Thioesters, aminolysis

Among the electrophilic handles proposed for head-to-tail and side-chain-to-tail cyclization of peptides on solid support by intrachain aminolysis with concurrent detachment of the product from the resin in the protected form (see Section 6.8.3.1.3), generally the oxime resin (also called Kaiser resin)1364 365 and a thioester resin[363l are recommended (see Scheme 14). In addition to the classical head-to-tail cyclization,[3431 the oxime resin is used for side-chain cyclizations as well as for the synthesis of multicyclic peptides vide infra). Due to its dual functions, the oxime resin can be employed only with Boc/Bzl chemistry it is not compatible with Fmoc/tBu chemistry where the basic N -deprotection leads to free amino groups and thus to premature cyclization reactions. To avoid this premature cleavage of the... 

In a similar manner, the thioester resin[363l is used only with Boc/Bzl chemistry, and the intramolecular aminolysis of the thioester by the amino group occurs upon deprotection and neutralization with DIPEA (3.0 equiv) and is catalyzed by DMAP (0.1 equiv) in DMA (cyclization time 2-7 d). 

Preparation of the thioacids and their esters via SPPS is mainly restricted to the Boc methodology. The popular Fmoc approach is limited by aminolysis of the thioester bond during removal of the Fmoc group by piperidine. However, a modified Fmoc-deprotecting mixture (1-methyl pyrrolidine/hexamethyleneimine/HOBt/DMSO/NMP 25 2 2 35.5 35.5) gave the final desired peptide ester with 24% yield J24 ... 

Aminolysis of a series of aryl 2,4-dinitrophenyl carbonates by a series of quinuclidines gave linear Br0nsted-type plots, the magnitudes of their slopes confirming their mechanisms as concerted.40 A comparison41 of the aminolysis, by primary amines, of 4-nitrophenyl phenyl carbonate (31 X = O) with its thiono analogue (31 X = S) is discussed in the section Thioacids, Thioesters, Thiolactones, and Thiocarbonates below. 

Likewise, the assumed O-methyl-serinyl-thioester intermediate reacts only slowly with the Aad thioester, but is readily released by aminolysis of free Val [reaction (14), with A2 either Cys or O-methyl-serine] or may react with the Val-thioester intermediate to be further processed [reactions (12) and (13), with O-methyl-serine for A2],... 

Further studies are needed on the kinetics of aminolysis of adenylates and thioesters and the possible migration of adenylates to contact reactive intermediates. Such surface diffusion or tunneling has to compete with the effective pantetheine-mediated covalent transport system. 

Fig. 3 Hypothetical free energy diagram proposed to account for NCA reactivity. Free energy levels of NCAs and of the transition state of their aminolysis reactions compared to those of reactions of other amino acid derivatives (AA-X). A Comparison with linear anhydrides (X = OAc) B Alkyl ester (X = OR), thioesters (X = SR) and p-nitrophenyl esters (X = ONp). The free energy regions corresponding to the possibility of pathways involving NCAs are shown hashed...

The sensitivity of thioester groups to aminolysis by piperidine has prevented the use of thioester-based linkers for Fmoc SPPS. Two groups d have adapted a modified form of Kenner s acylsulfonamide safety-catch linkerf for the synthesis of C-terminal thioester peptides using Fmoc protocols (Scheme 8). This approach is promising and future studies will more fully analyze the limitations of the method. 

To quantify the hydroxy-telechelic PMMA, LC was used to show that this two-step reaction yielded about 67% of telechelic oligomers [192], This unexpected low yield was explained by several factors during RAFT some disproportionation may occur leading to dead chains without any terminal thioesters also some side reactions occurred from aminolysis of the terminal thioester, leading to a hydrogen-terminated chain unable to be functionalized into a hydroxyl group. 

Chemical ligation methods are typically compatible with a wide range of reaction conditions. However, it is important to note that in addition to optimizing ligation rates, maintenance of the chemoselectivity of the reaction is critical. As a result, native chemical ligation is typically performed at a PH of 6.5-8.0 at 25-40°C to avoid the possibility of unwanted thioester reactivity such as aminolysis, hydrolysis, or epimerization of the C-terminal amino acid. To maintain pH control in the presence of high concentrations... 

The S—C=0 linkage is susceptible to attack by hard bases (18). The inherent reactivity is caused by the soft-hard union. Complex thioesters can be induced to cyclize to macrolides by the catalysis of mercuric salts (19, 20). Heavy metal ion-assisted aminolysis of thioesters (21) is also known. 

Interestingly, lipase-catalyzed acyl transfer onto SH-groups (corresponding to ester thiolysis in analogy to aminolysis) does not take place [289]. As a result, the resolution of sec-thiols is not feasible by this method and has to be performed via hydrolysis or alcoholysis of the corresponding thioesters [290-292]. 

Key words. Chiral, crown ether, enzyme model, peptide synthesis, chiral recognition, molecular recognition, thiolysis, aminolysis, thiol, thioester. 

In an earlier study, we pointed out some important aspects of aminolysis of thioester in our enzyme model. First, the fastest rate for aminolysis of thioester was obtained in the presence of equimolar amounts of acid and base catalysts. Second, the reaction proceeded in aprotic nonpolar solvents such as benzene, ethyl acetate, dichloromethane, and so on [7]. Thus, the peptide syntheses by the enzyme model have been performed in benzene buffered with equimolar amounts of pivalic acid and triethylamine as acid and base catalysts, respectively. Third, the superiority of intramolecular aminolysis over an intermolecular one was clearly demonstrated, despite the large membered cyclic intermediate expected for the intramolecular reaction. The host 10 could achieve the synthesis of the tetrapep-tide derivative (11) by formal turnover of the intra-complex thiolysis and the intramolecular aminolysis, but its efficiency as an enzyme model has remained to be improved [3]. 

For consideration of chiral recognition by the syn-iy t 12, the stability of the dipolar tetrahedral structure (26) was considered as the rate-determining intermediate for aminolysis of thioester [8]. The tetrahedral intermediate (26) formed across... 

Preparation of the future main chain occurred via activated PFP-ester chemistry, as discussed in Sect. 2. For this, PFPA was used as monomer in a RAFT polymerization. After removal of the thioester chain end by treatment with excess AIBN, the aminolysis using monoamine-functionalized fluorescent dye (Oregon Green Cadaverine) and prop-2-yn-l-amine and l-aminopropan-2-ol was conducted. Via an azide-alkyne 1,3-dipolar cycloaddition, an azide-capped cyclodextrin was attached onto the alkyne units along the main chain. As demonstrated in Scheme 10, the cyclodextrin moiety works as a supramolecular binding site for adamantyl residues. This supramolecular binding concept resulted ultimately in linear-g-(linear-hyperbranched) graft terpolymers. 

There are some thioesters that can be used for release of amides via aminolysis developed by Vlattas et al. in 1997 (Fig. 7) [90]. 

Thioester 84 has been used for aminolysis by several primary amines (beyond that it was used for Grignard addition) to give the corresponding amides in non-hydroxyUc solvents such as dioxane with 70-80% yield. For reaction of secondary amines with thioesters, more reactive thioesters like 82 and 83 were needed. 

The application of papain in peptide synthesis is well established [23-25]. Papain can be used for fhe preparation of di- and tripepfides in an aqueous medium wifh cosolvent addition (up to 40%) and at high pH to promote synthetic activity. The enzyme is a sulfhydryl protease with no homology to the trypsin or subtilase families of hydrolases. Since the catalytic nucleophile is a cysteine and because thioesters are relatively more prone to aminolysis than oxo-esters, the enzyme could be very attractive for synfhesis. However, unlike the case with the thiol variants of some serine hydrolases, fhe proteolytic activity is still high, and the broad substrate range of proteolysis makes peptide substrate and product hydrolysis more problematic than trypsin or chymotrypsin. Extensive enzyme engineering studies on papain are lacking, probably due to the laborious procedure for isolation of active papain from inclusion bodies formed in E. coli. 

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