Acknowledgements
We gratefully acknowledge the National University of Singapore
for financial support of this work (Academic Research Grant:
R143000408133, R143000408733 and R143000443112).
Notes and references
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Scheme 3 Isotope experiment.
carbonyl oxygen of chalcone 1a to increase the electrophilicity
of the conjugated system, which facilitates the later nucleophilic
addition to b-carbon. Deprotonation of the hydroxyl proton,
followed by intramolecular addition to the C C leads to the
formation of an enolate. The enolate then abstracts a proton
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Cu catalytic specie was believed to be regenerated. Lastly, 2a¢
is oxidized to 2a in the presence of CuI and molecular oxygen.
It is also possible for CuI to deprotonate the hydroxyl proton
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conjugate addition to form 2a. In this process, the CuI catalyzed
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step of the cascade sequence. Once 2a¢ was formed, it undergoes
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stable flavone 2a. To further approve our hypothesis, we started
an oxidation reaction from the intermediate 2a¢. As shown in
Scheme 4, intermediate 2a¢ could be oxidized to final product
2a under above optimized reaction condition in 18 h and with
93% yield.
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