9-Fluorenylmethoxycarbonyl-based solid-phase synthesis of peptide α-thioesters

Peptide thioesters play a key role in convergent protein synthesis strategies such as native chemical ligation, traceless Staudinger ligation, and Ag(+) -mediated thioester ligation. The Boc-based solid-phase synthesis provides a very reliable access to peptide thioesters. However, the acid lability of many peptide modifications and the requirements of most parallel peptide synthesizers call for the milder Fmoc-based solid-phase synthesis. The Fmoc-based synthesis of peptide thioesters is more cumbersome and typically proceeds with lower yields than the synthesis of peptide acids and peptide amides. The success of native chemical ligation and related technologies has sparked intensive research effort devoted to the development of new methods. The recent progress in this rapidly expanding field is reviewed.     

Synthesis of a thioester linker precursor for a general preparation of peptide C-terminal thioacids

A general procedure to prepare peptide thioacids by solid-phase peptide synthesis is presented. The method involves the synthesis of 4-[α-(S-acetyl)mercaptobenzyl]phenoxyacetic acid as general precursor. This reagent once attached to a solid support is derivatized with the Boc-amino acid of choice after deprotection of the thiol.     

Aspartyl methyl ester formation via aspartimide ring opening: a proposed modification of the protocols used in Boc- and Fmoc-based solid-phase peptide synthesis

Aspartimide formation is still an unresolved problem in the solid-phase peptide synthesis of aspartic acid-containing peptides, following either Boc- or Fmoc-based synthetic strategies. α-Aspartyl peptides of high purity can be obtained, despite aspartimide formation, by incorporating an additional step in the Boc- and Fmoc-based solid-phase peptide synthesis protocols, consisting of treatment of the peptide-resin with methanol in the presence of 2% DIEA (v/v) for 15 min immediately after completion of the peptide chain elongation.     

Preparation of tripeptide-bridged dicatechol ligands and their macrocyclic molybdenum(VI) complexes: fixation of the RGD sequence and the WKY sequence of urotensin ii in a cyclic conformation

Dicatechol ligands were prepared with caprylic acid (6-H(4)) or the naturally occurring RGD (23-H(4)) or WKY sequences (32-H(4)) as spacers. 6-H(4) was prepared by solution-phase amide coupling chemistry, while 16, the precursor of 23-H(4), was obtained by solution-phase and solid-phase preparation. In the latter case, a polystyrene resin with a hydrazine benzoate linker was used as the solid support. The last coupling step was performed simultaneously with cleavage of the peptide from the resin. The protecting groups of 16 were all removed in one step to yield the free ligand 23-H(4). The WKY-bridged derivative 32-H(4) was obtained by a similar solid-phase synthesis followed by deprotection. The reaction of all three ligands with dioxomolybdenum(VI) bis(acetylacetonate) afforded 19-membered metallamacrocycles in which the short peptides are conformationally fixed in a turn-type structure. Hereby, the side-chain functionalities of the peptides do not interfere in the metal complexation.     

糖肽的固相合成

目前糖肽的固相合成可以分为两种策略：构建单元策略和固相糖基化策略。本文对O-连接和N-连接糖肽固相合成的研究进展进行了综述。     

Solid-phase synthesis of lipidated peptides

A new flexible and efficient methodology for the solid-phase synthesis of lipidated peptides has been developed. The approach is based on the use of previously synthesized building blocks and overcomes the limitations of previously reported methods, since long doubly lipidated peptides can be synthesized by using this route. Furthermore, it was thus possible to prepare a large number of N- and H-Ras peptides bearing a wide range of reporter and/or linking groups--efficient tools for the investigation of biological processes. In terms of efficiency and flexibility this solid-phase method is superior to the solution-phase synthesis. It gives pure peptides in multimilligram amounts within a much shorter time and with superior overall yield.     

A Peptide Backbone Stapling Strategy Enabled by the Multicomponent Incorporation of Amide N-Substituents

The multicomponent backbone N-modification of peptides on solid-phase is presented as a powerful and general method to enable peptide stapling at the backbone instead of the side chains. This work shows that a variety of functionalized N-substituents suitable for backbone stapling can be readily introduced by means of on-resin Ugi multicomponent reactions conducted during solid-phase peptide synthesis. Diverse macrocyclization chemistries were implemented with such backbone N-substituents, including the ring-closing metathesis, lactamization, and thiol alkylation. The backbone N-modification method was also applied to the synthesis of α-helical peptides by linking N-substituents to the peptide N-terminus, thus featuring hydrogen-bond surrogate structures. Overall, the strategy proves useful for peptide backbone macrocyclization approaches that show promise in peptide drug discovery.     

Solution and solid-support synthesis of a potential leishmaniasis carbohydrate vaccine

The synthesis of a potential carbohydrate vaccine for the parasitic disease leishmaniasis is described. New solution- and solid-phase synthetic strategies were explored for the assembly of a unique tetrasaccharide antigen found on the Leishmania lipophosphoglycan. An initial solution-phase synthesis relied on thioglycosides as building blocks and the establishment of the central disaccharide from lactal via an oxidation-reduction sequence. A second approach was completed both in solution and on solid support. The solid-phase synthesis relied on assembly from monosaccharide units and was used to evaluate different glycosylating agents in the efficient installation of the galactose beta-(1-->4) mannoside. Glycosyl phosphates proved most successful in this endeavor. This first solid-phase synthesis of the Leishmania cap provided rapid access to the tetrasaccharide in 18% overall yield while requiring only a single purification step. The synthetic cap tetrasaccharide was conjugated to the immunostimulator Pam3Cys to create fully synthetic carbohydrate vaccine 1 and to the carrier protein KLH to form semisynthetic vaccine 2. Currently, both constructs have entered initial immunological experiments in mice targeted at the development of a vaccine against the parasitic disease leishmaniasis.     

On-resin cyclization of a head-to-tail cyclopeptide using an allyldimethylsilyl polystyrene resin pre-loaded by metathesis

Here, we report the solid-phase synthesis of a 17-mer cyclopeptide which is expected to have anti-angiogenic properties. The peptidic synthesis is performed on an allyldimethylsilyl polystyrene support loaded by metathesis with a conveniently functionalized d-Tyrosine amino acid. The linear peptide was assembled by standard Fmoc chemistry and on-resin cyclization was enabled after selective deprotection of the C-terminal group with 2% hydrazine/DMF at room temperature. Final cleavage was realized under mild acidic conditions allowing to obtain a cyclopeptide under partially protected form.     

Efficient solid-phase synthesis of peptide-based phosphine ligands: towards combinatorial libraries of selective transition metal catalysts

A new methodology for the solid-phase synthesis of peptide-based phosphine ligands has been developed. Solid supported peptide scaffolds possessing either primary or secondary amines were synthesised using commercially available Fmoc-protected amino acids and readily available Fmoc-protected amino aldehydes for reductive alkylation, in standard solid-phase peptide synthesis (SPPS). Phosphine moieties were introduced by phosphinomethylation of the free amines as the final solid-phase synthetic step, immediately prior to complexation with palladium(II), thus avoiding tedious protection/deprotection of the phosphine moieties during the synthesis of the ligands. The extensive use of commercial building blocks and standard SPPS makes this methodology well suited for the generation of solid-phase combinatorial libraries of novel ligands. Furthermore, it is possible to generate several different phosphine ligand libraries for every peptide scaffold library synthesised, by functionalising the scaffold libraries with different phosphine moieties. The synthesised ligands were characterised on solid support by conventional (31)P NMR spectroscopy and, cleaved from the support, as their phosphine oxides by HPLC, (1)H NMR, (31)P NMR and high resolution ESMS. Palladium(II) allyl complexes were generated from the resin bound ligands and to demonstrate their catalytic properties, palladium catalysed asymmetric allylic substitution reactions were performed. Good yields and moderate enantioselectivity was obtained for the selected combination of catalysts and substrate, but most importantly the concept of this new methodology was proven. Screening of ligand libraries should afford more selective catalysts.     

Enantioselective synthesis of non-natural aromatic alpha-amino acids

We present two complementary methods for the stereoselective synthesis of non-natural alpha-amino acids with aromatic or heteroaromatic side chains. One approach is based on the chemical transformation of methionine, whereas the other applies the stereoselective Myers alkylation of glycine. The resulting product types differ in the linker length between glycine and the aromatic substituent. Since methionine and pseudoephedrine are available in both absolute configurations, R- or S-configured enantiopure amino acids with either C(2) or C(3) linkers can be obtained on gram scales. In each case the key step of the synthesis is hydroboration of the unsaturated building blocks 9 and 17, followed by palladium-catalyzed Suzuki cross-coupling with aryl halides. Attention must in certain cases be paid to the stereochemical integrity when basic Suzuki conditions are applied. Our initial difficulties are reported as well as the final "racemization-proof" procedures. The protecting groups chosen for the alpha-amino acids should be compatible with solid-phase peptide synthesis. This was confirmed by the successful synthesis of a series of tripeptides.     

Total synthesis of pseudacyclins A–E by an on-resin head-to-side chain concomitant cyclization-cleavage reaction

Taking advantage of the nucleophile-sensitive ester link of oxime resin, a novel synthetic strategy was applied to the first synthesis of a type of cyclic peptides known as pseudacyclins A–E. The endocyclic ornithine side-chain part was incorporated by an on-resin acid-catalyzed concomitant cyclization-cleavage reaction after a selective deprotection of orthogonally protected ornithine. The synthetic methodology gives high macrocyclization yields and low oligomerization side-products. The combination used of solid-phase/solution-phase strategy was efficient to prepare pseudacyclins and could prove useful to prepare other natural cycle-tail peptides.     

Fullerene-derivatized amino acids: synthesis, characterization, antioxidant properties, and solid-phase peptide synthesis

A series of [60]fullerene-substituted phenylalanine (Baa) and lysine derivatives have been prepared by the condensation of 1,2-(4'-oxocyclohexano)fullerene with the appropriately protected (4-amino)phenylalanine and lysine, respectively. Conversion of the imine to the corresponding amine is achieved by di-acid catalyzed hydroboration. The reduction of the imine is not accompanied by hydroboration of the fullerene cage. The [70]fullerene phenylalanine derivative has also been prepared as have the di-amino acid derivatives. The compounds were characterized by MALDI-TOF mass spectrometry, UV/Vis spectroscopy, and cyclic voltammetry. 1H and 13C NMR spectroscopy allowed the observation of diastereomers. Fullerene-substituted peptides may be synthesized on relatively large scale by solid-phase peptide synthesis. The presence of the C60-substituted amino acid in a peptide has a significant effect on the secondary structures and self-assembly properties of peptides as compared to the native peptide. The antioxidant assay of Baa and a Baa-derived anionic peptide was determined to be significantly more potent than Trolox.     

Monofunctionally trans-diammine platinum(II)-modified peptide nucleic acid oligomers: a new generation of potential antisense drugs

A solid-phase approach is described that provides facile access to monofunctionally trans-PtII-modified PNA oligomers of arbitrary sequence for potential use both in antigene and antisense strategies. The approach includes the synthesis of a platinated building block 1 and its subsequent incorporation into three different PNA oligomers 5-7 by solid-phase synthesis. In a model cross-linking reaction one of the latter is found to recognize sequence-specifically a target oligonucleotide 8 and to cross-link to it. The resulting structure is the trans-PtII-cross-linked PNA/DNA duplex 9 as revealed by mass spectrometry in combination with a Maxam-Gilbert sequencing experiment.     

Solid-phase synthesis of the cyclic lipononadepsipeptide [N-Mst(Ser1), d-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin

An optimized solid-phase strategy for the preparation of the cyclic lipononadepsipeptide [N-Mst(L-Ser1), D-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin is reported. The strategy is based on the use of a mild orthogonal protection scheme and the incorporation of the nonproteinogenic amino acid (Z)-Dhb into the peptide chain as the dipeptide Fmoc-Thr(tBu)-(Z)-Dhb-OH. The didehydrodipeptide was synthesized by a water-soluble carbodiimide-induced beta-elimination of a protected dipeptide containing a residue of Thr with its free hydroxy side chain unprotected.     

Solid-phase synthesis of dihydrovirginiamycin S1, a streptogramin B antibiotic

We describe the first solid-phase synthesis of dihydrovirginiamycin S(1), a member of the streptogramin B family of antibiotics, which are nonribosomal-peptide natural products produced by Streptomyces. These compounds, along with the synergistic group A components, are "last line of defense" antimicrobial agents for the treatment of life-threatening infections such as vancomycin-resistant enterococci. The synthesis features an on-resin cyclization and is designed to allow production of streptogramin B analogues with diversification at positions 1', 1, 2, 3, 4, and 6. Several synthetic challenges known to hinder the synthesis of this class of compounds were solved, including sensitivity to acids and bases, and epimerization and rearrangements, through the judicious choice of deprotection conditions, coupling conditions, and synthetic strategy. This work should enable a better understanding of structure-activity relationships in the streptogramin B compounds, possible identification of analogues that bypass known resistance mechanisms, and perhaps the identification of analogues with novel biological activities.     

DEUSS: a perdeuterated poly(oxyethylene)-based resin for improving HRMAS NMR studies of solid-supported molecules

A novel resin called DEUSS (perdeuterated poly(oxyethylene)-based solid support) has been prepared by anionic polymerization of deuterated [D4]ethylene oxide, followed by cross-linking with deuterated epichlorohydrin. DEUSS can be suspended in a wide range of solvents including organic and aqueous solutions, in which it displays a high swelling capacity. As measured by proton HRMAS of the swollen polymer, the signal intensity of the oxyethylene protons is reduced by a factor of 110 relative to the corresponding nondeuterated poly(oxyethylene)poly(oxypropylene) (POEPOP) resin, thus facilitating detailed HRMAS NMR studies of covalently linked molecules. This 1H NMR invisible matrix was used for the solid-phase synthesis of peptides, oligoureas, and a series of amides as well as their characterization by HRMAS NMR spectroscopy. On-bead NMR spectra of high quality and with resolution comparable to that of liquid samples were obtained and readily interpreted. The complete absence of the parasite resin signals will be of great advantage, for example, for the optimization of multistep solid-phase stereoselective reactions, and for the conformational study of resin-bound molecules in a large variety of solvents.     

Solution- and solid-phase synthesis of natural product-like tetrahydroquinoline-based polycyclics having a medium size ring

A solid-phase synthesis of tetrahydroquinoline-derived polycyclic 4, having a medium size ring with an enamide functionality, was achieved from tetrahydroquinoline derivative 3 in five steps with overall 40-45% yield. An enantiopure, tetrahydroquinoline-derived beta-amino ester, 1, was converted into compound 2 that has a free phenolic hydroxyl group as an anchoring site for solid-phase synthesis. The solid-phase worked well for this sequence, in which the synthesis of the unsaturated eight-membered enamide lactam was obtained by a ring-closing metathesis approach. Compound 4 is a novel, natural product-like polycyclic derivative that could further be utilized in library generation for developing small molecule chemical probes.     

Organic Chemistry on Solid Supports

Until recently, repetitive solid-phase synthesis procedures were used predominantly for the preparation of oligomers such as peptides, oligosaccharides, peptoids, oligocarbamates, peptide vinylogues, oligomers of pyrrolin-4-one, peptide phosphates, and peptide nucleic acids. However, the oligomers thus produced have a limited range of possible backbone structures due to the restricted number of building blocks and synthetic techniques available. Biologically active compounds of this type are generally not suitable as therapeutic agents but can serve as lead structures for optimization. “Combinatorial organic synthesis” has been developed with the aim of obtaining low molecular weight compounds by pathways other than those of oligomer synthesis. This concept was first described in 1971 by Ugi.^[56f,g,59c] Combinatorial synthesis offers new strategies for preparing diverse molecules, which can then be screened to provide lead structures. Combinatorial chemistry is compatible with both solution-phase and solid-phase synthesis. Moreover, this approach is conducive to automation, as proven by recent successes in the synthesis of peptide libraries. These developments have led to a renaissance in solid-phase organic synthesis (SPOS), which has been in use since the 1970s. Fully automated combinatorial chemistry relies not only on the testing and optimization of known chemical reactions on solid supports, but also on the development of highly efficient techniques for simultaneous multiple syntheses. Almost all of the standard reactions in organic chemistry can be carried out using suitable supports, anchors, and protecting groups with all the advantages of solid-phase synthesis, which until now have been exploited only sporadically by synthetic organic chemists. Among the reported organic reactions developed on solid supports are Diels–Alder reactions, 1,3-dipolar cycloadditions, Wittig and Wittig–Horner reactions, Michael additions, oxidations, reductions, and Pd-catalyzed C? C bond formation. In this article we present a comprehensive review of the previously published solid-phase syntheses of nonpeptidic organic compounds.