Racemization variables

E. Fernandez, J.E. Ortiz, A. Perez Perez, E. Prats, D. Turbon, T. Torres, E. Arroyo Pardo, Aspartic Acid Racemization Variability in Ancient Human Remains Implications in the Prediction of Ancient DNA Recovery, Journal of Archaeological Science, 36,965 972 (2009). 

Radical R-X (RAAr, vinyl, Multiple Racemization Variable... 

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.)...

When platelike crystals of gly with well-developed 010 faces were partially dissolved in solvents containing variable amounts of resolved R a-amino acids, well-defined etch pits were formed only on the (010) face (Figure 30a). These pits exhibited twofold morphological symmetry with surface edges parallel to the a and c axes of the crystal. The enantiotopic (010) face dissolved smoothly (Figure 30b), exactly as it does when the crystal is dissolved in a solution of pure gly. As expected, S amino acids induced etch pits on the (010) face. Racemic... 

In psychopharmacology, interest in the properties of enantiomers has been aided by the need to improve the therapeutic efficacy and decrease the side effects and toxicity of drugs. For example, if the therapeutic activity resides entirely in one enantiomer (called a eutomer) then giving a racemic mixture which contains the active and the inactive enantiomer is clearly wasteful. Thus using the single enantiomer (isomer or eutomer) should enable the dose of the drug to be lowered, reduce the interpatient variability in the response and, hopefully, reduce the side effects and toxicity of the drug (see Table 3.4). 

It should be noted that the rate of racemization (or the rate of hydrogen exchange in Section 10.1.1) is exactly the same as the rate of enolization, since the reprotonation reaction is fast. Hence, the rate is typical of a bimolecular process and depends upon two variables, the concentration of carbonyl compound and the concentration of acid (or base). 

An intermediate case between the polymerization of enantiomerically pure and racemic monomers is the polymerization of a partially resolved mixture of enantiomers with variable optical purity. Two processes are distinguishable, depending on whether polymerization occurs in the presence of an achiral (or racemic) or of a chiral (optically active) catalyst. 

The most promising route towards variable isotacic PHB is a process in which PO is transferred to both enantiopure and racemic (3-BL in two parallel processes (Fig. 40). As mentioned, these monomers can be distilled off from the catalysts and polymerized directly. Thus, any degree of tacticity in the polymer can be adjusted by mixing racemic and enantiopure (3-BL, which would allow the preparation of tailor-made materials from the low-cost oil-based monomers PO and CO (cf. Sect. 8). 

There are only a few reports on chiral phase transfer mediated alkylations". This approach, which seems to offer excellent opportunities for simple asymmetric procedures, has been demonstrated in the catalytic, enantioselective alkylation of racemic 6,7-dichloro-5-methoxy-2-phenyl-l-indanone (1) to form ( + )-indacrinone (4)100. /V-[4-(tnfluoromethyl)phenylmethyl]cinchoninium bromide (2) is one of the most effective catalysts for this reaction. The choice of reaction variables is very important and reaction conditions have been selected which afford very high asymmetric induction (92% cc). A transition state model 3 based on ion pairing between the indanone anion and the benzylcinchoninium cation has been proposed 10°. 

All structures mentioned become chiral by appropriate mono-or disubstitution (C, or C2 in metacyclophanes with m = n = 2). However, according to the variable racemization (= inversion) barriers only for [2.2]metacyclophane derivatives con-formationally stable enantiomers result which permit optical resolutions. 

Slightly distorted chair conformations have also been assigned to the e-caprolactones (3) and (4) on the basis of their chiroptical properties (67JA5649). Both variable temperature NMR and racemization methods have been used to determine the rate of conformational interchange in dihydrodibenz[c, e]oxepins (5). Barriers were found to be in the range 38-71 kJ mol-1 for a single ortho R substituent (5 R = H) increasing to 117-146 kJ mol-1 for two ortho substituents (5, R, R H). 

FIGURE 7 Change in optical rotation (sohd line) during racemization of (-)-adrenaline in aqueous solution (1 M HC1) carried out under variable-temperature conditions. M T(K) = 309.6 + 0.001694f (dashed line). 

Alibrandi, G., Coppolino, S., D AUberti, S., Ficarra, P., MicaU, N., and Villari, A. (2002), Temperature-rate proUles by polarimetric variable-temperature kinetic experiments to study racemization reactions, I. Pharm. Biomed. Anal., 29,1025-1029. 

From the standpoint of synthesis of polysaccharides, the most significant aspect of a polymerization mechanism is whether or not it involves regio- and stereo-selective control. However, the structural and stereochemical problems with polysaccharide synthesis are generally simpler than those that obtain with a racemic, unsymmetrical monomer, such as (R, S)-propylene oxide. For example, a variable percentage of head-to-head and tail-to-tail sequences is found in... 

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