A two-phase bromination process using tetraalkylammonium hydroxide for the practical synthesis of α-bromolactones from lactones

A simple and efficient method for α-brominating lactones that affords α-bromolactones under mild conditions using tetraalkylammonium hydroxide (R₄N⁺OH^-) as a base was developed. Lactones are ring-opened with Br₂ and a substoichiometric amount of PBr₃, leading to good yields of the corresponding α-bromocarboxylic acids. Subsequent intramolecular cyclization over 1 h using a two-phase system (H₂O/CHCl₃) containing R₄N⁺OH^- afforded α-bromo lactones in good yields. This method can be applied at the 10 mmol scale using simple operations. α-Bromo-δ-valerolactone, which is extremely reactive and difficult to isolate, could be isolated and stored in a freezer for about one week using the developed method. Optimizing the solvent for environmentally friendly large-scale syntheses revealed that methyl ethyl ketone (MEK) was as effective. In addition, in situ-generated α-bromo-δ-valerolactone was directly converted into a sulfur-substituted functional lactone without difficulty by reacting it with a sulfur nucleophile in one pot without isolation. This new bromination system is expected to facilitate the industrial use of α-bromolactones as important intermediates.

Heterocyclic Chemistry Applied to the Design of N-Acyl Homoserine Lactone Analogues as Bacterial Quorum Sensing Signals Mimics

N-acyl homoserine lactones (AHLs) are small signaling molecules used by many Gram-negative bacteria for coordinating their behavior as a function of their population density. This process, based on the biosynthesis and the sensing of such molecular signals, and referred to as Quorum Sensing (QS), regulates various gene expressions, including growth, virulence, biofilms formation, and toxin production. Considering the role of QS in bacterial pathogenicity, its modulation appears as a possible complementary approach in antibacterial strategies. Analogues and mimics of AHLs are therefore biologically relevant targets, including several families in which heterocyclic chemistry provides a strategic contribution in the molecular design and the synthetic approach. AHLs consist of three main sections, the homoserine lactone ring, the central amide group, and the side chain, which can vary in length and level of oxygenation. The purpose of this review is to summarize the contribution of heterocyclic chemistry in the design of AHLs analogues, insisting on the way heterocyclic building blocks can serve as replacements of the lactone moiety, as a bioisostere for the amide group, or as an additional pattern appended to the side chain. A few non-AHL-related heterocyclic compounds with AHL-like QS activity are also mentioned.

Expanding the antibacterial selectivity of polyether ionophore antibiotics through diversity-focused semisynthesis

Polyether ionophores are complex natural products capable of transporting cations across biological membranes. Many polyether ionophores possess potent antimicrobial activity and a few selected compounds have ability to target aggressive cancer cells. Nevertheless, ionophore function is believed to be associated with idiosyncratic cellu-lar toxicity and, consequently, human clinical development has not been pursued. Here, we demonstrate that structurally novel polyether ionophores can be efficiently constructed by recycling components of highly abundant polyethers to afford analogues with enhanced anti-bacterial selectivity compared to a panel of natural polyether ionophores. We used classic degradation reactions of the natural polyethers lasalocid and monensin and combined the resulting fragments with building blocks provided by total synthesis, including halogen-functionalized tetronic acids as cation-binding groups. Our results suggest that structural optimization of polyether ionophores is possible and that this area represents a potential opportunity for future methodological innovation.

(2R)- and (2S)- 2-hydroxy- hexanoyl and octanoyl-l-homoserine lactones: New highly potent Quorum Sensing modulators with opposite activities

The synthesis and the QS modulation activity of diastereoisomerically pure 2-hydroxy-N-acyl-l-homoserine lactones (2-OH-AHLs) are unveiled. (2R)- and (2S)- 2-hydroxy-N-hexanoyl-l-homoserine lactone and 2-hydroxy-N-octanoyl-l-homoserine lactone have been identified as very potent QS agonists and antagonists on the Vibrio fischeri-quorum sensing system with opposite activities depending on the configuration of the carbon atom with the hydroxyl group. Flexible molecular docking showed that the (2R)-OH configuration in the antagonist isomer induces new hydrogen bonds with Tyr70 and Asp79, two importantly conserved residues in the LuxR protein family, while the (2S)-OH agonist configuration exhibits a binding mode comparable to the natural ligand 3-oxo-hexanoyl-l-homoserine lactone (OHHL). For the analogs with long alkyl chain 3a and 3b and aromatic analogs, all are antagonists with no effect of the configuration at C-2.

Four-Step Total Synthesis of (+)-Yaoshanenolides A and B

A highly concise bioinspired four-step total synthesis of yaoshanenolides A and B possessing tricyclic spirolactone with an unusual 5'H-spiro-[bicyclo[2.2.2]-oct[2]ene-7,2'-furan]-5'-one scaffold is reported. This synthesis features high-yielding aldol-type addition of γ-butyrolactone on to the aldehyde, exocyclic olefination of lactone derivative using Eschenmoser's salt, and highly facial- and endo-selective [4 + 2]-cycloaddition of fully functionalized 5-methylene-2(5H)-furanone with natural R-(-)-α-phellandrene. The approach allows access to yaoshanenolides A and B in four linear steps in 11 and 13% overall yield.

Alkyne-Substituted Fimbrolide Analogues as Novel Bacterial Quorum-Sensing Inhibitors

Gram-negative bacteria such as Pseudomonas aeruginosa use furanosyl diesters as autoinducers for quorum sensing (QS), a major regulatory and cell-to-cell communication system for social adaptation, virulence factor production, biofilm formation, and antibiotic resistance. A range of natural and synthetic brominated furanones, i.e. fimbrolide derivatives, have been found to act as inhibitors of QS-dependent bacterial phenotypes, complementing the bactericidal ability of traditional antibiotics. In this work, several novel acetylene analogues of fimbrolides were synthesised in moderate to high yields via Sonogashira coupling reactions of brominated furanones 4-bromo-5-(bromomethylene)furan-2(5H)-one 4 and 5-(dibromomethylene)-3-ethylfuran-2(5H)-one 5. The Sonogashira reaction of acetylenes on 4-bromo-5-(bromomethylene)furan-2(5H)-one 4 was favoured at the C5 methylene bromide over the C4 bromide substituent. On biological testing, the most potent compounds 13 and 14 showed 82 and 98 % bacterial quorum-sensing inhibitory (QSI) activity against Pseudomonas aeruginosa reporter strain respectively.

Synthesis and biofilm formation reduction of pyrazole-4-carboxamide derivatives in some Staphylococcus aureus strains

The ability of several N-phenyl-1H-pyrazole-4-carboxamide derivatives and other pyrazoles opportunely modified at the positions 3, 4 and 5, to reduce the formation of the biofilm in some Staphylococcus aureus strains (ATCC 29213, ATCC 25923 and ATCC 6538) were investigated. All the tested compounds were able, although to a different extent, to reduce the biofilm formation of the three bacterial strains considered. Among these, the 1-(2,5-dichlorophenyl)-5-methyl-N-phenyl-1H-pyrazole-4-carboxamide 14 resulted as the best inhibitor of biofilm formation showing an IC50 ranging from 2.3 to 32 μM, against all the three strains of S. aureus. Compound 14 also shows a good protective effect in vivo by improving the survival of wax moth larva (Galleria mellonella) infected with S. aureus ATCC 29213. These findings indicate that 14d is a potential lead compound for the development of new anti-virulence agents against S. aureus infections.

N,N'-alkylated Imidazolium-derivatives act as quorum-sensing inhibitors targeting the Pectobacterium atrosepticum-induced symptoms on potato tubers

Bacteria belonging to the Pectobacterium genus are the causative agents of the blackleg and soft-rot diseases that affect potato plants and tubers worldwide. In Pectobacterium, the expression of the virulence genes is controlled by quorum-sensing (QS) and N-acylhomoserine lactones (AHLs). In this work, we screened a chemical library of QS-inhibitors (QSIs) and AHL-analogs to find novel QSIs targeting the virulence of Pectobacterium. Four N,N′-bisalkylated imidazolium salts were identified as QSIs; they were active at the μM range. In potato tuber assays, two of them were able to decrease the severity of the symptoms provoked by P. atrosepticum. This work extends the range of the QSIs acting on the Pectobacterium-induced soft-rot disease.