Design, Synthesis and Biological Evaluation of Quorum Sensing Modulators

Pseudomonas aeruginosa is an opportunistic pathogen associated with the majority of hospital-acquired infections and lung infections in cystic fibrosis. P. aeruginosa uses an intercellular communication process termed quorum sensing to control formation of a drug-resistant biofilm and generation of virulence factors.

The system is driven by small signaling molecules, usually N-acylated homoserine lactones (AHLs). In the search for new treatment methods for biofilm infections the quorum sensing system has emerged as an attractive target. Recent advances in this area have proved that it is possible to intercept the communication system by synthetic non-native ligands and thereby lower the pathogenesis and antibiotic tolerance of a bacterial biofilm.

To identify new ligands with quorum sensing modulating activities, three types of AHL analogs were synthesized using different synthetic strategies. The effect of replacing the acyl chain of the natural AHL ligand with a dipeptide was investigated. An in silico screening of N-dipeptido homoserine lactones against the protein responsible for the control of quorum sensing in P. aeruginosa identified a number of hits.

The best hits were synthesized using a solid-phase strategy. Another library in which the amide bond was replaced with a triazole unit was synthesized by means of the copper- and ruthenium-catalyzed azide-alkyne cycloadditions. Finally, the synthesis of compounds with biaryl functionalities in the position of the acyl chain was carried out.

Overall, 17 compounds were identified as quorum sensing activators with EC50 values in the low micromolar range. Two build/couple/pair strategies for the synthesis of structurally diverse small molecules are presented. In the first strategy, the Petasis 3-component reaction (Petasis 3-CR) of hydrazides is applied in the coupling of functionalized building blocks.

Diversification by functional group pairing was envisioned to provide a diverse range of cyclized products. Triphosgene mediated carbonylative stiching of the 1,2-hydrazido alcohol from the Petasis 3-CR afforded a mixture of oxazolidinone and oxadiazolone products. Optimization afforded a method for the selective synthesis of either oxazolidinones or oxadiazolones and a small compound library was synthesized.

Ring-closing metathesis of Abstract iv appropriately situated alkene moieties incorporated in the Petasis 3-CR products yielded five different cyclized products. In the second strategy, a number of alkyne- and azidecontaing amino acid-derivatives were coupled and macrocyclic peptidomimetics were obtained after intramolecular ruthenium-catalyzed azide-alkyne cycloadditions.