Enantioselection origin

The simplest enantioselective process is the reaction of a racemic substrate with two identical substituents at the allylic termini (Scheme 10). Here, the enantioselectivity originates from regioselective nucleophilic attack at the two enantiotopic termini of the allyl system. If a C2-symmetric ligand is used, only one allyl-Pd intermediate has to be considered (if we disregard syn-anti isomerism) and an analysis of the possible regioselectivity-determining factors becomes relatively straightforward. 

Second, Sigamide was attached to Merrifield resin A 6. The polymer bound catalyst needs higher catalyst loadings (15mol%) and shows approx. 10% lower yields and selec-tivities. On the other hand, they also showed that the catalyst can be reused five times without any loss in activity. Performing blank experiments with only the resin, it was found that the lower enantioselectivities originate from the... 

Clearly, complete understanding of solvent effects on the enantioselectivity of Lewis-acid catalysed Diels-Alder reactions has to await future studies. For a more detailed mechanistic understanding of the origins of enantioselectivity, extension of the set of solvents as well as quantitative assessment of the strength of arene - arene interactions in these solvent will be of great help. 

Some of the developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds have origin in Diels-Alder chemistry, where many of the catalysts have been applied. This is valid for catalysts which enable monodentate coordination of the carbonyl functionality, such as the chiral aluminum and boron complexes. New chiral catalysts for cycloaddition reactions of carbonyl compounds have, however, also been developed. 

In recent years, together with enantioselective analysis, the determination of the natural abundance of stable isotopes by means of stable isotope ratio mass spectrometry (TRMS) can be very useful for the assignment of the origin of foods and food ingredients, and of authenticity evaluation (24). 

Enantioselective transport processes can be achieved either with solid or liquid membranes (Fig. 1-5). In this latter case, the liquid membrane can be supported by a porous rigid structure, or it can simply be an immiscible liquid phase between two solutions with the same character (aqueous or nonaqueous), origin and destination... 

Over the years, the original dihydroxylation procedure has been modified to operate catalytically, more rapidly, and in better yield. However, the last remaining task, making the dihydroxylation of a prochiral olefin enantioselective without losing all the other desirable features of the reaction, has only recently become a reality. 

Figure 2.9 Schematic summary of the directed evolution of enantioselective lipase variants originating from the WT PAL used as catalysts in the hydrolytic kinetic resolution of ester rac-1. CMCM = Combinatorial multiple-cassette mutagenesis [8c,22],...

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

Both cis- and trans-chrysanthemic nitriles and amides were resolved into highly enantiopure amides and acids by Rhodococcus sp. whole cells [85]. The overall enantioselectivity of reactions of nitriles originated from the combined effects of a higher (lJ )-selective amidase and a (IJ )-selective nitrile hydratase (Figure 6.29). Chrysanthemic acids are related to constituents of pyrethrum flowers and insecticides. 

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