Substituent effects on the photophysical properties of amino-aurone-derivatives

ABSTRACT

Aurones are potential candidates to be employed as fluorescent probes or as pharmacophores for biological applications. This work describes a density functional theory (DFT) and time-dependent -DFT study at the PBE0/6-31?+?G(d) level of theory to analyse the structural, electronic and photophysical properties of a series of new proposed 4′-amine-aurone derivatives in its E and Z isomeric conformations. The maximum absorption wavelength of the proposed aurones appears in the range 390???514?nm, while the most allowed emission pathways were computed in the range 493–530?nm. The bathochromic shift of these compounds with respect to the non-substituted aurone is modulated by the acceptor strength of the added 4-substituents, in addition to the ability of the substituents to localise the frontier molecular orbitals over the acceptor benzofuranone moiety without losing the tricyclic planarity, which favours the push–pull nature of these molecules. The influence of the 4-substituent is also evidenced in the Stokes shifts for the whole series; as the electron-withdrawing character of the 4-substituents enhances, higher is the polarisation of the structure resulting in higher Stokes shifts. As a result, -CF₃ and -NO₂ substituents were responsible of larger Stokes shifts, then compounds containing these substituents are proposed as potential fluorescence probes for useful applications in biological systems.     

A straightforward conversion of aurones to 2-benzoylbenzofurans: transformation of one class of natural products into another

The naturally occurring aurones (2-benzylidene-3(2H)-benzofuran-3-ones) can be easily converted to another class of natural products 2-benzoylbenzo[b]furans, via an effective reduction, acid-mediated rearrangement, and oxidation cascade. This easy conversion was conducted without purification of intermediates. This straightforward conversion may be considered as a possible biosynthesis pathway of 2-benzoylbenzo[b]furans in plants.     

Synthesis of aurones under neutral conditions using a deep eutectic solvent

Aurones are an interesting, but little studied member of the flavanoid family of natural products. Of the various methods available for their synthesis, the simplest involves the condensation of a coumaranone with an aldehyde. This reaction can be performed under acidic or basic conditions. We have recently discovered an effectively neutral set of conditions that employ the deep eutectic solvent comprised of choline chloride and urea as both solvent and catalyst. Modest to good yields can be achieved for a range of aldehydes, thereby facilitating further study of aurones from both a biological as well as a spectroscopic perspective.     

New Isoprenylated Aurones from the Flowers of Rosa damascena

A phytochemical investigation of the flowers of Rosa damascena resulted in the isolation of three new isoprenylated aurones, damaurones C–E ( 1 – 3 , resp.), together with six known compounds, 4 – 9 . The structures of the new compounds were elucidated by spectroscopic methods, including extensive 1D‐ and 2D‐NMR experiments.     

Rapid synthesis of aurones under mild conditions using a combination of microwaves and deep eutectic solvents

The combination of microwave heating and the deep eutectic solvent formed from choline chloride and urea has resulted in a new, essentially neutral, yet rapid method for the synthesis of a wide range of aurone derivatives. While isolated yields remain somewhat variable, in virtually every case, a significant increase in yield has been observed on going from conventional thermal heating to microwave heating. In addition, some compounds inaccessible using prior methods have become reproducibly available using this modification. Further application of the combination of DES and microwave heating is expected to be highly promising and of general utility.     

Crude peroxidase from onion solid waste as a tool for organic synthesis. Part I: Cyclization of 2′,3,4,4′,6′-pentahydroxy-chalcone into aureusidin

The potential of a crude peroxidase (POD) from onion solid waste as a biocatalyst for the synthesis of a naturally occurring aurone is described. The crude enzyme preparation effectively promotes the cyclization of 2′,3,4,4′,6′-pentahydroxy-chalcone (which is not a natural substrate of onion POD) into aureusidin.