic crystals of the piperidine and pyrrolidine complexes have chiral space groups before irradiation. The most important requirement for the racemic-to-chiral transformation is that the two molecules with R and S configurations crystallize in a chiral space group. Since the racemic compounds tend to make a pair around an inversion center in the process of crystallization, the racemic crystals, in general, have a center of symmetry. Otherwise, conglomerate crystals may be deposited from a racemic solution. Therefore, only several crystals with chiral space groups have been reported so far [38]. This may be a reason that such a racemic-to-chiral transformation has not been observed till now. 

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