Cyclic Deracemizations

To confirm this racemization mechanism, Crawford et al. added 5 mol% of the ketone to the reaction mixture and obtained the product in 78% yield in 98% . This DKR is therefore catalyzed by a carbonyl compound and can be compared to those shown in Section 5.2.4. 

In Section 5.3.1, chemoenzymatic cyclic deracemizations or stereoinversions are categorized according to the type of substrate, namely amino acids, hydroxy acids, 

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Figure 5.2 Cyclic deracemization process involving sequential enzyme-catalyzed oxidation and nonenzymatic reduction.

Figure 5.3 Progression of an ideal cyclic deracemization process.

Biocatalytic Deracemization Dynamic Resolution, Stereoinversion, Enantioconvergent Processes and Cyclic Deracemization, in Biocatalysts in the Pharmaceutical and Biotechnology industries, (ed. R.N. Patel), CRC Press, Boca Raton, pp. 27-51. 

During the past few years great efforts have been made to overcome the 50% threshold of enzyme-catalyzed KRs. Among the methods developed, deracemization processes have attracted considerable attention. Deracemizations are processes during which a racemate is converted into a non-racemic product in 100% theoretical yield without intermediate separation of materials [5]. This chapter aims to provide a summary of chemoenzymatic dynamic kinetic resolutions (DKRs) and chemoenzymatic cyclic deracemizations. 

It should be mentioned that the great majority of dynamic kinetic resolutions reported so far are carried out in organic solvents, whereas all cyclic deracemizations are conducted in aqueous media. Therefore, formally, this latter methodology would not fit the scope of this book, which is focused on the synthetic uses of enzymes in non-aqueous media. However, to fully present and discuss the applications and potentials of chemoenzymatic deracemization processes for the synthesis of enantiopure compounds, chemoenzymatic cyclic de-racemizations will also be briefly treated in this chapter, as well as a small number of other examples of enzymatic DKR performed in water. 

This approach transforms a racemic compound into the enantiomerically pure form of the same compound. Cyclic deracemizations have been achieved for... 

An important breakthrough was made very recently in this area. A chemoenzymatic method developed by Turner has allowed the cyclic deracemization of tertiary amines [80]. Enantiopure tertiary amines cannot be obtained via DKR. One of the variants obtained by directed evolution of the monoamine oxidase from Aspergillus niger showed high activity and enantioselectivity toward cyclic tertiary amines (Scheme 5.40). 

Simeo Y, KrontU W, Paber K. Biocatalytic deracemization dynamic resolution, stereoinversion, enantioconvergent processes, and cyclic deracemization. In Biocatalysis in the Pharmaceutical and Biotechnology Industries, Ed. Patel RN. CRC Press, Boca Raton, PL, 2007, p. 27. 

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