Cyclization, side chains/termini

Scheme 1). Introduction of a jt bond into the molecular structure of 1 furnishes homoallylic amine 2 and satisfies the structural prerequisite for an aza-Prins transform.4 Thus, disconnection of the bond between C-2 and C-3 affords intermediate 3 as a viable precursor. In the forward sense, a cation ji-type cyclization, or aza-Prins reaction, could achieve the formation of the C2-C3 bond and complete the assembly of the complex pentacyclic skeleton of the target molecule (1). Reduction of the residual n bond in 2, hydro-genolysis of the benzyl ether, and adjustment of the oxidation state at the side-chain terminus would then complete the synthesis of 1. 

The simplest approach to isosteric replacement of one or both sulfur atoms of the cystine disulfide with a methylene or ethylene moiety is given for natural bioactive peptides when one cysteine residue is located in the N-terminal sequence position and the related amino group or peptide extension is not involved in the bioactivity. This allows for direct side chain to backbone (N-terminus) cyclization via amide bonds with suitable 5-carboxyalkyl derivatives of the second cysteine residue, or with the oo-carboxy group of aminodicarboxylic adds containing an alkyl side chain that mimics the Ca to Ca spacer in cystine. Thereby, the length and degree of branching of the sulfide or alkyl spacer can additionally be varied. 

The choice of protecting groups and of the type of resin-anchor required for synthesis of cyclic peptides on solid supports not only depends upon the particular amino add sequence of the target molecule, but decisively upon the mode of cyclization, particularly whether the C-terminal carboxy group should act as bridgehead or not. Correspondingly, the synthetic routes can be subdivided into two main classes either based on the attachment of the C-terminus to the solid support by suitable anchors or where side-chain functionalities are exploited for this purpose. 

In natural bioactive peptides the modes of cyclization described previously may be prevented either by the lack of suitable side-chain functionalities for lactamization or because these as well as the amino and carboxy termini are crucially involved in the bioactivity itself, and thus cannot be modified. In order to overcome these potential limitations, the concept of backbone cyclization has been proposed.129 According to this, the cyclization is performed by a covalent interconnection of two backbone amides by artificial spacers or of one backbone amide by a correctly functionalized spacer with side-chain functions or with the N- and C-terminus of the peptide (Scheme 21). This type of strategy significantly increases the diversity of possible ring structures (see Scheme 22) and of their related libraries (see Section 6.8.4). Its potential for enhancing the stability of the related peptide derivatives toward proteolytic digestion,[417 419 potency,141942" and selectivity,11417-419 is well-established. 

As with the synthesis of cyclic peptides, various methods have been employed to effect head-to-tail cyclization of these peptides. The most straightforward method remains N-to-C-terminus amide bond formation in solution of the appropriately side-chain-protected peptide in the presence of carboxyl activating agents under basic conditions. 23 However, cyclodimerization reactions have also been employed to produce cyclic decameric peptides whereby two appropriately protected linear pentameric peptides are dimerized and cyclized in the presence of carboxyl activating agents under basic conditions in the same reaction. 23,24 The use of the cyclodimerization reactions, however, has limitations in that it can only be applied to produce symmetrical peptides, and that formation of the cyclic pentapeptide presents a side reaction and thus results in decreased yields of the desired product. 

Many biologically active peptides are cyclic in nature, and the SPS of this class of peptides, exemplified by 2.7, has also received attention with several different strategies for the final cyclization. The phenomenon of pseudodilution on bead in which each resin site is essentially isolated from its neighbors favors the intramolecular cyclization reaction compared to the intermolecular dimerization, which occurs in solution even at high dilutions. The SPS of cyclic peptides has recently been covered in two excellent reviews (31, 32). The technique of cyclative cleavage via the N- or the C-terminus (see Section 1.2.7) has been used, as has anchoring through amino acid side chains with sequential cyclization and peptide release. 

There are five commonly observed classes of cyclic peptides. The most common of these is a head-to-tail cyclic product, seen in Fig. 2, in which lactamization occurs between the carboxyl and amino termini. Alternatively, cyclization can be effected between a side chain and the carboxyl terminus or amino terminus of the peptide. Cyclization may also be achieved between two side chains, which often involves the use of an additional chemical spacer. The final method by which cyclization may be attained is through two backbone amide nitrogens. All these strategies have different requirements for the orthogonal protecting groups employed. 

The two most common reactions for the generation of cyclic peptides are (i) disulfide formation between cysteine or penicillamine residues through oxidation, and (ii) lactam formation between the amino and carboxy terminal ends (head-to-taU cyclization), the annino terminus and the side chain of an aspartic or glutanoic add residue, or between the side chains of a lysine (or another diamino acid) and aspartic or glutanoic acid residues. A special mode of the second type is backbone cyclization between an amino group (N-terminal or side chain) and a carboxyalkylated backbone nitrogen. Both reactions have been used to generate cyclic peptide libraries. 

Given the potential use of cyclic peptides as therapeutic agents [147-149], the synthesis of these compounds has been the object of extensive studies since the first development of SPPS. Cyclization increases proteolytic resistance and may result in enhanced biological activity compared to their linear counterparts [150, 151]. Cyclic peptides consist of distinct types of linkage (i) in the most common type, the N- and C-termini are joined ( head-to-tail ) (ii) a side-chain is linked to the C- or N-terminus (iii) two side-chains are joined (side-chain-to-side-chain). The linkage is usually an amide bond but can also be a disulfide or another type of functionality. 

COOH)/ Bu (side chains) protections. Trzeciak and Bannwarth [25] described the synthesis of the two cyclopeptides cyc/o(Lys-Arg-Ser-Lys-Gly-Asx), where Asx = Asp or Asn. Fmoc-Asp(OH)-OAl was coupled to the resin using either an amide or a hydroxy linker and TBTU as a condensing reagent (substitution level not reported) and the linear peptides were elon-.gated by standard Fmoc/tBu chemistry (TBTU activation). Subsequently, the allyl ester was cleaved with Pd(PPh3)4 and A -methylaniline in THF-DMSO-0.5 M HCl (2 2 1), the A-terminus was deprotected with piperi-dine-DMF (1 4), and the peptide was cyclized by TBTU/DIEA (1.5 equiv. each, 3 h) activation. Cleavage and deprotection with TFA-H2O (9 1) yielded the expected cyclic products (Scheme 5). 

A different orthogonal protection strategy was reported by Marlowe [75], who prepared a model peptide cyclized between the /V-terminus and an Asp side chain cyc/o(Glu-Ser-Thr-Arg-Pro-Met-Asp-NH2). The third level of orthogonal protection was obtained using a trimethylsilylethyl (TMSE) ester, which is selectively deprotected by fluoride ions. Accordingly, Fmoc-Asp(OTMSE)-OH was loaded onto Rink amine resin and the linear precursor peptide was constructed using Fmoc/rBu chemistry. Treatment... 

Intramolecular nucleophilic attack at the allyl carbon terminus via this allyl nitroso cationic species is feasible. Examples are represented by compounds 34 and 36, ° whose side-chain carboxylate or hydroxyl termini undergo cyclization in basic conditions to give the cyclized products 35 and 37 in reasonable yields (> 50%). 

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