Enantiomerization, fast

Still, in many cases photoderacemization of racemic mixtures has been successful. Irie et al. announced [54] the detection of the effect, but a detailed study of this system was published only as a patent [55] in CH2CI2 solutions of 1,l -binaphthyl they found values of [a]345 =-JX75-and 4- 0.7 for 1- and r-cpl with a 270-300 nm band of a 500 W Xe source at room temperature in ca. 5 min. Here two aromatic units are connected by a single bond. These systems are realizations of Kuhn s coupled oscillator model of optical activity [56]. The steric overlap provides chirality, ground state enantiomerization has AIv 92 kJ mol 1 [57], and the triplet state enantiomerizes fast, with 8 kJ mol 1. 

A kinetic resolution depends on the fact that the two enantiomers of a racemic substrate react at different rates with the enzyme. The process is outlined in Figure 6.1, assuming that the (S) substrate is the fast-reacting enantiomer (ks > ka) and Kic = 0-In ideal cases, only one enantiomer is consumed and the reaction ceases at 50% conversion. In most cases, both enantiomers are transformed and the enantiomeric composition ofthe product and the remaining starting material varies with the extent... 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

The pyrroline-iV-oxide 411 lost enantiomeric purity in the deprotection step. The THP protecting group could be deprotected under very mild conditions using Amberlyst 15 in methanol. However, the mixture was obtained in low yield accompanied by partial or total racemization as indicated by variation of specific rotation. Racemization also occurred during purification by silica gel chromatography or recrystallization. The lack of configurational stability of the nitrone 411 may be explained with the occurrence of a fast (not detectable by NMR), nitrone-hydroxyenamine tautomerism (Scheme 91). 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

The presence of the stereogenic centre at C(l) introduces an additional factor in the asymmetric epoxidation now, besides the enantiofacial selectivity, the diastereoselectivity must also be considered, and it is helpful to examine epoxidation of each enantiomer of the allylic alcohol separately. As shown in Fig. 10.2, epoxidation of an enantiomer proceeds normally (fast) and produces an erythro epoxy alcohol. Epoxidation of the other enantiomer proceeds at a reduced rate (slow) because the steric effects between the C(l) substituent and the catalyst. The rates of epoxidation are sufficiently significative to achieve the kinetic resolution and either the epoxy alcohol or the recovered allylic alcohol can be obtained with high enantiomeric purity [9]. 

Jimidar, M., Van Ael, W, Shah, R., Redlich, D., and De Smet, M. (2003). Fast method development and rapid analysis using a screening approach for enantiomeric separations in capillary electrophoresis./. Capillary Electrophor. Microchip Technol. 8, 101 — 110. 

Cyclopropyl carbanions are capable of maintaining their configuration whereas the CT-radical has been shown to reach inversion equlibrium with a rate constant of lO" s". ITie cyclopropyl bromide 13, and the corresponding iodide, are reduced in a single two-electron polarographic wave and the S +)-isomer yields the R(-)-hydrocarbon with 26% enantiomeric excess [67, 68]. Such a substantial retention of configuration during reduction of the carbon-bromine bond indicates a very fast second electron transfer process. Results from reduction of the cyclopropyl bro-... 

Capillary gas chromatography on optically active modified cyclodextrin phases is a simple, fast, accurate and highly sensitive method for the enantiomeric analysis of chiral volatile compounds. 

SCHEME 7. Test of the configurational stability of alkyl hthium carbenoids (a) fast equilibrium between enantiomeric lithium carbenoids (b) configurational stable hthium carbenoid 13... 

In work concerning the directed evolution of enantioselective enzymes, there was a need for fast and efficient ways to determine the enantiomeric purity of chiral alcohols, which can be produced enzymatically either by reduction of prochiral ketones (e.g., 26) using reductases or by kinetic resolution of rac-acetates (e.g., 19) by lipases (111). In both systems, the CD approach is theoretically possible. In the former case, an LC column would have to separate the educt 26 from the product (A)/(J )-20, whereas in the latter, (5)/(J )-20 would have to be separated from (S)/(R)-19. 

Y Martin-Biosca, C Garcia-Ruiz, ML Marina. Fast enantiomeric separation of uniconazole and diconazole by electrokinetic chromatography using an anionic cyclodextrin. Application to the determination of analyte-selector apparent binding constants for enantiomers. Electrophoresis 21 3240—3248, 2000. 

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