Reductive amination keto-acids

Some workers avoid delay. Pai)adium-on-carbon was used effectively for the reductive amination of ethyl 2-oxo-4-phenyl butanoate with L-alanyl-L-proline in a synthesis of the antihyperlensive, enalapril maleate. SchifTs base formation and reduction were carried out in a single step as Schiff bases of a-amino acids and esters are known to be susceptible to racemization. To a solution of 4,54 g ethyl 2-oxO 4-phenylbutanoate and 1.86 g L-alanyl-L-proline was added 16 g 4A molecular sieve and 1.0 g 10% Pd-on-C The mixture was hydrogenated for 15 hr at room temperature and 40 psig H2. Excess a-keto ester was required as reduction to the a-hydroxy ester was a serious side reaction. The yield was 77% with a diastereomeric ratio of 62 38 (SSS RSS)((55). 

Yet a third method for the synthesis of a-amino acids is by reductive amination of an a-keto acid with ammonia and a reducing agent. Alanine, for instance, is prepared by treatment of pyruvic acid with ammonia in the presence of NaBH As described in Section 24.6, the reaction proceeds through formation of an intermediate imine that is then reduced. 

Amino acids can be synthesized in racemic form by several methods, including ammonolysis of an a-bromo acid, alkylation of diethyl acetamido-malonate, and reductive amination of an cv-keto acid. Alternatively, an enantio-selective synthesis of amino acids can be carried out using a chiral hydrogenation catalyst. 

L)-Phosphinotricin 67, which is the active component of naturally occurring antibiotic biolaphos, was synthesized from the corresponding keto acid 66 via reductive amination catalysed by L-glutamate dehydrogenase (EDH) (Equation 32)7 ... 

From the very successful developments of the alcohol dehydrogenase technology for production of secondary alcohols and enzymatic reductive amination of keto-acids for production of amino acids, it is expected that we will also soon see applications for other enzymatic redox chemistries for example, reduction of unsaturated carbonyl compounds with... 

Recently, Borner and coworkers described an efficient Rh-deguphos catalyst for the reductive amination of a-keto acids with benzyl amine. E.e.-values up to 98% were obtained for the reaction of phenyl pyruvic acid and PhCH2COCOOH (entry 4.9), albeit with often incomplete conversion and low TOFs. Similar results were also obtained for several other a-keto acids, and also with ligands such as norphos and chiraphos. An interesting variant for the preparation of a-amino acid derivatives is the one-pot preparation of aromatic a-(N-cyclohexyla-mino) amides from the corresponding aryl iodide, cyclohexylamine under a H2/ CO atmosphere catalyzed by Pd-duphos or Pd-Trost ligands [50]. Yields and ee-values were in the order of 30-50% and 90 >99%, respectively, and a catalyst loading of around 4% was necessary. 

The investigation of the aminotransferase activity of apple ACS carried out by Feng et al reveals that it is able to reductively aminate PLP to PMP by transamination of some L-amino acids to their corresponding a-keto acids. The enzyme has shown substrate specificity with the preference of Ala > Arg > Phe > Asp. The addition of excess pyruvate causes a conversion of the PMP form of the enzyme back to the PLP form. The quite unstable PMP form of ACS can generate apoenzyme, which captures PLP to restore its physiologically active form. 

A pH-dependent chemoselective catalytic reductive amination of a-keto acids, affording a-amino acids with HCOONH4 in water, was achieved using the complex 31 or its precursor 28 as the catalyst [51]. The formation rates of alanine and lactic acid from pyruvic acid exhibited a maximum value around pH 5 and pH 3, respectively, and therefore, alanine was obtained quite selectively (96%) with a small amount of lactic acid (4%) at pH 5 (Scheme 5.18). A variety of nonpolar, uncharged polar and charged polar amino acids were also synthesized in high yields. 

We shall see later (see Section 15.6) that reductive amination of a keto acid is the way nature synthesizes amino acids, using the biological analogue of a complex metal hydride, namely NADPH (see Box 7.6). 

Scheme 2.13 Reductive amination of a-keto acid 29 using PDH.

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 a-keto acids are extremely versatile intermediates. For example, reduction of an arylpyruvic acid 450 yields the corresponding p-aryllactic acid 451, condensation of 450 with amines followed by reduction affords amino acid derivatives 454, ° and condensation of 450 with hydroxylamine yields a-oximi-noacids 452 as shown in Scheme 7.149. ... 

Fluorination of unsaturated 5(47/)-oxazolones 758 affords the expected difluori-nated derivatives 759. Basic hydrolysis of 759 yields a p-fluoro-a-keto acid 760 that is reductively aminated to give an e t/jw-p-fluoro-a-amino acid 761... 

Problem 21.2 Synthesize leucine, (CH,),CHCH,CH(NH,)COO, by (a) Hell-Volhard-Zelinsky reaction (Section 16.3) followed by ammonolysis. (b) Gabriel synthesis, (c) phthalimidomalonic ester synthesis, (d) reductive amination of a keto acid, (e) Strecker synthesis (addition of NH, HCN to RCH=0), (/) acetylaminomalonic ester. 

Since our initial communication (10) concerning the catalytic reduction of organic substrates by this homogeneous hydrogenation system, two further reports have come to our attention de Vries (2) demonstrated the reduction of sorbic acid to 2-hexenoic acid and Kang (8) described the reductive amination of a-keto acids. 

Reductive Amination of Keto Acids (L-tert-Leudne as Example)... 

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%. 

Reductive amination reactions of keto acids are performed with amino acid dehydrogenases. NAD-dependent leucine dehydrogenase from Bacillus sp. is of interest for the synthesis of (S)-fert.-leucine [15-17]]. This chiral compound has found widespread application in asymmetric synthesis and as a building block of biologically active substances. The enzyme can also be used for the chemoenzy-matic preparation of (S)-hydroxy-valine [18] and unnatural hydrophobic bran-ched-chain (S)-amino acids. NAD-dependent L-phenylalanine dehydrogenase from Rhodococcus sp. [19] has been used for the synthesis of L-homophenyl-alanine ((S)-2-Amino-4-phenylbutanoic acid) [9]. These processes with water-soluble substrates and products demonstrate that the use of coenzymes must not... 

The hydrazine 89 is made from the amine by nitrosation and reduction and the keto-acid 90 is available as levulinic acid. Now comes the big question when the Fischer indole synthesis is carried out on the hydrazone 91 which enamine is formed, the one we want 92, or the one we don t want 93 Since the Fischer indole is an acid- (or Lewis acid-) catalysed reaction we expect the more substituted enamine 92 to be favoured. 

We have developed the synthesis and conversion of 2-keto-6-hydroxyhexa-noic acid (2) to L-6-hydroxynorleucine (1) (Fig. 1) by a reductive amination... 

The synthesis of enantiopure amino-functionalized compounds such as a- and (3-amino acids or nonfunctionalized amines can be envisaged by the use of aldehydes, ketones, a- or (3-keto acids, or derivatives thereof as substrates for imine formation followed by, for example, diastereoselective Strecker reactions, reductions, or organometallic addition reactions. In the literature, diastereoselective syntheses based on a large variety of chiral auxiliaries, such as a-arylethylamines,4... 

Reductive amination was also conducted using cell extracts from E. coli strain SC16496 expressing PDHmod and cloned FDH from Pichia pastoris. Cells from a 15-L tank had 133 u/g FDH, 65u/g PDH (phenylpyruvate assay), and 12.7 u/g PDH (assayed with keto acid 3). The extract was used for conversion of 30g 3 to 4 in close to 100% yield, and this material, after filtration for protein removal, was converted to 2 by BOC protection. Further experiments showed that the E. coli extract could be used at 2.5% w/v concentration instead of the 12.5% concentration used for batches with Pichia pastoris extract. In subsequent experiments, the substrate input was increased to 100 g/ L and the reaction was carried out at pH 8.0. Cell extracts of E. coli strain SC16496 after polyethyleneamine treatment, clarification and concentration was used to complete the reaction in 30hrs with >96% yield and >99.9% ee of product 4. PDHmod and FDH expressed in E. coli have now been used to prepare several hundred kg of BOC-protected amino acid 2 to support the development of Saxagliptin (Hanson et al., 2007). 

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