Enantiomeric racemic compound

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic... 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

List possible advantages of using enantiomerically pure compounds as drugs, as opposed to using racemic mixtures. 

It is well known that certain microorganisms are able to effect the deracemization of racemic secondary alcohols with a high yield of enantiomerically enriched compounds. These deracemization processes often involve two different alcohol dehydrogenases with complementary enantiospedficity. In this context Porto ef al. [24] have shown that various fungi, induding Aspergillus terreus CCT 3320 and A. terreus CCT 4083, are able to deracemize ortho- and meta-fluorophenyl-l-ethanol in good... 

The simplest mesophase is the nematic phase. It is very fluid and involves highly disordered molecules having only short-range positional order, but with the molecules preferentially aligned on average in a particular direction (the director). If the constituent compound is racemic then it is possible to form a phase from the enantiomerically pure compound which is a chiral nematic phase. 

For a number of the systems, comparisons were made between the effects of enantiomeric composition in the monolayer and corresponding melting-point-composition curves for the crystals. All of the latter gave clear evidence of racemic compound formation in the crystals, and this type of pattern was repeated in the monolayer properties. 

On the other hand, if the enantiomeric purity of the original solid is less than that of the eutectic (as in the case of M2 in Fig. 25b), crystallization results in a decrease in enantiomeric purity. For example, when sufficient solvent has been added to correspond to point P2, the tie line shows that the solid N2 contains less of the predominant enantiomer D than M2 and is in equilibrium with E, which corresponds to a saturated solution of the eutectic solid, e. When the system reaches the composition represented by point Q2, the solid that crystallizes out is the racemic compound, R, which is in equilibrium with the saturated solution, U2, containing the racemic compound and enantiomer D. 

In the contemporary production of enantiopure compounds this feature is highly appreciated. Currently, kinetic resolution of racemates is the most important method for the industrial production of enantiomerically pure compounds. This procedure is based on chiral catalysts or enzymes, which catalyze conversion of the enantiomers at different rates. The theoretical yield of this type of reaction is only 50%, because the unwanted enantiomer is discarded. This generates a huge waste stream, and is an undesirable situation from both environmental and economic points of view. Efficient racemization catalysts that enable recycling of the undesired enantiomer are, therefore, of great importance. 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

Most of the zwitterionic compounds studied so far are chiral, with a chiral A5S/-silicate skeleton. Most of them have been isolated as racemic mixtures and in some cases as enantiomerically pure compounds, some of the optically active compounds being configurationally stable in solution. With these experimental investigations, in combination with computational studies, a new research area concerning the stereochemistry of molecular pentacoordinate silicon compounds has been developed. 

Relatively little attention has been paid to the conversion of racemic compounds into their enantiomerically pure versions in a single process, in other words a deracemization. For certain classes of chiral compounds such as secondary alcohols, this approach should provide many benefits, particularly to the pharmaceutical industry. Existing routes to high value intermediates in their racemic form may be modified to provide the equivalent homochiral product, thus reducing the extent of development chemistry required. In addition, the... 

It is well known that hydrogenation of dehydroamino acid derivatives derived from ring opening of unsaturated 5(4H)-oxazolones affords new racemic amino acids and, in some cases, enantiomerically pure compounds. On the other hand, a number of attempts have been made to hydrogenate the double bond of the unsaturated oxazolone itself. For example, 4-benzyl-2-methyl-5(4//)-oxazolone was prepared from 4-benzylidene-2-methyl-5(4H)-oxazolone using Raney Ni as a catalyst. This process is reported to be a general procedure to prepare saturated oxazolones directly (Scheme 7.194). 

Synthetic transformation of an enantiomerically pure compound into the target compound without a step causing racemization. If the starting material is a readily accessible natural product the term ex-chiral-pool synthesis, which was introduced by Seebach and Kalinowski3, is used (see Section A.2). 

The reasons for these choices are as follows. For most practical purposes a compound of a reasonably high degree of enantiomeric purity is the desired goal of an enantiosclcctivc reaction. The most convenient way to achieve this result is by recrystallization of the product or a derivative. This process, in the majority of cases, involves separation of the pure enantiomer from the racemate. The ee value, being equivalent to the percentage of the major isomer in the mixture with the racemic compound, defines the maximum yield of the pure isomer that can... 

The conglomerate shows a lower melting point (and hence, a higher solubility) than the individual enantiomers. From a melt or a solution with an enantiomeric ratio +1 1, the excess enantiomer crystallizes in pure form. The racemic compound may have a lower (curve 1) or a higher (curve 2) melting point (or solubility) than the corresponding enantiomers the eutectic mixture (E), however, always lies at a minimum. Finally, crystallization of pseudoracemates always yields enantiomerically impure samples. 

Additional enantiomerically pure 4,5-dihydroisoxazoles are prepared by separation of racemic compounds via chiral sulfoxides5-8, or by microbial reduction of 5-acetyl-4,5-di-hydroisoxazoles 34. 

The use of epoxides has expanded dramatically with the advent of practical asymmetric catalytic methods for their synthesis. Besides the enantioselective epoxida-tion of prochiral olefins, approaches for the use of epoxides in the synthesis of enantiomerically enriched compounds include the resolution of racemic epoxides. 

Fig. 17.14 Simultaneous stereoanalysis of Lavandula oil constituents, using enantio-MDGC (standard mixture), a Preseparation of racemic compounds unresolved enantiomeric pairs of octan-3-ol (6, 7), frcms-linalool oxide (1, 2), oct-l-en-3-ol (9, 10), ds-linalool oxide (3, 4), camphor (5, 8), linalool (17, 18), linalyl acetate (11, 12), terpinen-4-ol (15, 16) and lavandulol (13, 14). b Chiral resolution of enantiomeric pairs, transferred from the precolumn trans-linalool oxide 1 (2S,5S), 2 (2R,5R) ds-linalool oxide 3 (2R,5S), 4 (2S,5R) camphor 5 (IS), 8 (IR) octan-3-ol 6 R, 7S oct-1-en-3-ol PS, 10 R linalyl acetate 11 R, 12 S lavandulol 13 R, 14 S terpinen-4-ol 15 R, 16 S linalool 17 R, 18 S. [75]...

---