Resins, for SPPS

The acid chloride method has been successfully used for the synthesis of depsipeptides by O-acylation of hydroxy acids. Although Fmoc amino acid chlorides were recommended for such reactions, and can be used for the introduction of the first amino acid onto hydroxy resins for SPPS, oxazol-5(4//)-one formation promoted by the presence of the required tertiary amine leads to relatively low yields compared to other activation methods such as those involving UNCA groups. ... 

Barlos resin, an o-chlorotritylchloride resin for SPPS. The steric constraints of the o-chlorotrityl handle impedes dike-topiperazine formation on the dipeptide stage. Qeavage occurs upon treatment with 0.5% trifluoroacetic acid (TEA) in dichloromethane [K. Barlos et al., Int. J. Pept. Protein Res. 1991, 37, 513]. 

The most common resins for SPPS are based on polystyrene (PS), typically with 1 % cross-linking. A starting point for the production of many other resins is provided by chloromethyl-polystyrene (Merrifield resin), made either by chloromethylation of polystyrene or, better, by copolymerization to direcdy incorporate the chloromethyl moiety. Another important base resin is aminomethyl-polystyrene. Although chloromethyl-polystyrene can be used for Boc-SPPS, in by far most cases a dedicated linker (handle) is inserted between the base resin and the first amino acid. 

The biberty (Fig. 10), a monomode microwave reactor for automated SPPS, was recently introduced by the CEM Corporation [153]. Although this instrument was originally developed for SPPS, it also allows for a broader scale of solid-phase applications. The solid-phase vial is equipped with a polypropylene frit and cap at one end (the entire assembly fitting into the standard 10 mb CEM reaction vessel) to allow the processing of 0.1 to 1.0 mmol quantities of resin attached substrates. An integrated fiber optic probe provides... 

Table 6 Common Protection Schemes Used for SPPS and On Resin...

Peptides containing the general structure 84 (Scheme 39) underwent between 50-100% cleavage in 95%TFA/H20 at room temperature in 60 minutes, that is, under routine conditions for the removal of a peptide from the resin after SPPS. The hydrolysis yielded the N-acyl-A-methyl peptide fragment 88.12041 The same peptides were also cleaved in solution but at a much slower rate (13-18 h). The hydrolysis occurs only when a carbonyl group is present at the N-terminal side of the N-methylated residue. The hydrolysis was accompanied by racemization of the N-methylated residue. The suggested mechanism involves the formation of the oxazolonium tetrahedral intermediate 87. This hydrolysis was circumvented by cleavage of the peptide (that did not contain functional side chains) from the resin with 0.1% NaOH in 50% aq MeOH (rt, 60 min). 

The MBHA resin was acylated in CH2CI2 with Boc N-protected linker in the presence of BOP as coupling agent and DIPEA as base. After 4h and classical washings, the coupling was monitored by Kaiser s test. The Boc N-temporary protection was removed by treatment with TFA/CH2Q2 (1 1) for 1 h. The resin modified with the linker was ready for SPPS. 

Since azides are smoothly reduced on solid supports with SnCybenzenethiol/TEA (5 mmol/ 25 mmol/25 mmol for 1 mmol resin-bound azide rt, 2 this alternative procedure has been exploited for the synthesis of oligoureas on resin using a-azido acids.Similarly, azides are efficiently reduced to amines with dithiothreitol (DTT),P 1 a procedure that was successfully transferred to the reduction of a-azidoacyl peptides on resin for the SPPS with a-azido acids.In the case of sterically hindered azides, addition of small amounts of 2-sulfanylethanol to the DTT/DIPEA mixture or the use of DBU as a base enhances the reaction rates (Scheme Other reductions of azides include the use of H2S/pyridine/... 

Besides the classical polymer introduced by Merrifield (1%-crosslinked chloromethylated polystyrene), a broad variety of polymeric supports is available for SPPS and some of the most popular resins are summarized in Table 1. The chemical structures of some selected resins are presented in Figure 1 and electron micrographs of several examples are displayed in Figure 2. In addition to the solid supports listed in Table 1, there are several other carriers used in peptide synthesis such as the gel-type and macroporous poly(meth-acrylates), coated surfaces like polystyrene films on polyethylene (PEt) sheets, polystyrene-coated polyethylene or polytetrafluoroethylene, and modified glass surfaces. (For recent reviews on polymeric carriers see refs . )... 

Probably the one greatest advantage of SPPS is that all of the synthetic operations are carried out in a single vessel after addition of the resin carrying the first (C-terminal) protected amino acid. Thus, the many manipulations of peptide synthesis in solution, with conconoitant losses of material, are eliminated. Vessels for SPPS must ... 

These resins have their roots in the polyacrylamide resins introduced in the 1980s for SPPS using the fluorenylmethoxycarbonyl (Fmoc)-tert-butyl (tBu) strategy. They were developed following the concept that the insoluble support and peptide backbone should have comparable polarities [44]. 

Solid supports ultimately require a functional group, e.g., an amino, hydroxyl, chloromethyl, or other functionality, that can be a starting point for SPPS. In some cases, a suitable functionalized monomer is included in the polymerization mix that gives the final support. Alternatively, the desired functional group can be introduced by a postpolymerization transformation [15]. The classic example of functionalization of a solid support is the chlo-romethylation of cross-linked polystyrene to produce chloromethyl-resin, also known as Merrifield resin [31]. This may then be modified further to introduce other functional groups such as aminomethyl or hydroxymethyl... 

Hycram resin, hydroxycrotonoylamidomethyl resin, an allyl-based handle resin connection for SPPS of peptides and, especially, for glycopeptides, which is stable to base and mild acid. The cleavage of the final product from the resin occurs under neutral conditions by treatment with (Ph3P)4Pd/THF/morpholine, and does not affect Boc, Fmoc, or glycosidic bonds [H. Kunz, B. Dombo, Angew. Chem. Int. Ed. 1988, 27, 711]. 

Polystyrene resin, frequently used resin material for solid-phase peptide synthesis (SPPS). The polymeric support for SPPS must be chemically inert, mechanically stable, completely insoluble in the solvents used, and easily separated by filtration. For many applications a copolymer of polystyrene with 1% of divinyl benzene as crosslinker is used. The dry resin beads are able to swell up to the five-or sixfold volume in the different organic solvents mainly used for peptide synthesis (e.g., dichloromethane or dimethylfor-mamide). For SPPS the resin material must be chemically functionalized in order to allow for attachment of a handle/liker (e.g. Wang resin), or the first amino acid (—> Merrifield resin). Hydrophilic tentacle polymers gels (TentaGel) are obtained by grafting polyethylene glycol (PEG) chains with an arbitrary degree of polymerization onto porous polystyrene beads. 

Xanthenyl resin, a highly add-labUe resin suitable for SPPS of proteded peptide amide segments [P. Sieber, Tetrahedron Lett. 1987, 28, 2107]. 

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