Racemic amines amine oxidases

Figure 11.1 Enzymatic deracemization of racemic amines via a two-step, one-pot process utilizing an enantioselective amine oxidase in combination with ammonia-borane.

An elegant four-enzyme cascade process was described by Nakajima et al. [28] for the deracemization of an a-amino acid (Scheme 6.13). It involved amine oxidase-catalyzed, (i )-selective oxidation of the amino acid to afford the ammonium salt of the a-keto acid and the unreacted (S)-enantiomer of the substrate. The keto acid then undergoes reductive amination, catalyzed by leucine dehydrogenase, to afford the (S)-amino acid. NADH cofactor regeneration is achieved with formate/FDH. The overall process affords the (S)-enantiomer in 95% yield and 99% e.e. from racemic starting material, formate and molecular oxygen, and the help of three enzymes in concert. A fourth enzyme, catalase, is added to decompose the hydrogen peroxide formed in the first step which otherwise would have a detrimental effect on the enzymes. 

Truppo, M.D., N.J. Turner, and J.D. Rozzell, Efficient kinetic resolution of racemic amines using a transaminase in combination with an amino acid oxidase. Chem. Commun., 2009(16) 2127-2129. 

Chiral amines have been attracting attention as an important composition, particularly for pharmaceutical products. The organic synthetic methods of optically active amine compounds have been developed through the traditional resolution of racemic amines with the formation of diastereomer salts using an optically active mandelic acid or tartaric acid. Enzymatic synthesis has mainly used lipase and S- or R-stereoselective amine transaminase (AT) [29-31] (Figure 19.7). Turner et al. successfully synthesized chiral (R)- and (S)-amines by kinetic resolution using a combination of stereoselective AT and d- or L-amino acid oxidase (AAOx) [32] (Figure 19.7). However, the theoretical yield of the products has been limited to 50% in the kinetic resolution. 

Turner et al. first reported engineered S-stereoselective flavin-dependent monoamine oxidase variants bom Aspergillus niger for the deracemization of racemic amines producing the S-enantiomers of primary, secondary, and tertiary amines (Figure 19.8) [35]. On the other hand, Leisch et al. showed that wild-type cyclohexy-lamine oxidase from Brevibacterium oxydans IH-35A, which had been isolated and characterized by Hasegawa et al., oxidized the S-enantiomer of amines and successfully synthesized (f )-amine through a deracemization reaction [37]. 

Asymmetric synthesis of amines is a synthetically important research area due to the broad range of applications of chiral amines in fhe field of pharmaceuticals [134]. Amine oxidases are a versatile class of catalysts, which turned out to be very suitable for the preparation of chiral amines by means of desymmetrization or deracemization reactions. With respect to the latter, a racemic amine is enantioselectively oxidized by the amine oxidase, and simultaneously the in situ-formed imines or iminium ions, respectively, are reduced in a nonenantioselective chemical reduction process (with typically a borohydride) back to the racemic amine [135]. The key catalyst for such a... 

L-6-Hydroxynorleucine, a different key chiral intermediate used for synthesis of the vasopeptidase inhibitor Omapatrilat (Vanlev ), was prepared in 89% yield and > 99% optical purity by reductive amination of 2-keto-6-hydroxyhexanoic acid using glutamate dehydrogenase from beefliver (Hanson, 1999) (Figure 13.22). In an alternative process, racemic 6-hydroxynorleucine produced by hydrolysis of 5-(4-hydroxybutyl)hydantoin was treated with D-amino acid oxidase to prepare a mixture containing 2-keto-6-hydroxyhexanoic acid and L-6-hydroxynorleucine followed by the reductive amination procedure to convert the mixture entirely to L-6-hydroxynorleucine, with yields of 91-97% and optical purities of > 99%. 

A modification of this reaction concerns the availability of the keto acid substrate. To circumvent its complicated lengthy chemical synthesis, 2-keto-6-hydro-xyhexanoic acid was synthesized by treatment of racemic 6-hydroxynorleucine with D-amino acid oxidase and catalase (Fig. 37). The production of racemic 6-hydroxynorleucine occurs by hydrolysis from 5-(4-hydroxybutyl)hydantoin. d-Amino acid oxidase converts the D-enantiomer of racemic 6-hydroxynorleucine to the corresponding ketoacid which is reductively aminated to l 6-hydroxynorleucine by GluDH. 

Chemical synthesis and isolation of 2-keto-6-hydroxyhexanoic acid required several steps. In a second, more convenient process (shown in Scheme 17.4), the ketoacid was prepared by treatment of racemic 6-hydroxynorleucine (produced by hydrolysis of commercially available 5-(4-hydrox-ybutyl) hydantoin) with D-amino acid oxidase and catalase. After the ee of the remaining L-6-hydroxynorleucine had risen to >99%, the reductive amination procedure was used to convert the mixture containing 2-keto-6-hydroxyhexanoic acid and L-6-hydroxynorleucine entirely to... 

Deracemizatlon of racemic mexiletine (di-o-methyl-phenoxyisopropylamine) is successfully achieved in a simple two-step procedure. This work extends the utility of the TA to include deracemizatlon of amines and the asymmetric synthesis of ketones to bypass the limitations of kinetic resolution. The cofactor is recycled by amino acid oxidase [73] in the first deracemizatlon step and by dehydrogenases in the second asymmetric synthesis step [47a,74]. In addition, both enantiomers of mexiletine can be obtained by simply switching the order of the addition of stereospecific TAs. 

In a second, more convenient process, the keto acid was prepared by the treatment of racemic 6-hydroxynorleucine 10 (produced by the hydrolysis of 5-(4-hydroxybutyl) hydantoin) with (R)-amino acid oxidase (Figure 4.3). After ee of the unreacted (S)-6-hydroxynorleucine had reached 99.8%, the reductive amination procedure was used to produce (S)-6-hydroxynorleucine at 97% yield with 99.8% ee from racemic 6-hydroxynorleucine at lOOg/1 substrate concentration [34]. The (S)-6-hydroxynorleucine prepared by the enzymatic process was converted chemically to Vanlev 7 [35]. 

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