Enantiomeric pairs, separation factor

Another factor that remarkably affects the enantioresolution of given enantiomeric pairs has been shown to be the binding chemistry used for the silica immobilization of glycopeptides (see Section 2.2). This was illustrated in the case of ristocetin A, which was covalently bonded to silica microparticles by immobilization onto epoxy-activated silica under mild conditions [22], and compared with the corresponding commercially available CSP, where the macrocycle was immobilized as previously reported for vancomycin, rifamycin B, thiostrepton [7], and teicoplanin [30]. The comparison proved that immobilization of ristocetin A onto epoxy-activated silica could significantly improve the enantioselectivity and the resolution of the corresponding CSP in the separation of a-amino acids under RP conditions [22]. 

Enantioselective gas chromatography can provide three quite different kinds of information (1) the amount of each enantiomer present in a food, determined as the enantiomeric purity or the enantiomer excess, and the separation factor a for each pair of enantiomers (2) enantiospecific sensory evaluation using gas chromatography-olfactometry (GC-O) and (3) data used as part of an authenticity determination. 

In order to obtain the detailed feature of optical resolution of /<2C-[Co(AA)3] where AA represents a- or B-amino acid, a series of diastereomeric or enantiomeric mixtures of /ae-[Co(a-AA)3 n(B-AA)n] were prepared and each mixture was eluted by passing an aqueous solution of Napd-tart through the column packed with TSK cation-exchange resin. For /,ac-[Co(gly)3 n(B-ala)n], the retention volumes of the enantiomers were obtained in each chromatographic run. Thus, the ratio of the retention volumes of enantiomers, that is, the separation factor for A and A pairs of [Co(gly)2(B-ala)], [Co(gly) ( B-ala)2] and [CoCB-ala ] were obtained easily. 

In case a chiral amino acid, L- or D-serine is involved in complex formation, the procedure to obtain the separation factors for enantiomeric pairs, A(L)- A(D) and A(D)- A(l) is complicated. 

Trend of the Separation Factors of Enantiomeric Pairs Eluted with d-Tart... 

M Na2< -tart aqueous solution was used in place of Na2S0ij solution. The procedure to obtain the separation factor(a) of the enantiomeric pair is the same with that eluted with Na2S0. The results are shown in Table I. 

Table II. The Separation Factor of Enantiomeric Pairs Eluted with Sb d-tart...

The advantages are similar to those of the indirect method the additive can be chosen from a wide range and in some cases its chirality can be adapted to the separation problem the stationary phase is cheap. In fact, the reagent does not necessarily need to be optically pure (although it should not be a racemate), but decreasing enantiomeric purity reduces the separation factor. The price of the reagent can be high. The interaction between the chiral selector and the sample can be based on inclusion (e.g., with cyclodextrins), on complexation (e.g., with amino acid-copper additives), on ion pair formation (e.g.,... 

In the case of DNB amino acids, the relevance of the mechanism was established by the following results (i) Methyl esterification of the amino acid carboxylic group cancels all chiral recognition making the docking approach impossible (ii) A relationship between log a, the enanfioselecfivity factor, and log 2. the retention factor of the most retained enantiomer, was found depending on the amino acid side chain size. The retention factors, k, of the first eluting DNB amino acids were similar [41] (iii) The enanfioselecfivity factors and elution order, respectively, obtained on the quinine and quinidine CSPs are similar and opposite for the same enantiomeric pairs (iv) Native amino acid enantiomers are not separated (no n-n interaction) [41]. 

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

In contrast to (74), optically active l,2-dimethyl-2-norbornyl p-nitrobenzoate (79) gives active products the SN1 product (80) is formed with ca. 9% and the El product (81) with ca. 63% retention of configuration96. Interconversion of enantiomeric 1,2-dimethylnorbornyl cations apparently competes with the product forming steps. The different optical purities of (80) and (81) show that they are derived from different intermediates. The authors suggest that most, or all, of the El product is formed from an intimate ion pair and that the SN1 product is formed from a solvent-separated ion pair or a dissociated carbocation. Solvolysis is accompanied by ion pair return which results in racemization of (79) and equilibration of 180-labeled (79). The rate of racemization exceeds that of scrambling of 180 by a factor of ca. 2. Substrate re-formed by ion pair return must be at least as optically active as the El product (81). Therefore, and keq correspond to upper limits of 37% and 20% of the total return, respectively. Scrambling of 180 detects only a small fraction of the total ion pair return in the solvolysis of (79). 

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