Racemic compound modification

The racemic acid is not a primary product of plant processes but is formed readily from the dextrorotatory acid by heating alone or with strong alkaU or strong acid. The methods by which such racemic compounds can be separated into the optically active modifications were devised by Pasteur and were apphed first to the racemic acid. Racemic acid crystallizes as the dihydrate triclinic prisms. It becomes anhydrous on drying at 110°C... 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

Both modifications of the method depend upon a natural segregation of the molecules of each active component into their individual crystal lattices. Experience has shown that such segregation, though common for dissimilar solutes, seldom occurs in the solution of a racemic substance and then usually in a narrow range of conditions that cannot be predicted or attained readily. In the great majority of instances the molecules of both active components combine in equal numbers to form one species of crystals known as a racemic compound or combine in variable proportions to form a series of solid solutions. Also, when segregation does occur, the experimental procedure necessary to produce distinguishable crystals or a uniform deposit of one variety is usually troublesome and slow. Hence the method has assumed practical value only in a few instances in which all the circumstances are especially favorable. 

Our ongoing research on (aminomethyl)(lithiomethyl)silanes focuses on modification of the substituents at the nitrogen center. The unprotected 2-silyl-substituted pyrrolidine 5 is a very useful starting point for the synthesis of different A -substituted pyrrolidines like rac-6. The racemic compound rac-S and the enantioenriched compound (iS)-5 could be s)mthesized by reaction of the Af-Boc-protected compound rac-2 or (S)-2 trifluoroacetic acid (TFA) [10]. In subsequent reactions it was possible to introduce a methyl group at the nitrogen center by conversion of rac-S with methyl... 

There are three crystal modifications of racemic crystals, depending on the difference in strength of the intermolecular interactions between the enantiomeri-cally hetero- and homo-chiral pairs of the racemate the modifications are racemic compound, conglomerate, and racemic sohd solution (Fig. 5.2). Preferential crystallization can only be applied to a conglomerate. However, most racemic crystals are racemic compounds, and conglomerates are very rare (a few percent of racemic crystals in our experience). This means that we have to transform a racemic compound into a conglomerate in order to carry out the enantioseparation of the racemate by preferential crystallization. 

The prefix (RS) is used to denote a racemic modification. For example, (RS)-Sec butyl chloride. The symbols R and S are applied to compounds whose absolute stereochemistry has been determined. However, while applying the nomenclature to projection formulae of compounds containing several asymmetric centres Cahn, Ingold. Pielog procedures are supplemented by the following conversion rule. 

L-Dopa was produced industrially by Hoffrnann-LaRoche, using a modification of the Erlenmeyer synthesis for amino acids. In the 1960s, research at Monsanto focused on increasing the L-Dopa form rather than producing the racemic mixture. A team led by William S. Knowles (1917—) was successful in producing a rhodium-diphosphine catalyst called DiPamp that resulted in a 97.5% yield of L-Dopa when used in the Hoffrnann-LaRoche process. Knowles s work produced the first industrial asymmetric synthesis of a compound. Knowles was awarded the 2001 Nobel Prize in chemistry for his work. Work in the last decade has led to green chemistry synthesis processes of L-Dopa using benzene and catechol. 

Several prominent types of host molecule, such as the steroidal bile acids and the cyclodextrins, are chiral natural products that are available as pure enantiomers. Chemical modification of these parent compounds provides an easy route to the preparation of large numbers of further homochiral substances. Since all these materials are present as one pure enantiomer, it automatically follows that their crystalline inclusion compounds must have chiral lattice structures. It is not currently possible to investigate racemic versions of these compounds, but the examples discussed previously in this chapter indicate that very different behaviour could result. 

TYPICAL PROCESSES OF PREFERENTIAL CRYSTALLIZATION If a conglomerate derivative is found, a moderate supersaturated solution of the racemic modification is prepared. Then, appropriate quantity of the (+) or (-) seed crystal of the optically active compound is inoculated into the solution, and it is left standing or stirred gently to crystallize. If a certain amount of the optically active compound (e.g. 1 g) was crystallized out by the first inoculation, then we can obtain about 2 g each of the optically active compounds after adding 2 g each of the racemic modification and repeating the operation. 

The separation (resolution) of a racemic modification into its constituent enantiomers is normally achieved by converting the enantiomers in the racemate into a pair of diastereoisomers by reaction with a pure enantiomer (Figure 10.4.). Enantiomers of acids are used for racemates of bases whilst enantiomers of bases are used for racemates of acids (Table 10.1). Neutral compounds may sometimes be resolved by conversion to an acidic or basic derivative which is suitable for diastereoisomer formation. The diastereoisomers are separated using methods based on the differences in their physical properties and the pure enantiomers are regenerated from the corresponding diastereoisomers by suitable reactions. 

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

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