Wang resins, peptide synthesis

Solid-phase peptide synthesis. 1036-1038 PAM resin in. 1037 Wang resin in, 1037 Solvation, 370... 

Walden, Paul, 360 Walden inversion. 359-360 Wang resin, solid-phase peptide synthesis and. 1037 Water, acid-base behavior of, 50 dipole moment of, 39 electrostatic potential map of. 53 nucleophilic addition reactions of, 705-706 pKaof, 51-52... 

The investigation of minor groove-binding polyamides was greatly accelerated by the implementation of solid-phase synthesis [48]. Originally demonstrated on Boc-y9-Ala-PAM resin with Boc-protected monomers, it was also shown that Fmoc chemistry could be employed with suitably protected monomers and Fmoc-y9-Ala-Wang resin (Fig. 3.8) [49]. Recently, Pessi and coworkers used a sulfonamide-based safety-catch resin to prepare derivatives of hairpin polyamides [50]. Upon activation of the linker, resin-bound polyamides were readily cleaved with stoichiometric quantities of nucleophile to provide thioesters or peptide conjugates. 

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. 

One of the first dedicated applications of microwaves in solid-phase chemistry was in the synthesis of small peptide molecules, as described by Wang and coworkers [22]. As a preliminary test, the authors coupled Fmoc-Ile and Fmoc-Val, respectively, with Gly-preloaded Wang resin using the corresponding symmetric anhydrides (Scheme 7.1). 

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

JK Chang, M Shimuzu, S-S Wang. Fully automated synthesis of fully protected peptide hydrazides on recycling hydroxymethyl resin. J Org Chem 41, 3255, 1976. 

Solid-phase synthesis was performed using Wang resin (0.57 mmol-g 1 0.35 g, 0.2 mmol) with the coupling of Fmoc amino acids and Fmoc-Hse[PO(OPh)2]-OH performed using PyBOP (3 mol equiv) and NMM (5 mol equiv) in DMF. The peptide-resin was treated with TFA/H20/TIS (95 2.5 2.5 5 mL) at rt for 2h, the resin filtered under N2, and washed with TFA (4mL). The solvent was removed under reduced pressure, the peptide precipitated by the addition of Et20 (20 mL), triturated with Et20 (2 x 10 mL), and dried under high vacuum. 

The first applications of solid phase synthesis were to peptides, hence it is no surprise that there have been several reports of HRMAS studies of peptide systems attached to a support. One of the earliest reports of HRMAS in a supported sample was that of Wang-bound lysine, whose structure was determined by TOCSY and HMQC HRMAS NMR.38 More recently, HRMAS NMR has been used to identify several peptidomimetic inhibitors of hepatitis C virus NS3 protease while on the resin.79 However, it is perhaps a bit surprising that more has not been made of HRMAS in attacking problems of relevance to peptide synthesis, although most recent interest is moving that way. Combinatorial chemistry and solid phase organic chemistry has been a much more active area using HRMAS techniques. 

In a parallel study, Wipf and Fritch11041 have shown that also urethane-protected (Boc), and even amino acid segments, are tolerated as acyl compounds on the aziridine nitrogen. The best results were obtained with alkylcopper reagents derived from CuCN and an alkyl-lithium in the presence of boron trifluoride-diethyl ether complex. Some 6-alkylated compounds (11-15%) were isolated as well. This work was extended to a solid-phase procedure that resulted in resin-bound alkene isosteres that could immediately be used in further peptide synthesis.11051 For this purpose, the 2-nitrophenylsulfonyl (oNbs) group was used for nitrogen protection and aziridine activation. It could be readily cleaved with benzenethio-late, which was compatible with the acid-sensitive Wang linker used. 

Standard solid-phase peptide synthesis requires the first (C-terminal) amino acid to be esterified with a polymeric alcohol. Partial racemization can occur during the esterification of N-protected amino acids with Wang resin or hydroxymethyl polystyrene [200,201]. /V-Fmoc amino acids are particularly problematic because the bases required to catalyze the acylation of alcohols can also lead to deprotection. A comparative study of various esterification methods for the attachment of Fmoc amino acids to Wang resin [202] showed that the highest loadings with minimal racemization can be achieved under Mitsunobu conditions or by activation with 2,6-dichloroben-zoyl chloride (Experimental Procedure 13.5). iV-Fmoc amino acid fluorides in the presence of DMAP also proved suitable for the racemization-free esterification of Wang resin (Entry 1, Table 13.13). The most extensive racemization was observed when DMF or THF was used as solvent, whereas little or no racemization occurred in toluene or DCM [203]. 

Because no treatment with acid is required during peptide assembly, peptide synthesis with Fmoc amino acids can be conducted on acid-sensitive supports (e.g. Tenta-gel) and with acid-labile linkers. Wang resin is suitable for most purposes, but other supports, such as Sasrin or 2-chlorotrityl resin, can also be used. CPG, macroporous... 

As for Boc amino acids, all proteinogenic and several non-natural Fmoc amino acids esterified with Wang or similar resins are commercially available. Deprotection with 20% piperidine in DMF is usually complete within a few minutes, and the release of the fluorene derivative is easily monitored spectrophotometrically at 300-320 nm (see, e.g., [33]). The peak area can be used to determine the amount of chromophore released, which is proportional to the efficiency of the preceding coupling reaction. At the end of an automated peptide synthesis, inspection of the chromatogram of all Fmoc releases enables rapid assessment of the quality of the resin-bound peptide and quick location of positions in the peptide where coupling was unsuccessful or difficult. 

The A -diphenylmethylene protection has also been used in the solid-phase mode for the synthesis of either unnatural amino acidst 1 or peptides (Scheme 67)J 1 Thereby, both Merrifield or Wang resins were used and the best base proved to be the organic soluble, nonionic phosphazene bases of Schwesinger, e.g. 2-tert-(butylimino)-2-(ethylamino)-l,3-di-methyl-l,3,2-diazaphosphinane (BEMP). As electrophiles alkyl halides,aldehydes, and Michael acceptors have been used. 

To utilize the N -protected hydrazide approach for solid-phase synthesis, different hydrazide linkers for polystyrene resins were developed by Wang and MerrifieldP l (Table 2). Generally, the first N -protected amino acid is attached to the linker by treatment with DCC in presence of additives (HOBt, HOSu). The Bpoc groupf which is readily cleaved with 0.5% TFA in dichloromethane in 15 minutes at room temperature, was used as temporary N -protection. After completion of the peptide synthesis on the solid support, the hydrazides are cleaved from the resin with 50% TFA in 30 minutes. The dibenzocycloheptadienyl-hydrazide linker allows the synthesis of protected hydrazides by the Fmoc/tBu strategy, as the hydrazides are cleaved from the resin with 1% TFA in dichloromethane in less than 3 hours. 

For the Fmoc/tBu approach, several handles and resins are available for the synthesis of free peptide acids. Among the most conunonly used are the Trt(2-Cl)-Cl resin 2, the 4-alkoxybenzyl alcohol resin (Wang resin, 7),P 1 and the 4-(hydroxymethyl)phenoxyacetic acid (HMPAA, and 3-[4-(hydroxymethyl)phenoxy]propanoic acid (HMPPA, 9)1 han-... 

Human a-thrombin and the fluorogenic substrate (Tos-Gly-Pro-Arg-AMC HCI) were purchased from Sigma. Fmoc derivatives of amino acids were purchased from Advanced ChemTech and Novabiochem. N-a-Fmoc-N-y-trityl-L-Gln-Wang resin was purchased from Applied Biosystem Inc. The solvents used in peptide synthesis were obtained from Anachemia Chemical Inc. and Applied Biosystems Inc. 

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