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Advisor(s)
Abstract(s)
A large number of hydroxylated 2,3-diaryl-9H-xanthen-9-
ones have been synthesised by two different approaches,
starting either from 3-bromo-2-methyl-4H-chromen-4-one or
from (E)-3-bromo-2-styryl-4H-chromen-4-ones. The former
method involves Heck reactions between 3-bromo-2-methyl-
4H-chromen-4-one and styrenes, leading to (E)-2-methyl-3-
styryl-4H-chromen-4-ones; these condensed with benzaldehyde
to give (E,E)-2,3-distyryl-4H-chromen-4-ones, which
led to the desired 2,3-diaryl-9H-xanthen-9-ones under reflux
in 1,2,4-trichlorobenzene. 3-Bromo-2-styryl-4H-chromen-4-
ones were obtained either by aldol condensations between
3-bromo-2-methyl-4H-chromen-4-one and benzaldehydes,
or through Baker–Venkataraman rearrangements of 2-acetylphenyl
cinnamates, followed by one-pot bromination/
cyclisation with phenyltrimethylammonium tribromide. The
2,3-diaryl-9H-xanthene-9-ones were obtained in one-pot
transformations involving Heck reactions between (E)-3-
Introduction
Xanthones constitute one of the major classes of naturally
occurring oxygen-containing heterocyclic compounds
containing dibenzo-γ-pyrone rings.[1] They occur in two
major plant families, Guttiferae and Gentianaceae, and also
in some families of fungi and lichens.[2,3] Natural derivatives
can be hydroxylated, methoxylated or prenylated, among
other possibilities; the parent compound xanthone itself is
not known as a natural product.[3,4] The presence of aryl
groups on the xanthone core has only been reported for a
few synthetic derivatives, and as far as we know the literature
had never presented the synthesis of xanthones featuring
2,3-diaryl moieties before our work.[5,6]
Over the last decades these substances have been extensively
studied not only because they participate in several
biological functions but also as a consequence of their remarkable
antifungal,[7–9] anti-inflammatory,[10,11] antimalarial[
12–14] and antitumour activities,[15,16] and even as
promising antioxidant agents.[17–19] Structure–activity stud-
[a] Department of Vegetal Production and Technology, Escola
Superior Agrária de Bragança,
5301-855 Bragança, Portugal
Fax: +351-273-325405
E-mail: clems@ipb.pt
[b] Department of Chemistry & QOPNA, University of Aveiro,
3810-193 Aveiro, Portugal
Fax: +351-234-370084
E-mail: artur.silva@ua.pt
© 2009 Wiley-VCH Verlag GmbH 2642 & Co. KGaA, Weinheim Eur. J. Org. Chem. 2009, 2642–2660
bromo-2-styryl-4H-chromen-4-ones and styrenes, followed
by electrocyclisation and oxidation processes. The 2,3-diaryl-
3,4-dihydro-9H-xanthene-9-one intermediates were also isolated
under these conditions, and so when 5-methoxy-2-styryl-
4H-chromen-4-ones were used as starting materials the
1-hydroxy-6,7-diaryl-9H-xanthene-9-ones were also observed.
The second method is a general one, because it allowed
the synthesis of a great number of 2,3-diaryl-9Hxanthen-
9-ones with several substitution patterns, whereas
the first one is limited to certain derivatives. The last step in
the synthesis of hydroxylated 2,3-diaryl-9H-xanthen-9-ones
was the cleavage of the hydroxy protecting groups with boron
tribromide. The structures and stereochemistry of all new
compounds were established by NMR studies.
Description
Keywords
Xanthones Alcohols Oxygen heterocycles Heck reaction NMR spectroscopy
Citation
Santos, Clementina; Silva, Artur; Cavaleiro, José (2009). Efficient syntheses of new polyhydroxylated 2,3-diaryl-9h-xanthen-9-ones. European Journal of Organic Chemistry. ISSN 1434-193X. 16, p. 2642-2660
Publisher
Wiley