Solid-phase peptide synthesis linkers

Zheng A, Shan D, Shi X, Wang B. A model resin linker for solid-phase peptide synthesis which can be cleaved using two sequential mild reactions. J Org Chem 1999 64 7459-7466. 

Polystyrene resin with a hydroxymethylphenoxy linker (Wang resin)1 was originally developed for solid-phase peptide synthesis... 

One of the very first papers reporting about endo-linkers was published by Elmore et al. (Scheme 10.4) [13]. They described a new linker containing a phos-phodiester group (19) for solid-phase peptide synthesis using a Pepsyn K (polyacrylamide) resin. After completion of coupling and deprotection cycles, the phos-phodiester (20) was cleaved with a phosphodiesterase. In this way / -casomorphin. Leu-enkephalin and a collagenase substrate (21) were synthesized in high yields. 

In preparing these various libraries, extensive use is made of solid phase synthetic methods. These methods are all derived from the solid phase peptide synthesis (SPPS) method developed by Merrifield in 1963. When performing a large number of syntheses, it is preferable to perform the synthetic steps on a solid bead rather than completing the entire synthesis in the solution phase. The solid-phase technique makes byproduct removal and final compound purification easier. The organic chemistry literature contains a wealth of different types of solid-phase supports and novel linkers for attaching the synthetic substrate to the bead. 

A more versatile route to synthesize peptide thioesters is by the preparation of a peptide thioacid via the solid-phase method and then reaction with a halo derivative to form the thioester. This route can be achieved via the preparation of thioester linkers that can be applied in stepwise solid-phase peptide synthesis. The general structure of the N-protected amino thioacid attached to the linker is shown in Scheme 14. t65-80 ... 

The linker 37 with the first amino acid attached, compound 38, can be applied to stepwise solid-phase peptide synthesis. At the end of the synthesis, when the desired peptide sequence is completed, the thioacid-modified peptide fragment is cleaved from the solid support by HF and further S-alkylated with a N-bromoacetylated peptide to form an endothioester bond. The cleaved thioacid can also be reacted with an alkyl bromide to form the corresponding thioester. 

Esters of the PAM linker are slightly more resistant towards acids than the corresponding 4-alkylbenzyl esters [5,25-27] (Table 3.1). The PAM linker is particularly well suited for solid-phase peptide synthesis using A-Boc amino acids because less than 0.02% cleavage of the peptide from the support occurs during the acidolytic deprotection steps [27], Esters of both the 4-alkylbenzyl alcohol and PAM linkers can also be cleaved by nucleophiles (see Sections 3.1.2 and 3.3.3). 

The two most commonly used types of allyl alcohol linker are 4-hydroxycrotonic acid derivatives (Entry 1, Table 3.7) and (Z)- or ( )-2-butene-l, 4-diol derivatives (Entries 2 and 3, Table 3.7). The former are well suited for solid-phase peptide synthesis using Boc methodology, but give poor results when using the Fmoc technique, probably because of Michael addition of piperidine to the a, 3-unsaturated carbonyl compound [167]. Butene-l,4-diol derivatives, however, are tolerant to acids, bases, and weak nucleophiles, and are therefore suitable linkers for a broad range of solid-phase chemistry. 

Support-bound triacylmethanes (e.g. 2-acetyldimedone) readily react with primary aliphatic amines to yield enamines. These are stable towards weak acids and bases, and can be used as linkers for solid-phase peptide synthesis using either the Boc or Fmoc methodologies, as well as for the solid-phase synthesis of oligosaccharides [456]. Cleavage of these enamines can be achieved by treatment with primary amines or hydrazine (Entries 2 and 3, Table 3.23 see also Section 10.1.10.4). 

Benzyl carbamate protection (Cbz or Z group see Table 10.15) was initially chosen by Merrifield for solid-phase peptide synthesis [255], The strongly acidic conditions required for its solvolysis (30% HBr in AcOH, 25 °C, 5 h) demanded the use of an acid-resistant nitrobenzyl alcohol linker. Z-protection of the a-amino group in solid-phase peptide synthesis was, however, quickly abandoned and replaced by the more acid-labile Boc protection. Benzyl carbamates can be cleaved by strongly ionizing... 

The first solid-phase peptide synthesis reported by Merrifield had some disadvantages, which were corrected in later versions of this synthesis. The main improvements were the replacement of benzyloxycarbonyl protective groups by the TFA-labile Boc protection, and the use of TFA-resistant linkers cleavable by HF. 

Solid-phase peptide synthesis is based on the sequential addition of protected amino acids onto an insoluble support. Addition proceeds from carboxy terminus to amino terminus. The first amino acid is attached to a solid support by a linker and, if necessary, side-chain amino acid function is protected throughout chain assembly. The carboxy group of the in-coming, acylating amino acid is activated for coupling while its amino group is protected temporarily for each coupling step and then deprotected for the next cycle. The... 

Scheme 2.7.2. Convergent solid-phase peptide synthesis of the library 15 of cyclic neoglyco-peptides on amino-functionalized TentaGel without employing a linker [17]. (All = allyl, Aloe = allyloxycarbonyl, Bal = / -alanine,...

Trityl resins are particularly suitable for immobilization of nucleophilic substrates such as acids, alcohols, thiols, and amines. They are quite acid-sensitive and are cleavable even with acetic acid this is useful when acid-labile protecting groups are used. The stability of trityl resin can be tailored by use of substituted arene rings, as shown by chlorotrityl resin, which furnishes a more stable linker than the trityl resin itself. Steric hindrance also prohibits formation of diketopiperazines during the synthesis of peptides. Orthogonality toward allyl-based protective groups was demonstrated in the reverse solid-phase peptide synthesis of oligopeptides [30] (Scheme 6.1.4). 

Over the years, a number of cleavable linkers that are acid labile, base labile, or photol-abile have been developed for solid-phase peptide synthesis. (This topic has been covered in detail by several review papers [34-36].) For libraries that require the linker to be cleaved before screening, most of these conventional linkers can be used. Several unconventional linkers have been found to be particularly useful and user-friendly for combinatorial applications (see Fig. 1). Among them are methionine-containing linker [37] and safety-catch benzylhydrylamine linker 1 [38], Bray et al. [39] have utilized an orthogonal peptide-resin linker 2 which allows the final deprotection and removal of contaminating chemicals and the peptide is later released into an aqueous buffer. Hoffmann and Frank [40] recently described a novel safety-catch linker 3 based on the intramolecular catalytical... 

The purpose of this chapter is to delineate strategic considerations and provide practical procedures to enable non-experts to synthesize peptides with a reasonable chance of success. This chapter focuses on Fmoc chemistry, which is now the most commonly employed strategy for solid phase peptide synthesis (SPPS). Protocols for the synthesis of fully deprotected peptides are presented, together with a review of linkers and supports currently employed for SPPS. The principles and the different steps of SPPS (anchoring, deprotection, coupling reaction, and cleavage) are all discussed, along with their possible side reactions. 

Key Words Solid phase peptide synthesis side reaction coupling anchoring deprotection cleavage linker. 

Gu, W. and Silverman, R. B. (2003) New stable backbone linker resins for solid-phase peptide synthesis. Org. Lett. 5, 415 18. 

Alsina, J., Yokum, T. S., Albericio, F., and Barany, G. (2000) A modified backbone amide linker (BAL) solid-phase peptide synthesis strategy accommodating prolyl, A-alkylamino acyl, or histidyl derivatives at the C-terminus. Tetrahedron Lett. 41, Hll-11%0. 

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