The reaction of triptycene haloquinones with alkoxides. An unusual route to pentiptycene quinones

Article abstract of DOI:10.1016/S0040-4039(03)00755-X

Triptycene haloquinones 3 react with sodium alkoxides in refluxing alcohol to afford, besides the expected substitution products, pentiptycene quinone 4. This approach to 4 is compared with a Diels-Alder strategy to the same compound.

Full text of DOI:10.1016/S0040-4039(03)00755-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3767–3770
The reaction of triptycene haloquinones with alkoxides.
An unusual route to pentiptycene quinones
Spyros Spyroudis* and Nikoletta Xanthopoulou
Laboratory of Organic Chemistry, Chemistry Department, University of Thessaloniki, Thessaloniki 54124, Greece
Received 27 January 2003; revised 11 March 2003; accepted 21 March 2003
Abstract—Triptycene haloquinones 3 react with sodium alkoxides in refluxing alcohol to afford, besides the expected substitution
products, pentiptycene quinone 4. This approach to 4 is compared with a Diels–Alder strategy to the same compound. © 2003
Elsevier Science Ltd. All rights reserved.
Triptycene quinones form an interesting class of com-
pounds. The rigid structure of triptycene combined with
the redox potential of the quinone ring confer some
unique properties to triptycene quinone derivatives.
Triptycene quinones exhibit interesting intramolecular
charge transfer characteristics,1 they are precursors to
liquid crystalline triptycene derivatives,2 and they form
three-dimensional supramolecules.3 Triptycene quinone
and derivatives also find application as acceptors, with
porphyrin derivatives4 and tetrathiafulvalene5 serving
as donors, for the synthesis of electron-transfer com-
pounds. Also, in a preclinical study6 it was shown that
some triptycene quinones decrease the viability of
leukemic cells in vitro.
Also recently, an unusual reaction of triptycene
diquinones with amines was reported to afford deriva-
tives with potent anticancer and antimalarial
activities.13
In the context of our previous study12 we examined the
reaction of triptycene chloroquinone 3, with MeONa/
MeOH. Quinone 3 (X=Cl) is prepared in one step
from a Diels–Alder reaction of anthracene 1 with two
equivalents of chlorobenzoquinone 2 in 96% yield. Its
reaction with the alkoxide would hopefully have pro-
vided an alternative route to triptycene hydroxy-
quinones, a reaction known for other halogenated
quinones.14 In refluxing MeOH the reaction afforded
the expected dimethoxy- and chloromethoxy-triptycene
quinones 5 and 6, anthracene, and, quite unexpectedly,
pentiptycene quinone, 4. This compound was identical
to that prepared by an independent route, namely a
double Diels–Alder reaction of anthracene with benzo-
quinone and subsequent oxidation of the resulting dihy-
droxy derivative.7 This unusual reaction pathway seems
to be of general character since anthracene 1 and
pentiptycene quinone 4 were isolated, in varying yields
(5–10% for anthracene and 28–32% for 4), when using
different haloquinones and alkoxides (Scheme 1). The
same compounds were obtained, although in lower
yields (10–15% for 4), even when KOH/MeOH was
used as reagent. Yields of alkoxyquinones 5 and 6 vary
between 13 and 20%.
Pentiptycene quinones, 4, bearing two triptycene units,
are particularly promising reagents for the preparation
of polymeric chemosensors,7 materials with monolayer
assembly structures,8 fluorescent chemosensors for
metal ions,9 electron-donor porphyrin quinone diads
and triads10 and building blocks for the construction of
novel chain and channel networks.11
Although there is increasing interest in the use of these
compounds for the construction of molecules with
intriguing properties, less is known about their chem-
istry. Recently we reported the preparation of trip-
tycene hydroxyquinone and its transformation through
phenyliodonium chemistry to the corresponding
cyclopentenedione analogue, a potential dienophile.12
The formation of anthracene indicates that a retro-
Diels–Alder reaction takes place. This reaction is a
typical one as it has been observed with a variety of
triptycene quinones under basic conditions.15 In order
to explain the formation of 4 the reaction was repeated
in the presence of substituted (1,5-dichloro-, 9,10-
Keywords: triptycene quinones; pentiptycene quinones; haloquinones.
* Corresponding author. Tel.: +2310-997833; fax: +2310-997679;
e-mail: sspyr@chem.auth.gr
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00755-X

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Scheme 2. Suggested reaction pathway for the formation of
pentiptycene quinone 4.
Scheme 1. Reaction of triptycene haloquinones with alkox-
ides.
dimethyl- and 1,4-dimethoxy-) anthracenes (1–3 mmol).
In all cases the reaction products were the same as
those in Scheme 1 and no cross pentiptycene quinones
were isolated. This finding indicates that the formation
of 4 takes place through some internal process. Other-
wise one would normally expect cross derivatives, at
least with dimethyl and dimethoxyanthracenes, which
were reported to be more reactive than anthracene in
Diels–Alder reactions with maleic anhydride and ben-
zyne.16 The same results were also obtained when the
reaction was conducted in the presence of other dienes
such as 2,3-dimethylbutadiene, isoprene and furan.
On the basis of the above results it is possible that
nucleophilic attack of an alkoxide anion gives rise to
enolate 7 through an initially formed charge transfer
complex.17 The latter either affords anthracene in a
retro-Diels–Alder reaction or reacts with 3 to give
Diels–Alder adduct 8. Finally 8 is dehydrohalogenated
to pentiptycene quinone 4 (Scheme 2).
Scheme 3. Diels–Alder synthesis of 1,4-dimethoxy-pentip-
tycene quinone and pentiptycene diquinone.
demethylated to triptycene diquinone 12 by
As mentioned, when the reaction of triptycene halo-
quinones 3 with alkoxides takes place in the presence of
1,4-dimethoxyanthracene no cross derivative was iso-
lated. In order to verify that such a derivative can exist
we prepared it by an independent method (Scheme 3).
Ce(NH4)2(NO3)6 (CAN), in 69% yield.
The formation of pentiptycene quinones from trip-
tycene haloquinones seems to be a general pheno-
menon. Dimethoxytriptycene chloroquinone 13, pre-
pared from dimethoxyanthracene 9 and chlorobenzo-
Diels–Alder reaction of 1,4-dimethoxyanthracene 9
with triptycene quinone 1012 in refluxing acetic acid
afforded directly the desired 1,4-dimethoxypentiptycene
quinone 11 in 62% yield. Quinone 11 was oxidatively
quinone
2 through the Diels–Alder enolisation–
oxidation sequence, upon reaction in MeOH/MeONa
afforded one of the tetramethoxypentiptycene quinone
regioisomers, 15a or 15b, in 10% yield. The other

S. Spyroudis, N. Xanthopoulou / Tetrahedron Letters 44 (2003) 3767–3770
3769
products of the reaction were dimethoxyanthracene 9
(7%) and tetramethoxytriptycene quinone 14 in 10% yield
(Scheme 4). Again 15a (or 15b) was demethylated to the
corresponding pentiptycene triquinone, 16a or 16b in 30%
yield.
Triptycene chloroquinones with substituents on the
bridge exhibited the same reactivity. The cycloaddition
reaction of 9-methylanthracene 18 with chlorobenzo-
quinone 2 afforded in three steps (addition-enolization-
oxidation) a mixture of the isomeric chloroquinone
adducts 19. The treatment of 19 with MeONa/MeOH
gave, besides the usual retro Diels–Alder product methyl-
anthracene 18, an equimolecular mixture of the two
dimethylpentiptycene quinone isomers 21 in 12% yield
(Scheme 6). Dimethoxytriptycene quinone, 20, was also
isolated in 7% yield from the reaction.
In order to confirm the structures of 15, a different
approach to these compounds was attempted. A Diels–
Alder cycloaddition reaction of dimethoxyanthracene 9
with dimethoxytriptycene quinone 1718 in refluxing acetic
acid afforded on this occasion a mixture of the two
possible regioisomers 15a and 15b in 31% yield, which
was also demethylated to the mixture of 16a and 16b
(Scheme 5).
Again, pentiptycene quinones 21 were prepared by a
cycloaddition methodology. Methyltriptycene quinone
2219 reacted with methylanthracene 18 in refluxing acetic
acid to afford the same mixture of pentiptycene quinones
21 in 32% yield. This time a complicated mixture of
endo-exo dihydroadducts was also isolated in 4% yield
(Scheme 7).
In summary, the reaction of triptycene haloquinones with
alkoxides provides a short route to pentiptycene
quinones, through an unusual reaction pathway. This
Scheme 4. The triptycene haloquinone-alkoxide route to tetra-
methoxypentiptycene quinones and pentiptycene triquinones.
Scheme 6. Preparation of pentiptycene quinones with methyl
substituents on the bridge.
Scheme 5. The Diels–Alder route to tetramethoxy-pentiptycene
quinones.
Scheme 7. The Diels–Alder route to 21.

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route and the Diels–Alder strategy, with comparable
yields, are the two main approaches to these interesting
building blocks, which find many applications in Chem-
istry.
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