7-Ketoacids, reduction

According to de Duve, thioesters on the young Earth were capable of a broad spectrum of chemical reactions for example, of phosphorolysis leading to acylphos-phates (Fig. 7.8a) or of reductive thioester cleavage, which (after carbonylation) made possible the synthesis of ketoacids (Fig. 7.8b). 

Another model is based on the fact that the genetic code shows a number of regularities, some of which have already been mentioned above. It is suspected that codons beginning with C, A or U code for amino acids which were formed from a-ketoacids (or a-ketoglutarate, 1-KG), oxalacetate (OAA) and pyruvate. This new model, which is quite different from the previous models, assumes that a covalent complex formed from two nucleotides acted as a catalyst for chemical reactions such as the reductive amination of a-ketoacids, pyruvate and OAA. More recent analyses suggest that the rTCA cycle (see Sect. 7.3) could have served as a source of simple a-ketoacids, including glyoxylate, pyruvate, OAA and a-KG. a-Ketoacids could, however, also have been formed via a reductive acetyl-CoA reaction pathway. The bases of the two nucleotides specify the amino acid synthesized and were retained until the modern three-letter codes were established (Copley et al., 2005). 

Aminotransferases are widely distributed enzymes in nature, participating in a number of metabolic pathways (7-11). They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor. In effect, this statement implies the reductive animation of a keto moiety, although a redox process is not directly involved. 

The reaction of 3-ketoacids with allyl carboxylates is also believed to proceed via a palladium enolate intermediate.126 Less than complete stereospecificity is also observed in these reactions (equation 163). Interestingly, the bicyclic lactone substrate employed to ascertain the stereointegrity of this reaction, in addition to being incapable of any syn-anti isomerization, cannot epimerize the starting material by car-boxylate attack at the metal. The observed stereochemical leakage could be due to epimerization of the intermediate allyl complex (equation 164) or reductive elimination of an allylpalladium enolate (retention) (equation 165). 

In another example <2002BML705>, the reduction of the keto group of intermediate a-keto tetrazoles 356 to an a-hydroxy group of tetrazole 357 (Equation 66) is an important stage in the directional synthesis of potent tripeptide a-ketoacid inhibitors of the hepatitis C virus NS3/NS4A serine protease (cf. Section 6.07.12.1.1). 

Chemical synthesis and isolation of 2-keto-6-hydroxyhexanoic acid required several steps. In a second, more convenient process (Fig. 2), the ketoacid was prepared by treatment of racemic 6-hydroxynorleucine [produced by hydrolysis of 5-(4-hydroxybutyl)hydantoin (3)] with D-amino acid oxidase and catalase. After the e.e. of the remaining L-6-hydroxynorleucine had risen to >99%, the reductive animation procedure was used to convert the mixture containing 2-keto-6-hydroxyhexanoic acid and L-6-hydroxynorleucine entirely to L-6-hy-droxynorleucine with yields of 91-97% and e.e. of >98%. Sigma porcine kidney D-amino acid oxidase and beef liver catalase or Trigonopsis variabilis whole cells (source of oxidase and catalase) were used successfully for this transformation [22],... 

Figure 3 Preparation of chiral synthon for vasopeptidase inhibitor enzymatic reductive amination of ketoacid acetal (5) to amino acid acetal (4).

Formate dehydrogenase has been reported to have a pH optimum of 7.5-8.5 [25], The pH optimum for the reductive amination of (5) by an extract of T. intermedins was found to be about 8.7. Reductive amination reactions were carried out at pH 8.0. A summary of laboratory-scale batches is shown in Table 2. The time course for a representative batch showing conversion of ketoacid (5) to amino acid (4) is presented in Figure 5 using E. coli/C. boidinii heat-dried cells. 

Lithium aluminium hydride reduction of 235 followed by mesylation afforded 236. The latter was oxidized with osmium tetroxide and sodium metaperiodate to yield the cyclobutanone 237. Treatment of 237 with acid afforded in 48% yield the ketoacid (238), which was esterified with diazomethane to 239. The latter was converted to the ketal 240 by treatment with ethylene glycol and /7-toluenesulfonic acid. Compound 240 was reduced with lithium aluminium hydride to the alcohol 241. This alcohol had been synthesized previously by Nagata and co-workers (164) by an entirely different route. The azide 242 was prepared in 80% yield by mesylation of 241 and treatment of the product with sodium azide. Lithium aluminium hydride reduction of 242 gave the primary amine, which was converted to the urethane 243 by treatment with ethyl chloroformate. The ketal group of 243 was removed by acidic hydrolysis and the resulting ketone was nitro-sated with N204 and sodium acetate. Decomposition of the nitrosourethane with sodium ethoxide in refluxing ethanol afforded the ketone 244 in 65% yield. The latter had been also synthesized previously by Japanese chemists (165). The ketone 244 was converted to the ketal 246 and the latter to 247... 

Reductive amination of ketones and aldehydes is one of the best methods for synthesizing amines (Section 19-18). It also forms amino acids. When an a-ketoacid is treated with ammonia, the ketone reacts to form an imine. The imine is reduced to an amine by hydrogen and a palladium catalyst. Under these conditions, the carboxylic acid is not reduced. 

Show how the following amino acids might be formed in the laboratory by reductive amina-tion of the appropriate a-ketoacid. 

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. 

A similar effect could be observed when we investigated the reduction of 3-ketoacid esters by Saccharomy-ces cerevisiae and the hydrolysis of 3-acetoxyacid esters by Candida utilis (17). As shown in Figure 4 the chiral... 

The configuration of 3-hydroxyacid esters obtained by yeast reduction of 3-ketoprecursors depends on the chain length of the acids (.18. >1.9). Zhou et al. (20)demonstrated that the structure of the alcohol esterified in 3-ketoacid esters also influences the configuration of the 3-hydroxycompounds that are formed. The fact, that the optical purity of the products depends on the concentration of the 3-ketocompounds (21) indicates the presence of competing enzymes leading to opposite enantiomers at different rates. 

Figure 4. Enantiomeric composition of 3-hydroxyacid esters formed by reduction of 3-ketoacid esters (baker s yeast) and hydrolysis of 3-acetoxyacid esters (Candida utilis).

Ethyl 3-hydroxybutanoate, isolated from yellow passion fruit, mainly consisted of the (S)-enantiomer (82 %), comparable to the product, obtained by yeast reduction of the corresponding 3-ketoacid ester (see above). In contrast, ethyl 3-hydroxybutanoate in purple passion fruit and mango mainly consisted of the (R)-enantiomer (69 % and 78 %). 

Several pyridazines were prepared unintentionally by the attempted Wolff-Kishner reduction of ketoacids. It has been claimed that the ease of pyridazine formation during the Wolff-Kishner-Huang-Minlon reduction of aromatic 1,4-ketoacids depends on the nature of... 

The reaction is rather general for ketones with ammonia, primary, and even secondary amines [10] amino acids [12,13] have thus been prepared from a-ketoacids by reductive amination. 

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