Optically pure amino acid production

Table 1 Enzymatic Process for Optically Pure Amino Acid Production...

The diastereoselective alkylation of /V-acyloxazolidinones enolates was examined first. Lithium enolates of 107 were reacted with a variety of alkyl halides, and alkylation products were formed with excellent diastereoselectivities (94-99% de). Hydrolysis gave optically pure carboxylic acids, and the chiral auxiliary was recovered for reuse almost quantitatively.105-106 Highly diastereoselective bromination was also achieved by reaction of the boron enolate of 107 with /V-bromosuccinimide (NBS) (98% de). Optically pure amino acids could be accessed by simple synthetic transformations (Scheme 24.26).106... 

Among the advantages of transaminases as biocatalysts for the production of optically pure amino acids are as follows ... 

Production of optically pure amino acids using their amides as racemic precursors and L-aminopeptidases as catalysts is a well-established commercial process. To overcome the limit of 50% conversion, D-aminopeptidases and amino acid amide racemases were also developed [182]. 

Another group of precursors for the production of optically pure amino acids include their hydantoin derivatives. An intensive search during the past two decades has led to some effective hydantoin racemases which are already used on pilot-scale processes [183-185]. 

Smirnova et al. demonstrated the determination of the insecticide, carbaryl, using a two-chip system. The first chip (for the hydrolysis of carbaryl) had a simple Y-shaped channel while the second chip (for the diazo couphng reaction between hydrolyzed products and 2,4,6-trimethylaniline)—the extraction required special channel shapes with a partial surface— modification obtained by using capillary-restricted modification (CARM) (Figure 35.11). " Determination of carbaryl pesticide in water with sufficient sensitivity was carried out with an analysis time of 8 min. In a similar manner, Honda et al. developed a combination of a tube-type enzyme-immobilized microreactor and a microextractor with partial surface modification to produce optically pure amino acids. 

Pedrazzoli (1957) catalytically reduced (VII) (Fig. 17) to unequal amounts of diastereomeric products which yielded nearly optically pure amino acids when hydrolyzed. Catalytic hydrogenation of the (—)-bornyl ester corresponding to (VII), however, yielded almost equal amounts of enantiomeric amino acids. Also, Yamada et al. (1962) hydrogenated (VIII) and obtained nearly... 

Dihydropyrimidinases (EC 3.5.2.2) are involved in the reductive pathway of pyrimidine degradation, catalyzing the hydrolysis of 5,6-DHU and 5,6-dihydrothymine to the corresponding Namino adds. However, dihydropyrimidinases have been more commonly known as hydantoinases [32, 33], as this enzyme can be used in the production of optically pure amino acids starting from racemic mixtures of 5-monosubstituted hydantoins using the so-called hydantoinase process, ... 

This microreactor enabled a highly enantioselective reaction for a racemic amino acid derivative. The integration of enzyme microreactor (CLEA-CEM) and microextractor provided efficient continuous production of optically pure amino acids. 

Hydantoinase process, outlined in Fig. 1, includes two hydrolases—hydantoin-hydrolyzing enzyme (hydantoinase) and AT-carbamoyl amino acid-hydrolyzing enzyme (carbamoylase)—and is one of the most efficient and versatile methods for the production of optically active a-amino acids. DL-5-Monosubstituted hydantoins, which are used as common precursors for the chemical synthesis of DL-a-amino acids [1], are the starting material of this enzymatic process. Keto-enol tautomerism is a typical feature of the hydantoin structure. Under neutral conditions, the keto form is dominant in alkaline solution, enolization between the 4 and 5 positions can occur, as has been concluded from the fact that optically pure hydantoins readily racemize. This feature is of practical relevance for the complete conversion of racemic hydantoin derivatives to optically pure L- or D-a-amino acids without any chemical racemization step. A variety of hydantoinase and carbamoylase with different stereospecificity were found. They are D-specific hydantoinase (D-hydantoinase), L-specific hydantoinase (L-hydantoinase), none-specific hydantoinase (DL-hydantoinase), D-specific carbamoylase (D-carbamoylase), and L-specific carbamoylase (L-carbamoylase). With the combination of these enzymes, optically pure amino acids are obtained from DL-5-monosubstituted hydantoins (Fig. 2). The wide substrate range of hydantoinases and carbamoylases also gives generality to the hydantoinase process. 

Alkylation of amino acids.1 This base is preferable to LDA for dideprotonation of di-f-butyl N-formylaspartate (1) to give 2. The dianion is alkylated by CH3I, CH2 CHCH2Br, and C6H5CH2Br to give a mixture of / - and a-alkylated aspartate derivatives (3 and 4) in a ratio of ca. 7 2. As expected, the /3-alkylated a-amino acid esters 3 are optically pure surprisingly, the products 4 are also optically active and, after crystallization, can be obtained in optical yields of 60%. 

One example is the optically active amino acid derivative (S)-20n which contains a bipyridyl substituent (Scheme 3.14). The alkylation reaction in the presence of the cinchona alkaloid catalyst 33 proceeds with 53% ee (83% yield of (S)-20n) and gave the desired enantiomerically pure a-amino acid ester (S)-20n in >99% ee after re-crystallization [43]. Subsequent hydrolysis of the optically pure (S)-20n furnished the desired unprotected a-amino acid 35. A different purification method, subsequent enzymatic resolution, reported by Bowler et al., furnished the a-amino acid product 35 with enantioselectivity of 95% ee [44],... 

One of the most widely used enzymatic methods for D-amino acid production is the hydantoinase process [4]. The great advantage of this process is that, potentially, any optically pure D-amino acid can be obtained using the corresponding substrate from a wide spectrum of D,L-5-monosubstituted hydantoins, which are readily accessible by chemical synthesis [5]. In this cascade of reachons the chemically synthesized D,L-5-monosubstituted hydantoin ring is first hydrolyzed by a stereoselective hydantoinase enzyme (D-hydantoinase). Further hydrolysis of the resulting N-carbamoyl D-amino acid to the free D-amino acid is catalyzed... 

Although initially prepared and evaluated as a racemate, the NMDA antagonist activity was likely to reside primarily in a single enantiomer. The stereoselective nature of the NDMA receptor is well established, albeit not completely understood. Consequently, several attempts have been undertaken to develop synthetic protocols that would allow preparation of optically active compounds. Early reports of preparation of optically active co-amino-o-carboxyalkylphosphonic acids describe the preparation of (.S )-A P-3 from an optically active amino nitrile prepared by reaction of diethyl 1-formylphosphonate with hydrogen cyanide and (5)-(-)-a-methylbenzylamine. Acid hydrolysis, enrichment of the diastereomers by fractional recrystaUization, and debenzylation lead to the isolation of (.S )-A P-3 in 86% enantiomeric excess. " Recently reported procedures, which use chemoenzymatic processes, offer a more convenient and mild approach for the production of optically pure aminophosphonic acids. Enzymatic hydrolysis of amides using penicillinacylase (EC... 

Among the various enzymes capable of producing optically-active amino acids, transamination reactions, catalyzed by enzymes known as aminotransferases or transaminases, have broad potential for the synthesis of a wide variety of enantio-merically pure (R)- and (S)-compounds containing amine groups. Indeed, various examples of the use of aminotransferases for the production of d- and L-amino acids, both naturally-occurring and non-natural, have been published17 151. In addition, certain aminotransferases have been found to act on amines, and methods for the production of enantiomerically pure amines by transamination have been described116-211. This method allows for yields of up to 100% whereas routes based on hydrolases require external racemization to reach such yield levels. In this section we will focus on the application of aminotransferases. 

One product only can result when the two components consisting of the pure optically active amino acids are combined together, e.g.—... 

All biotechnological processes depend on enzymes - either in whole-cell living systems or isolated out of their biological context. In fact, purified enzymes are established in processing food and textiles and are supplements in feed and detergents - all products of everybody s daily life. Whole-cell systems particularly in the field of specialty chemicals provide a broad range of methods and processes. For many years, enantiomerically pure amino acids for food, feed and pharma industries have been produced by microbial fermentation. Ambitious R D resulted in enzymes especially modified and optimized for a desired chemical reaction such as biocatalysis of optically active amines, alcohols, epoxides and more. ... 

Enantioselective cleavage of non-peptide amide bonds is also important in the production of optically active amino acids (Scheme 3.12). Carboxy-peptidases often are the enzymes of choice in this area of work these enzymes catalyse the hydrolysis of an amide function which is close to a carboxylic acid group. The rate of hydrolysis is usually increased if R (Scheme 3.12) is an aromatic unit or a large aliphatic moiety. For example, thrco-jS-phenylserine R = PhCH(OH) has been resolved by incubation of the racemic JV-trifluoroacetate with carboxypeptidase-A, with the optically pure (L)-enantiomer being obtained in a good yield. 

Clemente-Jimenez, J.M., Martinez-Rodriguez, S., Rodriguez-Vico, F., and Heras-Vazquez, F.J. (2008) Optically pure alpha-amino acids production by the Hydantoinase Process . Recent Patents Biotechnol., 2 (1), 35 46. 

Only about 20% of the optically active pharmaceuticals are sold as pure enantiomers (6). This has resulted in an increasing interest in stereoselective syntheses based on chiral intermediates. The production of these so-called auxiliaries ultimately requires enantiomerically pure natural substances, with optically active amino acids playing an important part as chiral pool. Consequently, efficient analytical procedures for control of optical purity are needed to supplement modem procedures for asymmetric syntheses. 

In optically pure a-amino acid production from DL-5-monosubstituted hydantoins, the wide applicability to a broad substrate range is valuable especially for the production of D-a-amino acids [70] and unnatural L-a-amino acids (Fig. 9), e.g., D-p-hydroxyphen-ylglycine [71], D-phenylglycine [71], substituted L-phenyManine such as L-p-chlorophen-ylalanine [72] and p-trimethylsilylphenylalanine [73,74], L-a- and p-naphthylalanine [75], an A -methyl-D-aspartate receptor antagonist, (2R, 4R, 55)-2-amino-4,5-(l,2-cyclohexyl)-7-... 

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