Soluble Non-Cross-Linked Poly(norbornene) Supports for Peptide Synthesis with Minimal Reagents

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.

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.

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.

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 rink-amide soluble support: high purity conotoxins and other peptides accessed with minimal reagents

The use of peptides as therapeutics has been growing due to their biocompatibility. Solid phase peptide synthesis typically used to access these peptides requires excess reagents and/or microwave irradiation to drive reactions to completion because the reaction medium is heterogeneous. Reported herein is a soluble polynorbornene support containing rink amide attached sites for synthesizing oligopeptides and conotoxins in high purity using only 1.2 to 2 equivalents of coupling reagents. The support can be isolated as a precipitate from the reaction medium by adding ether. The loading capacity of the support can be easily determined by spectroscopy and can also be tuned by varying the monomer ratio. This support is promising for accessing peptides as the methodology uses minimal reagents and by-products can be easily separated.

Conditional changes enhanced production of bioactive metabolites of marine derived fungus Eurotium rubrum

Metabolites of marine derived fungus Eurotium rubrum MPUC136 differed between cultivation on wheat medium and Czapek-Dox agar medium. Melanin synthesis inhibitory activity of crude extract of culture on wheat medium showed stronger activity than that of crude extract of culture on Czapek-Dox agar medium. A new diketopiperazine compound isoechinulin D (1) and eight reported diketopiperazines (2-9) were isolated from the crude extract of wheat medium. The structure of 1 was established using NMR, MS and IR methods. 2-5 inhibited melanogenesis using B16 melanoma cells (IC₅₀=68, 2.4, 83, 9.1μM each). Structure-Activity-Relationships of diketopiperazines (1-10) indicated the importance of the prenyl groups at C-2, C-5 and C-7, the vinyl group at C-12 to C-25 and the sp² carbons at C-8 and C-9. Isolated compounds (1-9) were not or slightly observed from the extracts of Czapek-Dox agar medium by HPLC analysis, suggesting that different cultivation processes could affect metabolism and enhance bioactivities.

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.

An improved soluble polynorbornene support for peptide synthesis

A soluble polynorbornene support containing an oligoether linker as well as alkyl and oligoether side chains has been developed and used to synthesize Leu⁵-Enkephalin in 52% overall yield using only 1.2 equivalents of coupling reagents.