The microwave-assisted synthesis19 offers considerable
advantages over conventional heating because of rapid
heating and substantial rate enhancements of a wide
range of organic reactions. Cleaner reactions are also
commonly achieved, together with improvement in yield
and selectivity.20 The increasing demand of clean and
efficient chemical synthesis makes solvent-free reactions,
which when combined with microwave irradiation give
a more eco-friendly approach required from both eco-
nomic and environmental standpoints. It is because of
with this demand that different reports on the use of
microwaves to enhance several solvent-free reactions
have been published by our group.21
Solvent-Free Synthesis of Functionalized
Flavones under Microwave Irradiation
Julio A. Seijas,* M. Pilar V a´ zquez-Tato,* and
Raquel Carballido-Reboredo
Departamento de Qu ı´ mica Org a´ nica, Universidad de
Santiago de Compostela, Facultad de Ciencias, Aptdo. 280,
2
7080-Lugo, Spain
qoseijas@lugo.usc.es; pilarvt@lugo.usc.es
Received July 29, 2004
Recently there have been several reports on the ap-
plication of microwave irradiation to the synthesis of
22
flavonoids and neoflavonoids or 4-substituted cou-
marins;23 however, no attention has been paid to the
development of a two-component single-stepped reaction
to prepare flavones. Mentzer24 reported the reaction
between phenols and â-ketoesters to yield flavones under
very harsh conditions (250 °C for long reaction times) and
extended this method to a wide number or flavones,
despite the low yield in these thermal cyclocondensations.
Eco-friendly direct solvent-free synthesis of flavones is
achieved by microwave irradiation of phloroglucinol and
â-ketoesters. Heating with microwaves versus under clas-
sical conditions was shown to be higher yielding, cleaner,
and faster. The reaction goes through a cycloaddition of an
R-oxo ketene intermediate followed by an uncatalyzed
thermal Fries rearrangement.
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1
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0.1021/jo048685z CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/04/2005
J. Org. Chem. 2005, 70, 2855-2858
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