Peptides, cyclic thioesters

Key words Backbone amide linker, Handle, Peptide, Aldehyde, Thioester, 1 7-Alkyl amide. Cyclic... 

Workers in three laboratories have studied ligation via cyclic transition-state intermediates. The couplings described in Section 5.1.10.1 to form the amide or thioester bond involve second-order intermolecular reactions of two peptide components, which are necessarily... 

The linear peptide thioester was dissolved in 0.2 M phosphate buffer (pH 7.2, 0.1-0.6 mM soln) containing 50% DMF and TCEP (6 equiv). After completion of the reaction (3-24 h, monitored by HPLC) the cyclic peptide was purified by preparative HPLC (small amounts of byproducts result from hydrolysis of the peptide thioester). 

Thioacid- and thioester-modified peptides have been used in the silver-mediated condensations of peptide fragments. 65-70 Thioester-functionalized peptides have also been utilized as the starting precursors for the formation of cyclic peptides 71,72 as well as alkyl and aryl substituted amides. 73 ... 

In this method, the cysteine-thioester cyclization generates a cyclic peptide 86a (see Scheme 23) with a Xaa-Cys bond whose thiol moiety is then used for tethering to the core through an S-alkylation reaction.191 The requirement for the cyclization reaction is a linear precursor 84 containing both an N-terminal Cys and a C-terminal thioester. Such a peptide precursor can be conveniently synthesized by a stepwise solid-phase synthesis on a thioester resin 81 using Boc chemistry (Scheme 22). Cleavage by HF after assembly of the peptide sequence will produce the desired precursor with an N-terminal Cys and a C-terminal thioester 84. The crude peptide is then purified by RP-HPLC and the purified unprotected peptide is then circularized in aqueous conditions buffered at pH > 7.0. 

Scheme 23 Synthesis of Cyclic Peptide Building Blocks through End-to-End Cysteine-Thioester Cycliza-tion,87n61...

Besides C-terminal and side-chain attachment of the first amino acid derivative onto the solid support, there is a third possibility, which involves attachment of a backbone amide nitrogen to an appropriate handle/support, as shown in Scheme This strategy represents a novel and general concept for the solid-phase preparation of peptides having a variety of C-terminal functionalities, including peptide alcohols, A,A-dialkylamides, esters, thioesters, aldehydes, and head-to-tail cyclic peptides. 

Figure 14.9 Synthesis of cyclic peptides, thioesters and p-nitroanilides using the BAL linker.

The isolated TE domain from the tyrocidine (tyc) NRPS has recently been shown to catalyze the macrocyclization of unnatural substrates to generate a variety of cyclic peptides. In conjunction with standard solid-phase peptide synthesis, Walsh and coworkers demonstrated a broad substrate tolerance for peptidyl-N-acetylcysteamine thioesters by the tyrocidine TE [41,42], Cyclization of peptide analogs, where individual amino acids were replaced with ethylene glycol units, was observed with high efficiency. In addition, hydroxyacid starter units were readily cyclized by the isolated TE domain to form nonribosomal peptide-derived macrolactones. More recently, Walsh and coworkers have demonstrated effective cyclization of PEGA resin-bound peptide/polyketide hybrids by the tyrocidine TE domain [43], Utilization of a pantetheine mimic for covalent attachment of small molecules to the resin, serves as an appropriate recognition domain for the enzyme. As peptide macrocyclizations remain challenging in the absence of enzymatic assistance, this approach promises facile construction of previously unattainable structures. 

The oxidation-reduction method, developed initially by Mukaiyama et al. [133] and related to the previously described organophosphorus methods, has permitted a variety of important solid-phase applications. The mechanism of the activation is complex and involves the oxidation of the triaryl/ alkyl-phosphine to the oxide as well as reduction of the disulfide to the mercapto derivative. However, different active species, such as 81 (Fig. 11), the 2-pyridyl thioester, or even the symmetrical anhydride, have been postulated to form. For the intermediate 81, the peptide bond formation may proceed through a (cyclic transition state. The method has been used for conventional stepwise synthesis [134], acylation of the first protected amino acid to a hydroxymethyl resin, and to achieve segment condensation on a solid support in the opposite direction (N C) [135,136]. Lastly, it has been used for efficient grafting of a polyethylene glycol (molecular weight 2000) derivative to an aminomethyl resin to prepare PEG-PS resins [137]. 

The constrained conformation of cyclic peptides often results in increased exo-and e (iopeptidase resistance, enhanced binding affinity, and in certain cases, increased cell penetration compared to their linear counterparts. Numerous strategies, both in solution and solid-phase, have been reported for the synthesis of cyclic peptides [56-58] NCL, the reaction of a C-terminal peptide thioester with an... 

Hackenberger and Kleineweischede reported a traceless Staudinger ligation for the head-to-tail macrocyclization of peptides without a deprotection step (Fig. 8) [63]. In this strategy, a phosphine tethered to a thioester at the C-terminus of a peptide reacts intramolecularly with an azide at the N-terminus to form the cyclic peptide. 

Chen et al. developed a strategy for preparing cyclic peptides via in situ generation of a thioester resulting from disulfide reduction subsequent NCL results in the desired peptide (Fig. 12) [76]. This strategy was used to synthesize linear glycopeptides, which, after thioester formation, resulted in cyclization to form a model glycopeptide in 73% yield (Scheme 8). 

The synthesis of branched peptides using masked side-chain thioester derivatives of Asp and Glu which are compatible with Fmoc-SPPS is an important goal. Boll et al. synthesized cyclic and branched chain peptides using bis (2-sulfanylethyl)amido (SEA) side-chain derivatives of Asp and Glu via Fmoc SPPS [77]. The tail-to-side-chain cyclization via an in situ reduction of both acyclic and cyclic disulfides with tris(2-carboxyethyl)phosphine (TCEP) triggered the SEA intramolecular ligation. Glu derivatives cyclized more readily than the Asp analogues and without formation of side products (Scheme 9). 

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