Amino Acids via Enzymatic Resolutions

In the following discussions the individual biocatalytic methods applied in the production of L-amino acids are described in more detail. 

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Scheme 2.15 Enzymatic resolution of iV-acyl amino acids via the acylase method...

Servi, S., Viani, F., and Tessaro, D. (2012) Naphthyl-L-a-amino acids via chemo-enzymatic dynamic kinetic resolution. Tetrahedron Asymmetry, 23 (13), 938-944. 

The product of a NHase/amidase cascade reaction is an acid, which is the same as the single enzymatic reaction performed by a nitrilase. However, the NHases usually have different substrate specificities than nitrilases, making them more suitable for the production of certain compounds. Although most organisms have both NHase and amidase activity (see earlier text), it is sometimes preferable, in a synthetic application, to combine enzymes from different organisms. The reasons for this are the enantioselectivity of the amidase or specific activity or substrate specificity of either of the enzymes. In this way, products with different enantiomeric purity can be obtained. Recently, a coupling of a NHase with two different amidases with opposite enantiopreference together with an -amino-a-caprolactam racemase that allows the formation of small aliphatic almost enantiopure (R)- or (S)-amino acids via dynamic kinetic resolution processes has been described [52]. 

Finally, their main use remains the diastereoselective synthesis of p-hydroxy-a-amino-acids via two possible strategies, an enzymatic asymmetric synthesis or an enzymatic resolution (deracemisation by cleavage of the undesired enantiomer). ... 

We prepared the (5)-enantiomer of cericlamine via enzymatic resolution of a-methyl-3,4-dichlorophenylalanine amide (28) and subsequent reduction of the amino acid as described in Scheme 11 [61]. Racemic a-methyl-3,4-dichlorophenylalanine amide (28) is prepared by phase transfer-catalyzed benzylation of the benzaldehyde Schiff base of alanine amide (27) followed by acidic workup. Since this substrate is nearly insoluble in water, the enzymatic resolution using the amidase from O. anthropi NCIMB 40321 is performed at pH 5.3. At this pH there is a sufficient amount of the substrate in solution and still approximately 50% of the amidase activity left to allow the enzymatic hydrolysis... 

The main application of the enzymatic hydrolysis of the amide bond is the en-antioselective synthesis of amino acids [4,97]. Acylases (EC 3.5.1.n) catalyze the hydrolysis of the N-acyl groups of a broad range of amino acid derivatives. They accept several acyl groups (acetyl, chloroacetyl, formyl, and carbamoyl) but they require a free a-carboxyl group. In general, acylases are selective for i-amino acids, but d-selective acylase have been reported. The kinetic resolution of amino acids by acylase-catalyzed hydrolysis is a well-established process [4]. The in situ racemization of the substrate in the presence of a racemase converts the process into a DKR. Alternatively, the remaining enantiomer of the N-acyl amino acid can be isolated and racemized via the formation of an oxazolone, as shown in Figure 6.34. 

Another interesting example of resolution through formation of diastereo-mers is the isolation of four stereoisomers of 3-amino-2-methyl-3-trifluoro-methyl butanoic acid [55]. In this process, the chemical-enzymatic method by the combination of chemical and enzymatic reaction is a very convenient. At first, -phenylacetyl derivatives 61a and 61b were prepared in excellent isolated yields via the Schotten-Baumann procedure. After these materials were hydrolysed with penicillin acylase (EC 3.5.1.11) from Escherichia coli until attainment of 50% conversion, enzymatically unconverted -phenylacetyl derivatives 62 a and 62 b (organic layer) and amino acids 63 b and 63 d (aqueous layer) were separated. Acidic hydrolysis of unconverted materials produced other stereoisomers 63 a and 63 c in high optical pure form. 

This eoncept has been known for a long time in pure enzymatic synthesis, e.g. amino acid synthesis via hydantoins [1] or oxazolidinones [2]. Cyanohydrins [3] and lactols [4] are prone to in situ racemization as well and may serve as substrates in kinetic resolutions. 

The enantioselective synthesis of 1 has also been achieved by a number of methods including enzymatic resolution of a keto ester precursor to the racemate followed by conversion of the ester to an amino group, enzymatic resolution of an amino azide precursor followed by reduction, enzymatic resolution through 0-acylation of a racemic A -benzylcarbamate derivative of and the resolution via the formation of an amide with a homochiral amino acid. Bioconversion of in-... 

Enzymatic resolution has been successfully applied to the preparation of optically active gem-difluorocyclopropanes (see Scheme 12.4). We succeeded in the first optical resolution of racemic gm-difluorocyclopropane diacetate, trans-43, through lipase-catalyzed enantiomer-specific hydrolysis to give (R,R)-(-)-44 with >99% ee (see equation 9, Scheme 12.4) [4a], We also applied lipase-catalyzed optical resolution to an efficient preparation of monoacetate cw-46 from prochiral diacetate m-45 (see equation 10, Scheme 12.4) [4a], Kirihara et al. reported the successful desymmetrization of diacetate 47 by lipase-catalyzed enantiomer-selective hydrolysis to afford monoacetate (R)-48, which was further transformed to enantiopure amino acid 15 (see equation 11, Scheme 12.4) [19]. We demonstrated that the lipase-catalyzed enantiomer-specific hydrolysis was useful for bis-gem-difluorocyclopropane 49. Thus, optically pure diacetate (R,S,S,R)-49 and (S,R,R,S)-diol 50, were obtained in good yields, while meso-49 was converted to the single monoacetate enantiomer (R,S,R,S)-51 via efficient desymmetrization (see equation 12, Scheme 12.4) [4b, 4e], Since these mono- and bis-gm-difluorocyclopropanes have two hydroxymethyl groups to modify, a variety of compounds can be prepared using them as building blocks [4, 22],... 

The enantioselective hydrolysis of racemic N-acetylated a-amino acids d,l-1 at De-gussa represents a long established large-scale process for the production of L-ami-no acids, l-2 [4]. This enzymatic resolution requires an L-aminoacylase as the biocatalyst. The starting materials for this process are readily available, since racemic N-acetyl amino acids d,l-1 can be economically synthesized by acetylation of racemic a-amino acids with acetyl chloride or acetic anhydride under alkaline conditions via the so-called Schotten-Baumann reaction [5]. The enzymatic resolution reaction of N-acetyl d,L-amino acids, d,l-1, is achieved by a stereospecific L-aminoacylase which hydrolyzes only the L-enantiomer and produces a mixture of the corresponding L-amino acid, l-2, acetate, and N-acetyl D-amino acid, d-1 (Fig. 4) [6],... 

Enzymatic Resolution of Racemic a-Methyl Phenylalanine Amides. The chiral amino acids (22) and (23) (Fig. 6A) are intermediates for the synthesis of (33-receptor agonists (30,31). These are available via the enzymatic resolution of racemic a-methyl phenylalanine amide (24) and a-methyl-4-methoxy-phenylalanine amide (25), respectively, by an amidase from Mycobacterium neoaurum ATCC 25795 (32). Wet cells (10% wt/vol) completed the reaction of amide (24) in 75 min with a... 

The recycling of the undesired enantiomer from the enzymatic resolution is of crucial importance particularly on an industrial scale [107]. The classical chemical method consists of the thermal racemization of an amino acid ester at about 150-170°C. Milder conditions can be employed for the racemization of the corresponding amides via intermediate formation of Schiff bases with aromatic aldehydes such as benzaldehyde or salicylaldehyde (Scheme 2.14). More recently, intense research has been devoted to the use of isomerase enzymes (such as amino acid racemases [108]) aiming at the development of dynamic resolution processes. 

R = alkyl or aryl R = short-chain alkyl R = H or acyl Scheme 2.11 Enzymatic resolution of amino acid esters via the esterase method... 

Scheme 2.14 Enzymatic resolution of amino acid amides via the amidase method...

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