Linkers, nucleophile-labile

The wide use of acid-labile linkers and protecting groups in peptide SPS has reduced efforts toward the development of base-labile linkers. The commonly used SP Fmoc peptide coupling protocols require Fmoc deprotection under basic conditions during the synthesis, thus ruling out base-labile linkers. However, base-labile linkers are popular in oligonucleotide SPS and will be described in Section 2.2. Other examples of base- or nucleophile-labile linkers are shown in Fig. 1.10. 

The oc-mercaptobenzhydryl linker 27 [71] was developed for the synthesis of thioacids by Boc synthesis. Acylation of the thiol group with Boc-amino acids leads to a resin-bound thioester. Subsequent peptide chain extension and then treatment with HF cleaves the ester-linker bond, releasing the C-terminal peptide thioacid. Unfortunately, owing to the nucleophile lability of thioesters, this approach cannot be applied to Fmoc-based methods. 

The cleavage of organic compounds from nucleophile-labile resins usually relies on the addition-elimination chemistry at the carbonyl group of the carboxylic acid derivative (i.e. an ester or thioester) that mediates the linkage between the assembled compound and the linker-resin. The overall reaction is a nucleophilic substitution that involves the release into the solution of the compound of interest with the attacking nucleophile generally incorporated. The linker-resin generally acts... 

Figu re 15.2 Nucleophile labile resins and linkers. An ester/thioester derivative is shown (R-CO-Nu will be the released product, R is generally alkyl or aryl, unless otherwise indicated). Linkers are also displayed anchored to suitably functionalized resins. 

The successful assembly of organic compounds on a solid support represents only part of the challenge in SPOS. After completion of synthetic sequence, the compounds must be cleaved from linkers attached to polymer by a chemical or photochemical reaction, for example, treatment of a polymer-bound compound with acids, bases, nucleophiles, redox reagents, and even photons. Acid-labile linker and amine-cleavable Marshall linker are two major classes of hnkers used in combinatorial synthesis. 

The Marshall Unker [23] has been widely used to synthesize compounds that can be cleaved by primary and secondary amines to afford the corresponding amides. Marshall linker was used in the synthesis of three or more diversity-site hbraries because it allowed the addition of one more diversity element at the cleavage step. While the original reported linker [23] involved the oxidation of the Unker before cleavage, the efficient release of the resin-bound compounds using nucleophiles from the unoxidized linker has been reported [16, 24]. Similarly to the acid-labile linkers, the kinetics of the cleavage reaction and time required for this reaction directly affect the synthesis efficiency, purity and yield of the final products. A cleavage study was carried out on seven resin-bound thiophenol esters (34—40) on Marshall Unker with 3 amines (41-43) (Scheme 12.11 and Tab. 12.4). 

After capping (i.e. acetylation of the remaining hydroxyl groups), the resulting functionalized cellulose can be used for the synthesis of peptides using both the Boc and Fmoc methodologies. Because cellulose is hydrolyzed by hydrogen fluoride (but not by TFA), nucleophilic saponification must be used to cleave the peptides from the support [209]. Alternatively, the support can be derivatized with a TFA-labile linker [211]. 

Primary and secondary aliphatic amines can be linked to polymeric supports by acid-labile linkers or by linkers sensitive to nucleophiles. Linkers cleavable by light or by transition metal catalysis have also been described. The main types of linker for amines are sketched in Figure 3.24. 

Polystyrene-derived phenylboronic acids have been used for the attachment of diols (carbohydrates) as boronic esters [667]. Cleavage was effected by treatment with acetone/water or THF/water. This high lability towards water and alcohols severely limits the range of reactions that can be performed without premature cleavage of this linker. Arylboronic acids esterified with resin-bound diols can be oxidatively cleaved to yield phenols (Entry 8, Table 3.36). Alcohols have also been prepared by nucleophilic allylation of aldehydes with polystyrene-bound, enantiomerically enriched allyl-silanes [668], as well as by Pummerer reaction followed by reduction of resin-bound sulfoxides [669]. 

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). 

Seven noncommercial acid-labile linkers have been reported recently in the literature and are shown in Figs. 1.8. (1.7-1.10) and 1.9 (1.11-1.13). The THP (tetrahy-dropyran) linker 1.7 (63), which is easily grafted onto Merrifield resin, has been used to support primary alcohols, secondary alcohols, hydroxylamines, and carboxylic acids. It is stable to strong nucleophiles and basic conditions and can be cleaved by... 

The Kenner sulfonamide-based SC linker 1.27 was supported on PS resin (84) allowing the attachment of carboxylic acids or amino acids to the sulfonamide function. After synthetic elaboration, treatment with diazomethane produces the A-methylacylsulfonamide, which can be cleaved with nucleophiles such as 0.5 N NHs-dioxane or hydrazine-MeOH, 0.5 N NaOH, releasing amides, hydrazides, or carboxylic acids, respectively. A modification using iodoacetonitrile produces the more labile A-cyanomethyl derivative, which can be cleaved completely with stoichiometric amounts of amines to release the corresponding amides into solution. 

The safety catch principle for the solid-phase preparation of C-ternninal modified peptides requires the bond between the handle and the first residue to be stable to the normal conditions of SPPS. However, at the end of the synthesis, a chennical transformation of the linker substituent makes the key bond labile to nucleophiles. The 4-sulfanylphenol 44b ti and the sulfonamide 45 (Kenner) resins are two examples of safety catch resins. Thus, in the first case, which is only compatible with the Boc/Bzl strategy, once the peptide chain is elongated, treatment of the peptide-resin with hydrogen peroxide converts the sulfide into the corresponding sulfone, which makes the bond labile to nucleophiles. In the second case, N-methylation with diazomethane leads to an N-methylated peptidyl-sulfonannide-resin, from which peptides can also be cleaved by nucleophiles. 

The hydroxyl version of the Rink amide linker, known as the Rink acid resin (25), was developed as a tool for the preparation of protected peptide fragments [13]. The peptide-linker ester bond is labile to extremely weak acids, such as HOBt or acetic acid, allowing peptides bearing t-butyl-based side-chain protection to be cleaved intact. Conversion of the hydroxyl group into chloride [66] or trifluoroacetyl [67] provides linkers that have been used for immobilization of various nucleophiles, including alcohols, N-protected hydroxylamines, phenols, purines, amines, anilines and thiols [66-68], The stability of the cation derived from this tinker is such that even thiols and amines can be cleaved from this tinker with TFA (Figure 14.12). 

For small molecule synthesis, linker-functional group combinations that are labile to TFA tend to be selected. In the absence of nucleophilic functionalities such as those described above within the target molecule, simple mixtures of TFA and dichloro-methane tend to be used for release of the product from the support as this mixture is volatile and so makes work up of the reaction a simple matter of evaporation. 

Below, the usefulness of nucleophile- and base-labile linkers and resins is reviewed in the preparation of various organic compounds. First, intermolecular nucleophilic displacement (saponification, transesterification and amminolysis and related reactions) are analyzed. Finally, the obtention of cyclic products by intramolecular nucleophilic reactions is reviewed. 

---