Solution-phase-peptide synthesis via the group-assisted purification (GAP) chemistry without using chromatography and recrystallization

Development of a nitrogen-bound hydrophobic auxiliary: application to solid/hydrophobic-tag relay synthesis of calpinactam

In the last couple of decades, technologies and strategies for peptide synthesis have advanced rapidly. Although solid-phase peptide synthesis (SPPS) and liquid-phase peptide synthesis (LPPS) have contributed significantly to the development of the field, there have been remaining challenges for C-terminal modifications of peptide compounds in SPPS and LPPS. Orthogonal to the current standard approach that relies on installation of a carrier molecule at the C-terminus of amino acids, we developed a new hydrophobic-tag carbonate reagent which facilitated robust preparation of nitrogen-tag-supported peptide compounds. This auxiliary was easily installed on a variety of amino acids including oligopeptides that have a broad range of noncanonical residues, allowing simple purification of the products by crystallization and filtration. We demonstrated a de novo solid/hydrophobic-tag relay synthesis (STRS) strategy using the nitrogen-bound auxiliary for total synthesis of calpinactam.

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

Tag-assisted liquid-phase peptide synthesis (LPPS) is one of the important processes in peptide synthesis in pharmaceutical discovery. Simple silyl groups have positive effects when incorporated in the tags due to their hydrophobic properties. Super silyl groups contain several simple silyl groups and play an important role in modern aldol reactions. In view of the unique structural architecture and hydrophobic properties of the super silyl groups, herein, two new types of stable super silyl-based groups (tris(trihexylsilyl)silyl group and propargyl super silyl group) were developed as hydrophobic tags to increase the solubility in organic solvents and the reactivity of peptides during LPPS. The tris(trihexylsilyl)silyl group can be installed at the C-terminal of the peptides in ester form and N-terminal in carbamate form for peptide synthesis and it is compatible with hydrogenation conditions (Cbz chemistry) and Fmoc-deprotection conditions (Fmoc chemistry). The propargyl super silyl group is acid-resistant, which is compatible with Boc chemistry. Both tags are complementary to each other. The preparation of these tags requires less steps than previously reported tags. Nelipepimut-S was synthesized successfully with different strategies using these two types of super silyl tags.

Asymmetric Reactions of N-Phosphonyl/Phosphoryl Imines

The asymmetric reactions of imines continued to attract the attention of the scientific community for decades. However, the stereoselective reactions of N-phosphonyl/phosphoryl imines remained less explored as compared to other N-substituted imines. The chiral auxiliary-based asymmetric-induction strategy with N-phosphonyl imines could effectively generate enantio- and diastereomeric amine, α,β-diamine, and other products through various reactions. On the other hand, the asymmetric approach for the generation of chirality through the utilization of optically active ligands, along with metal catalysts, could be successfully implemented on N-phosphonyl/phosphoryl imines to access numerous synthetically challenging chiral amine scaffolds. The current review critically summarizes and reveals the literature precedence of more than a decade to highlight the major achievements existing to date that can display a clear picture of advancement as well drawbacks in this area.

Zambrano-Rodríguez P. The Use of Peptides in Veterinary Serodiagnosis of Infectious Diseases: A Review

Peptides constitute an alternative and interesting option to develop treatments, vaccines, and diagnostic tools as they demonstrate their scope in several health aspects; as proof of this, commercial peptides for humans and animals are available on the market and used daily. This review aimed to know the role of peptides in the field of veterinary diagnosis, and include peptide-based enzyme-linked immunosorbent assay (pELISA), lateral flow devices, and peptide latex agglutination tests that have been developed to detect several pathogens including viruses and bacteria of health and production relevance in domestic animals. Studies in cattle, small ruminants, dogs, cats, poultry, horses, and even aquatic organisms were reviewed. Different studies showed good levels of sensitivity and specificity against their target, moreover, comparisons with commercial kits and official tests were performed which allowed appraising their performance. Chemical synthesis, recombinant DNA technology, and enzymatic synthesis were reviewed as well as their advantages and drawbacks. In addition, we discussed the intrinsic limitations such as the small size or affinity to polystyrene membrane and mention several strategies to overcome these problems. The use of peptides will increase in the coming years and their utility for diagnostic purposes in animals must be evaluated.

Strategies for improving antimicrobial peptide production

Antimicrobial peptides (AMPs) found in a wide range of animal, insect, and plant species are host defense peptides forming an integral part of their innate immunity. Although the exact mode of action of some AMPs is yet to be deciphered, many exhibit membrane lytic activity or interact with intracellular targets. The ever-growing threat of antibiotic resistance has brought attention to research on AMPs to enhance their clinical use as a therapeutic alternative. AMPs have several advantages over antibiotics such as broad range of antimicrobial activities including anti-fungal, anti-viral and anti-bacterial, and have not reported to contribute to resistance development. Despite the numerous studies to develop efficient production methods for AMPs, limitations including low yield, degradation, and loss of activity persists in many recombinant approaches. In this review, we outline available approaches for AMP production and various expression systems used to achieve higher yield and quality. In addition, recent advances in recombinant strategies, suitable fusion protein partners, and other molecular engineering strategies for improved AMP production are surveyed.

A Practical Peptide Synthesis Workflow Using Amino-Li-Resin

Herein we report a practical approach for peptide synthesis using second-generation fibrous polyacrylamide resin (Li-resin, “Li” is coming from the name of its inventor, Yongfu Li). This resin with the corresponding handle was used for solid phase peptide synthesis (SPPS) using a fluorenylmethoxycarbonyl (Fmoc) approach. We reveal that the most appropriate mixing and filtration strategy when using amino-Li-resin in SPPS is via shaking and gravity filtration, instead of mechanical stirring and suction filtration used with other resins. The strategy was demonstrated with the SPPS of H-Tyr-Ile-Ile-Phe-Leu-NH₂, which contains the difficult sequence Ile-Ile. The peptide was obtained with excellent purity and yield. We are confident that this strategy will be rapidly implemented by other peptide laboratories.

Liquid-Phase Peptide Synthesis (LPPS): A Third Wave for the Preparation of Peptides

Since the last century, peptides have gained wide acceptance as drugs, with almost 100 already in the market and a large number in the pipeline. In this context, peptide synthesis has grown massively as a stringent field for pharmaceuticals around the globe. Three methodologies, namely, classical solution peptide synthesis (CSPS), solid-phase peptide synthesis (SPPS), and liquid-phase peptide synthesis (LPPS), have made significant contributions to the field. This review provides a comprehensive and integrated vision of LPPS as the third wave for peptide synthesis. LPPS combines the advantages of CSPS and SPPS, where peptide elongation is carried out in solution and the growing peptide chain is supported on a soluble tag, which confers characteristic properties. LPPS protocols allow the large-scale production of peptides and reduce the use of excess reagents and solvents, thus meeting the principles of green chemistry. In this review, tags associated with LPPS are broadly discussed under the following headings: polydisperse polyethylene glycol (PEG), membrane-enhanced peptide synthesis (MEPS), fluorous technology, ionic liquids (ILs), PolyCarbon, hydrophobic polymers, and group-assisted purification (GAP). It also highlights the signature accomplishments of LPPS tags and the limitations of the same.

Silicon-based hydrophobic tags applied in liquid-phase peptide synthesis: protected DRGN-1 and poly alanine chain synthesis

Two types of silicon-based hydrophobic tags, including a siloxy group containing tag and an arylsilyl group containing tag, were developed for applying them in tag-assisted liquid-phase peptide synthesis (Tag LPPS) to synthesize long peptides.

Quick Access to High-Purity Peptide Drugs Bradykinin, Leuprolide Analogue, 2(PZ-128), and Rapastinel with Minimal Reagents

Peptide drugs bradykinin, a leuprolide analogue, 2(PZ-128), and rapastinel are synthesized in 56-77% yield using heating-assisted liquid-phase peptide synthesis on a soluble polynorbornene support. These drugs of commercial utility and complex structures are obtained in 2-5.5 h with no epimerization and >95% purity using only 1.2 equivalents of amino acids and coupling reagents. The peptide yield and purity are comparable or superior to the reported methods.

Liquid Phase Peptide Synthesis via One-Pot Nanostar Sieving (PEPSTAR)

Herein, a one‐pot liquid phase peptide synthesis featuring iterative addition of amino acids to a “nanostar” support, with organic solvent nanofiltration (OSN) for isolation of the growing peptide after each synthesis cycle is reported. A cycle consists of coupling, Fmoc removal, then sieving out of the reaction by‐products via nanofiltration in a reactor‐separator, or synthesizer apparatus where no phase or material transfers are required between cycles. The three‐armed and monodisperse nanostar facilitates both efficient nanofiltration and real‐time reaction monitoring of each process cycle. This enabled the synthesis of peptides more efficiently while retaining the full benefits of liquid phase synthesis. PEPSTAR was validated initially with the synthesis of enkephalin‐like model penta‐ and decapeptides, then octreotate amide and finally octreotate. The crude purities compared favorably to vendor produced samples from solid phase synthesis.

Amide-assisted α-C(sp3)–H acyloxyation of organic sulfides to access α-acyloxy sulfides

The direct acyloxyation of 2-(alkylthio)benzamide has been established via an amide-assisted α-C(sp3)–H functionalization in the presence of Selectfluor by using simple carboxylic acid and its corresponding salt as acyloxy sources.

An overview of recent progress in modern synthetic approach—combinatorial synthesis

Background

In recent times, a powerful tool of combinatorial synthesis has been used for the preparation of large chemical entities through a small set up of reactions between different building blocks using solid-phase and solution-phase techniques. This method reduced the time and cost of the drug discovery process substantially.

Main text

Thousands of compounds are synthesised in a few reactions through combinatorial synthesis instead of getting a few compounds in the traditional method. This method also helps to identify chemical lead of the compounds and optimise them through the biological screening using a high-throughput method. There is no review concerning the recent research finding of combinatorial synthesis. Hence, an attempt had been made on the latest research findings (2002–2020) of newly synthesised compounds using combinatorial synthesis and their biological activities.

Conclusion

To the best of our knowledge, the current review has completely analysed the importance of combinatorial synthesis and furnished an overview of solid-phase and solution-phase techniques as well as helped mankind by improving higher productivity at low cost, lead identification and optimization and preventing environmental pollution.

Graphical abstract

Greener liquid-phase synthesis and the ACE inhibitory structure-activity relationship of an anti-SARS octapeptide

A high-efficiency strategy for resin-free and large scale liquid phase synthesis of the anti-SARS octapeptide AVLQSGFR is described. Herein, tri(4'-diphenylphosphonyloxylbenzoyl phenyl)phosphate (TDPBP) derivatives were designed as C-terminal supports to aid octapeptide intermediate purification without the need for chromatographic separation. Furthermore, the ACE inhibitory structure-activity relationship (SAR) of the anti-SARS octapeptide and its alanine-scanning analogues was systematically studied by in vitro assay and 3D-QSAR via molecular docking. This paper provides a new strategy for the development of peptide-based drugs. Simultaneously, a study on the ACE inhibition and structure-activity relationship of the anti-SARS octapeptide also lays a foundation for further understanding how the anti-SARS octapeptide acts as an ACE inhibitor.

Recent advances in synthetic and medicinal chemistry of phosphotyrosine and phosphonate-based phosphotyrosine analogues

Phosphotyrosine-containing compounds attract significant attention due to their potential to modulate signalling pathways by binding to phospho-writers, erasers and readers such as SH2 and PTB domain containing proteins. Phosphotyrosine derivatives provide useful chemical tools to study protein phosphorylation/dephosphorylation, and as such represent attractive starting points for the development of binding ligands and chemical probes to study biology, and for inhibitor and degrader drug design. To overcome enzymatic lability of the phosphate group, physiologically stable phosphonate-based phosphotyrosine analogues find utility in a wide range of applications. This review covers advances over the last decade in the design of phosphotyrosine and its phosphonate-based derivatives, highlights the improved and expanded synthetic toolbox, and illustrates applications in medicinal chemistry.

Synthesis of Tri(4-formylphenyl) Phosphonate Derivatives as Recyclable Triple-Equivalent Supports of Peptide Synthesis

To seek the novel application of organophosphorus compounds, the designed tri(4-formylphenyl) phosphonate (TFP) derivatives were successfully synthesized herein, which were used as C-terminal protecting groups of amino acid or greener triple-equivalent supports in liquid-phase peptide synthesis (LPPS). Through the support-aided precipitation effect of TFP derivatives, the peptide intermediates during peptide synthesis were separated and collected via rapid precipitation and facile filtration without chromatographic purification. Furthermore, the TFP derivative support can be directly recycled for reuse without further regeneration after being sheared from the target peptide.

Liquid-Phase Total Synthesis of Plecanatide Aided by Diphenylphosphinyloxyl Diphenyl Ketone (DDK) Derivatives

Plecanatide is an oral guanylate cyclase-C agonist for the treatment of gastrointestinal disorders. The large-scale supply of plecanatide is restrained primarily by its industrial manufacture. Herein we developed diphenylphosphinyloxyl diphenyl ketone (DDK) derivatives as greener supports with unique precipitation-inducing properties to aid the liquid-phase total synthesis of plecanatide without the use of chromatography. Plecanatide could be obtained in high yield, and the ultimately sheared DDK derivative residue could be directly recycled or regenerated for reuse.

Resin-free peptide synthesis mediated by tri(4-benzoylphenyl) phosphate (TBP) derivatives as small-molecule supports

A series of novel tri(4-benzoylphenyl) phosphate (TBP) derivatives with unique precipitation-inducing properties were synthesized and used as C-terminal protecting groups of amino acids and recyclable supports in peptide synthesis.

A Metal-Free Three-Component Reaction of trans-β-Nitrostyrene Derivatives, Dibromo Amides, and Amines Leading to Functionalized Amidines

A mild, metal-free, and multicomponent route for the preparation of N-acyl amidines from nitroalkene derivatives, dibromo amides, and amines has been developed that accesses an initial α,α-dibromonitroalkane intermediate that can undergo C-C bond cleavage. This protocol offers an alternative approach toward N-acyl amidines and features the rapid construction of amidine frameworks with high diversity and complexity. The procedure also accesses bisamidine and α,β-unsaturated amidines which are challenging targets by traditional methods.

A one-pot protocol for the synthesis of β-ketosulfones from α,α-dibromoketones

A one-pot debromination/sulfonylation strategy was developed for the direct transformation of α,α-dibromoketones into β-ketosulfones in the presence of sulfinate salts in a methanol solution.

A Practical and Diastereoselective Synthesis of Dihydrofurocoumarin from Pyridinium Ylides in Aqueous Medium

A series of 2,3-dihydrofurocoumarins were prepared via a three-component reaction of aldehyde, pyridinium salt and 4-hydroxycoumarin in aqueous ethanol. This method avoids using excessive pyridine and trimethylamine, and has operational simplicity and avoids tedious column chromatography purification. Both aryl aldehydes and aliphatic aldehydes are applicable to this protocol.

Preparation of Optically Pure Tertiary Phosphine Oxides via the Addition of P-Stereogenic Secondary Phosphine Oxide to Activated Alkenes

Functionalized P,C-stereogenic tertiary phosphine oxides were prepared by the addition of (RP)-menthyl phenylphosphine oxide to activated olefins, in high drP and drC, and were isolated in excellent yields. The reaction was readily catalyzed by Ca(OH)2 or occurred with gentle heating. A wide range of substrates, including vinyl ketones, esters, nitriles, and nitro alkenes, can be used in the reaction.

GAP Peptide Synthesis via Design of New GAP Protecting Group: An Fmoc/tBu Synthesis of Thymopentin Free from Polymers, Chromatography and Recrystallization

A novel method for Fmoc/tBu solution-phase peptide synthesis and the development of a new benzyl-type GAP protecting group is reported. This new GAP protecting group is utilized in place of a polymer support, facilitating C→N Fmoc peptide synthesis without chromatography, recrystallization, or polymer supports. The GAP group can be added and removed in high yield, and was used to synthesize over 1 gram of the immunostimulant, thymopentin, in high overall yield (83%) and purity (99%).

Asymmetric aza-Morita–Baylis–Hillman reactions of chiral N-phosphonyl imines with acrylates via GAP chemistry/technology

A series of aza-Morita–Baylis–Hillman acrylates have been synthesized by using chiral N-phosphonyl imines and GAP chemistry/technology.

Acid-Triggered Colorimetric Hydrophobic Benzyl Alcohols for Soluble Tag-Assisted Liquid-Phase Synthesis

Simple screening of acid-triggered reactions of methoxybenzyl alcohols led to the development of a novel colorimetric hydrophobic benzyl alcohol (HBA) tag. HBA tag-3 (14) retained high solubility in less polar solvents and excellent precipitation properties in polar solvents. Our routine procedure for tag-assisted liquid phase peptide synthesis was applied using HBA tag-3 (14), and an effective synthesis of β-sheet breaker peptide iAβ5 (4) was achieved. The tagged peptides showed a vivid blue color under acidic conditions both on TLC plates and in solution, enabling quantitative assay.

N-Phosphonyl/phosphinyl imines and group-assisted purification (GAP) chemistry/technology

Group-assisted purification (GAP) chemistry, which can provide various chiral amines and other functionalities without the use of column chromatography or recrystallization; products are consistently obtained with excellent stereocontrol.