- Studies of the selective O-alkylation and dealkylation of flavonoids. XVIII. A convenient method for synthesizing 3,5,6,7-tetrahydroxyflavones
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In the demethylation of 6-hydroxy-3,4',7-trimethoxy-5-(tosyloxy)flavone with anhydrous aluminum bromide, the 5-tosyloxyl group was eliminated with bromination to give 8-bromo-3,6,7-trihydroxy-4'-methoxyflavone as the main product. When anhydrous aluminum chloride was used in the demethylation of the acetate, the 5-tosyloxyl group was cleaved prior to the demethylation to give 5,6,7-trihydroxy-3,4'-dimethoxyflavone. Demethylation of 6-hydroxy-4',5,7-trimethoxy-3-(tosyloxy)flavone and its acetate with the bromide or chloride afforded the 5,6,7-trihydroxyflavone without the cleavage of the 3-tosyloxyl group, but was not suitable for the general synthesis of the 3,5,6,7-tetrahydroxyflavones because of the difficulty in removing the protecting group. Consequently, it was found that the direct demethylation of 3,6-dihydroxy-5,7-dimethoxyflavones with anhydrous aluminum chloride-sodium iodide in acetonitrile was the most useful general method for synthesizing 3,5,6,7-tetrahydroxyflavones. Additionally, the reported structures of two natural flavones were revised.
- Horie,Kobayashi,Kawamura,Yoshida,Tominaga,Yamashita
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- Preparation of two sets of 5,6,7-trioxygenated dihydroflavonol derivatives as free radical scavengers and neuronal cell protectors to oxidative damage
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An unusual class of 5,6,7-trioxygenated dihydroflavonols (3a-e and 4a-j) were designed and prepared. Their antioxidative properties were assessed by examining their capacities in several in vitro models, including superoxide anion and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, rat liver homogenate lipid peroxidation inhibition, PC12 cells protection from oxidative damage, and xanthine oxidase inhibition. These dihydroflavonols displayed positive quenching abilities towards O2{radical dot} - and DPPH free radicals, in which the majority exhibited superior antioxidant properties to Vitamin C. cis-Configurated compound (±)-3e demonstrated remarkable inhibition to LPO with an IC50 value of 1.9 ± 0.3 μM, which was apparently stronger than that of quercetin (IC50 = 6.0 ± 0.4 μM). trans-Configurated dihydroflavonol (±)-4h exhibited significant protective effect on PC12 cells against oxidative damage with an EC50 value of 41.5 ± 5.3 μM, more effective compared to that of quercetin (EC50 = 81.8 ± 8.7 μM). The 6-OH-5,7-dimethoxy analogue (±)-3d showed significant inhibition of xanthine oxidase with an IC50 value of 16.0 ± 0.8 μM, which is superior to that of allopurinol (IC50 = 23.5 ± 2.0 μM). In addition to the hypothesized action mechanism of the bio-active compounds, 3D modeling was used to analyze the relationship between the minimized-energy structures and antioxidant activities.
- Gong, Jingxu,Huang, Kexin,Wang, Feng,Yang, Leixiang,Feng, Yubing,Li, Haibo,Li, Xiaokun,Zeng, Su,Wu, Xiumei,Stoeckigt, Joachim,Zhao, Yu,Qu, Jia
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supporting information; experimental part
p. 3414 - 3425
(2009/09/27)
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