Total synthesis and structural validation of cyclodepsipeptides solonamide A and B

Total synthesis and a systematic structure-activity relationship study of WAP-8294A2

WAP-8294A2 is a cyclic peptide antibiotic with novel structure and excellent activity against Gram-positive pathogens. Herein, we report the total synthesis of complex macrocyclic peptide WAP-8294A2 (W1), ent-analogue W2, deoxy analogue W3 and de-methyl analogue W4 using a solid-phase synthetic route followed by a final stage solution-phase cyclization reaction. Exploitation of this process allowed the synthesis of eleven alanine-scanning analogues and eight lysine-scanning analogues. The antimicrobial activity of these analogues was evaluated in vitro against Gram-positive bacteria. Based on the MIC results, a primary systematic structure-activity relationship has been established.

Synthetic strategies to access staphylococcus auto-inducing peptides as quorum sensing modulators

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of staphylococcus virulence and has been the focus of tremendous interest in the development of effective therapies for pathogenic bacterial infections. With regards to chemotherapeutic based strategies, the significant proportion of currently reported agr-system modulating molecules are mimics of the native ArgC substrate, which is a thioester-based macrocyclic peptide know as the auto-inducing peptide. Over the past two decades, more than two-hundred synthetic analogues have been reported. This review traces the development of the synthetic strategies employed to synthesise these analogues with a particular focus on macrocyclisation. At present these synthetic approaches can be clustered into five broad categories (1) solution-phase cyclisation, (2) immobilised carbodiimide assisted cyclisation, (3) concomitant on-resin cleavage and macrocyclisation, (4) Boc-compatible chemoselective thioesterification, and (5) Fmoc-compatible chemoselective thioesterification. The advantages and limitation provided by each of the approaches are compared and contrasted with a view towards potential reaction scale-up.

Solid-state conformations of linear depsipeptide amides with an alternating sequence of α,α-disubstituted α-amino acid and α-hydroxy acid

Depsipeptides and cyclodepsipeptides are analogues of the corresponding peptides in which one or more amide groups are replaced by ester functions. Reports of crystal structures of linear depsipeptides are rare. The crystal structures and conformational analyses of four depsipeptides with an alternating sequence of an α,α-disubstituted α-amino acid and an α-hydroxy acid are reported. The molecules in the linear hexadepsipeptide amide in (S)-Pms-Acp-(S)-Pms-Acp-(S)-Pms-Acp-NMe₂ acetonitrile solvate, C₄₇H₅₈N₄O₉·C₂H₃N, (3b), as well as in the related linear tetradepsipeptide amide (S)-Pms-Aib-(S)-Pms-Aib-NMe₂, C₂₈H₃₇N₃O₆, (5a), the diastereoisomeric mixture (S,R)-Pms-Acp-(R,S)-Pms-Acp-NMe₂/(R,S)-Pms-Acp-(R,S)-Pms-Acp-NMe₂ (1:1), C₃₂H₄₁N₃O₆, (5b), and (R,S)-Mns-Acp-(S,R)-Mns-Acp-NMe₂, C₃₀H₃₇N₃O₆, (5c) (Pms is phenyllactic acid, Acp is 1-aminocyclopentanecarboxylic acid and Mns is mandelic acid), generally adopt a β-turn conformation in the solid state, which is stabilized by intramolecular N-H...O hydrogen bonds. Whereas β-turns of type I (or I') are formed in the cases of (3b), (5a) and (5b), which contain phenyllactic acid, the torsion angles for (5c), which incorporates mandelic acid, indicate a β-turn in between type I and type III. Intermolecular N-H...O and O-H...O hydrogen bonds link the molecules of (3a) and (5b) into extended chains, and those of (5a) and (5c) into two-dimensional networks.

Synthesis of novel sulfide-based cyclic peptidomimetic analogues to solonamides

Eight new sulfide-based cyclic peptidomimetic analogues of solonamides A and B have been synthesized via solid-phase peptide synthesis and S_N2’ reaction on a Morita–Baylis–Hillman (MBH) residue introduced at the N-terminal of a tetrapeptide. This last step takes advantage of the electrophilic feature of the MBH residue and represents a new cyclization strategy occurring. The analogues were prepared in moderate overall yields and did not show toxic effects on Staphylococcus aureus growth and were not toxic to human fibroblasts. Two of them inhibited the hemolytic activity of S. aureus, suggesting an interfering action in the bacterial quorum sensing similar to the one already reported for solonamides.

Structure-Activity Relationship Studies of Small Molecule Modulators of the Staphylococcal Accessory Gene Regulator

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of Staphylococcus virulence. The agr-system serves a crucial role in pathogenesis by triggering substantive gene expression alterations to up-regulate the production of a wide variety of virulence determinants such as exoenzymes (proteases, lipases, nucleases) and downregulate the expression of surface binding proteins. Accordingly, the agr-system represents a compelling target for the development of antivirulence therapeutics as potential adjuncts, or alternatives, to conventional bactericidal and bacteriostatic antibiotics. Despite this potential, to date, no agr-system inhibitors have progressed to the clinic; however, several promising lead compounds have been identified through screens of synthetic and natural product libraries. On the basis of the molecular components within the agr-system, the current contingent of regulating compounds can be clustered into three broad groups, AgrA-P3 activation inhibitors, AgrB-AgrD processing inhibitors, and AgrC-AIP interaction inhibitors. This review aims to provide an overview of the development, structure-activity-relationships, and limitations of compounds within each of these groups in addition to the current opportunities for developing next-generation anologs.

Total Synthesis of Asperphenins A and B

The first total synthesis of asperphenins A and B has been accomplished in a concise, highly stereoselective fashion from commercially available materials (15 steps, 9.7% and 14.2% overall yields, respectively). The convergent route featured the judicious choice of protecting groups, fragment assembly strategy and a late-stage iron-catalyzed Wacker-type selective oxidation of an internal alkene to the corresponding ketone.

Chirality sensing of bioactive compounds with amino alcohol unit via circular dichroism

The aim of the present work was to test various chiroptical techniques, including in particular the in situ dirhodium methodology, to assign the absolute configuration of 1,2- and 1,3-amino alcohols. As models, we selected mainly compounds that have both an additional strongly absorbing and interfering chromophoric system and application in medicinal chemistry. Determination of the absolute configuration (AC) of the tested molecules such as cinchona alkaloids, Tamiflu, and others was carried out using a combination of electronic and vibrational circular dichroism (ECD, VCD) spectroscopy. It has been demonstrated that both 1,2- and 1,3-aminol moieties are subject to the same sector rule correlating stereostructure of formed Rh₂ -complex with chiroptical properties, and that the changes in the position of the stereogenic center do not affect its proper use.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

The agr Inhibitors Solonamide B and Analogues Alter Immune Responses to Staphylococccus aureus but Do Not Exhibit Adverse Effects on Immune Cell Functions

Staphylococcus aureus infections are becoming increasingly difficult to treat due to antibiotic resistance with the community-associated methicillin-resistant S. aureus (CA-MRSA) strains such as USA300 being of particular concern. The inhibition of bacterial virulence has been proposed as an alternative approach to treat multi-drug resistant pathogens. One interesting anti-virulence target is the agr quorum-sensing system, which regulates virulence of CA-MRSA in response to agr-encoded autoinducing peptides. Agr regulation confines exotoxin production to the stationary growth phase with concomitant repression of surface-expressed adhesins. Solonamide B, a non-ribosomal depsipeptide of marine bacterial origin, was recently identified as a putative anti-virulence compound that markedly reduced expression of α-hemolysin and phenol-soluble modulins. To further strengthen solonamide anti-virulence candidacy, we report the chemical synthesis of solonamide analogues, investigation of structure–function relationships, and assessment of their potential to modulate immune cell functions. We found that structural differences between solonamide analogues confer significant differences in interference with agr, while immune cell activity and integrity is generally not affected. Furthermore, treatment of S. aureus with selected solonamides was found to only marginally influence the interaction with fibronectin and biofilm formation, thus addressing the concern that application of compounds inducing an agr-negative state may have adverse interactions with host factors in favor of host colonization.

Cyclodepsipeptides: a rich source of biologically active compounds for drug research

Faced with the need to find new drugs for all kinds of diseases, science sees that Nature offers numerous classes of compounds showing an impressively high biological potential. Among those are the cyclodepsipeptides, hybrid structures composed of amino and hydroxy acids. In the past decades numerous cyclodepsipeptides have been isolated and their potential as drugs has been studied extensively. For several cyclodepsipeptides total syntheses both in solution and on solid-phase have been established, allowing the production of combinatorial libraries. In addition, the biosynthesis of specific cyclodepsipeptides has been elucidated and used for the chemoenzymatic preparation of nonnatural analogues. This review summarizes the recent literature on cyclic tetra- to decadepsipeptides, composed exclusively of α-amino- and α-hydroxy acids.