Synthesis of anthracene and azaanthracene fluorophores via [2+2+2] cyclotrimerization reactions

Article abstract of DOI:10.1021/ol8019549

(Chemical Equation Presented) A highly convergent [2+2+2] cyclotrimerization approach to anthracenes and 2-azaanthracenes has been developed. It allows for the facile introduction of the anthracene moiety on alkyne and nitrile bearing molecules and the rapid construction of compound arrays. This is showcased in the assembly of a collection of fluorophores and their photochemical evaluation.

Full text of DOI:10.1021/ol8019549

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4661-4664
Synthesis of Anthracene and
Azaanthracene Fluorophores via
[2+2+2] Cyclotrimerization Reactions
Yan Zou,† Douglas D. Young,† Alejandra Cruz-Montanez, and Alexander Deiters*
Department of Chemistry, North Carolina State UniVersity, Campus Box 8204,
Raleigh, North Carolina 27695-8204
alex_deiters@ncsu.edu
Received August 20, 2008
ABSTRACT
A highly convergent [2+2+2] cyclotrimerization approach to anthracenes and 2-azaanthracenes has been developed. It allows for the facile
introduction of the anthracene moiety on alkyne and nitrile bearing molecules and the rapid construction of compound arrays. This is showcased
in the assembly of a collection of fluorophores and their photochemical evaluation.
Anthracenes are core structures employed in a variety of
practical applications, including potential therapeutics,1 opti-
cal devices,2 and polymeric materials.3 Most importantly,
anthracenes have interesting fluorescent properties and have
received attention as imaging agents for cellular processes.4
Recently, we5,6 and others7 discovered significant micro-
wave effects on transition-metal catalyzed [2+2+2] cyclo-
trimerization reactions, enabling their application as effective
tools for the synthesis of arrays of carbo- and heterocyclic
structures. Here, we describe the application of this meth-
odology to the rapid assembly of fluorophores based on an
anthracene and an azaanthracene scaffold. Although several
synthetic routes to anthracenes have been reported,8,9 the
synthesis of 2-azaanthracenes is an undeveloped field.
Moreover, we discovered that the synthesized 2-azaan-
thracenes have very unique fluorescent properties in contrast
to regular anthracenes.
A classical [2+2+2] cyclotrimerization reaction involves
the transformation of three or two alkynes and/or a nitrile to
benzenes and pyridines (Scheme 1). The reactions are
† Both authors contributed equally to this work.
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(c) Tan, W. B.; Bhambhani, A.; Duff, M. R.; Rodger, A.; Kumar, C. V.
Photochem. Photobiol. 2006, 82, 20. (d) Piao, W. H.; Wong, R.; Bai, X. F.;
Huang, J.; Campagnolo, D. I.; Dorr, R. T.; Vollmer, T. L.; Shi, F. D.
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10.1021/ol8019549 CCC: $40.75
Published on Web 09/25/2008
2008 American Chemical Society

Scheme 1
.
General [2+2+2] Cyclotrimerization Reaction to
Scheme 3
.
Azanthracene Synthesis via [2+2+2]
(Fused) Benzenes and Pyridines
Cyclotrimerization
typically conducted under Co, Ni, Ru, or Rh catalysis,
although other transition metals have been used as well.10
To avoid chemoselectivity issues in the cyclotrimerization
step, one of the alkynes can be immobilized on a solid-
support,5,11 or two alkynes can be tethered together. The latter
leads to the synthesis of fused benzene and pyridine rings
and has been applied in the assembly of a wide range of
aromatic structures.
We envisioned the application of this strategy to the
synthesis of anthracenes, and azanthracenes and thus as-
sembled the known diyne 19 in three steps from com-
mercially available o-xylene dibromide with an overall yield
of 72%. The precursor 1 was cyclotrimerized with a set of
six alkynes containing alkyl chains, benzenes, hydroxy
groups, nitriles, and imides. These reactions were conducted
with 10 mol% (PPh3)2Ni(CO)2 in toluene at 120 °C under
microwave irradiation (300 W) in 10 min, delivering the
tricyclic compounds 2-7 in 66-86% yield (Scheme 2).
aPyr ) pyridine.
The anthracences 8-13 were generated in yields of
70-79% through a rapid microwave-assisted oxidation with
2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) (Scheme 2).
This two-step procedure provides a flexible and facile
approach to the introduction of an anthracene moiety into a
wide range of alkynes.
We subsequently investigated the feasability of this route
toward the synthesis of 2-azaanthracenes, a mostly unex-
plored class of compounds. Here, the diyne 1 was reacted
with nitriles bearing a variety of functional groups, including
alkyl and alkene chains, hydroxy groups, benzene, and
pyridine rings. The reactions were conducted under Cp-
Co(CO)2 catalysis in toluene using microwave irradiation
(300 W), delivering the cyclotrimerization products 14-19
in 80-94% yields. The change in catalyst system was
necessary to achieve cyclotrimerization reactions toward
Scheme 2
.
Anthracene Synthesis via [2+2+2]
Cyclotrimerization
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aOxidized to aldehyde. bPth ) phthalimide.
Reactions conducted thermally in the absence of microwave
irradiation displayed diminished yields (e.g., 35% in the case
of compound 4 under otherwise identical conditions). By-
products observed in some reactions resulted from the
dimerization of 1, as well as cyclotrimerization of 1 with
two monoalkyne molecules.
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1313. (b) Young, D. D.; Senaiar, R. S.; Deiters, A. Chem.-Eur. J. 2006,
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4662
Org. Lett., Vol. 10, No. 20, 2008

pyridines.7e-k In the case of thermal heating, but in the
absence of microwave irradiation, very low product yields
were obtained under otherwise identical conditions (e.g., 8%
in case of compound 18). The subsequent DDQ oxidation
step proceeded smoothly and yielded the azaanthracences
20-25 in 53-85% yield. To investigate the effects of a
permanently positively charged nitrogen center on the
fluorescent properties of azaanthracenenes, and to increase
their solubility in an aqueous environment, we methylated
the azaanthracenes 20-25 in neat methyl iodide at 60 °C to
obtain the salts 26-31 in quantitative yields.
A prominent feature of anthracenes is their intrinsic
fluorescence,12 a property that is largely unexplored in case
of the corresponding 2-azaanthracenes. The developed array
approach to both compound classes prompted us to inves-
tigate the fluorescent properties of the synthesized molecules,
since combinatorial approaches to fluorophores have been
successfully applied to biological imaging problems.13 We
were especially interested in their application as environ-
mentally sensitive probes, and therefore investigated the
dependence of their fluorescence spectra on (a) the polarity
of the solvent, (b) the pH of the solvent, and (c) the presence
of different metal cations. These experiments were conducted
in a 96-well format (Figure 1).
Figure 2. Fluorescence spectra (350 nm excitation wavelength) of
(A) representative azaanthracenes 20 and 26 demonstrating the
effect of quaterinization on the fluorescence properties in DMSO
and H2O; (B) the representative azaanthracene 22 in the presence
of different solvents and different pH; and (C) the pyridylazaan-
thracene 25 in the presence of different divalent metal cations.
Figure 1. Synthesized azaanthracenes arrayed in a 96-well plate,
exposed to a variety of conditions (see text) and excited with 365
nm UV light.
S1-S3). A significant change in fluorescence intensity was
observed for most of the azaanthracenes 20-25 between
protic (H2O at pH 4-10) and aprotic (DMSO) solvents
(Figure 1). More drastic changes in fluorescence emission
were visually observed in other nonpolar solvents such as
CH2Cl2 and toluene (see Supporting Information, Figures S2
and S3); however, these solvents were incompatible with the
96-well microtiter plates employed in the fluorescence
measurements. This phenomenon has previously been in-
vestigated for 2-azaanthracene (20), finding that a polar
solvent causes a broadening of the π f π1* energy levels
resulting in a bathochromic shift.14 A loss of solvent
sensitivity was observed upon quaternization of the nitrogen
center in 26-31, and a moderate broadening of the emission
spectrum occurred when compared to the non-quaternized
While the anthracenes 9-12 did not show any significant
changes in fluorescence under different conditions, many of
the azaanthracenes 20-25 exhibited environmental sensitiv-
ity (Figure 1). This can be explained by the ability of the
nitrogen center to undergo coordination to the solvent, to
protons, and to metal ions. Moreover, the azaanthracenes
20-25 exhibited generally higher fluorescence levels than
the anthracenes 8-13 (see Supporting Information, Figures
(13) (a) Rosania, G. R.; Lee, J. W.; Ding, L.; Yoon, H. S.; Chang, Y. T.
J. Am. Chem. Soc. 2003, 125, 1130. (b) Schiedel, M. S.; Briehn, C. A.;
Bauerle, P. Angew. Chem., Int. Ed. 2001, 40, 4677. (c) Yee, D. J.; Balsanek,
V.; Sames, D. J. Am. Chem. Soc. 2004, 126, 2282.
(14) Zvolinskii, V. P. Russ. Phys. J. 1977, 20, 701.
(15) Piantadosi, C. A.; Suliman, H. B. J. Biol. Chem. 2006, 281, 324.
Org. Lett., Vol. 10, No. 20, 2008
4663

analogs (Figure 2A). Additionally, a visible bathochromic
shift in emission could be detected upon quaternization from
blue to green (see compounds 20-25 and 26-31 in Figure
1). At low pH (<4), protonation of the azaanthracenes 20-25
led to a general bathochromic shift (∼480 to 520 nm) of
their fluorescence emission spectra, similar to their quater-
nized analogs 26-31 (Figure 2B). No changes in fluores-
cence were observed between pH 7-10. As expected, the
azaanthracene 25 demonstrated a fluorescence sensitivity
toward metal ions, as a result of the chelating ability of the
bipyridyl motif. An increase in the fluorescence intensity of
25 was observed in the presence of different divalent metal
Figure 3. (A) HEK-293T cells incubated with azaanthracene 24
(20 µM) for 12 h were imaged at 365 nm UV irradiation (100 ×
magnification). The slight green color is a result of cell autofluo-
rescence. (B) Confocal microscopy image of azaanthracene 18
crystallized inside a single human embryonic kidney cell (400 ×
magnification, false color image).
ions, suggesting a trend in binding affinity Cu2+ < Mg2+
≈
Zn2+ (Figure 2C).
We also examined the application of the synthesized
compounds for the fluorescent labeling of mammalian cells.
Compounds 20-31 (20 µM) were incubated with human
embryonic kidney (HEK-293T) cells overnight in standard
DMEM growth media. To our surprise, compound 24
selectively forms ∼200 nm sized particles within the cells
but not outside the cells, as confirmed by confocal micros-
copy using a Leica TCS SP1 laser scanning confocal
microscope (Figure 3). By imaging the cells on varying
planes of focus, the upper and lower membranes are apparent
with the aggregates inside of the cell, not on the surface (see
supporting video, Supporting Information). No observable
effect on the cell phenotype and the cell viability was
observed (Promega Cell-Titer Blue assay).15
phenomena and lead to the application of these fluorophores
in a biological context.
Acknowledgment. This research was supported by the
Donors of the American Chemical Society Petroleum
Research Fund and the Department of Chemistry at North
Carolina State University. D.D.Y. acknowledges a graduate
research fellowship from the ACS Medicinal Chemistry
Division. A.C.M. was supported by the NCSU REU program.
We also acknowledge T. J. Styslinger (NCSU) for his
contributions to the work.
In summary, we have developed a rapid route to an-
thracenes and azaanthracenes via microwave assisted [2+2+2]
cyclotrimerization reactions. These compounds have unique
photochemical and biological properties and can act as
environmentally sensitive dyes, metal sensors, pH sensors,
and cellular stains. Future work will reveal further interesting
Supporting Information Available: General reaction
protocols, analytical data, as well as H NMR spectra and
fluorescent properties for compounds 2-31. This information
is available free of charge via the Internet at http://pubs.acs.org.
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Org. Lett., Vol. 10, No. 20, 2008