Fmoc solid-phase synthesis of peptide thioesters using an intramolecularn,S-acyl shift

[reaction: see text] We describe the Fmoc solid-phase synthesis of peptide thioesters based on the alkylation of the safety-catch sulfonamide linker with a protected 2-mercaptoethanol derivative. The thioester is generated on the solid phase after the peptide chain assembly as a consequence of an intramolecular N,S-acyl shift. Depending on the stability of the spacer separating the sulfonamide linker from the resin toward TFA, treatment of the peptidyl resin with TFA led to a soluble or supported deprotected thioester.     

Transfer allyl esters to thioesters in solid phase condition: synthesis of peptide thioesters by Fmoc chemistry

A method to transfer allyl esters to thioesters under a solid phase condition has been developed to synthesize peptide thioesters. A Fmoc chemistry has been applied to synthesize the peptide allyl esters, which are selectively transferred to the expected peptide thioesters under solid phase synthesis conditions successfully.     

Fmoc synthesis of peptide thioesters without post-chain-assembly manipulation

An operationally simple method for the synthesis of peptide thioesters is developed using standard Fmoc solid-phase peptide synthesis procedures. The method relies on the use of a premade enamide-containing amino acid which, in the final TFA cleavage step, renders the desired thioester functionality through an irreversible intramolecular N-to-S acyl transfer.     

Facile, fmoc-compatible solid-phase synthesis of peptide C-terminal thioesters

A short route to peptide C-terminal thioesters was developed that does not require the use of special linkers or resins and is compatible with standard Fmoc chemistry. Following conventional solid-phase peptide synthesis, an excess of Me(2)AlCl and EtSH in dichloromethane cleaves peptides from Wang or Pam resins to give the corresponding thioesters directly in good yield and purity.     

Convenient synthesis of N-methylamino acids compatible with Fmoc solid-phase peptide synthesis

N(alpha)-Methylamino acid containing peptides exhibit interesting therapeutic profiles and are increasingly recognized as potentially useful therapeutics. Unfortunately, their synthesis is hampered by the high price and unavaibility of many N(alpha)-methylamino acids. An efficient and practical preparation of N(alpha)-methyl-N(alpha)-(o-nitrobenzenesulfonyl)-alpha-amino acids without extensive purification is described. The procedure is based on the well-known N-alkylation of N(alpha)-arylsulfonylamino esters which was improved by using dimethyl sulfate and DBU as base. Ester cleavage is efficiently achieved by using an S(N)2-type saponification with lithium iodide, avoiding racemization observed with lithium hydroxide hydrolysis. Compatibility of the synthesized N(alpha)-methylamino acids with Fmoc solid-phase peptide synthesis is demonstrated by using normal coupling conditions to efficiently prepare N-methyl dipeptides. The described procedure allows the preparation of N(alpha)-methylamino acids in a very short period of time and a rapid synthesis of N-methyl peptides using Fmoc solid-phase peptide synthesis.     

Convenient synthesis of acetonide-protected 3,4-dihydroxyphenylalanine (DOPA) for Fmoc solid-phase peptide synthesis

We report a facile approach to the synthesis of acetonide and Fmoc-protected 3,4-dihydroxyphenylalanine (DOPA), Fmoc-DOPA(acetonide)-OH. By protecting the amino group of DOPA with a phthaloyl group and the carboxyl group as a methyl ester, acetonide protection of the catechol of DOPA derivative was realized in the presence of p-toluenesulfonic acid. Following removal of protecting groups, the intermediate was converted to Fmoc-DOPA(acetonide)-OH, which was successfully incorporated into a short DOPA-containing peptide, derived from marine tubeworm cement proteins Pc1 and Pc2.     

Synthesis of photoactive p-azidotetrafluorophenylalanine containing peptide by solid-phase Fmoc methodology

[structure: see text] N-Fmoc-L-p-azidotetrafluorophenylalanine was prepared from achiral starting materials using an acetamidomalonate synthesis and enzymatic resolution. A photoactive peptide containing this fluorinated residue could be assembled using solid-phase Fmoc chemistry.     

Direct Fmoc-chemistry-based solid-phase synthesis of peptidyl thioesters

Attachment of a growing peptide chain to a glycylaminomethyl resin via a thioglycinamide bond is compatible with Fmoc-chemistry solid-phase peptide synthesis. Subsequent S-alkylation of the thioamide gives a thioimide that, on treatment with aqueous trifluoroacetic acid, releases the peptide from the resin in the form of a C-terminal thioester.     

Preparation of peptide thioesters using Fmoc strategy through hydroxyl side chain anchoring

In the course of the chemical synthesis of human protein mitogaligin, we present here a simple method to prepare peptide thioesters using Fmoc chemistry. The hydroxyl side chain of serine was reacted with a trichloroacetimidate Wang resin to anchor it on solid phase. After peptide elongation and orthogonal unmasking of the C-terminus, the amino thioester was introduced under optimized conditions to avoid epimerization.     

N-to-C solid-phase peptide and peptide trifluoromethylketone synthesis using amino acid tert-butyl esters

Solid-phase peptide synthesis in the N-to-C direction, opposite to the classical C-to-N direction of peptide synthesis, provides the synthetically versatile C-terminal carboxyl group for further modification into C-terminally modified peptide mimetics. These are of general interest as potential bioactive agents, particularly as protease inhibitors. Elaboration of peptide mimetics on the solid-phase would facilitate synthesis of peptide mimetic combinatorial libraries. This report describes an effective strategy for solid-phase inverse peptide synthesis based on readily available amino acid tert-butyl esters. The potential of this approach for peptide mimetic synthesis is demonstrated by the solid-phase synthesis of two peptide trifluoromethylketones.     

A designed well-folded monomeric four-helix bundle protein prepared by Fmoc solid-phase peptide synthesis and native chemical ligation

The design and total chemical synthesis of a monomeric native-like four-helix bundle protein is presented. The designed protein, GTD-Lig, consists of 90 amino acids and is based on the dimeric structure of the de novo designed helix-loop-helix GTD-43. GTD-Lig was prepared by the native chemical ligation strategy and the fragments (45 residues long) were synthesized by applying standard fluorenylmethoxycarbonyl (Fmoc) chemistry. The required peptide-thioester fragment was prepared by anchoring the free gamma-carboxy group of Fmoc-Glu-allyl to the solid phase. After chain elongation the allyl moiety was orthogonally removed and the resulting carboxy group was functionalized with a glycine-thioester followed by standard trifluoroacetic acid (TFA) cleavage to produce the unprotected peptide-thioester. The structure of the synthetic protein was examined by far- and near-UV circular dichroism (CD), sedimentation equilibrium ultracentrifugation, and NMR and fluorescence spectroscopy. The spectroscopic methods show a highly helical and native-like monomeric protein consistent with the design. Heat-induced unfolding was studied by tryptophan absorbance and far-UV CD. The thermal unfolding of GTD-Lig occurs in two steps; a cooperative transition from the native state to an intermediate state and thereafter by noncooperative melting to the unfolded state. The intermediate exhibits the properties of a molten globule such as a retained native secondary structure and a compact hydrophobic core. The thermodynamics of GuHCl-induced unfolding were evaluated by far-UV CD monitoring and the unfolding exhibited a cooperative transition that is well-fitted by a two-state mechanism from the native to the unfolded state. GTD-Lig clearly shows the characteristics of a native protein with a well-defined structure and typical unfolding transitions. The design and synthesis presented herein is of general applicability for the construction of large monomeric proteins.     

A convenient synthesis of O-alkyl thioesters from esters

O-Alkyl thioesters may be prepared from the corresponding carboxylic esters by successive treatment with lithium diisopropylamide, chlorotrimethylsilane, and hydrogen sulfide.     

Enhancement in the rate of conversion of peptide Cys-Pro esters to peptide thioesters by structural modification

We previously reported that the peptide containing a Cys-Pro ester (CPE) moiety is spontaneously transformed into a peptide thioester via an N to S acyl shift followed by diketopiperazine formation. In an attempt to identify more reactive structures for the formation of a peptide thioester, we modified the CPE structure, in which the Pro residue in the CPE moiety was replaced with N-substituted glycine derivatives. These peptides were transformed into a peptide thioester more rapidly. Alternatively, the addition of an amino acid residue at the C-terminus of the CPE moiety also accelerated thioester formation.     

Solid-phase synthesis of peptide thioacids through hydrothiolysis of resin-bound peptide thioesters

We report that solid-phase hydrothiolysis is an efficient method to convert resin-bound peptide thioesters to thioacids in aqueous buffer by using a total PEG-based resin. Also demonstrated is the use of the so-prepared peptide thioacids in chemoselective amide bond formation reactions.     

High-pressure-promoted Fmoc-aminoacylation of N-ethylcysteine: preparation of key devices for the solid-phase synthesis of peptide thioesters

In this study synthesis of Fmoc-aminoacyl-N-ethyl-S-triphenylmethylcysteine, a new N→S acyl migratory device for the preparation of peptide thioesters by Fmoc solid-phase peptide synthesis (SPPS) is described. Condensation of Fmoc-aminoacyl fluoride and N-ethyl-S-triphenylmethylcysteine allyl ester, readily prepared from known S-triphenylmethylcysteine allyl ester, was efficiently promoted in CH₂Cl₂ under high-pressure (800 MPa). When the reaction was performed with the additive DIEA, considerable epimerization at the chiral centers occurred, affording a mixture of diastereomers. When the preparation procedure for N-ethyl-S-triphenylmethylcysteine allyl ester was changed and the additive DIEA in the high-pressure reaction was excluded, Fmoc-aminoacyl-N-ethyl-S-triphenylmethylcysteines was obtained as a single stereoisomer without epimerization. The Fmoc-l-leucine adduct thus prepared was deallylated and used for the SPPS of a known decapeptide. A remarkable increase (44%) in the overall yield of the decapeptidethioester was achieved, compared to the 7% obtained by the stepwise on-resin Leu-Cys condensation method.     

Continuous-flow solid-phase peptide synthesis

We describe a simple manually operated synthesizer for solid-phase peptide synthesis (SPPS). The synthesis is performed on standard polystyrene-based resin in a flow reactor under low-pressure conditions. The usefulness of the present configuration of SPPS is exemplified on the synthesis of two octapeptide and one decapeptide amides.