Racemic pair

Quantitative Analysis of Selectivity. One of the principal synthetic values of enzymes stems from their unique enantioselectivity, ie, abihty to discriminate between enantiomers of a racemic pair. Detailed quantitative analysis of kinetic resolutions of enantiomers relating the extent of conversion of racemic substrate (c), enantiomeric excess (ee), and the enantiomeric ratio (E) has been described in an excellent series of articles (7,15,16). 

Figure 3.8 Second-dimension chiral cyclodextrin capillary column separation of a non-racemic pair of nonachlorobomane compounds extracted from dolphin blubber, shown with expanded attenuation in the inset. The primary separation (not shown) was performed on an apolar primary capillary column. Reproduced from H.-J. de Geus et al. J. High Resol. Chromatogr. 1998, 21, 39 (59).

The data indicated that MCTA can be successfully applied for the enantiomeric separation of a wide variety of racemic pairs using aqueous organic mobile phases [137],... 

Let us now move to the insertion of another methylene, forming the following segments. The first set contains racemic pairs. 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

Concentrations and Equilibrium Constants for Racemic Pairs in the Reaction Mixture [Co-( )-pn3]Cl3 (16)... 

Paul Pfeiffer discovered a very interesting stereochemical phenomenon, which now bears his name — the Pfeiffer effect this has received a good deal of attention.30 When an optically active substance which is stable in solution is added to a solution of a labile chiral substance, the optical rotation of the solution changes, reaching a new level in some hours. Several theories have been advanced to explain the phenomenon, the most satisfactory based on the supposition that the optically active ion or molecule forms an association with one isomer of the racemic pair of the labile substance and thus shifts the dextro—levo equilibrium. In general it is not possible to use this as a means of resolution, for when the added optically active substance is removed from the labile material, the latter immediately racemizes. 

The dihydroxo-bridged binuclear complexes are among the most common, structurally well-characterized polynuclear complexes. For symmetrical bidentate nonbridging ligands, LL, the cation (LL)2M(OH)2M(LL)2"+ may exist in three isomeric forms a meso isomer (A,A) with configuration A and A, respectively, at the two metal centers, and a racemic pair of isomers (A,A and A, A) with configuration A or A at each of the two metal centers (cf. Fig. 2). If the bidentate... 

In summary, therefore, after on-line chiral HPLC separation, NMR spectroscopy has been used to characterise compounds in terms of the cis- and trans-isomers and to identify the racemic pairs on the basis of their identical NMR spectra. In addition, HPLC-circular dichroism (HPLC-CD) was used to identify... 

Prediction of the in vitro activity for the racemic pair of compounds leads to a less clear interpretation of the data. Is only one enantiomer active If so, should the biological activity value be doubled, assuming only half of the assayed material is active If the enantiomers are unequally active, how should the data be treated This final model was used to predict the activities of each enantiomeric pair, providing some insight into the solution to these questions. 

The remaining naturally-occurring O-methylinositols are derivatives of myoinositol, which can theoretically give rise to six monomethyl ethers. Two of these, the 2-methyl (LXXII) and the 5-methyl ether (LXXI), must be meso compounds. The other four must be optically active and constitute two racemic pairs (one enantiomorph of each is shown as LXXIII and LXXIV) they are, therefore, designated (+)- and (—)-l- and (+)- and (—)-4-0-methy 1-myoinositol, by the principle of lowest numbering. 

Definition An ensemble of molecules is chemically achiral under given observation conditions if and only if it is composed of racemic pairs of enantiomers and achiral molecules. 

The possibility of separating dilevalol from labetalol was considered as an option at the commencement of the dilevalol project. However, it quickly became clear, from work carried out in both Research and Development, that this approach might not be a viable option. Although the racemic pairs (RR + SS and RS + SR) were separable by crystallization, and although the optical resolution of the RR and SS enantiomers could be achieved through salt formation with a chiral acid, the direct yield of dilevalol was less than 20%. Nevertheless, it was recognized that if the recovery and recycling of the waste streams from the physical and optical resolutions could be carried out efficiently, considerable economies would be obtained (Scheme 1). 

Unlike the racemic pair ( + )A( + )B and ( —)A( —)B, the diastereoisomeric pair (4-)A( + )B and (+ )A( — )B can also be separated by simple processes (which do not involve chiral agents or influences)—for example, crystallization or chromatography. This is the basis for Pasteur s resolution of ( )-tartaric acid and for countless other similar resolutions. (For a recent account of strategies in optical resolutions, see [30].)... 

Two enantiomeric carbocations are formed. Each carbocation can react with H2O from either the top or the bottom to yield a total of four stereoisomers. The same argument used in Problem 9.21 can be used to show that the (15,3/ ) and (l/ ,35) enantiomers are formed in equal amounts, and the (15,35) and (IR,3R) isomers are formed in equal amounts. The result is a non-50 50 mixture of two racemic pairs. 

The morphine molecule possesses five asymmetric centers at carbons 5(R), 6(S), 9(R), 13(5), and 14(1 ), and it is this geometry that affords the familiar opioid pharmacological actions. The C-9 to C-13 ethanamino bridge restricts the number of possible optical isomers to 16 (i.e., eight racemic pairs). 

A remarkable number of papers dealing with the stereochemical aspects of Mannich synthesis are present in the literature, some of them reporting the use of optically active enantiomeric reactants, instead of the commonly employed racemic pairs. Interesting asymmetric syntheses are also described. 

Fig. 113. The carbonyl group reduction of a-chiral P-aminoketones (only one enantiomer of the racemic pair is represented).

Non 59 S( mixture orf a racemic pair Noii SOrSO mixture of n raoemir pair... 

The addition of sulfenyl halides across a double bond is a well-known reaction and is mainly used for the synthesis of //-halo sulfides. The first report on the steric course of this reaction concerned the addition of 2,4-dinitrobenzenesulfenyl chloride to ( )- and (Z)-2-phenyl-2-butene15. The addition to each isomer produces different crystalline derivatives as racemic pairs. 

The chirality of the (Z)-isomer of 2,2 -(44a) and 3,3 -dimethylbianthrone (44b) was established by observing a splitting of resonances in the presence of Eu(hfc)3 . It was possible to distinguish the racemic pair from the meso-isoma as well. The methyl resonances were the most convenient to monitor. The helical chirality of annulenones, e.g. 45, was confirmed by the doubling of certain resonances in the H NMR spectrum in the presence of Eu(dcm)3 . 

Kinetic Resolution. The selective reaction of one member of a racemic pair with a chiral reagent is the basis for a kinetic resolution. This reaction provides recovered starting material in one enantiomeric series with a product in the opposite series. 

Non-50 50 mixture of a racemic pair Non-50 50 mixture of a racemic pair Non-50 50 mixture of two racemic pairs optically inactive... 

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