Kinetic resolutions dynamic resolution

Kinetic Resolution, Dynamic Kinetic Resolution, and Desymmetrization... 

Asymmetric synthesis can refer to any process which accesses homochiral products. We will focus on asymmetric synthesis from racemic or prochiral starting materials in the presence of an enantioselective catalyst (enzyme). There are four general methodologies commonly applied kinetic resolution, dynamic kinetic resolution, deracemization and... 

Conventional Kinetic Resolution Dynamic Kinetic Resolution... 

Figure 1.3 Kinetic and dynamic resolution of racemic substrates...

The asymmetric processes discussed so far in this chapter have focused on reactions that create non-racemic, chiral products from achiral reagents by selective reaction at one prochiral face or position over the other. However, these principles can also be applied to reactions that separate enantiomers of an existing racemic mixture, channel both enantiomers of such a mixture to a single enantiomeric product, or that select between reaction at one of two diastereotopic functional groups in an achiral substrate. These reactions are also synthetically valuable and are called kinetic resolutions, dynamic kinetic resolutions, and desymmetrizations. An understanding of these reactions draws from the principles established so far in this chapter, but they also require some additional principles to be established that apply in a specific way to these classes of asymmetric transformations. Thus, the remainder of Chapter 14 introduces the fundamentals of these classes of asymmetric catalysis. 

Example of Dynamic Kinetic Resolutions Dynamic Kinetic Resolution of 1,3-Dicarbonyl Compounds Through Asymmetric Hydrogenation... 

Kinetic and Dynamic Resolution of rac-Nitriles. a-Hydroxy and a-amino acids can be obtained from the corresponding a-hydroxynitriles (cyanohydrins) and a-aminonitriles [684], which are easily synthesized in racemic form from the corresponding aldehyde precursors by addition of hydrogen cyanide or a Strecker synthesis, respectively (Schemes 2.107 and 2.108). In aqueous systems, cyanohydrins are stereochemically labile and undergo spontaneous racemization via HCN elimination, which furnishes a dynamic resolution process. From aliphatic rac-cyanohydrins, whole cells of Torulopsis Candida yielded the corresponding (S)-a-hydroxy acids [685], while (R)-mandelic add is produced from rac-mandelraiitrile... 

Application of Enzymes in Kinetic Resolutions, Dynamic Kinetic Resolutions and Deracemization Reactions... 

Other principal techniques for the preparation of one special enantiomer are kinetic resolution, dynamic kinetic resolution, and dynamic kinetic asymmetric transformations. 

Paizs C, Tahtinen P, Lundell K, Poppe L, Irimie F-D, Kanerva LT. Preparation of novel phenylfuran-based cyanohydrin esters hpase-catalysed kinetic and dynamic resolution. Tetrahedron Asymm. 2003 14 1895-1904. 

Scheme 9.20 Dynamic kinetic resolution of racemic epoxide...

Hydantoinases belong to the E.C.3.5.2 group of cyclic amidases, which catalyze the hydrolysis of hydantoins [4,54]. As synthetic hydantoins are readily accessible by a variety of chemical syntheses, including Strecker reactions, enantioselective hydantoinase-catalyzed hydrolysis offers an attractive and general route to chiral amino acid derivatives. Moreover, hydantoins are easily racemized chemically or enzymatically by appropriate racemases, so that dynamic kinetic resolution with potential 100% conversion and complete enantioselectivity is theoretically possible. Indeed, a number of such cases using WT hydantoinases have been reported [54]. However, if asymmetric induction is poor or ifinversion ofenantioselectivity is desired, directed evolution can come to the rescue. Such a case has been reported, specifically in the production of i-methionine in a whole-cell system ( . coli) (Figure 2.13) [55]. 

Figure 5.1 Schematic illustration of (a) dynamic kinetic resolution, (b) deracemization, and (c) enantioconvergent processes.

Figure 6.1 Kinetic resolution k ac — ) dynamic kinetic resolution (krac ks)-...

Figure 6.30 Nitrilase-catalyzed dynamic kinetic resolution of cyanohydrins.

Figure 6.38 Dynamic kinetic resolution of amino acid amides.

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