Protection of the indole nucleus of tryptophan in solid-phase peptide synthesis with a dipeptide that can be cleaved rapidly at physiological pH

The synthesis of a new derivative of tryptophan Fmoc-Trp(Boc-Sar-Sar)-OH is described. Fmoc-Trp(Boc-Sar-Sar)-OH is introduced into peptides by solid-phase peptide synthesis and during treatment with TFA at the end of the synthesis, the Boc group is cleaved and the peptide is obtained with the indole nucleus modified with the sarcosinyl-sarcosinyl (Sar-Sar) moiety. This modification will introduce a cationic charge that improves the solubility of the peptide during HPLC purification. The Sar-Sar moiety is cleaved upon exposure to physiological pH. The Boc-Sar-Sar group might, therefore, also find use in the design of pro-drugs of indole-containing compounds.     

A carbamoyl-protective group for tyrosine that facilitates purification of hydrophobic synthetic peptides

The Boc-N-methyl-N-[2-(methylamino)ethyl]carbamoyl group (Boc-Nmec) is reported as a new side chain-protective group for tyrosine in Fmoc solid-phase peptide synthesis. Tyrosine is incorporated into the peptide as Fmoc-Tyr(Boc-Nmec)-OH by standard coupling methods. During the cleavage of the peptide from the resin with TFA the Boc group is simultaneously cleaved while the cationic N-methyl-N-[2-(methylamino)ethyl]carbamoyl group remains attached to the tyrosine residue, thereby increasing the solubility of the peptide. After purification of the peptide, the Nmec protective group can be cleaved under neutral or mild alkaline conditions via an intramolecular cyclization reaction.     

Propargyloxycarbonyl as a protecting group for the side chains of serine, threonine and tyrosine

Propargyloxycarbonyl group is used as a protecting group for the hydroxyl groups of serine, threonine and tyrosine. The propargyloxycarbonyl derivatives of these hydroxy amino acids are stable to acidic and basic reagents commonly employed in peptide synthesis. The deprotection of the O-Poc derivatives using tetrathiomolybdate does not affect commonly used protecting groups such as N-Boc, N-Cbz, N-Fmoc, methyl and benzyl esters. The di-and tripeptides synthesized using O-Poc derivatives of serine, threonine and tyrosine are stable, isolable compounds and give the hydroxy peptides in good yields when treated with tetrathiomolybdate.     

Hydrazine hydrate: A new reagent for Fmoc group removal in solid phase peptide synthesis

In solid-phase peptide synthesis using the Fmoc/tBu strategy (SPPS-Fmoc/tBu), an orthogonal protection scheme of amino acids is used; specifically, the alpha-amine group is protected by the 9-fluorenylmethyloxycarbonyl (Fmoc) group, which is removed by weak bases, while side chains are protected by groups that are acid labile. We demonstrated that hydrazine hydrate is an efficient reagent for eliminating the Fmoc group in SPPS-Fmoc/tBu. First, experimental conditions were established for Fmoc group removal from Fmoc-Val-OH in solution. It was determined that the Fmoc group was completely removed with 16% hydrazine hydrate in DMF after 60?min at rt. Second, SPPS-Fmoc/tBu using hydrazine hydrate for Fmoc group removal was standardized. The Fmoc group removal was completed using 16% hydrazine hydrate in DMF for 10?min at rt (twice). When the reaction of Fmoc group removal was microwave-assisted, the reaction only required 30?s to efficiently remove the Fmoc group in SPPS-Fmoc/tBu. The method reported here can be routinely used, and it is equivalent to conventional SPPS-Fmoc/tBu methodologies where 4-methylpiperidine or piperidine is used.     

An efficient approach for monosulfide bridge formation in solid-phase peptide synthesis

An efficient approach for the synthesis of cyclic peptides containing unnatural thioether side-chain bridges, based on the use of (2S)-9-fluorenylmethyl-2-[(tert-butoxycarbonyl)amino]-4-iodobutanoate and its homologue 5-iodopentanoate, derived from Boc-l-Asp-OFm and Boc-l-Glu-OFm, respectively, is reported. The synthesis was performed by a tandem combination of solid-phase peptide synthesis and microwave-assisted cyclization strategy.     

A convenient semicarbazide resin for the solid-phase synthesis of peptide ketones and aldehydes

Use of a semicarbazide resin for the solid-phase preparation of peptide ketones and aldehyde led to optimal results in terms of both purity of the final product and overall yield. This resin was prepared without complication by activation of the commercial available aminomethyl polystyrene with CDI at room temperature, followed by treatment with tert-butyl carbazate. Furthermore, the TNBSA colorimetric assay has been adapted for checking the incorporation of the carbonyl moiety onto hydrazine-based resins.     

Methyl esters: an alternative protecting group for the synthesis of O-glycosyl amino acid building blocks

The glycosyl amino acids α-GalNAc-Ser and α-GalNAc-Thr are fundamental building blocks for glycopeptide synthesis, Schmidt’s synthesis method often being chosen for this purpose. Methyl esters used as orthogonal carboxylic acid protecting group in this procedure were found to be an efficient and inexpensive alternative to other groups. The mild selective methyl ester deprotection by LiI improved the efficiency of the synthesis method.     

3,3-Dimethoxypropylsulfonyl Group: A new versatile protecting and activating group for amine synthesis

3,3-Dimethoxypropylsulfonyl (Dimps) chloride was prepared and used as a new versatile sulfonating agent for ammonia, primary and secondary amines to afford corresponding Dimps-amides in excellent yields. The resulting N-nonsubstituted and N-monosubstituted Dimps-amides, activated amines, were alkylated satisfactorily under new Mitsunobu conditions. The Dimps group was removed by treatment in aqueous solution under acidic followed by basic conditions. Furthermore, epilachnene, the defensive droplets from the Mexican bean beetle, Epilachna varivestis, was synthesized utilizing this Dimps methodology in short steps.     

Amino acid bromides: their utilization for difficult couplings in solid-phase peptide synthesis

Use of N-protected-α-amino acid bromides for facile solid-phase synthesis of peptides (SPPS) containing extremely sterically hindered non-proteinogenic amino acids is presented. Amino acid bromides (Aaa-Br), generated in situ, were used for the synthesis of long chain homopeptides containing up to eight successive α-MeVal or Aib residues. SPPS of a heteropeptide containing a very bulky amino acid building block is also described. The choice of suitable N-protections is discussed.     

Preparation of a core-shell type MBHA resin and its application for solid-phase peptide synthesis

MBHA (4-methylbenzhydrylamine) resin has been extensively used as a solid support for the synthesis of peptide amides. Herein, we prepared the core-shell-type MBHA resin by benzotriazole-catalyzed amidoalkylation and partial hydrolysis. The core-shell structure of the MBHA resin was confirmed by two-photon microscopy and its synthetic performance in solid-phase peptide synthesis (SPPS) was evaluated.     

Total synthesis of bidensyneosides A2 and C: remarkable protecting group effects in glycosylation

Bidensyneosides are a group of five recently identified polyacetylenic glucosides from Bidens parviflora WILLD, a traditional Chinese medicinal plant that contains rich bioactive natural products. It was shown that bidensyneosides inhibited both histamine release and nitric oxide production. The synthesis of bidensyneoside A2 (2) and C (4) as well as 3-deoxybidensyneoside C (5) are described. These syntheses establish a synthetic entry to the bidensyneosides and confirm the stereochemistry at C3. Furthermore, a remarkable protecting group effect on orthoester formation was observed during the glycosylation reaction.     

A new colorimetric test for detection of hydroxyl groups in solid-phase synthesis

A new colorimetric test (methyl red-DIC test) for monitoring the presence of hydroxyl groups on resins for use in solid-phase synthesis was developed. The resin became orange-red when the carboxyl group of methyl red formed ester linkages with the resin’s hydroxyl groups. This new test can be used not only for qualitative analysis but also quantitative analysis by using pictures of the relationship between color depth and hydroxy loading rates.     

Design and synthesis of a new polymer-supported Evans-type oxazolidinone: an efficient chiral auxiliary in the solid-phase asymmetric alkylation reactions

Wang resin-supported Evans' chiral auxiliary (23) was designed based on a novel polymer-anchoring strategy, which utilizes the 5-position of the oxazolidinone ring, and its new synthetic route applicable to multi-gram preparation in just a day was developed. Solid-phase Evans' asymmetric alkylation on 23-derived N-acylimide resin and following lithium hydroperoxide-mediated chemoselective hydrolysis afforded the corresponding α-branched carboxylic acids in desired high stereoselectivities (up to 97% ee) and moderate to good overall yield (up to 70%, for 3 steps), which were comparable to those of the conventional solution-phase methods. Furthermore, recovery and recycling of the polymer-supported chiral auxiliary were successfully achieved without decreasing the stereoselectivity of the product. Therefore, this is the first successful example that the solid-phase Evans' asymmetric enolate-alkylation was efficiently performed on the solid-support, and it is concluded that the connection to the solid-support via the 5-position of the oxazolidinone ring is an ideal strategy in the solid-phase Evans' chiral auxiliary.     

Simple and efficient solid-phase synthesis of unprotected peptide aldehyde for peptide segment ligation

We describe an efficient solid-phase synthesis of C-terminal peptide aldehyde. Making use of the stability of the PAM linker towards both acid and base conditions, a pentapeptide was synthesized starting from a PAM resin according to Fmoc/tBu chemistry. The side-chains were deprotected by TFA. The peptide was cleaved by aminolysis with aminoacetaldehyde-dimethylacetal leading to a C-terminal masked aldehyde. The unprotected peptide aldehyde was then coupled to amino-oxy derivatives by chemoselective ligation in aqueous solution.     

Synthesis of a highly hydrophobic cyclic decapeptide by solid-phase synthesis of linear peptide and cyclization in solution

<正>A general method was described to synthesize a highly hydrophobic cyclic peptide,cyclo[LWLWLWLWLQ]where underlines indicate D-configuration of the amino acid,by a two-step solid-phase/solution synthesis strategy.The linear decapeptide was assembled by standard Boc chemistry on solid-phase and subsequently cyclized in solution with high efficiency and reproducibility. In subsequent purification by semi-preparative HPLC,50%(v/v) DMF/H_2O was employed as the solvent to overcome the difficulty of solubilization for the hydrophobic cyclic decapeptide and achieved a total yield of 30-35%with a purity of over 98%.     

Trifluoroacetyl as a protecting group for HYNIC: stability in the presence of electrophiles and application in the synthesis of Tc-radiolabelled peptides

TrifluoroacetylHYNIC-peptides have recently been shown to label directly with ^99mTc as efficiently as their non-trifluoroacetylated analogs. In this work, the trifluoroacetyl (Tfa) moiety has been evaluated as a protecting group for HYNIC against reaction with strong electrophiles. Fmoc-(trifluoroacetylHYNIC)-lysine, the chosen model starting material, was found to be resistant against acetaldehyde and benzylchloroformate challenges, at 1 mol equiv and a 1000 M excess, respectively. In contrast, the Fmoc-(HYNIC)-lysine derivative, with a free hydrazine group, was quantitatively converted to the corresponding hydrazone after a 1 h incubation with acetaldehyde. Fmoc-(trifluoroacetylHYNIC)-lysine was also found to be stable over a wide pH range (3.6-10) to the acetaldehyde challenge. High efficiency ^99mTc-radiolabelling (99%) was achieved in the presence of acetaldehyde using Fmoc-(trifluoroacetylHYNIC)-lysine, as compared to a poor radiolabelling yield (34%) obtained with the non-trifluoroacetylated analog. These findings firmly establish the trifluoroacetyl group as a convenient and effective protecting group for HYNIC, and as a promising alternative to currently available labelling strategies.     

A novel, base-labile fluorous amine protecting group: synthesis and use as a tag in the purification of synthetic peptides

A new, base-labile fluorous tag based on the Msc amine protecting group was synthesized. Its use in the purification of synthetic peptides by fluorous HPLC or fluorous SPE was demonstrated.     

A new polymer-supported Evans-type chiral auxiliary derived from α-hydroxy-β-amino acid, phenylnorstatine: synthesis and application in solid-phase asymmetric alkylation reactions

Based on a new anchoring strategy, a polymer-supported chiral oxazolidinone was prepared starting from (2R,3S)-3-amino-2-hydroxy-4-phenylbutanoic acid (phenylnorstatine, Pns) and Wang resin. Solid-phase asymmetric alkylation on this resin proceeded in high diastereoselectivity comparable to that of conventional solution-phase model experiments. This study suggests that anchoring through the 5-position of oxazolidinone is highly suited to achieving diastereoselective alkylation reactions on solid-support.     

2-(4-Sulfophenylsulfonyl)ethoxycarbonyl group: a new water-soluble N-protecting group and its application to solid phase peptide synthesis in water

Solid phase peptide synthesis is carried out in organic solvents, creating environmental problems after disposal. To avoid this problem, we aimed to perform solid phase peptide synthesis in water. A new water-soluble N-protecting group, 2-(4-sulfophenylsulfonyl)ethoxycarbonyl (Sps) group, was designed and Sps-amino acids were prepared. To evaluate the utility of this technique, Leu-enkephalin amide was prepared by solid phase synthesis using Sps-amino acids in water.     

Structure‐enantioselectivity relationships for the study of chiral recognition in peptide enantiomer separation on cinchona alkaloid‐based chiral stationary phases by HPLC: Influence of the N‐terminal protecting group

Eleven different N‐terminal protecting groups (acetyl, benzoyl, FMOC, etc.) were employed for the HPLC separation of oligoalanine peptide enantiomers containing up to six amino acids. Isocratic HPLC separations were performed using a hydro‐organic buffered mobile phase and 4 mm ID columns containing three different chiral anion exchange stationary phases based on cinchona alkaloid‐derived chiral selectors. For most peptides successful separations could be obtained with all protecting groups, although those comprising aromatic moieties were found to yield higher enantioselectivities than those with aliphatic residues, since they are capable of undergoing favourable π‐π interactions with the selector. Systematic investigations concerning the presence or absence of structural features of related protecting groups showed that the use of protecting groups that are optimally adjusted to the binding pocket of the chiral selector effects a significant gain in enantioselectivity. At the same time these studies provided new insights into the chiral recognition mechanism.