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Synthesis of Flavonoids by Mentzer’s Reaction:Enhancement by Microwaves and Ionic Liquids |
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[E003]
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Synthesis of Flavonoids by Mentzer’s Reaction:Enhancement by Microwaves and Ionic Liquids |
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Julio A. Seijas,* M. Pilar Vázquez-Tato,* Raquel Carballido-Reboredo
Departamento de Química Orgánica. Facultad de Ciencias. Universidad de Santiago de Compostela. Campus de Lugo. Aptdo. 280. 27080-Lugo. Spain
Flavonoids due to their interest as biological markers and antioxidant properties have been synthesized following many strategies.1 Our group, during a project dealing with the phytochemical study of the flora of Galicia, needed to prepare some compounds and after reviewing some of the classical methods, we considered Mentzer’s reaction,2 where a phenol like phloroglucinol (1) is heated (240-250ºC/1.5h) in the presence of ethyl benzoylacetate (2) yielding flavanone chrisin (20% yield, scheme1), a good candidate to be explored for enhancing by means of microwave irradiation.
Scheme 1
After trying several reaction conditions we found that a 59% yield could be achieved when a mixture of phloroglucinol and ethyl benzoylacetate (in 1:1.2 molar ratio) were irradiated for 2 minutes in a microwave oven (80% of 1000W total output, ETHOS D-Millestone), longer reaction times did not increase yields.
We also check the influence of adding an ionic liquid to the reaction media. Ionic liquids3 are known to enhance a wide variety of organic reactions and not only constitute an environmentally friendly alternative to classic organic solvents but are as well sometimes a helpful tool in MORE (microwave organic reactions enhancement) chemistry. The number of ionic liquids commercially available is large and is increasing, N-Butyl-N-Methylimidazolium chloride BMIMCl was chosen to avoid any interference with Pechmann's condensationas had been described previously.4
Scheme 2
The irradiation of phloroglucinol (1), ethyl benzoylacetate (2) and BMIMCl in a molar ratio 1:1.2:0.22, gave after 1 minute, 69% of chrisin5 (scheme 2). Higher ratios of ionic liquid led to decomposition of reaction products. To our knowledge this is the first report on the enhancement of Mentzer reaction by the use of microwaves or microwave/ionic liquids, which open a new way to prepare flavanones, actually we are exploring the scope of the reaction with different substitutions in the aromatic rings.
Acknowledgements
XUNTA DE GALICIA for financial support: PGIDT01PXI26203PR and PR405 A 098/59-0
1.- Bohn, B. A. "Introduction to flavonoid", Harwood Academic Pub. 1998.
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