N-acylamino acid ester

In the third transition state (TS3), the neutral catalyst is recovered by transferring the proton back from the catalyst to the substrate. In other words, the (former) azlactone ether oxygen atom deprotonates the tertiary ammonium ion. For proton transfer, again a low-barrier hydrogen bond is formed (N—O distance 2.479A, <(0,H,N) = 166.2°). In the product complex, the catalyst is neutral and the N-acylamino acid ester is bound in its iminol form to the catalyst (Product [iminol]). Finally, an additional 66.6kJ/mol are gained by the subsequent iminol-amide tautomerization (Product [amide]). 

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]. 

Most of the experiments on incorporating amino acid esters into proteins during the plastein reaction have been carried out with papain, indicating that it is one of the best enzymes for this purpose. Other enzymes such as chymotrypsin (40) or carboxypeptidase Y from Sac-charomyces cerevisiae (41) are potent catalysts for peptide synthesis in homogeneous systems using N-acylamino acid esters of peptides as substrates and amino acid derivatives or peptides as nucleophile components. Adding organic co-solvents favored peptide bond synthesis (42,43). 

Carbonic anhydrase - an enzyme termed for its ability to catalyze the hydration of carbon dioxide forming hydrogen carbonate - can also be employed. In contrast to the above-mentioned biocatalytic systems, it exhibits the opposite enantiopre-ference by hydrolyzing the D-N-acylamino acid esters [123]. 

The method of N-chlorination of amino acid esters 40 and AT-acylamino acid esters with tert-butyl hypochlorite and subsequent elimination of hydrogen chloride is of general use in the preparation of DHA derivatives 41/42 122-1241 (Scheme 15). 

Phenolic esters are obtained similarly. The presence of a nitro group in the aromatic nucleus and the use of pyridine as solvent facilitates the reaction. This reaction is recommended for the characterization of phenols. 2,4,5-Trichlorophenyl-, pentachlorophenyl-, 4-nitrophenyl- and thiophenyl esters of N-acylamino acids are prepared in this manner. These aromatic esters are used in the stepwise lengthening of peptides, du Vigneaud and coworkers synthesized lysine vasopressin from a nonapeptide which they prepared stepwise using the nitrophenyl ester method. Room temperature esterification of dicarboxylic acids and diphenols are also carbodiimide mediated using the 1 1 complex derived from DMAP and p-toluenesulfonic acid as catalyst Methacrylic acid is also esterified with phenols using carbodiimides and DMPA to mediate the reaction. ... 

The mixed anhydride obtained from an N-acylamino acid (1) and diphenylketene, for example (2), is attacked by a phenol to give an optically pure aryl ester (3)."... 

The reagent (1) also effects synthesis of dipeptides from an N-acylamino acid and an amino acid ester hydrochloride. Yields are in the range of 70-80%. The optical yield in the one case reported was 86%. 

Derivatization Most amino acids are nonvolatile and must be derivatized prior to separation by GC. Amino acids are both esterified and acylated to produce N(0,S)-acylamino acid esters, with the most common being (N(0,5)-trifluoroacetyl- -butylesters. Other alcohols of low relative molecular mass have also been employed for the esterification including methanol, propanol, isopropyl alcohol, butanol, or isobutyl alcohol. Several different acylating reagents have been explored as well, including acetic anhydride, trifluoroacetic anhydride, penta-fluoroproprione anhydride, or heptafluorobutyric anhydride. 

A synthesis of acylenamino carboxylic acids from acylamino acids has been described by Poisel and Schmidt 310,312,313). Acylamino acid esters are N-chlorinated with t-butylhypochlorite in the presence of catalytic quantities of base. A full equivalent of base can then be used to eliminate HCl, with the initial formation of acylimino esters as unstable primary products which then either add solvent (alcohol) to form a-acylamino-a-alkoxy acid derivatives (22), or (in the absence of protic solvents) rearrange to the acylenamino acid esters (21). The a-acylamino-a-methoxy carboxylic acid (22) esters can also be converted... 

This reaction has been studied in some detail by H. Poisel and U. Schmidt 310, 312, 314), using several acylamino acid esters, a-Methoxy derivatives (22) were obtained from the N-chloro compounds on elimination of HCl with CHaO /CHaOH, while treatment with non-nucleophilic bases in aprotic solvents afforded acylenamino acid esters (21). Acylimino acids were undoubtedly primary elimination products, although it proved impossible to detect them. 

Tajima obtained a-acylamino-a-alkoxy esters from a-acylamino acid esters reacting with DCCD/ROH (476). The intermediate azlactones underwent autoxidation which was followed by DCCD-supported hydroxy-alkoxy-exchange. An electrochemical synthesis of N-acyl-2-alkoxyproline derivatives was reported by Horikawa et al. (477). 

Belleau and Malek47 introduced the use of this reagent (EEDQ) to couple acylamino acids with amino acid esters, and Dunstan and Hough46 were able to couple the peracetates of maltosyl-, lactosyl-, and cello-biosyl-amine (55, 47, and 57, respectively) with 1-benzyl N-(benzyl-... 

Stannyl esters have been prepared to protect a —COOH group in the presence of an —NH2 group [(n-Bu3Sn)20 orn-Bu3SnOH, C6H6, reflux, 88%]. Stannyl esters of A-acylamino acids are stable to reaction with anhydrous amines, and to water and alcohols 2 aqueous amines convert them to ammonium salts.2 Stannyl esters of amino acids are cleaved in quantitative yield by water or alcohols (PhSK, DMF, 25°, 15 min, 63% yield or HOAc, EtOH, 25°, 30 min, 77% yield).2... 

Active Esters. The synthesis of reactive esters of acylamino acids and their use in lengthening a peptide chain by reaction at the amino end is a justifiably popular method racemization is avoided, yields are good, and purification of the products is relatively easy. Esters of N-hydroxy-succinimide (l), N-hydroxypiperidine (ll) and 8-hydroxyquinoline (III) have received particular attention during I966. 

The practical value of N-hydroxysuccinimide (HOSu) esters of acylamino acids is illustrated by their use in the syntheses of insulin components... 

Acylamino-l-thioethers N-Acylcarboxylic acid thioamides Acylisothiocyanates Cyanothioformic acid esters Methylthiomethyl iminoesters l-(Organothio)alkoximes... 

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