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Chromone derivatives as novel dienes in Diels-Alder reactions
Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, C.F.R.A.C.; Santos, L.M.N.B.F.; Cavaleiro, José; Silva, Artur
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.
2-[(1E,3E)-4-Arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels-Alder reactions - experimental and computational studies
Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, Carlos F.R.A.C.; Santos, Luís M.N.B.F.; Cavaleiro, José; Silva, Artur
The synthesis and reactivity of 2-[(1E,3E)-4-arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels–Alder (DA) reactions with several electron-poor and electron-rich dienophiles under microwave irradiation was studied. The optimized reaction conditions were achieved with N-methylmaleimide as the dienophile and Sc(OTf )3 (OTf = triflate) as a Lewis acid under microwave-assisted and solvent-free conditions. The Lewis
acid improved the reaction yields as it prevented the adducts obtained from undergoing a second DA reaction; thus, the formation of a bisadduct was avoided. The α, :γ,δ-diene of the starting chromones was the most reactive, and the computational results confirmed the experimental findings. Theoretical calculations also provided a rationale for the unexpected lack of reactivity shown by some dienophiles. The adducts prepared were dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); however, the aza adducts were sensitive to the highly energetic reaction conditions necessary for the aromatization.
2-[(1E,3E)-4-Arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels–Alder reactions: experimental and computational studies
Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, Carlos F.R.A.C.; Santos, Luís M.N.B.F.; Cavaleiro, José; Silva, Artur
The synthesis and reactivity of 2-[(1 £,3£)-4-arylbuta-1 ,3-dien-l-yl]-4H-chromen-4-ones 1 as
dienes in Diets-Alder (DA) reactions towards several electron-poor and electron-rich
dienophiles under microwave irradiation was studied. The optimized reaction conditions were
achieved by using N-methylmaleimide2 as dienophile and Sc(0Tf)3 as Lewis acid under
microwave-assisted and solvent-free conditions. The Lewis acid improved the reaction yields
since it prevented the adducts obtained to undergo a second DA reaction, thus avoiding
formation of a bisadduct. The a,~:y ,o-diene of the starting chromones showed to be the most
reactive and the computational results confirmed the experimental findings. Theoretical
calculations also allowed explaining some unexpected lack of reactivity by some dienophiles.
The adducts prepared were dehydrogenated by using DDQ, however, the aza-adducts showed
to be sensitive to the high energetic reaction conditions necessary to perform the
aromatization.
Solubility and solvation of monosaccharides in ionic liquids
Publication . Teles, Ana Rita R.; Dinis, Teresa B.V.; Capela, Emanuel V.; Santos, Luís M.N.B.F.; Pinho, Simão; Freire, Mara G.; Coutinho, João A.P.
Herein, solubility experimental data for six monosaccharides, viz. D-(+)-glucose, D-(+)-mannose, D-(-)-fructose, D-(+)-galactose, D-(+)-xylose and L-(+)-arabinose, in four ionic liquids (ILs), at temperatures ranging from 288.2 to 348.2 K, were obtained aimed at gathering a better understanding of their solvation ability and molecular-level mechanisms which rule the dissolution process. To ascertain the chemical features that enhance the solubility of monosaccharides, ILs composed of dialkylimidazolium or tetra-alkylphosphonium cations combined with the dicyanamide, dimethylphosphate or chloride anions were investigated. It was found that the ranking of the solubility of monosaccharides depends on the IL; yet, D-(+)-xylose is always the most soluble while D-(-)-fructose is the least soluble monosaccharide. The results obtained show that both the IL cation and the anion play a major role in the solubility of monosaccharides. Finally, from the determination of the respective thermodynamic properties of solution, it was found that enthalpic contributions are dominant in the solubilization process. However, the observed differences in the solubilities of monosaccharides in 1-butyl-3-methylimidazolium dicyanamide are ruled by a change in the entropy of solution.
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Funding agency
Fundação para a Ciência e a Tecnologia
Funding programme
COMPETE
Funding Award Number
PEst-C/QUI/UI0081/2013