- Regioselective hydroxylation of 2-hydroxychalcones by dimethyldioxirane towards polymethoxylated flavonoids
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The flavone nucleus is part of a large number of natural products and medicinal compounds. In this presentation the novel regioselective hydroxylation of hydroxyarenes with DMD is described. The results showed further that flavonoids with 5-hydroxy group were selectively oxyfunctionalized at the para-position C8 carbon atom by DMD. Finally, according to this methodology, the naturally occurring isosinensetin, tangeretin, sinensetin, nobiletin, natsudaidain, gardenin B, 3,3′,4′,5,6,7,8- heptamethoxyflavone, quercetin and its derivatives were synthesized.
- Chu, Han-Wei,Wu, Huan-Ting,Lee, Yean-Jang
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p. 2647 - 2655
(2007/10/03)
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- Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. VI. Demethylation of 8-Hydroxy-5,7-dimethoxyflavones with Anhydrous Aluminum Chloride or Bromide in Acetonitrile
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The demethylation of seven 8-hydroxy-5,7-dimethoxyflavones with anhydrous aluminum chloride or bromide in acetonitrile was studied and the following results were obtained. (1) Anhydrous aluminum chloride in acetonitrile selectively split the 5-methoxyl group without cleavage of the 7-methoxyl group, and seven 5,8-dihydroxy-7-methoxy-flavones were quantitatively synthesized by the demethylation. (2) In the demethylation with anhydrous aluminum bromide, the 5- and 7-methoxyl groups on 4',5,7-trimethoxy- and 3',4',5,7-tetramethoxy-8-hydroxyflavone were selectively split to give the corresponding 5,7,8-trihydroxyflavones in good yield.However, the demethylation was not accessible to the synthesis of 5,7,8-trihydroxy-3',4',5'-trimethoxyflavone, 4',5,7,8-tetrahydroxy-3'-methoxy-, and 3',5,7,8-tetrahydroxy-4'-methoxyflavone, because of the cleavage of the methoxyl groups on the B ring.The synthesized flavones were employed for the identification of the two natural flavones, which were proposed to be 4',5,8-trihydroxy-7-methoxyflavone and 4',5,8-trihydroxy-3',7-dimethoxyflavone.The former, salvitin, was revised to 4',5,7-trihydroxy-6-methoxyflavone and the latter seemed to be 4',5,7-trihydroxy-3',8-dimethoxyflavone.
- Horie, Tokunaru,Kourai, Hiroki,Fujita, Nobuhisa
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p. 3773 - 3780
(2007/10/02)
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- O-methylation of flavonoids by cell-free extracts of calamondin orange
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Cell-free extracts of calamondin orange (Citrus mitis) catalysed the O-methylation of almost all hydroxyls of a number of flavonoids, indicating the existence in citrus tissues of ortho, meta, para and 3-O-methyltransferases. The latter, hitherto unreported enzyme, catalysed the formation of 3-O-methyl ethers of galangin and quercetin. The stepwise O-methylation of a number of compounds, especially quercetin and quercetagetin, tends to suggest a coordinated sequence of O-methylations on the surface of a multienzyme complex. The methyl acceptor abilities of the flavonoid substrates used are discussed in relation to their hydroxyl substitution patterns and their negative electron density distribution.
- Brunet, Gunter,Ibrahim, Ragai K.
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p. 741 - 746
(2007/10/02)
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