Total Synthesis of L-156,373 and an oxoPiz Analogue via a Submonomer Approach

Synthesis, derivatization, and conformational scanning of peptides containing N-Aminoglycine

N-alkylated glycine residues are the main constituent of peptoids and peptoid-peptide hybrids that are employed across the biomedical and materials sciences. While the impact of backbone N-alkylation on peptide conformation has been extensively studied, less is known about the effect of N-amination on the secondary structure propensity of glycine. Here, we describe a convenient protocol for the incorporation of N-aminoglycine into host peptides on solid support. Amide-to-hydrazide substitution also affords a nucleophilic handle for further derivatization of the backbone. To demonstrate the utility of late-stage hydrazide modification, we synthesized and evaluated the stability of polyproline II helix and β-hairpin model systems harboring N-aminoglycine derivatives. The described procedures provide facile entry into peptidomimetic libraries for conformational scanning.

Stabilizing Pseudouridimycin: Synthesis, RNA Polymerase Inhibitory Activity, and Antibacterial Activity of Dipeptide-Modified Analogues

Pseudouridimycin (PUM) is a microbially produced C-nucleoside dipeptide that selectively targets the nucleotide addition site of bacterial RNA polymerase (RNAP) and that has a lower rate of spontaneous resistance emergence relative to current drugs that target RNAP. Despite its promising biological profile, PUM undergoes relatively rapid decomposition in buffered aqueous solutions. Here, we describe the synthesis, RNAP-inhibitory activity, and antibacterial activity of chemically stabilized analogues of PUM. These analogues feature targeted modifications that mitigate guanidine-mediated hydroxamate bond scission. A subset of analogues in which the central hydroxamate is replaced with amide or hydrazide isosteres retain the antibacterial activity of the natural product.

Concise Total Synthesis and Biological Evaluation of Pargamicin A and its Diastereomer, Piperazic Acid-containing Cyclopeptides

We have accomplished the total synthesis, structure determination, and biological evaluation of pargamicin A and one of its diastereomers. Two key tripeptide segments were synthesized using a linear peptide elongation process that includes the direct coupling of a poorly nucleophilic piperazic acid derivative. The resulting tripeptides were coupled using triphosgene/collidine at ambient temperature leading to a precursor for the final cyclization step. T3P-promoted macrolactamization under high-dilution conditions, followed by the removal of the benzyl protecting group was used to furnish two putative structures of pargamicin A. Comparison of the 1 H and 13 C NMR spectra and the antibacterial activity of the natural and synthetic products successfully revealed that the absolute configuration of the N-hydroxy-Ile residue of pargamicin A is 2S,3S. A biological evaluation of synthetically obtained pargamicin A and its diastereomer suggested that the stereostructure of the cyclic peptide scaffold of the natural product plays a crucial role in determining the strength of its antibacterial activity.

Total Synthesis of Pargamicin A

We report the total synthesis and configurational assignment of pargamicin A, a highly oxidized nonribosomal peptide that potently inhibits the growth of drug-resistant bacteria. Our synthetic approach relies on late-stage piperazine ring formation and careful selection of condensation reagents to assemble the densely substituted hexapeptide backbone. This work enables the synthesis of pargamicin congeners for the development of structure-activity relationships and informs strategies for accessing other sterically congested piperazic acid-containing natural products.

Marine Cyclic Peptides: Antimicrobial Activity and Synthetic Strategies

Oceans are a rich source of structurally unique bioactive compounds from the perspective of potential therapeutic agents. Marine peptides are a particularly interesting group of secondary metabolites because of their chemistry and wide range of biological activities. Among them, cyclic peptides exhibit a broad spectrum of antimicrobial activities, including against bacteria, protozoa, fungi, and viruses. Moreover, there are several examples of marine cyclic peptides revealing interesting antimicrobial activities against numerous drug-resistant bacteria and fungi, making these compounds a very promising resource in the search for novel antimicrobial agents to revert multidrug-resistance. This review summarizes 174 marine cyclic peptides with antibacterial, antifungal, antiparasitic, or antiviral properties. These natural products were categorized according to their sources—sponges, mollusks, crustaceans, crabs, marine bacteria, and fungi—and chemical structure—cyclic peptides and depsipeptides. The antimicrobial activities, including against drug-resistant microorganisms, unusual structural characteristics, and hits more advanced in (pre)clinical studies, are highlighted. Nocathiacins I–III (91–93), unnarmicins A (114) and C (115), sclerotides A (160) and B (161), and plitidepsin (174) can be highlighted considering not only their high antimicrobial potency in vitro, but also for their promising in vivo results. Marine cyclic peptides are also interesting models for molecular modifications and/or total synthesis to obtain more potent compounds, with improved properties and in higher quantity. Solid-phase Fmoc- and Boc-protection chemistry is the major synthetic strategy to obtain marine cyclic peptides with antimicrobial properties, and key examples are presented guiding microbiologist and medicinal chemists to the discovery of new antimicrobial drug candidates from marine sources.

Total Synthesis of Pseudouridimycin

Pseudouridimycin (1), a potent antibiotic against both Gram-positive and Gram-negative bacteria including multi-drug-resistant strains with a new mode of action isolated from Streptomyces sp., was synthesized by a convergent strategy from 5'-amino-pseudouridine 5 and N-hydroxy-dipeptide 26 in 23% total yield. The key intermediate 26 was synthesized by hydroxylaminolysis of the nitrone derived from glutamine and subsequent glycylation with glycine chloride. The synthetic method provides an efficient and practical way for the synthesis of N-hydroxylated peptidyl nucleoside.

Diastereoselective Synthesis of (3R,5R)-γ-Hydroxypiperazic Acid

We report an asymmetric synthesis of the (3R,5R)-γ-hydroxypiperazic acid (γ-OHPiz) residue encountered in several bioactive nonribosomal peptides. Our strategy relies on a diastereoselective enolate hydroxylation reaction and electrophilic N-amination to provide the acyclic γ-OHPiz precursor. This orthogonally protected α-hydrazino acid intermediate is amenable to late-stage diazinane ring formation following incorporation into a peptide chain. We determined the N-terminal amide rotamer propensity of the γ-OHPiz residue and showed that the γ-OH substituent enhances trans-amide bias relative to piperazic acid.

Synthesis and conformation of backbone N-aminated peptides

The chemical modification of peptides is a promising approach for the design of protein-protein interaction inhibitors and peptide-based drug candidates. Among several peptidomimetic strategies, substitution of the amide backbone maintains side-chain functionality that may be important for engagement of biological targets. Backbone amide substitution has been largely limited to N-alkylation, which can promote cis amide geometry and disrupt important H-bonding interactions. In contrast, N-amination of peptides induces distinct backbone geometries and maintains H-bond donor capacity. In this chapter we discuss the conformational characteristics of designed N-amino peptides and present a detailed protocol for their synthesis on solid support. The described methods allow for backbone N-amino scanning of biologically active parent sequences.

A Reliable Enantioselective Route to Mono-Protected N1-Cbz Piperazic Acid Building Block

The chiral N1-Cbz, N2-H derivative of the piperazic acid monomer is a valuable building block in the total synthesis of natural products, comprising this nonproteinogenic amino acid. In that context, we wish to report an improved synthetic protocol for the synthesis of both (3R)- and (3S)-piperazic acids bearing the carboxybenzyl protecting group (Cbz) selectively at the N1 position. Our method builds on previously reported protocols, circumventing their potential shortcomings, and optimizing the ultimate selective deprotection at the N2 position, thus, offering an efficient and reproducible pathway to suitably modified piperazates in high optical purity.

N-Hydroxy peptides: solid-phase synthesis and β-sheet propensity

Backbone amide hydroxylation of peptide strands enhances β-hairpin folding.

N-Amino peptide scanning reveals inhibitors of Aβ42aggregation

The aggregation of amyloids into toxic oligomers is believed to be a key pathogenic event in the onset of Alzheimer's disease. Peptidomimetic modulators capable of destabilizing the propagation of an extended network of β-sheet fibrils represent a potential intervention strategy. Modifications to amyloid-beta (Aβ) peptides derived from the core domain have afforded inhibitors capable of both antagonizing aggregation and reducing amyloid toxicity. Previous work from our laboratory has shown that peptide backbone amination stabilizes β-sheet-like conformations and precludes β-strand aggregation. Here, we report the synthesis of N-aminated hexapeptides capable of inhibiting the fibrillization of full-length Aβ₄₂. A key feature of our design is N-amino substituents at alternating backbone amides within the aggregation-prone Aβ_16–21 sequence. This strategy allows for maintenance of an intact hydrogen-bonding backbone edge as well as side chain moieties important for favorable hydrophobic interactions. An N-amino scan of Aβ_16–21 resulted in the identification of peptidomimetics that block Aβ₄₂ fibrilization in several biophysical assays.

Structure-based design of backbone-aminated peptides affords novel β-strand mimics that inhibit amyloid-beta fibrillogenesis.

Total synthesis of the proposed structure of talarolide A

The proposed structure of talarolide A, a cycloheptapeptide featuring a hydroxamate moiety within the peptide backbone, was successfully synthesized.