Deracemizations

A number of previous reviews [2-6] have dealt with both deracemization and enantioconvergent processes and hence this chapter will focus primarily on the recent literature. 

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

Assuming that the enzymatic reaction is highly enantioselective, then even after only four cycles the enantiomeric excess will have reached 93.4% whereas after seven catalytic cycles the enantiomeric excess is >99% (Figure 5.3). This type of deracemization is really a stereoinversion process in that the reactive enantiomer undergoes stereoinversion during the process. One of the challenges of developing this type of process is to find conditions under which the enzyme catalyst and chemical reactant can coexist, particularly in the case of redox chemistry in which the coexistence of an oxidant and reductant in the same reaction vessel is difficult to achieve. For this... 

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

Figure 5.3 Progression of an ideal cyclic deracemization process.

Figure 5.4 Deracemization of a-amino acids using D-amino acid oxidase in combination with sodium borohydride.

Recently Turner and coworkers have sought to extend the deracemization method beyond a-amino acids to encompass chiral amines. Chiral amines are increasingly important building blocks for pharmaceutical compounds that are either in clinical development or currently licensed for use as drugs (Figure 5.7). At the outset of this work, it was known that type II monoamine oxidases were able to catalyze the oxidation of simple amines to imines in an analogous fashion to amino acid oxidases. However, monoamine oxidases generally possess narrow substrate specificity and moreover have been only documented to catalyze the oxidation of simple, nonchiral... 

Figure 5.8 Deracemization of a-methylbenzylamine by using a variant monoamine oxidase in combination with NH3 BH3.

Subsequently Turner and coworkers were able to show that the Asn336Ser variant possessed broad substrate specificity, with the ability to oxidize a wide range of chiral amines of interest [19]. They also discovered a second mutation, Ile246Met, which conferred enhanced activity toward chiral secondary amines as exemplified by the deracemization of racemic 1-methyltetrahydroisoquinoline (MTQ) (9) (Figure 5.9)[20j. 

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

In a related approach, Adam ef al. used glycolate oxidase with D-lactate dehydrogenase for the deracemization of a wide range of racemic a-hydroxy acids (20) (Figure 5.13) [23]. 

Microbial Deracemization of Secondary Alcohols Using a Single Microorganism... 

It is well known that certain microorganisms are able to effect the deracemization of racemic secondary alcohols with a high yield of enantiomerically enriched compounds. These deracemization processes often involve two different alcohol dehydrogenases with complementary enantiospedficity. In this context Porto ef al. [24] have shown that various fungi, induding Aspergillus terreus CCT 3320 and A. terreus CCT 4083, are able to deracemize ortho- and meta-fluorophenyl-l-ethanol in good... 

Chadha et al, have published a series of papers on the deracemization of P-hydroxyesters using whole cells of Candida parapsilosis. For example, deracemization of racemic ethyl 2-hydroxy-4-phenylbutanoic acid (22 R = H) yielded the (S) enantiomer in 85-90% yield and >99% ee (Figure 5.15) [26]. 

Likewise, C. parapsilosis was investigated for substrate tolerance in the deracemization reactions with aryl a-hydroxy esters (23) (Figure 5.16) [29]. A range of... 

Figure 5.15 Deracemization of (3-hydroxyesters using Candida parapsilosis.

Carnell et al. discovered that whole cells of Cunninghamella echinulata NRRL1384 were able to deracemize racemic N-(l-hydroxy-l-phenylethyl)benzamide (24) to produce the (R) enantiomer (Figure 5.17) [30]. The deracemization involves fast, highly (S)-selective oxidation, followed by slower, partially (R)-selective reduction of the ketone (25). Optimization by removing competing extracellular amidase/prote-ase activity resulted in 82% yield and 92% ee. 

Deracemization of Alcohols Using Two Enzyme/Microorganism Systems... 

An alternative approach to the microbial deracemization of secondary alcohols is to use two different microorganisms with complementary stereoselectivity. Fantin et al. studied the stereoinversion of several secondary alcohols using the culture supernatants of two microorganisms, namely Bacillus stearothermophilus and Yarrowia lipolytica (Figure 5.18) [31]. The authors tested three main systems for deracemization. First, they used the supernatant from cultures of B. stearothermophilus, to which they added Y. lipolytica cells and the racemic alcohols. Secondly, they used the culture supernatant of Y. lipolytica and added B. stearothermophilus cells and the racemic alcohols. Finally, they resuspended the cells of both organisms in phosphate buffer and added the racemic alcohols. The best results were obtained in the first system with 6-penten-2-ol (26) (100% ee and 100% yield). The phosphate buffer system gave... 

Figure 5.17 Deracemization of alcohol (24) using Cunninghamella echinulata.

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