Esters tripeptide

A general step ahead in polycondensation was achieved by the application of the active ester method by DeTar et al.19) and Kovacs et al.291 Very soon, the nitrophenyl ester, the pentachlorophenyl ester, or the hydroxysucdnimido ester were used exclusively. The esters of the protected tripeptides could be purified by crystallization, then the N-protecting group was split off and the free peptide esters were purified again. Addition of base starts the polycondensation, resulting quickly in the formation of a viscous solution at low temperature. 

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

The only nucleophile is the free amine In (22) and the most reactive electrophile is the p-nitro-phenyl ester in (21). These combine to give the tripeptide (23). 

Tripeptide esters can also be synthesized by this method. Dissociation of the ben-zotriazolide into benzotriazole and isocyanate has been suggested for the reaction mechanism.[47]... 

For instance, in a synthesis of N-Ras lipopeptide 8, the choline ester in the palmitoylated tripeptide 5 was removed selectively and in high yield by means of the butyryl choline esterase (BChE). Efficient cou-... 

Janetka and Rich (78) have utilized the considerable stability of ruthenium-77-arene systems in the synthesis of cyclic tripeptides as analogs of the protease inhibitor K-13 (cf. 34, Scheme 28). Their approach involves the construction of linear tripeptide complexes (35) using diimide/HOBt coupling of [Ru(Cp)(Boc-p-Cl-PheOH)]PF6 (Cp = 775-C5H5) with the appropriate dipeptide ester. Cyclization of 35 affords the biphenyl ether 36, which on photolysis (350 nm) gives 34. 

Once it is part of a cyclic dipeptide, the prolyl residue becomes susceptible to enantiomerization by base (see Section 7.22). The implication of the tendency of dipeptide esters to form piperazine-2,5-diones is that their amino groups cannot be left unprotonated for any length of time. The problem arises during neutralization after acidolysis of a Boc-dipeptide ester and after removal of an Fmoc group from an Fmoc-dipeptide ester by piperidine or other secondary amine. The problem is so severe with proline that a synthesis involving deprotection of Fmoc-Lys(Z)-Pro-OBzl produced only the cyclic dipeptide and no linear tripeptide. The problem surfaces in solid-phase synthesis after incorporation of the second residue of a chain that is bound to the support by a benzyl-ester type linkage. There is also the added difficulty that hydroxymethyl groups are liberated, and they can be the source of other side reactions. 

The traditional method for preparing activated esters of A -protected dipeptides is combination of the A-protected amino acid with the amino acid ester (Figure 7.16). The latter is obtained by A-deprotection of the diprotected amino acid in an acidic milieu. Coupling is achievable using the carbodiimide, mixed-anhydride, and acyl-azide methods. Success with this approach indicates that the esterified residues react preferentially with the other derivatives and not among themselves. The chain cannot be extended to the protected tripeptide ester because the dipeptide ester cyclizes too... 

FIGURE 8.7 Synthesis of a protected tripeptide containing a 2-hydroxy-4-methoxybenzyl-protected peptide bond.38 (A) Acylation of the carboxy-terminal residue, (B) removal of both protecting groups, (C) O-acylation of the benzyl-protector by the symmetrical anhydride of the amino-terminal residue, and (D) migration of the protected amino-terminal residue from the oxygen atom to the amino group of the dipeptide ester. 

The utility of carbodiimide reagents was too important to consider abandoning them. Instead, it rapidly became obvious that carbodiimide activation could be used to prepare in situ active esters. Numerous nucleophilic additives were discovered and prepared for use in these reactions. The most important of these additives was 1,2,3-benzotriazol-l-ol (HOBt, 1), first reported for use in peptide synthesis by Konig and Geiger in 1970.[2(l As an example of a typical result, 1.2 equivalents of HOBt was added to the DCC coupling of Boc-Leu-Phe-OH to H-Val-OtBu in DMF as a solvent. Less than 1% of the l-d-l epimerized tripeptide was formed. When the reaction was carried out in the absence of HOBt, the amount of l-d-l product formed was 14.3%. Addition of HOBt to DCC reactions converts the intermediate G-acylisourea (2) (and any symmetrical anhydride) into the HOBt active ester 3 (Scheme 8). 

A series of derivatives of the ester 37, having amino acid or peptide residues linked through the carboxyl group of the lactic acid residue, has been described. Their exact structures depend on the growth conditions and the bacterial strain used. The chain attached may contain L-alanine 7,151,154,155 the dipeptide L-alanyl-D-glutamic acid 156,157 the tripeptides L-alanyl-y-D-glutamyl-L-lysine,157-160 L-al-... 

Azacyclols arising from amide-amide interaction have been extensively investigated. The p-nitrophenyl ester (60) of the linear tripeptide N-benzyloxycarbonyl-L-alanyl-L-phenylalanyl-L-proline undergoes a double cyclization when left in an aqueous buffer-dioxane (1 1) solution for 1 h, to produce cyclol (61) (7 ICC 1605). The hydroxyl group of the cyclol could be converted to the methyl ether by treatment with methyl iodide-silver oxide. The structure of the cyclol (61) could be confirmed by X-ray crystallography of the corresponding p-bromobenzyloxycarbonyl derivative (7 ICC 1607). 

A tetracyclic azacyclol (66) has been obtained from the p-nitrophenyl ester of the linear tripeptide prolylphenylalanylproline (81TL3671). However, the tripeptide active ester Phe-Pro-Pro-ONp with an altered sequence did not lead to the cyclol (66). This suggests that in the former reaction the piperazinedione (65) is an intermediate (Scheme 20). 

In the above examples, the quaternary cyclolic carbon is located at the junction of a 5-membered and a 6-membered ring. However, when attempts were made to create a stable cyclol with two 6-membered rings [n = 2, in structure (52)], the cyclization either did not occur, or led to further transformation products. Thus, cyclol (67) could not be obtained from the N-protected linear tripeptide active ester Cbz-/3-Ala-Phe-Pro-ONp, incorporating a /3-amino acid [82JCS(P1)1311]. Cyclization of the same sequence having a free NH2 at the N-terminus gave the cyclotripeptide (68) with a 10-membered ring. 

Sheppard, 1963) and tripeptide oxazoline [18] spontaneously yields cyclol [19] when dissolved in ethyl acetate (Jones et al., 1963). The cyclol structure was confirmed by an X-ray structure determination for compound [21a] which was obtained by treatment of the peptide-nitrophenyl ester [20a] with 1 1 carbonate bicarbonate buffer in aqueous dioxan [21ft] was obtained similarly from [20ft] (Lucente and Romeo, 1971 Cerrini et al., 1971). 

E. Bayer, K. Albert, H. Wikkisch, W. Rapp and B. Hemmasi, Carbon-13 nmr relaxation times of a tripeptide methyl ester and its polymer-bound analogs, Macromolecules, 1990, 23, 1937 1940. 

Amides. Metal ions catalyze the hydrolysis of a variety of amides, including acylamino acids, dipeptides and tripeptides, and amino acid amides. In all these compounds it is possible for a metal ion to complex with one or more ligand groups, either amine or carboxylate ion functions, in addition to the amide group. Thus the structural prerequisites for the metal ion catalysis of amide hydrolysis are the same as those for ester hydrolysis. 

Piperazine-2,5-dione formation can occur not only in the case of dipeptides esters or amides containing a free amino group, but also during the activation of N-blocked or protected IV-alkyl amino acids containing dipeptides or even tripeptides. Thus, the activated dipeptide Z-Gly-Pro-ONp (76) afforded the Z-protected piperazine-2,5-dione 77 when subjected to buffered dioxane solution at pH 8 (Scheme 29)J159 ... 

This tripeptide was a key intermediate in an attempted preparation of c[3-(benzyloxymethyl)-2-nitrobenzoyl-Thr-D-Val-Pro-Sar-(Me)Val-] by the [2+3]-segment condensation strategy peptide and cyclization between Sar and (Me)Val. This side reaction was circumvented by using the dipeptide H-D-Val-Pro-OtBu in a [3+2]-segment condensation strategy. Contrary to its benzyl ester, this dipeptide did not form the piperazine-2,5-dione because of presence of the bulky tert-butyl ester.[165 1661 Cyclization was performed between Pro and Sar. 

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