Enantiomer racemic compound

Individual enantiomers Racemic compound I, Mandelic acid [31]... 

One of the initial routes considered for the preparation of isomers was the enzymatic resolution using 5 -nucleotidase and alkaline phosphatase that would allow access to both enantiomers, racemic compound. The chemically synthesized monophosphate derivative ( )-121 was resolved using 5 -nucleotidase from Crotalus atrox venom, and the resulting mixture was separated by chromatography and purified on silica gel to give (+)-123 with a 90% ee. Hydrolysis of the remaining monophosphate (-)-122 with alkaline phosphatase from E. coli afforded Epivir [155]. 

The first observation of the enantioselective properties of an albumin was made in 1958 (28) when it was discovered that the affinity for L-tryptophan exceeded that of the D-enantiomer by a factor of approximately 100. This led to more studies in 1973 of the separation of DL-tryptophan [54-12-6] C22H22N2O2, on BSA immobilized to Sepharose (29). After extensive investigation of the chromatographic behavior of numerous racemic compounds under different mobile-phase conditions, a BSA-SILICA hplc column (Resolvosil-R-BSA, Macherey-Nagel GmvH, Duren, Germany) was... 

The original commercial source of E was extraction from bovine adrenal glands (5). This was replaced by a synthetic route for E and NE (Eig. 1) similar to the original pubHshed route of synthesis (6). Eriedel-Crafts acylation of catechol [120-80-9] with chloroacetyl chloride yields chloroacetocatechol [99-40-1]. Displacement of the chlorine by methylamine yields the methylamine derivative, adrenalone [99-45-6] which on catalytic reduction yields (+)-epinephrine [329-65-7]. Substitution of ammonia for methylamine in the sequence yields the amino derivative noradrenalone [499-61-6] which on reduction yields (+)-norepinephrine [138-65-8]. The racemic compounds were resolved with (+)-tartaric acid to give the physiologically active (—)-enantiomers. The commercial synthesis of E and related compounds has been reviewed (27). The synthetic route for L-3,4-dihydroxyphenylalanine [59-92-7] (l-DOPA) has been described (28). 

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. 

Meso compounds contain chirality centers but are achiral overall because they have a plane of symmetry. Racemic mixtures, or racemates, are 50 50 mixtures of (+) and (-) enantiomers. Racemic mixtures and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point. 

A quasi-racemate is a molecular compound that is related to a true racemic compound by a small structural change in one of the enantiomers (Fredga, 1944). 

Fig. 23 Tie lines associated with different systems (1) a solid phase D (pure enantiomer) in the presence of mother liquor of variable composition, (2) a solid phase L, (solvated enantiomer) in mother liquor of variable composition, (3) a solid phase R (pure racemic compound) in mother liquor of variable composition, (4) a solid phase Rs (solvated racemic compound) in mother liquor of variable composition, (5) two solid phases, one enantiomer and the racemic compound (or two enantiomers if E is on SR, i.e., for a conglomerate) in mother liquor of fixed composition E (eutectic), and (6) the tie lines do not converge one solid phase is present (solid solution of D and L) in mother liquor of variable composition. (Reproduced with permission of the copyright owner, John Wiley and Sons, Inc., New York, from Ref. 141, p. 177.)...

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. 

If solvent is added to either of the solid eutectics represented by e or e in Fig. 25a or b, the undissolved solid retains this composition while the saturated solution maintains the composition E or E, respectively. Again, Gibbs phase rule [145,146] can provide further insight into these systems. If the solid enantiomers are solvated, the compositions of the equilibrium solids are displaced symmetrically along the DS or LS axes to an extent determined by the stoichiometry of the solvates. Similarly, if the racemic compound is solvated, the stoichiometry of the equilibrium solid is displaced from R along the line RS to an extent determined by the stoichiometry of the solvate. 

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. 

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. 

As illustrated for compounds 77 and 78 in Scheme 18, different methods were applied for the syntheses of 77-79 (79 was obtained analogously to 78 according to method a). The racemic products 77a 0.7CH3CN, 78 CH3CN, and 79 were isolated as crystalline solids. In addition, crystals of the racemic compound 77b (an isomer of 77a) were obtained. For the solvent-free compound 78 formation of enantiomorphic crystals was observed. The crystals studied by X-ray diffraction contained (just by accident) the (A)-enantiomer. 

The force-area curves for racemic and (5)-(+)-2-tetracosanyl acetate recorded with a barrier speed of 5 cm/min are shown in Figures 17 and 18, respectively. Again, both enantiomers showed identical monolayer behavior. The film balance behavior of the racemic acetate was indistinguishable from that of the pure enantiomers at temperatures above about 27°C however, below this temperature the force-area curves differed markedly even at low surface pressures, which indicates that racemic compound formation occurs at relatively large areas per molecule. 

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-... 

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. 

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