Enantioselectivity Quick

Eigure 1 shows the plots for the relation between A AG and E at three different temperatures 50, 0, and —50°C. At the lower temperature (—50°C), the curve flattens quickly, and E value of 100 requires AAG = 2047calmoU, less than those of 2506 (at 0°C) and 2965 cal moF (at 50°C), and thus a small difference in transition-state energy (AAG ) between the enantiomers gives a large effect on the enantioselectivity. Thus, lowering the temperature increases the... 

This achievement was unique in two respects 1) it was the first example of industrial application of a homogeneous enantioselective catalysis methodology and 2) it represented a rare example of very quick convergence of basic knowledge into commercial application. The monophosphine ligand CAMP was shortly replaced by the related diphosphine ligand DIPAMP which improved the selectivity for the I-DOPA system up to 95% ee [45]. 

In particular, Diels-Alder adducts from the enantioselective reaction of 2-bromo-acrolein and 2-chloroacrolein with a variety of dienes are of exceptional synthetic versatility. Readers are advised to consult the review article by Corey and Guzman-Perez.54 For the purpose of quick reference, chiral ligands commonly used in Diels-Alder reactions are listed in Table 5-6. 

The use of chiral copper complexes in asymmetric synthesis was inaugurated in 1966 when the first homogeneous asymmetric metal-catalyzed reaction was reported a copper catalyzed cyclopropanation (2). At the end of 1999, more than 25 distinct reactions were reported wherein the use of a chiral copper complex had induced an enantioselective transformation. The field grew quickly and the best is most likely yet to come. 

Another colorimetric assay for testing the enantioselectivity of lipases or esterases in ester hydrolysis reactions is based on a different principle (75). To simulate the state of competitive conditions of an enzymatic process, the so-called Quick-ii-Test... 

The dioxirane epoxidation of a prochrral alkene will produce an epoxide with either one new chirality center for terminal alkenes, or two for internal aUcenes. When an optically active dioxirane is nsed as the oxidant, expectedly, prochiral alkenes should be epoxi-dized asymmetrically. This attractive idea for preparative purposes was initially explored by Curci and coworkers in the very beginning of dioxirane chemistry. The optically active chiral ketones 1 and 2 were employed as the dioxirane precursors, but quite disappointing enantioselectivities were obtained. Subsequently, the glucose-derived ketone 3 was used, but unfortunately, this oxidatively labile dioxirane precursor was quickly consumed without any conversion of the aUcene . After a long pause (11 years) of activity in this challenging area, the Curci group reported work on the much more reactive ketone... 

Janes, L. and Kazlauskas, R. (1997) Quick E. A fast spectrophotometric method to measure the enantioselectivity of hydrolases. J. Org. Chem., 62,4560-4561. 

A direct comparison of catalysis of olefin epoxidation with a homogeneous chemical catalyst (Mn salen), an enzyme (CPO), and an antibody resulted in sufficiently high enantioselectivity for all three catalysts, a higher turnover number for the enzyme, and a slightly higher substrate/catalyst ratio for the homogenous catalyst. Criteria for comparison should be quantitative and include catalyst lifetime as well as volumetric productivities, but have been found to depend on the different needs of laboratory synthetic chemists, who need a broadly specific catalyst quickly, versus those of process chemists, who often control catalyst availability and can allow narrow specificity (provided their substrate is accepted) but need high productivity. 

Natural products having chiral tertiary amine functions were tested among the first catalysts in asymmetric MBH reactions [24, 60]. The importance of the proton donor capacity of the catalyst in the rate and selectivity of the MBH reaction was recognized very quickly, and attention was turned to genuine a-amino alcohol structures, such as the compounds listed in Scheme 5.8 [61]. Results were modest, however. Apart from the earlier discussed (R)-3-HDQ, which catalyzed the MBH reaction at atmospheric pressure (though with no enantioselectivity),... 

The first highly enantioselective examples of this catalysis strategy were reported by MacMillan et al. in 2000 (Ahrendt et al. 2000 also see Wilson et al. 2005 Northrup and MacMillan 2002b), shortly after our first report on the proline-catalyzed intermolecular aldol reaction had appeared. The MacMillan group has quickly established that Diels-... 

In laboratory microcosms, ira 5-permethrin was selectively degraded compared to the other diastereomer, cw-permethrin, by six bacterial strains [19]. These strains also preferentially biotransformed 15-cw-bifenthrin over their antipodal l/ -cw-enantiomers, which were more toxic to daphnids [19]. Enantioselectivity was more pronounced for cw-permethrin than for cw-bifenthrin, and was strain-dependent. The (—)-enantiomer of both pyrethroids was preferentially depleted in sediments adjacent to a plant nursery, suggesting that in situ microbial biotransformation was enantioselective [20]. Although all enantiomers of permethrin were hydrolyzed quickly in C-labeled experiments in soils and sediments, the degradates of both cis- and irara-permethrin s -enantiomers were mineralized more quickly than those of the 5-enantiomer, while degradation products of cA-permethrin were more persistent than those of the trans-isomex [185]. Enantioslective degradation of fenvalerate in soil slurries has also been reported [83]. These smdies underscore how enantiomer-specific biotransformation can affect pyrethroid environmental residues, the toxicity of which is also enantiomer-dependent [18-20]. 

Enantioselective oxidation of achiral selenides and their subsequent rearrangement would provide optically active alcohols. Chiral alkyl arylselenoxides racemize quickly in the presence of moisture via an acid-catalyzed formation of an achiral hydrate. However, bulky ortho substituents in the aryl moiety slow down the rate of racemization by sterically inhibiting the formation of this hydrate. 

Likewise, once the initial screening of ADHs was performed, we took a closer look at the catalytic properties of the new ADH. We selected two secondary alcohols as substrates for the enzymatic oxidation and measured the rate of NADH formation for both enantiomers individually, so the enantiodiscrimination of the enzyme could be estimated. While this is not a true enantioselectivity value since the competition factor between enantiomers has been eliminated by making separate reactions for each isomer instead of the racemic mixture, it gives an estimate and allows a quick identification of highly selective enzymes. Table 5 shows the results of this screening. Most of the enzymes found were selective for the S-alcohol isomer, except AD99, which shows reversed selectivity towards the 7 -alcohol. 

In contrast to the enantiomers of the carbinols, e g. HHD or THP, the enantiomers of si-lanols, e.g. sila-procyclidin, racemize quickly in aqueous solution [40] and are therefore unsuitable for the pharmacological investigation of enantioselectivity. 

With the water wheel it is not merely a question of one enantiomer reacting and the other one not. It is, rather, a question of one reacting quickly (kfast) and the other reacting slowly (ksk)W). We will see much more of something called the s value later. This is the selectivity factor and is quite simple s = kfast/ kslow or, in other words, the relative rate. Consider two enantiomers of an alcohol being enantioselectively acetylated by some means. One reacts fast and one more slowly. 

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