Linker Succinate

New amino acid-tetrahydropterin conjugates were synthesized from A2-isobutyryl-6,7-dimethyl-5,6,7,8-tetrahydropterin (169) by either direct acylation (170) or via a succinate linker (171) followed by removal of protecting groups and ammonia treatment to deblock selectively the isobutyryl group (Scheme 26) <89MI 718-08). 

Diacid-based linkers, such as the succinic linker 21, have been described to prepare alcohols. The procedure involves the esterification of the starting alcohol with succinic anhydride and DMAP to yield the hemiester that is anchored to an amino containing-resin by means of an amide bond. The bound alcohol is then elaborated and finally released with a nucleophile. Oligosaccharides have been assembled following this approach and released with aqueous ammonia or sodium methoxide in methanol-dioxane [73, 74]. Peptide alcohols have also been prepared with the succinic linker on BHA resin and released by treatment with NH3 in MeOH for 72-96h or hydrazine in DMF for 24h [75]. Similarly, hydroquinone-0,0 -diacetic acid (linker 22) has been used to link nucleosides to polystyrene or CPG supports. Cleavage of oligonucleotides was carried out with aqueous ammonia [76]. Other diacids with a similar function have also been described [77]. 

Moving toward longer aliphatic carboxylates is a succinate FIJYAZ [1 2(0411404)3(1120)2] H2O (Fig. 8). A 3D framework is observed in this structure, the neutrality of which is anticipated from the 2 3 Ln/linker ratio. Bridging bidentate and bridging tridentate coordination is observed in this compound, with each displayed by the single, unique succinate linker. Glutarate... 

Minko and co-workers linked paclitaxel via succinate linker to PAMAM G4 dendrimers and could show that the drug could be released when the ester bonds were hydrolyzed by esterases. The dendritic carrier was more efficient in cell internalization and showed better cytotoxicity than the free drug and a linear PEG-drug conjugate which proves that dendritic nanocarriers are a highly potent drug delivery platform [82]. 

Succinic anhydride also is a convenient extender for creating spacer arms on chromatography supports. Supports derivatized with amine-terminal spacers may be succinylated to totally block the amine functionalities and form terminal carboxylic acid linkers for coupling amine-containing affinity ligands (Cuatrecasas, 1970). 

Attachment of suitable linkers to the surface of silica can be achieved by transesterification with (3-aminopropyl)triethoxysilane, which leads to the support 2 (Figure 2.8) [198-200]. Alternatively, silica can be functionalized by reaction with alkyltri-chlorosilanes [201]. For the solid-phase synthesis of oligonucleotides, supports with a longer spacer, such as that in 3, have proven more convenient than 2 [202-206]. Supports 3, so-called LCAA-CPG (long chain alkylamine CPG [194,195]), are commercially available (typical loading 0.1 mmol/g) and are currently the most commonly used supports for the synthesis of oligonucleotides. For this purpose, protected nucleosides are converted into succinic acid monoesters, and then coupled to LCAA-CPG. CPG functionalized with a 3-mercaptopropyl linker has been used for the solid-phase synthesis of oligosaccharides [207]. 

In 1996, Gauglitz and coworkers coated surfaces with various amino-and carboxy-substituted polymers [198], The polymers tested were branched poly-(ethyleneimine), a,co-amino-functionalized PEG, chitosan, poly(acrylamide-co-acrylic acid) and an amino-modified dextran. The amino-substituted polymers were immobilized on glass by first immobilizing an aminosilane, followed by succinic anhydride/A-hydroxysuccinimide linker chemistry. Poly(acrylamide-co-acrylic acid) was directly coupled to an aminosilanized surface. When probed with 1 mg mL 1 ovalbumin solution, nonspecific adsorption was lowest for the dextran derivative. Notably, nonspecific adsorption increased in most cases when a hydrophobic hapten (atrazine) was coupled to the polymer-modified surface. 

An ester bond is widely used, and the succinate 2.29 (91) is one of the most popular linkers. It is introduced onto the LCAA-CPG support as the 3 -monosuccinate ester of the first nucleoside, and it is cleaved under basic conditions in the final SPS step. The inclusion of a sarcosine spacer as in 2.30 (92) increases the linker stability... 

The commercially available 3-amino-1,2,4-dithiazole-5-one, (3), has been attached to a hydroxyl resin via a succinic acid linker, and has been used as an efficient sulfur-transfer reagent for the solution-phase synthesis of phos-phorothioates. DNA synthesis scale-up to lOOg of a 20-mer phosphorothioate has been examined " key to this was the purification of the product, which was carried out using a high efficiency polymeric anion exchange chromatographic media. 

Oxalic acid linker is more base-stable than succinic acid linker. The linker remains on the resin. 

A succinyl anchor is commonly cleaved by concentrated aqueous ammonia at ambient temperature for 1-2 h. It is possible to largely preserve the integrity of the succinyl linker and retain most of the N,P-deprotected oligonucleotide on the support by using ethanolamine deprotection [210]. The main problem with the succinate ester attached to the primary amino group on polymer is its propensity for base-catalyzed cycfization into the corresponding succinimide [211], similarly to the aspartimide formation in the Fmoc peptide synthesis. This unwanted side-reaction can be prevented by joining the succinate to the secondary amine, for... 

A simple ammonia-labile linker (Figure 19.11) has been prepared by Roland et al. [272] from 4-hydroxybenzyl alcohol, dimethoxytritylated at the aliphatic hydroxyl and attached through the phenol to the succinylaminopropyl CPG (60-90 pmol per g of DMTr). After the oligonucleotide assembly, the 3 -phosphate or phospho-rothioate is produced after succinate hydrolysis followed by the elimination of quinone methide. The support has been used to make a library of 3 -thiophos-phorylated dinucleotides. 

In the case of mPEG N-succinimidyl succinate, the reaction also leads to a stable amide linkage to protein amino groups, but the linker to PEG bears an ester group which is hydrolyzed in vivo by esterases [16]. 

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