N-acyl amino acids

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. 

Figure 6.34 Resolution of N-acyl amino acids by acylases.

T Wieland, H Bernhard. On the synthesis of peptides. Part 3. The use of anhydrides of N-acylated amino acids and derivatives of inorganic acids. Ann Chem 572, 190, 1951. 

Interesting cyano, polyoxyethylene, and O-acetyl functionalized N-acyl-amino acid derivatives can be obtained from functionalized olefins (Table 1). Diamidocarbonylation products may also be synthesized in moderate yields from terminal diolefins [13-15]. 

N-Acylated amino acids (sodium lauiyl sarcosinatc)... 

In Section 5.03.6.2, a stereoselective synthesis of L-homophenylalanine from the racemic AAacetylated amino acid is described. The authors, however, found that substrate solubility limited the utility of this procedure. Having found an L-N-carbamoylase in Bacillus kaustophilus, they introduced the gene for this enzyme together with that for the N-acyl amino acid racemase from D. radiodurans into E. coli for coexpression. These cells, permeabilized with 0.5% toluene, were able to deliver L-homophenylalanine in 99% yield and were able to be used for multiple reaction cycles. 

Aside from the Maillard reaction, other covalent modifications of amino acids and proteins are possible within the caries lesion, which merit future investigation. For example, certain oral microorganisms excrete y-glutamyl transferases. These enzymes catalyse the formation of cross-links between glutamic acid and lysine residues of proteins. In addition, N-acyl amino acids are present in plaque, which adsorb to mineral surfaces. 

Numerous examples of the preparation of tetramic acids from N-acylated amino acid esters by a Dieckmann-type cyclocondensation have been reported (Entries 7-9, Table 15.4). Deprotonated 1,3-dicarbonyl compounds and unactivated amide enolates can be used as carbon nucleophiles. In most of these examples, the ester that acts as electrophile also links the substrate to the support, so that cyclization and cleavage from the support occur simultaneously. The preparation of five-membered cyclic imi-des is discussed in Section 13.8. 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

G. M. Whitesides, Kinetic resolution of unnatural and rarely occurring amino adds enantioselective hydrolysis of N-acyl amino acids catalyzed by acylase I,... 

In cases of extensive branching, as for the alkylbenzenesulfonates, the CnH2n+2 1°33 series is significantly perturbed compared to the straight chain alkyl sulfates, but it is sufficiently abundant to show perturbations in the loss pattern. These perturbations may be interpretable for identifying branch points. N-Acylated amino acids also show a suppressed remote charge site loss series because the preferred fragmentations are the decarboxylation of the parent anion and formation of the carboxylate anion of the amino acid (8). 

Another Variation on the Theme Does N-acyl Amino Acid Racemase Represent Evolution in Action ... 

Substituted oxazolinones (see Scheme 4.23, p. 78) were synthesized directly by heating amino acids with TFA anhydride at 150°C for 10 min or by the action of dicyclo-hexylcarbodiimide on N-acylated amino acids [266]. Oxazolinones of eleven amino acids were chromatographed on 0.325% EGA on Chromosorb G at 40—140°C. 2-H-, 2-methyl-and 2-phenyloxazolinones of Leu were analysed on OV-17 together with 2-trifluoro-methyloxazolinone, which has the shortest retention time and is very volatile. 

Indeed, this concept proved successful and after optimization of the catalyst structure and of the reaction conditions, a number of azlactones 6 could be transformed to highly enantiomerically enriched N-acyl amino acid esters 9 of high enantiomeric purity (Berkessel et al. 2005, 2006). Some of the results are summarized in Scheme 5. 

A straightforward approach to avoid low yields is to perform the reaction as a dynamic kinetic resolution. Racemisation can be achieved chemically [33] or enzymatically, indeed a number of N-acyl amino acid racemases have been described and it has been demonstrated that they could be employed together with the l-N-acyl amino acylase for the production of optically pure methionine [81]. 

Enzymatic Kinetic Resolution of N-Acyl Amino Acids Coupled with Racemization by N-Acyl Amino Acid Racemase Acylases are enzymes hydrolysing the N-acetyl derivatives of amino acids. They require the free carboxylate for activity and have long been used for the kinetic resolution of amino acids. The unreacted enantiomer is usually racemized in a separate step by treatment with acetic anhydride. While acylases from hog kidney have an L-specificity, bacterial acylases with L- and D-specificity of various origins have been isolated and used for the kinetic resolution of N-acetyl amino acids. An industrial process for the production of L-Met and other proteinogenic and non-proteinogenic L-amino acids such as L-Val, L-Phe, L-Norval, or L-aminobutyric acid has been established. Currently, several hundred tons per year of L-methionine are produced by this enzymatic conversion using an enzyme membrane reactor [46]. 

N-Acylated amino acids, in the presence of watercoupling reagents (dicyclohexylcarbodiimide or the excess of TFAA), form other cyclic derivatives — azlactones (2,4-disubstituted oxazolin-5-ones) ... 

Gerencevic, N., Prostenik, M. n-Acyl amino acids in the Dakin-West reaction replacements of the acyl groups. Bulletin Scientifique, Section A Sciences Naturelles, Techniques et Medicales (Zagreb) 1970, 15,158. 

Esterification catalyst. Sulfuryl chloride is an effective esterification catalyst, particularly useful for peptides since no racemization occurs."" The N-acylated amino acid or peptide is dissolved in methanol containing a trace of the catalyst, and the mixture is allowed to stand for 48 hrs. at 20°. Yields are generally in the range 85-95%. Benzyl esters are prepared similarly except that sy m-tetrachloroethane is used as solvent. 

Chenault, H. K. Dahmer, J. Whitesides, G. M., Kinetic Resolution of Unnatural and Rarely Occurring Amino Acids Enantioselective Hydrolysis of N-Acyl Amino Acids Catalyzed by... 

CSPs incorporating BSA have been successfully employed to separate the enantiomers of acidic and neutral compounds [161-163], such as N-acylated amino acids, aromatic amino acids and sulfoxides. Retention and enantioselectivity can be optimized by adjusting pH, ionic strength and organic modifier content of the mobile phase. 

The properties of the N-acyl amino acid derivative and the amino acid itself can be important if crystallization is employed to ensure high optical purity. However, in the experience of the authors, it is seldom a problem. Only 3-pyridylalanine could not be purified to high optical purity by recrystallization. Use of enzymes to enhance enantiomeric excess as well as allowing the use of mild hydrolysis conditions has been advocated by others [12]. Amino acids are surprisingly stable to acid-induced racemization. 

The most interesting report with respect to preparative applications concerns a ligase antibody obtained by immunization against phosphodiester hapten 5 (Scheme 2) [22]. This antibody catalyzes the coupling of an N-acylated amino-acid 4-nitrophenylester such as 6 with an L-tryptophan ester or amide such as 7 with useful selectivity and rate. 

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