Rink-acid resin

The trialkoxy benzhydrol linker, developed by Rink in 1987 [46] ( Rink acid resin , Figure 3.4) is a further acid-labile linker for carboxylic acids. Esters of this linker can, like trityl esters, be cleaved with acids as weak as acetic acid or HOBt [47], and care must be taken to avoid loss of the product during synthetic operations. 

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). 

Dimethoxyphenylhyciroxymethyl)phenoxynnethyt resin (Rink acid resin)... 

Rink acid resin is highly sensitive to acid. Great care must be exercised not to introduce acid during the synthesis to prevent premature cleavage. 

In another application, the group of Berteina-Raboin demonstrated the solid-supported synthesis of the indole core of melatonin analogues under microwave irradiation (Scheme 7.6) [26]. A benzoic acid derivative was coupled to Rink amide resin by... 

A more recent publication by Weigand and Pelka has disclosed a polymer-bound Buchwald-Hartwig amination [40], Activated, electron-deficient aryl halides were coupled with conventional PS Rink resin under microwave irradiation. Subsequent acidic cleavage afforded the desired aryl amines in moderate to good yields (Scheme 7.22). Commercially available Fmoc-protected Rink amide resin was suspended in 20% piperidine/N,N-dimethylformamide at room temperature for 30 min to achieve deprotection. After washing and drying, the resin was placed in a silylated microwave vessel and suspended in dimethoxyethane (DME)/tert-butanol... 

The cyclization of IV-allyl-o-haloanilines was adapted to the solid phase for both indoles [332, 333] and oxindoles [334]. For example, as illustrated below, a library of l-acyl-3-aIkyl-6-hydroxyindoles is readily assembled from acid chlorides, allylic bromides, and 4-bromo-3-nitroanisole [332], Zhang and Maryanoff used the Rink amide resin to prepare Af-benzylindole-3-acetamides and related indoles via Heck cyclization [333], and Balasubramanian employed this technology to the synthesis of oxindoles via the palladium cyclization of o-iodo-N-acryloylanilines [334], This latter cyclization route to oxindoles is presented later in this section. 

In a later study, an insoluble Rink amide resin was employed with the linkage of the glucuronic-acid-based acceptor through an amide bond (Scheme 4.68) [347,378]. 

Gilley CB, Kobayashi Y (2008) 2-nitrophenyl isocyanide as a versatile convertible isocyanide rapid access to a fused y-lactam (3-lactone bicycle. J Org Chem 73 4198 204 Chen JJ, Golebiowski A, Klopfenstein SR, West L (2002) The universal Rink-isonitrile resin applications in Ugi reactions. Tetrahedron Lett 43 4083 085 Hulme C, Peng J, Morton G, Salvino JM, Herpin T, Labaudiniere R (1998) Novel safety-catch linker and its application with a Ugi/De-BOC/Cyclization (UDC) strategy to access carboxylic acids, 1, 4-benzodiazepines, diketopiperazines, ketopiperazines and dihydroqui-noxalinones. Tetrahedron Lett 39 7227-7230... 

The monomeric peptides [Cys-I], [Cys,Cys(Acm-II], [Cys(Acm),Cys-III], and [Cys-IV] were synthesized by Fmoc chemistry on Rink-amide resin as 5-Mob derivatives and cleaved/deprotected with 1M TMSBr/thioanisole in TFA in the presence of m-cresol and 1,2-ethanedithiol as scavengers. Following gel filtration on Sephadex G-10 with 1M AcOH as solvent and HPLC purification the peptides were obtained in 30—40% yield. Each product was characterized by LSIMS, HPLC, and amino acid analysis. 

A common goal of library synthesis is to maximize diversity around a core, so the synthetic route described above was extended to accommodate a variety of substituted aryl groups. The modified route is shown in Scheme 11. The solid-phase starting material was Rink amide resin,36 which had been loaded with y-bromocrotonic acid (attempts to load with other suitable fragments, such as fumaraldehydic acid, or the pentafluorophenol-activated ester, failed). Substituted ort o-iodoanilines could then be used to alkylate... 

In a subsequent paper on the use of the Stille reaction to form biaryls,58 the reaction was carried out under ambient conditions to allow robotic automation of the process. Attachment of the tin species to resin was very straightforward, in that 4-tri-n-butylstannylphenyl acetic acid was linked to Rink amide resin simply using a DIC coupling. Loading of this species was determined by tin elemental analysis and correlated with a quantitative ninhydrin test of free amines remaining on the support (Scheme 24). 

To Rink amide resin iieylated with. 1-iodobenzyloxy acetic acid (0.2 g, 0.094 mmol) were added DMA (4.7 mL), sodium acetate (23 mg, 0.28 mmol, 3 equiv.), B114NCI (56 mg, 0.20 mmol, 2 equiv.), and dimethyl itaconate (119 mg, 0.75 mmol,... 

Mannich Reaction on a Resin-Bound Indole Scaffold (89) (Fig. 18).50 Commercially available indole 5- or 6-carboxylic acid supported on Rink amide resin is suspended in HOAc/l,4-dioxane (1 4). A secondary amine (1.5 equiv.) and formaldehyde (1.5 equiv.) are added. The suspension is shaken for 1.5 h at RT. The resin is then filtered, washed with appropriate solvents, and dried under vacuum to obtain (90). 

Chen and co-workers at Procter and Gamble developed a traceless synthesis of 2,5-diketopiperazines [18b] by employing the universal Rink-isocyanide resin. The Ugi-4CR between the resin, aldehydes, amines, and N-Fmoc-protected a-amino acids afforded the resin-bound dipeptide derivatives 131 which were N-deprotected on treatment with piperidine in DMF. Cyclization by heating with 10% AcOH in DCE smoothly provided the desired diketopiperazines 132 in good yields (Scheme 2.47). 

The synthesis of a linear precursor peptide of the HSV-a-[Tyr ]-conotoxin chimera (see Note 8) was performed by solid phase methodology on an Applied Biosystems Mod. 431A peptide synthesizer (Applied Biosystems Paris, France). Fmoc-based chemistry was used for the preparation of peptide using Rink-amide resin with 0.43 mmol/g loading. Four equivalents of Fmoc amino acid derivatives and HBTU/HOBt were used for coupling. The following amino acid derivatives were used in the synthesis Fmoc-Cys(Trt)-OH, Fmoc-Gly-OH, Fmoc-Val-OH, Fmoc-Pro-OH, Fmoc-Cys(Acm)-OH, Fmoc-Asn(Trt)-OH, Fmoc-Tyr( Bu)-OH. The synthetic protocol for Fmoc chemistry was as follows ... 

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