Radical-initiated oxygenation of flavonols by dioxygen

In the presence of free radicals, such as 2,2,6,6-tetramethyl-1-piperidinyl oxyl (TEMPO) and 2,6-di-tert-butyl-a-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexa dien- 1-ylidene)-p-tolyloxyl (galvinoxyl), flavonols undergo catalytic oxyg enation to the corresponding depsides (phenolic carboxylic acid eaters), wi th concomitant evolution of CO. The oxygenolysis was performed in aprotic s olvents (DMF, MeCN) and was followed by Glc. The results of oxygenation of 4 ' -substituted flavonols show that the formation of flavonoxy species is the key step in the activation process of the substrate, and that electron- releasing substituents enhance the reaction rate. (C) 2001 Elsevier Science B.V. All rights reserved.