P1 Glutamine isosteres in the design of inhibitors of 3C/3CL protease of human viruses of the Pisoniviricetes class.

Viral infections are one of the leading causes of acute morbidity in humans and much endeavour has been made by the synthetic community for the development of drugs to treat associated diseases. Peptide-based enzyme inhibitors, usually short sequences of three or four residues, are one of the classes of compounds currently under development for enhancement of their activity and pharmaceutical properties. This review reports the advances made in the design of inhibitors targeting the family of highly conserved viral proteases 3C/3CLpro, which play a key role in viral replication and present minimal homology with mammalian proteases. Particular focus is put on the reported development of P1 glutamine isosteres to generate potent inhibitors mimicking the natural substrate sequence at the site of recognition.'

Synthetic Studies towards the Calcium-Dependent Lipopeptide Antibiotic Cadaside B.

In the current global crisis of antimicrobial resistance, antimicrobial peptides represent a promising source of alternative antibiotics. Recently discovered cadaside B, a novel calcium-dependent antibiotic, exhibits potent antimicrobial activity towards Gram-positive pathogens including multi-drug resistant strains. These properties, coupled with a novel structure, non-cytotoxicity, and low likelihood of developing resistance render cadaside B an important synthetic target. Herein, a synthetic strategy towards cadaside B is reported with the key steps involving on-resin depsipeptide bond formation and solution-phase macrolactamization. Good agreement of the synthetic cadaside B MS/MS fragmentation pattern was observed with the natural product, but a different 1 H NMR spectrum and absence of antimicrobial activity suggest an undetected epimerization event took place during the synthesis. Herein the findings of our synthetic journey and suggestions for future directions are presented.

A Concise Synthetic Strategy Towards the Novel Calcium-dependent Lipopeptide Antibiotic, Malacidin A and Analogues.

Malacidin A is a novel calcium-dependent lipopeptide antibiotic with excellent activity against Gram-positive pathogens. Herein, a concise and robust synthetic route toward malacidin A is reported, employing 9-fluorenylmethoxycarbonyl solid-phase peptide synthesis of a linear precursor, including late-stage incorporation of the lipid tail, followed by solution-phase cyclization. The versatility of this synthetic strategy was further demonstrated by synthesis of a diastereomeric variant of malacidin A and a small library of simplified analogues with variation of the lipid moiety.

Synthesis and antiproliferative activity of C- and N-terminal analogues of culicinin D.

Culicinin D (1), a 10 amino acid peptaibol containing several unusual residues, has been shown to exhibit potent anticancer activity. Previous work in our group towards developing a structure-activity relationship (SAR) for this peptaibol has concentrated on replacement of the synthetically challenging AHMOD (3) and AMD (4) residues, resulting in the discovery of analogues with equivalent or better potency and simplified synthesis. The SAR of this peptaibol is extended in this work by investigating the effect of the N-terminal lipid tail and C-terminal amino alcohol, revealing the key contribution of each of these moieties on antiproliferative activity in a panel of breast and lung cancer cell lines.

Alanine scan-guided synthesis and biological evaluation of analogues of culicinin D, a potent anticancer peptaibol.

Culicinin D (1), a 10 amino acid peptaibol originally isolated from Culicinomyces clavisporus, exhibits potent activity against a range of cancer cell lines. Building on our previous work exploring the structure-activity relationship (SAR) of the unusual (2S,4S,6R)-AHMOD residue, a series of analogues of culicinin D were prepared to further investigate the SAR of these peptaibols. Alanine scanning of a potent and readily accessible analogue 23 revealed the effect of each residue on antiproliferative activity, and a small panel of analogues were prepared to explore the SAR of the non-natural amino acid residue (2S,4R)-AMD. Results from the alanine scan were used to design an expanded library of culicinin D analogues, leading to the discovery of cyclohexylalanine analogue 52, which exhibited better antiproliferative activity than the natural product 1.

Synthesis and SAR Analysis of Lipovelutibols B and D and Their Lipid Analogues.

The first syntheses of the cytotoxic peptides lipovelutibols B and D are described. While lipovelutibol D was prepared using solid-phase peptide synthesis followed by an O-N acyl migration to install the C-terminal amino alcohol, a different strategy was required to access lipovelutibol B and a series of N-terminal lipid analogues of the natural products. A cytotoxicity structure-activity relationship study revealed that the lipovelutibol D framework, whereby serine is substituted for alanine in the fifth position, provided the most potent analogues. Modification of the lipid tail was generally well tolerated, with longer alkyl chains enhancing analogue cytotoxicity.

Synthesis and antiproliferative activity of culicinin D analogues containing simplified AHMOD-based residues.

Culicinin D is a 10 amino acid peptaibol containing a rare and synthetically challenging (2S,4S,6R)-AHMOD residue, that exhibits potent antiproliferative activity against MDA-MB-468 cells. An SAR study focusing on replacement of the AHMOD residue was undertaken, culminating in the revelation that a 6-hydroxy or 6-keto substituent was essential to retain potent low nanomolar antiproliferative activity.

Investigations of the key macrolactamisation step in the synthesis of cyclic tetrapeptide pseudoxylallemycin A.

The total synthesis and structural confirmation of naturally occurring all l-cyclic tetrapeptide pseudoxylallemycin A is reported. X-ray crystallography revealed that the linear precursor adopted an all-trans (ttt) extended linear conformation, while its cyclic derivative adopts a trans,cis,trans,cis (tctc) conformation. Two kinetically favoured cyclic conformers prone to hydrolysis initially formed rapidly during cyclisation, with subsequent conversion to the thermodynamically stable tctc macrocycle taking place slowly. We postulate the initial unstable cyclic product undergoes an unprecedented nucleophilic ring opening with either the T3P or PyAOP by-products to give the linear ttt structure as a reactivated species and through a series of equilibria is slowly consumed by cyclisation to the thermodynamic product pseudoxylallemycin A. Consumption of the reactivated species by formation of pseudoxylallemycin A requires a trans-cis isomerism to occur and necessitates moderately increased reaction temperatures. Cyclisation with T3P was found to provide the greatest stereoretention. Synthesis and X-ray crystallography of the C-terminal epimer demonstrated its cyclisation to be kinetically favoured and to proceed without epimerisation despite also bearing an all-trans backbone.

Synthesis of AHMOD-containing aminolipopeptides, unique bioactive peptaibiotics.

An interesting family of bioactive aminolipopeptides contain the unusual building block 2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid (AHMOD). This building block has presented a significant challenge to synthesis of this family of aminolipopeptides due to the sensitive ß-hydroxyketone motif present. The discovery and identification, syntheses, and eventual incorporation of synthetically challenging AHMOD into selected aminolipopeptides is described herein.

N-alkylsulfonylimines as dipolarophiles in cycloaddition reactions.

First described in the late 1960s, N-alkylsulfonylimines are heterocumulenes that participate in reactions with a range of 1,3-dipoles to afford interesting 3-, 4-, 5-, and 6-membered heterocycles. The distribution of adducts obtained suggests that multistage, stepwise mechanistic pathways rather than a concerted process are in operation.