The direct acylation of quercetin (I) with 3-chloro-2,2-dimethylpropanoyl chloride (II) gives a complex reaction mixture. The synthesis of different acylated quercetin with from mono- to tetra-O-substituted functions was achieved in a simple procedure wherein the yield of isomers depended on the stoichiometric ratio of reagents. The crude reaction mixtures were analysed (LC-MS) and compared with the isolated products. Unambiguous structural characterisation of isomeric quercetin derivatives was confirmed by NMR analysis. In addition, the quercetin dimer can be obtained in a high yield in the simple procedure. The anti-oxidant activity and aldose reductase inhibition of the compounds were screened with the aim of providing bi-functional remedies to treat diabetic complications and other diseases where oxidative stress and the polyol pathway are key etiological factors.

The ability of flavonoids to affect multiple key pathways of glucose toxicity, as well as to attenuate inflammation has been well documented. In this study, the inhibition of rat lens aldose reductase by 3,7-di-hydroxy-2-[4-(2-chloro-1,4-naphthoquinone-3-yloxy)-3-hydroxyphenyl]-5-hydroxy-chromen-4-one (compound 1), was studied in greater detail in comparison with the parent quercetin (compound 2). The inhibition activity of 1, characterized by IC 50 in low micromolar range, surpassed that of 2. Selectivity in relation to the closely related rat kidney aldehyde reductase was evaluated. At organ level in isolated rat lenses incubated in the presence of high glucose, compound 1 significantly inhibited accumulation of sorbitol in a concentration-dependent manner, which indicated that 1 was readily taken up by the eye lens cells and interfered with cytosolic aldose reductase. In addition, compound 1 provided macroscopic protection of colonic mucosa in experimental colitis in rats. At pharmacologically active concentrations, compound 1 and one of its potential metabolite 2-chloro-3-hydroxy-[1,4]-naphthoquinone (compound 3) did not affect osmotic fragility of red blood cells.

The human intracellular enzyme AKR1B1 belongs to the aldo-keto reductase superfamily. The AKR1B1-catalyzed reduction of aldehydes is part of the intracellular inflammatory pathway leading to the activation of NF-κB and the expression of pro-inflammatory genes. The present study is aimed at determining the inhibition of AKR1B1 brought about by an extract of artichoke leaves (bracts), and the effects of this extract and three participating compounds on the expression of AKR1B1, COX-2, and MMP-2 proteins in THP-1 cells. It seeks to identify the ability of the test substances to modulate the lipopolysaccharide (LPS)-induced activation of NF-κB in cells and the intracellular oxidant effect of test substances after incubation with LPS. Low concentrations of the extract inhibit the enzyme AKR1B1. After stimulation by LPS, the extract attenuated the activity of NF-κB in THP-1 cells, but no changes in the expression of AKR1B1 were recorded. The extract diminished the expression of the inflammation-related enzymes COX-2 and MMP-2, probably by inhibiting the activity of NF-κB. The extract significantly diminished the intracellular reactive oxygen species after a brief LPS incubation, which may also have reduced intracellular inflammation. The diminished activity of NF-κB in the cells could be linked to the inhibition of the activity of AKR1B1. Copyright © 2017 John Wiley & Sons, Ltd.