Deracemization of racemic amines

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.

Figure 14.34 Deracemization of racemic amines by repeated cycles of enzyme catalyzed enantioselective oxidation followed by nonselective chemical reduction.

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]. 

The s)mthesis of all kinds of amino acids has been studied extensively in the last decades. Due to enhanced enzymatic and pharmacodynamic stability, as well as their diverse structures and biochemical properties, amino acids were maintained as chiral building blocks in numerous peptidomimetics, in single-enantiomer drugs, and also in various fields of agriculture [79,80]. Due to the wide substrate specificity of transaminases, these enzymes are suitable as biocatalysts for the amination of keto acids or deracemization of racemic amines to produce enantiopure amino acids [2,54,81]. 

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. 

With morphinan 20 in hand, the stage was set for the deracemization and functionalization of the D ring of the alkaloid. Resolution of racemic amine 20 with dibenzoyl tartrate (Scheme 2) afforded the isomer with correct configuration at C9 and Cl3 but epimeric at Cl4. The identity of the synthetic material was unambiguously confirmed... 

Scheme 11.13 Deracemization of racemic amino acids by cascade reactions, including DAAOs-catalyzed reactions and reductive amination reactions catalyzed either by amino acid dehydrogenases (a) or transaminases (b).

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. 

The deracemization of a number of pharmaceutically valuable building blocks has been carried out by biocatalytic processes. This includes epoxides, alcohols, amines and acids. DKR involves the combination of an enantioselective transformation with an in situ racemisation process such that, in principle, both enantiomers of the starting material can be converted to the product in high yield and ee. The racemization step can be catalysed either enzymatically by racemases, or non-enzymatically by transition metals. 

The method is of general applicability in the deracemization of secondary alcohols and amines and consists of a Upase-catalyzed irreversible acylation and in situ racemization of the non-reacted enantiomer catalyzed by a ruthenium catalyst. 

The synthesis of optically pure L-phenylglycine via the deracemization of mandelic acid was reported via three steps (racemization, enantioselective oxidation and stereoselective reductive amination). Racemization by mandelate racemase combined with simultaneous oxidation and reduction reactions with cofactor recycling gave the amino acid in 97% ee and 94% yield (Scheme 4.43) [96]. 

The asymmetric amination of ketones is by far the most preferred approach for the preparahon of chiral amines using co-TAs. However, alternative methodologies may be considered if the carbonyl precursor is unstable or the synthesis of the racemic amine is easier to provide better results in economic and/or yield terms. Using racemic amines deracemization strategies allow the preparation of the desired amines in enantiopme form and a theoretical 100% yield [84- ]. This can be achieved by the combination of two stereocomplementary w-TAs. In the first step, the enantioselec-tive deamination of the racemic amine affords enantiopure untouched amine (50%) and the corresponding ketone (50%). In the second step, an enantiocomplementary -TA catalyzes the asymmetric amination of the ketone, leading to the optical pure amine in 100% theoretical yield (Scheme 2.18). 

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... 

Recently, Turner et al. have shown tertiary amines can also be used as substrates by using a further variant of MAO-N (MAO-N-D5) which had also been developed by directed evolution techniques (Scheme 2.33). For example, racemic N-methyl-pyrrolidine 65 was subjected to deracemization, via the intermediate iminium ion... 

The term deracemization covers reactions in which two enantiomers are inter-converted by a stereoinversion process such that a racemate can be transformed to a non-racemic mixture without any net change in the composition of the molecule. Deracemization reactions usually involve a redox process, for example, the interconversion of chiral secondary alcohols via the ketone or alternatively the interconversion of amino acids/amines via the corresponding imine (Scheme 4.37). 

Other routes to enantiomerically-pure amines have been put forward. Knowing that CAL-B would selectively acylate just the R enantiomer of the racemic a-methyl primary amine 22, Stephane Gastaldi, Gerard Gil and Michele P. Bertrand of the Universite Paul Cezanne devised (Organic Lett. 2007, 9, 837) a thiyl-based method for equilibrating 22, leading to a net deracemizing acylation. 

For the preparation of 2-naphthyl alanine, a one-pot, two-step enzyme cascade was invented with d-AAO from Rhodotorula gracilis and an L-aminotransferase from E. coli [50]. Detail information about the deracemization steps was provided in Section 29.3.1 and shown in Scheme 29.3. Starting with the racemic 2-naphthyl alanine, the AAO generated the corresponding a-keto acid, which served as rfie substrate in the reductive amination step to enantiopure 2-naphthyl alanine. An irreversible amino donor, cysteine sulfinic acid, was used. 

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