Active Ester Functionalized Azobenzenes as Versatile Building Blocks

Photoswitchable glycoligands targeting Pseudomonas aeruginosa LecA

Biofilm formation is one of main causes of bacterial antimicrobial resistance infections. It is known that the soluble lectins LecA and LecB, produced by Pseudomonas aeruginosa, play a key role in biofilm formation and lung infection. Bacterial lectins are therefore attractive targets for the development of new antibiotic-sparing anti-infective drugs. Building synthetic glycoconjugates for the inhibition and modulation of bacterial lectins have shown promising results. Light-sensitive lectin ligands could allow the modulation of lectins activity with precise spatiotemporal control. Despite the potential of photoswitchable tools, few photochromic lectin ligands have been developed. We have designed and synthesized several O- and S-galactosyl azobenzenes as photoswitchable ligands of LecA and evaluated their binding affinity with isothermal titration calorimetry. We show that the synthesized monovalent glycoligands possess excellent photophysical properties and strong affinity for targeted LecA with K d values in the micromolar range. Analysis of the thermodynamic contribution indicates that the Z-azobenzene isomers have a systematically stronger favorable enthalpy contribution than the corresponding E-isomers, but due to stronger unfavorable entropy, they are in general of lower affinity. The validation of this proof-of-concept and the dissection of thermodynamics of binding will help for the further development of lectin ligands that can be controlled by light.

In-situ Oxidation and Coupling of Anilines towards Unsymmetric Azobenzenes Using Flow Chemistry

Molecular switches, especially azobenzenes, are used in numerous applications, such as molecular solar thermal storage (MOST) systems and photopharmacology. The Baeyer-Mills reaction of anilines and nitrosobenzenes has been established as an efficient synthetic method for non-symmetric azobenzenes. However, nitrosobenzenes are not stable, depending on their substitution pattern and pose a health risk. An in-situ oxidation of anilines with Oxone® was optimized under continuous flow conditions avoiding isolation and contact. The in-situ generated nitrosobenzene derivatives were subjected to a telescoped Baeyer-Mills reaction in flow. That way azobenzenes with a broad substituent spectrum were made accessible.

Synthesis of a Series of 12-Membered Azobenzene Macrocycles and Tuning of the Half-Life of the Thermal Z-E Isomerization

Azobenzene macrocycles (AzMs) represent a class of azobenzene that are typically photoswitchable with good switching yields of E and Z isomers at certain photostationary states. Here, the synthesis and versatile functionalization of 12-membered AzMs is presented to obtain various meta- and para-aryl-substituted AzMs in high yields of 71-98%. At different positions in the periphery, these substituents significantly impact on the thermal half-lives of the less-stable Z isomers. Para-substitution leads to faster thermal relaxation than meta-substitution, and electron-donating groups lead to a faster relaxation than electron-withdrawing groups.