Efficient syntheses of new polyhydroxylated 2,3-diaryl-9H-xanthen-9-ones
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Overview
abstract
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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.
publication date
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Research
keywords
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Alcohols
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Heck reaction
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NMR spectroscopy
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Oxygen heterocycles
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Xanthones
Identity
Digital Object Identifier (DOI)
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