Racemic compound mixture

If the racemic compound (mixture of enantiomers in 1 1 ratio) is reacted with equimolar amount of a derivative of one of the enantiomers (e.g. R, having opposite chemical character) a quasi-enantiomeric mixture is formed. 

Our strategy consisted of the following steps A mixture of potential chiral selectors is immobilized on a solid support and packed to afford a complete-library column , which is tested in the resolution of targeted racemic compounds. If some separation is achieved, the column should be deconvoluted to identify the selector possessing the highest selectivity. The deconvolution consisted in the stepwise preparation of a series of sublibrary columns of lower diversity, each of which constitute a CSP with a reduced number of library members. 

Plouvier then prepared the previously unknown racemic form of proto-quercitol by mixing equal weights of the two enantiomers. The melting point (237°C.) of the mixture was not depressed, and its (presumably solid state) infrared spectrum reportedly (36) was identical with that of either active form. It thus appears that DL-proto-quercitol exists as a solid solution, not a racemic compound or conglomerate. 

Racemic Compounds existing as a racemate, or a 50-50 mixture of two enantiomers also denoted as dl or ( + ). Racemates are also called racemic mixtures. 

Synthesis of the common intermediate C (4), and its further conversion to 2 and 3 is illustrated in Scheme 7-3. Two racemic compounds, ( )-7 and ( + )-10, are prepared from readily available starting materials 5 and 8, respectively (Scheme 7-2). Coupling of 7 and 10 gives a mixture of diastereomers 11. An intramolecular aldol reaction of 11 catalyzed by D-proline yields diastereomers 12 and 13 in equal molar ratios (about 36% ee for each diastereomer). Compound 12, the desired ketone, is converted to 14, which is further purified by crystallization to give the compound in the desired stereochemistry in sterically pure form. Reduction of the ketone carbonyl group and subsequent methoxy... 

Olivetol 4.74 g (or equimolar amount of analog), 4.03 g (+) cis or trans p-methadien (2,8)-ol-l (the racemic compound can be used but yield will be one-half), 0.8 g p-toluenesulfonic acid in 250 ml benzene reflux two hours (or use 0.004 Moles trifluoracetic acid and reflux five hours). Cool, add ether, wash with NaHC03 and dry, evaporate in vacuum/to get about 9 g of mixture (can chromatograph on 3 50 g silica gel-benzene elutes the THC benzene ether 98 2 elutes an inactive product then benzene ether 1 1 elutes unreacted olivetol evaporate in vacuum to recover olivetol). 

A quasi-racemate or pseudo-racemate is a true racemate like molecular compound formed between optical antipode of different (but related) compounds. The quasi-racemate also has a melting point ciin c resembling the curve of a true racemate but with quasi-racemic compounds the curves are unsymmetrical, because the melting points of the components are different as shown in Fig. (9.3). The curve A represents the melting point of a true-racemate formed by mixing (+) mandelic acid XXII and (-) hexahydromandelic acid XXIII while B represents that of a mixture of (+) XXII and (+) hexa hydro-mandelic acid XXIII. 

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

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. 

Phase-composition diagrams may also be plotted for mixtures of optically pure samples that are structurally similar, but not identical. Such plots have been employed frequently as an empirical correlation to absolute configuration, which is referred to as the method of quasiracemates. The details, variations, limitations, and numerous examples of this method have been presented by others (33). We merely note here that if a phase-composition diagram of the racemic compound type were obtained for a particular pair of enantiomers having known configuration, and if the phase-composition diagram of mix-... 

Partial or complete separation of enantiomers within a racemic mixture as a result of unequal rates of reaction with another agent. The latter reagent, catalyst, solvent, or micelle must itself exhibit chirality, resulting in its stereoselective or stereospecific action on the racemic compound. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

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... 

Racemate - Traditionally, a 1 1 mixture of enantiomers was denoted racemic mixture. In order to describe precisely the solid state according to distinct phases racemates (or racemic compounds), conglomerates and solid solutions were distinguished. Thus, the term race-mate had a very specific function. However, currently the term racemate is almost exclusively used in the general sense of racemic mixture. It appears sensible to comply to current usage and reserve the term racemic compound to a distinct solid phase. 

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. 

It must be emphasized that only conglomerates can be resolved into the enantiomers by direct crystallization. For racemic compounds, pure enantiomers can be crystallized only from partially resolved mixtures (vide infra). Which type is present in a given case is best decided by trial and error. For a complete list of conglomerates forming chiral compounds, see reference 5. 

For racemic compounds (Figure 4), pure enantiomers are obtained by crystallization only if the composition of the mixture (M) lies between D (or l) and E (eutectic composition). In the opposite case (M ), the racemic compound crystallizes from the solution. When E is located closer towards R, the potential yield of pure enantiomer increases. An extreme, but not uncommon case, is encountered if E is located very close to d (or l). The racemic mixture crystallizes from the solution (or melt) and the mother liquor contains practically pure enantiomer. An appropriate derivatization of the partially resolved compound is often helpful. 

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]...

---