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Chromones are a group of heterocyclic compounds widespread in Nature and an interesting scaffold
because they have been involved in a range of chemical transformations [1]. Following our previous
studies on Diels‒Alder (DA) reactions of chromone derivatives [2], novel 2-[(1E,3E)-4-arylbuta-1,3-dien-
1-yl]-4H-chromen-4-ones 1 were synthesized to further study the reactivity of their two diene systems
(α,β:γ,δ versus 2,3:α,β) in DA reactions (Fig.1).
A series of dienophiles were reacted with chromones 1 under microwave-assisted conditions. The best
yields of adducts 2 were achieved using N-methylmaleimide as dienophile and Sc(OTf)3 as Lewis acid
in solvent-free conditions. The crucial role of Sc(OTf)3 relies on the chelation with carbonyl group of
adducts 2 avoiding both the occurrence of a second DA reaction and the formation of bisadducts 6. The
α,β:γ,δ-diunsaturated system of chromones 1 showed to be the most reactive diene for maleimides,
dicarboxylates and azo-dicarboxylates, affording adducts 2, 4 and 5, respectively. Adducts 2 were
dehydrogenated to the corresponding flavone derivatives 3 using DDQ as oxidizing agent (Fig.1).
Computational calculations confirmed the experimental findings on DA reactions and helped to explain
the unexpected lack of reactivity of some dienophiles.
All the experimental procedures and extensive 1D and 2D NMR characterization of the final adducts
and intermediates will be presented and discussed.
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Albuquerque, Hélio; Santos, Clementina M.M.; Lima, CFRAC; Santos, LMNBF; Cavaleiro, José; Silva, Artur (2016). Chromone derivatives as novel dienes in Diels-Alder reactions. In XXII Encontro Luso-Galego de Química. Bragança: Sociedade Portuguesa de Química. ISBN 978-989-8124-17-3