N-Alkyl cysteine-assisted thioesterification of peptides

A new method for the preparation of peptide thioester by the post-solid phase peptide synthesis (SPPS) approach was developed. A series of N-alkyl cysteine derivatives were prepared and used as the C-terminus residue of the peptides prepared by the Fmoc SPPS. The synthetic peptides released from resin by TFA were readily converted to the peptide thioester in aqueous 3-mercaptopropionic acid (MPA) without significant side reactions.     

Dibutylphosphate (DBP) mediated synthesis of cyclic N,N′-disubstituted urea derivatives from amino esters: a comparative study

The N,N′-disubstituted urea derivatives such as amino acid hydantoins and dihydrouracil derivatives were prepared starting from natural and unnatural amino acid esters using dibutylphosphate (DBP). During the attempted synthesis of N-heterocycles with larger than six-membered rings containing the N,N′-disubstituted urea functionalities, three unexpected products namely squamolone, N-methyl pyrrolidine-2-one, and diketopiperazine were isolated.     

Green solid-phase peptide synthesis 4. γ-Valerolactone and N-formylmorpholine as green solvents for solid phase peptide synthesis

Herein, we report the use of γ-valerolactone (GVL) and N-formylmorpholine (NFM) as DMF substitutes in polystyrene based SPPS. The solubility of selected amino acids and coupling reagents were studied in GVL and NFM, followed by their use in the successful synthesis of Aib-enkephalin pentapeptide (H-Tyr-Aib-Aib-Phe-Leu-NH₂) and Aib-ACP decapeptide (H-Val-Gln-Aib-Aib-Ile-Asp-Tyr-Ile-Asn-Gly-NH₂).     

A convenient microwave-assisted synthesis of N-glycosyl amino acids

Optimization of coupling reactions of glycosylamines with Fmoc-protected aspartic acid, by microwave approach, is described. Different reaction conditions, quantities of substrates and solvents were tested to develop simple and reproducible methodologies. The best results were obtained using new triazine-based coupling reagents with a monomode microwave Discover® BenchMate™ instrument (CEM). The N-glycosyl amino acids were then deprotected to achieve final products for SPPS.     

Stepwise solid phase synthesis of uridylylated viral genome-linked peptides using uridylylated amino acid building blocks

The uridylylated amino acid building blocks 2-cyanoethyl-(N^α-9-fluorenylmethoxy-carbonyl-tyrosin-4-yl)-(2′,3′-di-O-acetyluridin-5′-yl) phosphate and 2-chlorophenyl-(N^α-fluorenyl-methoxycarbonyl-serin-3-yl)-(2′,3′-di-O-acetyluridin-5′-yl) phosphate have been used successfully in an on-line SPPS of the VPgpU from the polio, coxsackie and cowpea mosaic virus.     

Stereoselective synthesis of N-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides

The stereoselective synthesis of N-acyl- and N-Boc-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides is described. These reactions effect formation of two bonds in a single operation and proceed with generally high levels of diastereoselectivity. In contrast to previously described reactions of γ-(N-arylamino) alkenes, these transformations proceed in high yield and high regioselectivity with both electron-rich and electron-deficient aryl bromides as well as vinyl bromide substrates.     

A new protecting group for tryptophan in solid-phase peptide synthesis which protects against acid-catalyzed side reactions and facilitates purification by HPLC

The indole nucleus of Z-Trp-OBzl is modified by acylation of the indole nitrogen using Boc-N-methyl butyric acid followed by catalytic hydrogenation and introduction of the Fmoc group. The resulting derivative, Fmoc-Trp(Boc-Nmbu)-OH, is incorporated into peptide chains via solid-phase peptide synthesis (SPPS). After assembly of the peptide chain, the Boc group is cleaved by treatment with TFA. The peptide is isolated with the tryptophan residue modified with a cationic 4-(N-methylamino) butanoyl group, which improves the solubility of the peptide during HPLC purification. On treatment of the purified peptide at pH 9.5, the Nmbu group undergoes an intramolecular cyclization reaction; this results in the fully deprotected peptide and N-methylpyrrolidone.     

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.     

An efficient synthesis of a novel analog of octreotide with an unnatural l-lysine-like tetrazolyl amino acid

A new cyclic octreotide-like octapeptide was prepared by incorporation of an unnatural tetrazolyl amino acid, an analog of Fmoc-l-lysine, into the peptide chain. The new tetrazolyl amino acid, (S)-2-{[2-(9H-fluoren-9-yl)acetoxy]amino}-6-(1H-tetrazol-1-yl)hexanoic acid, was obtained by azidation of Fmoc-l-lysine trifluoroacetate with sodium azide in the presence of triethyl orthoformate in glacial acetic acid. The linear peptide sequence was prepared using efficient Fmoc/t-Bu solid-phase peptide synthesis (SPPS). Cyclization of the octapeptide was carried out via oxidation with iodine. The structure and purity of the cyclic octapeptide were confirmed by LC–MS, MALDI-TOF MS/MS analysis as well as 1D/2D ¹H and ¹³C NMR spectroscopy.     

A simple approach for the synthesis of new classes of dithiocarbamate-linked peptidomimetics

An efficient protocol for the synthesis of a new series of dithiocarbamate-linked peptidomimetics is described. The in situ generated dithiocarbamic acid intermediate formed by the reaction of an amino acid ester and carbon disulfide in the presence of triethylamine was treated with N-protected amino alkyl iodide to afford title compounds 3a-g in good to moderate yields. The synthesis of N-Fmoc-protected tripeptidomimetics 4a-e containing two dithiocarbamate linkages is also described. The protocol was further extended to synthesize N,N′-orthogonally protected dithiocarbamate-linked dipeptidomimetics 7a-c as well. The mild reaction conditions and non-toxic reagents are the advantages of the present method.     

Quaternary β2,2‐Amino Acids: Catalytic Asymmetric Synthesis and Incorporation into Peptides by Fmoc‐Based Solid‐Phase Peptide Synthesis

β‐Amino acid incorporation has emerged as a promising approach to enhance the stability of parent peptides and to improve their biological activity. Owing to the lack of reliable access to β^2,2‐amino acids in a setting suitable for peptide synthesis, most contemporary research efforts focus on the use of β³‐ and certain β^2,3‐amino acids. Herein, we report the catalytic asymmetric synthesis of β^2,2‐amino acids and their incorporation into peptides by Fmoc‐based solid‐phase peptide synthesis (Fmoc‐SPPS). A quaternary carbon center was constructed by the palladium‐catalyzed decarboxylative allylation of 4‐substituted isoxazolidin‐5‐ones. The N?O bond in the products not only acts as a traceless protecting group for β‐amino acids but also undergoes amide formation with α‐ketoacids derived from Fmoc‐protected α‐amino acids, thus providing expeditious access to α‐β^2,2‐dipeptides ready for Fmoc‐SPPS.     

Straightforward, racemization-free synthesis of peptides with fairly to very bulky di- and trisubstituted glycines

Several fully protected tri- and pentapeptides containing a central symmetrical α,α-dialkyl glycine residue, with the alkyl group varying from methyl or ethyl to benzyl, were synthesized in good yields by a strategy based on the Ugi-Passerini reaction. Each Ugi-Passerini adduct was selectively cleaved and the product submitted to an assisted N,N′-dicyclohehylcarbodiimide coupling to an amino acid or dipeptide ester, respectively. Tripeptides as the above but containing a 4-methoxybenzyl group at the nitrogen atom of the central residue were also synthesized in fair to good yields by N-[(1H-benzotriazol-1-yl)-(dimethylamino)methylene]-N-methylmethanaminium hexafluorophosphate N-oxide assisted couplings. The results reported here show that our strategy is appropriate for routine synthesis of peptides incorporating these moieties.     

Efficient functional molecule incorporation method to functionalized peptide nucleic acid (PNA): use in synthesis of labeled PNA oligomers

A novel efficient synthetic method for a functionalized PNA (peptide nucleic acid) is described, in which a functional molecule is incorporated in place of a nucleobase. Novel ω-AA-^BocPNA-OH (20-24, AA=amino acid) were designed as PNA precursor monomer units into which functional molecules could be incorporated efficiently. Compounds 20-24 reacted quantitatively with OSu (N-hydroxysuccinimidyl) active ester derivatives and isothiocyanate derivatives of commercial functional molecules to give target functionalized PNA monomer units 25-53. Various types of functionalized PNA monomer units could be efficiently incorporated into multiple predetermined positions in a PNA oligomer by SPPS (solid phase peptide synthesis) in the same way as for the four A(Cbz), G(Cbz), C(Cbz), and T PNA monomer units.     

A novel approach for the one-pot synthesis of linear and angular fused quinazolinones

A convenient and inexpensive one step methodology has been developed for the synthesis of linear and angular fused quinazolinones. The protocol, which uses amino heterocycles and o-bromo benzyl/naphthyl bromides as reactants, CuI as catalyst, Cs₂CO₃ as base, l-proline as ligand, and DMF as solvent, proceeds via nucleophilic aromatic substitution of the N-heteroaromatic cationic intermediate followed by in situ aerial oxidation at the benzylic position to the quinazolinone scaffold.     

Ligand-free CuI-catalyzed selective C–N coupling of aliphatic amino group of α-amino acid amides with ortho-dihaloarenes

Many synthetic methods of racemic N-aryl (including N-(2-haloaryl)) α-amino acid amides have been reported, but there are only a few for eutomers and none for inactivated halo(hetero)arenes. Selective C–N coupling of aliphatic amino group of α-amino acid amides is a direct and valuable method, but still unresolved. Here we disclose a simple and cheap ligand-free CuI-catalyzed selective synthesis of chiral N-(2-halo(hetero)aryl) amino acid amides without obvious racemization. A rational mechanism is proposed.     

Rapid and high-yield solution-phase synthesis of DOTA-Tyr-octreotide and DOTA-Tyr-octreotate using unprotected DOTA

An improved method for the solution-phase derivatization of Tyr³-Lys⁵(Dde)-octreotide (TOC(Dde)) and Tyr³-Lys⁵(Dde)-octreotate (TATE(Dde)) with the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-N′,N″,N?,N?-tetraacetic acid) has been developed. The fully protected parent peptides were assembled via solid-phase peptide synthesis (SPPS) using Fmoc-strategy. After cleavage from the solid support, disulfide bond formation was carried out using H₂O₂. Both TOC(Dde) and TATE(Dde) were successfully coupled with DOTA in the presence of NHS, EDCI and DIPEA in a water/DMF solvent system. Yields of the coupling reaction were >98% within only 2 h with no detectable formation of sideproducts. This method for the preparation of DOTATOC, DOTATATE and other DOTA-peptide conjugates is therefore a rapid and economic alternative to the currently used methods.     

Palladium-catalyzed highly regioselective and stereoselective arylation of electron-rich allylamines with aryl bromides

A palladium-catalyzed, highly efficient Heck arylation of electron-rich N,N-diprotected allylamine derivatives with a wide range of aryl bromides under ligand-free conditions has been developed. In the presence of Pd(OAc)₂ and an appropriate additive, the reaction proceeds with excellent regioselectivity and stereoselectivity, leading exclusively to the γ-arylated (E)-allylamine products in good to excellent yields. It was found that the choice of solvent, olefin, additive and temperature has an important influence on the reaction. Worthy of note is that good results were observed only when using N,N-diprotected allylamines containing carbamate moiety, and the steric properties of allylamines also have important impacts on the regiocontrol. The use of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or HQ (hydroquinone) as the additive is also crucial for securing a faster reaction rate. This method provides a straightforward approach for the efficient synthesis of various γ-arylated, linear (E)-allylamines.     

Separation of chelating agents as copper complexes by capillary zone electrophoresis using quaternary ammonium bromides as additives in N-methylformamide

This study presents the use of quaternary ammonium bromides as additives in N-methylformamide (NMF) for the separation and quantification of chelating agents as copper complexes by capillary zone electrophoresis (CZE). The new quaternary ammonium bromides were synthesized in our laboratory and used for the first time for CZE applications performed in NMF media. The methods were developed and optimized for determination of six chelating agents (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), nitrilotriacetic acid (NTA) and triethylenetetraaminehexaacetic acid (TTHA)) as copper complexes. Among the tested electrolyte additives in NMF media (pH_app 10.2) dimethyldioctylammonium bromide (DMDOAB), dimethyldinonylammonium bromide (DMDNAB) and dimethyldidecylammonium bromide (DMDDAB), at a concentration of 20 mmol L⁻¹ improved the separation of the copper complexes. The optimized methods require only 12 min for one analysis, and the detection limits for copper complexes of DMDNAB, the best-performing additive, were ≤24 μmol L⁻¹. Relative standard deviations (R.S.D.) for migration times were ≤2.5, ≤2.1, ≤3.1% and for peak areas, ≤3.1, ≤3.0, ≤3.2% for DMDOAB, DMDNAB and DMDDAB used as additives, respectively. All three methods were successfully applied to the analysis of natural and wastewater samples. No matrix effects from these samples were observed. The interaction between quaternary ammonium bromides and copper complexes is discussed.     

Selective substitutions in the C10-methyl group in erythromycin derivatives

A method for chemical modifications of the relatively unreactive C10-methyl group in the erythromycin macrolactone ring has been developed. Erythralosamine was protected as an N-oxide in the N,N-dimethylamino group and reacted with NBS in acetic acid to provide two regioisomeric allylic bromides. The same amine was formed from both isomers on nucleophilic substitution. Both regioisomeric bromides in cross-coupling reactions under Stille conditions provided the same product from substitution in the 10-methyl group via a common π-allylic palladium complex. Under Negishi conditions with trimethylalane, the Pd-catalysed cross-coupling provided the 10-ethyl homologue. X-ray analyses were used to confirm the structure of erythralosamine, and to determine the structures of the allylic bromides from erythralosamine N-oxide.     

Synthesis of new ribosylated Asn building blocks as useful tools for glycopeptide and glycoprotein synthesis

We performed the first synthesis of new Asn derivatives bearing α- or β-ribose as pure anomers, linked by an N-glycosidic bond, on the side chain of the Asn residue orthogonally protected for Fmoc/^tBu SPPS, by an efficient five-step strategy with a global yield of 73% starting from d-ribose. These building blocks are obtained in a large scale and can be useful tools for glycopeptide and glycoproteins synthesis.