Achiral phase

Figure 11.3 Typical configuration for the on-line coupling of an achiral and chiral cliro-matograpliic system by means of a switching valve. The non-enantio-resolved solute is isolated on the achiral phase and then stereochemically separated on the chiral phase. Reprinted from G. Subramanian, A Practical Approach to Chiral Separation by Liquid Chromatography, 1994, pp. 357-396, with permission from Wiley-VCH.

Figure 8.18 Smectic dimer of Watanabe, possessing an odd number of methylene units in linking group. This material self-assembles into intercalated smectic structure very similar to B6 banana phase. As for B6 phase, this achiral phase is also neither ferroelectric nor antiferroelectric.

In the case of diastereomeric mixtures of chiral hydroperoxides, standard chromatography on achiral phase can be employed to separate the diastereomers. As one example for the preparation of optically pure hydroperoxides via this method, the ex-chiral pool synthesis of the pinane hydroperoxides 11 is presented by Hamann and coworkers . From (15 )-cw-pinane [(15 )-cw-10], two optically active pinane-2-hydroperoxides cA-lla and trans-llb were obtained by autoxidation according to Scheme 17. Autoxidation of (IR)-c -pinane [(17 )-cw-10] led to the formation of the two enantiomers ent-lla and ent-llh. The ratio of cis to trans products was 4/1. The diastereomers could be separated by flash chromatography to give optically pure compounds. 

There are several characteristics of diastereomeric chiral separations (also known as indirect enantiomeric separations) that are worth mentioning. Achiral phases that are cheaper, more rugged, and widely commercially available are used. The elution order can be controlled by choice of the chirality of the derivatizing agent. This feature is useful for the analysis of trace levels of enantiomers. The separation can be designed such that the minor enantiomer is eluted first, allowing for more accurate quantitation. 

Enantioseparation can be achieved on a conventional achiral stationary phase by the inclusion of an appropriate chiral additive into the mobile phase. It is theoretically predicted that the enantioselectivity in CEC with a chiral additive may be higher than that using a chiral column with the same chiral selector [10]. Lelievre et al. compared an HP-/3-CyD column and HP-/3-CyD as an additive in the mobile phase with an achiral phase (ODS) to resolve chlortalidone by CEC [22]. It was demonstrated that resolution on ODS with the chiral additive was superior on the CSP however, efficiency was low. With an increasing amount of acetonitrile, the peak shape was improved and the migration time was decreased. We achieved the separation of salsolinol by the use of CEC with /3-CyD as a chiral... 

A different but experimentally simpler solution is to use a chiral selector (called often in such a case chiral additive) dissolved directly in the mobile phase. The diastereomeric associates thus formed differ in their mobility or adsorption, and in favorable cases they can be separated on an achiral phase. This approach was applied in the 1980s by Pettersson et al. who used native cinchona alkaloids in ion-pairing chromatography [ 51 ]. N owadays, ion-pairing technique is of considerable interest in a number of electrophoretic techniques (see below). 

Until recently, little success had been achieved in developing a highly enantioselective version of the Darzens reaction. Several investigations of chiral phase-transfer catalysts for this condensation, in which low or modest asymmetric induction is obtained, have been reported. These include the use of N-alky -N-methylephedrinium halides, the quinine-derived salt (120), and polyamino acids. A related study has examined the use of achiral phase-transfer catalysts in the condensations of carbonyl compounds and the asymmetric chloromethylsulfonate ester (121). The same group of researchers subsequently reported similar studies employing the sulfonamides (122)-(124). ... 

The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

Actually, such bowl phases are still to be found. However, polar achiral phases have been observed in the so-called polyphilic compounds [8]. The rod-like molecules of these compounds consist of distinctly different chemical parts, a hydrophilic rigid core (a biphenyl moiety) and hydrophobic perlluoroalkyl- and alkyl-chains at opposite edges. Such molecules form polar blocks that, in turn, form a polar phase manifesting pyroelectric and piezoelectric properties with a field-induced hysteresis characteristic of ferroelectric phases. 

B], B2,..., B Unknown Series of achiral phases formed by banana- or bent-shape molecules. The phase symmetry reduces with suffix n. Manifest spontaneous brake of mirror symmetry and interesting ferroelectric and antiferroelectric properties... 

In a stack of subsequent layers the tilt may be constant (synclinic structure) or alternating (anticlinic structure). Both synclinic and anticlinic multilayer stacks can further be subdivided into ferroelectric and antiferroelectric structures. The molecular projections (Mito the tilt plane 7y are shown in Fig. 13.29b. In ferroelectric (symbol F) phases spmitaneous polarization has the same direction in each layer (synclinic chiral SmCsPp and anticlinic achiral SmCAPp phases). In the antiferroelectric (symbol A) phases the direction of polarization alternates (achiral synclinic SmCsPA and chiral anticlinic SmCAPA phases). In fact we have a conglomerate of chiral and achiral phases both in either synclinic or anticlinic form. 

The hlted chiral phase is somewhat special because the reduced symmetry of the liquid crystal phase leads to a different strucmre from the achiral phase. As described in Chapter 3, the chiral phase has a hehcal strncmre as a result of the spiralling... 

Tsai, W. L, Hermann, K., Hug, E., Rohde, B. and Dreiding, A. (1985) Enantiomer-differentiation induced by an enantiomeric excess during chromatography with achiral phases, Heh. Chinu Acta 68, 2238-2243. 

Brief Overview of Synthetic Purification by Achiral-Phase ... 

Self-Disproportionation of Enantiomers During the Purification by Achiral-Phase HPLC... 

Vanin et al. studied order proflles of the perdeuterated laurate chain in chiral and achiral phases by means of deuterium NMR. The quadrupole splittings measured for the sequence of CD2 moieties are all quite similar for the nematic and chiral nematic phase the conclusion was that chiral distortions of the micellar shape must be very small. Their proposal for the mechanism behind the phase chirality is analogous to the one for polypeptides an asymmetric point charge distribution which in this case was speculated to be created by the chiral dopant on the micellar surface [19]. Radley and Tracey prepared samples from a racemic detergent and observed high fluidity, whereas those consisting of some enantiomeric excess showed a significantly increased viscosity [30]. But this is no proof for the association of micelles to necklaces. 

The second group of catalysts [Vllla-f] consists of resins carrying a protected galactose residue and a number of different achiral phase transfer catalysts. In these the stereochemical relationship of the optically active species and the catalyst itself is not well defined and the aim has been simply to load macromolecular chains as heavily as possible with both species. 

In the niid-1980 s indacranone (17) came to the group, and we faced the challenge of enantiospecifically preparing a tetrasubstituted carbon center. Initially, we established the stereogenic center in intermediate 18 via an achiral phase-transfer reaction employing methyl iodide, aliquat 336, toluene and 50%... 

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