Hydrolysis of N-acylamino Acids

The hydrolytic enzymes amidases are useful for the hydrolysis ofN-acylamino acids for the synthesis of amino acids and in the formation of amide bonds in polypeptides and proteins. In fact, this method is the resolution of amino acids.  

Some other applications in use of amidases are also given (Table 4)  

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Figure 10.16 Subtilisin-catalyzed hydrolysis of N-acylamino acid esters.

Subtilisin is an endoprotease that has been used in the enantioselective hydrolysis of N-acylamino acid esters (Figure 10.16) into the corresponding (S)-amino acid derivatives. An organic solvent, such as acetonitrile, is often added to improve the solubility of the amino acid derivative, and this function can also be performed by an ionic liquid mixture [133, 134, 135]. 

The enzymatic hydrolysis of N-acylamino acids has been known for a century and was first detected in aqueous kidney preparations 3. Based on the finding that this enzymatic hydrolysis proceeds enantiospecifically 2, Greenstein and coworkers developed a general and very attractive procedure for the resolution of a vast number of racemic N-acylated amino acids to the corresponding L-amino acids catalyzed by aminoacylase (E.C. 3.5.1.14) whereas the N-acetyl-D-amino acid does not react13 (Fig. 12.3-1). 

Hydrolysis of N-Acylamino Acids 757 Non-proteinogenic amino acids... 

One especially interesting method for resolving amino acids is based on the use of enzymes called deacylases. These enzymes catalyze the hydrolysis of N-acylamino acids in living organisms. Since the active site of the enzyme is chiral, it hydrolyzes only A/ acylamino acids of the l configuration. When it is exposed to a racemic mixture of A/ acylamino acids, only the derivative of the L-amino acid is affected and the products, as a result, are separated easily ... 

One especially interesting method for resolving amino acids is based on the nse of enzymes called deacylases. These enzymes catalyze the hydrolysis of N-acylamino acids in living organisms. Since the active site of the enzyme is chiral, it hydrolyzes only... 

Many of the ligands that have been advocated for analogous reductions provide high enantioselectivity and some can provide good turnover numbers and frequencies [4, 5], Knowles catalyst often results in an ee of about 94-95% if the reaction mixture is monitored. However, crystallization of the N-acylamino acid product often results in enantioenrichment [10, 11]. In addition, hydrolysis of the amide to provide the amino acid itself also provides an opportunity for enantioenrichment (Fig. 3) [10]. 

A major chemical effect of y-rays on simple peptides such as the N-acylamino acids under oxygen-free conditions, both in the solid state and in concentrated aqueous solution, leads to formation of labile amidelike compounds which are readily degraded on mild hydrolysis to yield ammonia as a characteristic product. Several classes of nitrogen-deficient products are formed concomitantly with the ammonia. Earlier communications have discussed certain limited aspects of the radiolytic lability of simple peptides in the solid state and in concentrated solutions (9, 10, 18). The radiation chemistry of these systems is more complex than that involved in the radiolysis of simple peptides in dilute oxygen-free aqueous solution under which conditions main-chain degradation is of minor importance (10). In this paper we report detailed experimental evidence... 

Melphalan and the racemic analog have been prepared by two general routes (Scheme I). In Approach (A) the amino acid function is protected, and the nitrogen mustard moiety is prepared by conventional methods from aromatic nitro-derivatives. Thus, the ethyl ester of N-phthaloyl-phenylalanine was nitrated and reduced catalytically to amine I. Compound I was reacted with ethylene oxide to form the corresponding bis(2-hydroxyethyl)amino derivative II, which was then treated with phosphorus oxychloride or thionyl chloride. The blocking groups were removed by acidic hydrolysis. Melphalan was precipitated by addition of sodium acetate and was recrystallized from methanol. No racemization was detected [10,28—30]. The hydrochloride was obtained in pure form from the final hydrolysis mixture by partial neutralization to pH 0.5 [31]. Variants of this approach, used for the preparation of the racemic compound, followed the same route via the a-acylamino-a-p-aminobenzyl malonic ester III [10,28—30,32,33] or the hydantoin IV [12]. 

Amino-l,2,4-oxadiazolcs (11) arc excellent sources of amidines. When they are treated with 1,3-dicarbonyl derivatives they are converted into enaminones (12), which cyclize to imidtizolcs in the presence of bases (Scheme 2.2.5). This reaction utilizes the well-known general attack of a nucleophilic group in the side chain at N-2 of the oxadiazole ring. Yields of 2-acylamino-4-acylimidazoles usually lie between 60 and 80% [21]. The free amino derivatives are readily isolated after acid hydrolysis. 

The orientation of reaction of aminopyridazines with electrophiles, and further reaction of the intermediates is illustrated by a study with 4-amino-5-aroylpyridazines. Acylation and sulfonylation occurs on the amino nitrogen, while alkylation occurs, as expected, at N-1 the acylamino derivatives are relatively unstable, but the toluenesulfonamide is quite stable to acid and to base. Methylation of the toluenesulfonamide gives a mixture of ring and amino alkylated products with the latter predominating. Hydrolysis of the ring A-alkylated compounds gives 4(l//)-pyridazinones (Scheme 57). Selective amino-alkylation of the 4-amino-5-aroylpyridazines is achieved via the imidate by reduction and subsequent oxidation of the alcohol <85H(23)265l>. 

Taurides (or taurates) are acylamino alkane sulfonates which have chemical structures close to isethionates. They are prepared by the reaction of a fatty acid chloride with N-methyl taurine (CH3-NH-CH2-CH2-S03-Na). Oleic acid chloride or coconut fatty add chloride are suitable reactants. At variance with the above fatty acid isethionates, taurides are less prone to hydrolysis. Taurides have good foaming and emulsifying properties and are insensitive to water hardness. 

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