Racemate systems

Other racemization systems that may be amenable to conversion to deracemization processes in future have recently been reported by Faber and coworkers [33]. Resting cells of L. paracasei have been used for biocatalytic racemization of open-chain and cyclic dialkyl-, alkyl-aryl-, and diaryl-substituted acyloins (29/30) (Figure 5.20). Both... 

The stability limits of these monolayers are given in Table 6, and demonstrate the fact that the more highly expanded racemic film is also the more stable system over the temperature range studied. However, the general trend that can be observed in the 11/A isotherms and the monolayer stability limits is that, as the enantiomeric films become increasingly stable, their isotherms begin to take on the characteristics of the racemic system. Coupled with the observation of the temperature and compositional dependence of the... 

The latter point is illustrated by the surface shear viscosities of the homochiral and heterochiral films at surface pressures below the monolayer stability limits. Table 7 gives the surface shear viscosities at surface pressures of 2.5 and 5 dyn cm -1 in the temperature range given in Fig. 19 (20-40°C). Neither enantiomeric nor racemic films flow under these conditions at the lower temperature extreme, while at 30°C the racemic system is the more fluid, Newtonian film. However, in the 35-40°C temperature range, the racemic and enantiomeric film systems are both Newtonian in flow, and have surface shear viscosities that are independent of stereochemistry. These results are not surprising when one considers that (i) when the monolayer stability limit is below the surface pressure at which shear viscosity is measured, the film system does not flow, or flows in a non-Newtonian manner (ii) when the monolayer stability limit is above the surface pressure... 

Enantiomeric discrimination and its relation to film component reorganization upon compression can also be observed in dynamic surface tension hysteresis loops. Figure 26 shows the WjA isotherms generated upon five successive compression/expansion cycles (from II = 0 to lOdyncm-1) of racemic and enantiomeric films containing 17 mole percent palmitic acid. The hysteresis loops, obtained on the apparatus described in Section 2 (p. 63), show that the first compression/expansion cycle of the racemic system is repeated in each successive cycle. Upon expansion of the film from the maximum surface pressure back to Odyncm-1, the racemic film returns to its original state without detectable reorganization of the components. However, the... 

Conversely, the racemic film system appears to be solubilized by the achiral fatty acid component. At compositions of 10-33% palmitic acid, the ESP of the racemic system varies linearly with film composition, indicating that the monolayer in equilibrium with the racemic crystal is a homogeneous mixture of racemic SSME and palmitic acid. At compositions of less than 33% palmitic acid, the ESP is constant, indicating that three phases consisting of palmitic acid monolayer domains, racemic SSME monolayer domains, and racemic SSME crystals exist in equilibrium at the surface. 

How are the smafl-to-microscale excesses of one enantiomer over the other, produced by any of the scenarios outlined above, capable of generating a final state of enantiomeric purity In 1953 Frank [16] developed a mathematical model for the autocatalytic random symmetry breaking of a racemic system. He proposed that the reaction of one enantiomer yielded a product that acted as a catalyst for the further production of more of itself and as an inhibitor for the production of its antipode. He showed that such a system is kinetically unstable, which implies that any random fluctuation producing a transient e.e. in the 50 50 population of the racemic... 

Tissot, O., Gouygou, M., Dallemer, F., Daran, J.-C. B., and Balavoine, G. G. A. (2001) The combination of spontaneous resolution and asymmetric catalysis a model for the generation of optical activity from a fully racemic system. Angew. Chem. Int. Ed., 40, 1076-1078. 

Racemic and enantiomorphous 2-D and 3-D crystals display different physical and chemical properties. This difference has been utilized to enhance chirality in non-racemic systems that self-assemble in racemic and enantiomorphous crystallites. Morowetz [196] has elaborated a mathematical model that considers an evaporation/crystallization process where the racemate is less soluble than the pure enantiomorphous crystal and the enantiomer (in excess) is concentrated in the solution. A similar enrichment of chirality has... 

Some of these catalyze the smooth racemization of chiral secondary alcohols at room temperature. However, a major problem which needed to be solved in order to design an effective combination of ruthenium catalyst and lipase in a DKR of secondary alcohols was the incompatibility of many of the ruthenium catalysts and additives, such as inorganic bases, with the enzyme and the acyl donor. For example, the ruthenium catalyst may be susceptible to deactivation by the acetic acid generated from the acyl donor when it is vinyl acetate. Alternatively, any added base in the racemization system can catalyze a competing selective transesterification of the alcohol, resulting in a decrease in enantioselectivity. Consequently, considerable optimization of reaction protocols and conditions was necessary in order to achieve an effective DKR of secondary alcohols. 

Three specimens were tested in each system, except for the racemic system in which two specimens were tested. 

Substrate-Induced Diastereoselectivity in Acyclic Racemic Systems... 

Conglomerates and racemates can be easily distinguished from each other on the basis of their melting point phase diagrams [25]. A conglomerate system will exhibit a single eutectic minimum in its phase diagram at the mole fraction of 0.5, since at this enantiomeric ratio the system will consist of an equimolar mixture of two crystalline enantiomers that melts as if it were a pure system. Qn the other hand, a racemate system will exhibit two eutectics on either side of the mole fraction of 0.5 since both enantiomers will be foimd in the unit cells of the crystallized solid. 

To illustrate how a racemate system can be turned into a conglomerate system, and to demonstrate how one may recognize the existence of a conglomerate system, we will consider the example of ibuprofen, or 2-(4-isobutylphenyl)propanoic acid. The single crystal structures of enantio-merically pure [44] and racemic [45] ibuprofen have been published, and the powder diffraction patterns of the enantiomerically pure and racemic forms are shown in Fig. 9.3. The existence of a conglomerate system is indicated if the diffraction patterns of enantiomerically pure and racemic... 

Asymmetric Synthesis in the Racemic System.—Once we had a suitable model system, we moved to the next step of the planning, which required the construction of a crystalline phase isomorphous to (2), but composed of non-chiral molecules or a racemic mixture of molecules. One possible solution to this problem came from the observation that the crystals of many compounds containing the chiral s-butyl group have a strong tendency to disordering about the chiral centre (see Section 4, p. 218). When this is the case, the resolved enantiomer and its racemate generally pack in isostructural crystals. 

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