Synthesis of 1,2,5,6- and 1,4,5,8-anthracenetetrone: Building blocks for π-conjugated small molecules and polymers

Article abstract of DOI:10.1080/00397911.2018.1483027

Reliable reactions for the synthesis of two interesting anthracenetetrones have been identified and optimized. Both syntheses start from dihydroxy-9,10-anthraquinones and were selected for maximized efficiency and minimized workload. Work-up of all reactions can be achieved without column chromatography, which facilitates further scale-up. So far, both target compounds are considerably underexplored despite their promising molecular structure for use in devices and in organic synthesis, especially as building blocks for π-conjugated compounds. The crystal structure of 1,4,5,8-anthracentetrone is reported.

Full text of DOI:10.1080/00397911.2018.1483027

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Synthesis of 1,2,5,6- and 1,4,5,8-
anthracenetetrone: Building blocks for π-
conjugated small molecules and polymers
Florian Glöcklhofer, Berthold Stöger & Johannes Fröhlich
To cite this article: Florian Glöcklhofer, Berthold Stöger & Johannes Fröhlich (2018): Synthesis
of 1,2,5,6- and 1,4,5,8-anthracenetetrone: Building blocks for π-conjugated small molecules and
polymers, Synthetic Communications, DOI: 10.1080/00397911.2018.1483027
To link to this article: https://doi.org/10.1080/00397911.2018.1483027
© 2018 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group.
View supplementary material
Published online: 17 Aug 2018.
Submit your article to this journal
Article views: 200
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=lsyc20

R
SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2018.1483027
Synthesis of 1,2,5,6- and 1,4,5,8-anthracenetetrone:
Building blocks for p-conjugated small molecules
and polymers
a
b
a
€
€
€
Florian Glocklhofer
, Berthold Stoger
and Johannes Frohlich
b
aInstitute of Applied Synthetic Chemistry, TU Wien, Vienna, Austria; X-Ray Centre, TU Wien,
Vienna, Austria
ABSTRACT
ARTICLE HISTORY
Received 8 May 2018
Reliable reactions for the synthesis of two interesting anthracenete-
trones have been identified and optimized. Both syntheses start
from dihydroxy-9,10-anthraquinones and were selected for maxi-
mized efficiency and minimized workload. Work-up of all reactions
can be achieved without column chromatography, which facilitates
further scale-up. So far, both target compounds are considerably
underexplored despite their promising molecular structure for use
in devices and in organic synthesis, especially as building blocks
for p-conjugated compounds. The crystal structure of 1,4,5,8-
anthracentetrone is reported.
KEYWORDS
Anthracenetetrone;
anthraquinone; building
block; crystal
structure; tetrone
GRAPHICAL ABSTRACT
Introduction
Anthracenetetrones are interesting compounds for technological applications and further
conversion. For example, 1,4,9,10-anthracenetetrone (Scheme 1, top), which is currently
the best investigated anthracenetetrone, was used to modify electrodes for supercapaci-
tors,[1] was applied as acceptor in charge-transfer complexes,[2–4] it served as a precursor
in the synthesis of antitumor compounds,[5] and afforded push–pull chromophores in
cycloaddition reactions with electron-rich alkynes.[6] Investigations of this anthracenete-
trone are facilitated by its facile synthesis in a single high-yielding reaction that uses
€
CONTACT Florian Glocklhofer
florian.gloecklhofer@tuwien.ac.at
Institute of Applied Synthetic Chemistry,
TU Wien, Getreidemarkt 9/163, Vienna 1060, Austria.
Supplemental data for this article can be accessed on the publisher’s website.
Color versions of one or more of the figures in the article can be online at www.tandfonline.com/lsyc.
ß 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.

€
F. GLOCKLHOFER ET AL.
2
Scheme 1. Facile synthesis of 1,4,9,10-antracenetrone starting from 1,4-dihydroxy-9,10-anthraquinone
(top), molecular structure of 1,2,5,6- and 1,4,5,8-anthracenetetrone (bottom).
low-cost 1,4-dihydroxy-9,10-anthraquinone as the starting material (Scheme 1, top).[7]
Different to other anthracenetetrones, also the crystal structure of this compound has
been reported.[8]
In contrast, the synthesis of 1,2,5,6- and 1,4,5,8-anthracenetetrone (Scheme 1, bottom)
is more challenging.[9–12] This may explain why these compounds are less investigated
despite the large number of available reactions for converting quinones, including our
new conversion into cyanoarenes (demonstrated for 5,7,12,14-pentacenetetrone and a
range of other para- and ortho-quinones).[13–16] Beside other syntheses, the two tetrones
are particularly interesting for the synthesis of p-conjugated small molecules and poly-
mers, but they have been used in only a few reactions so far.
1,2,5,6-Anthracenetetrone was used for condensation with aromatic ortho-diamines, a
reaction first demonstrated by Boldt.[9] This reaction yields fully aromatic compounds
by connecting the building blocks via pyrazine units. It has later been shown that
the corresponding polycondensation with tetraamines yields fully conjugated ladder pol-
ymers.[17] The polymers have not yet been tested in devices, but Bunz et al. recently
optimized the condensation for larger ortho-diamines and tested the resulting com-
pounds in organic light-emitting diodes.[18] Regarding applications, environmentally
benign methods for (poly)condensations, which afford highly crystalline materials, open
up new opportunities.[19,20]
1,4,5,8-Anthracenetetrone, on the other hand, was used as a dienophile in double
Diels–Alder cycloaddition reactions, which – depending on the diene – can yield macro-
cyclic compounds,[21] substituted 5,7,12,14-pentacenetetrones,[22] and pentacene precur-
sors.[23] The absorption and electrochemical properties of this tetrone, which is the most
stable anthracenetetrone according to a computational study,[24] were reported decades
ago.[11,25] In a more recent study, the binding of this tetrone on graphene was computa-
tionally predicted to be the strongest of several electroactive compounds for green
battery electrodes.[26] Such strong binding benefits the cycling performance of batteries.
It was our aim to facilitate the use and investigation of 1,2,5,6- and 1,4,5,8-anthrace-
netetrone beyond computational studies by identifying and evaluating reliable synthetic

R
SYNTHETIC COMMUNICATIONSV
3
Scheme 2. (i) Reduction of 2,6-dihydroxy-9,10-anthraquinone (1a) to 2,6-dihydroxyanthracene (2a).
Conditions: NaBH4, 1 M Na2CO3 solution, under argon, RT, overnight. (ii) Oxidation to 1,2,5,6-anthrace-
netetrone (3a). Conditions: benzeneseleninic acid anhydride, dry THF, under argon, 50ꢁC, 3 h. (iii)
Conversion of 1,8-dihydroxy-9,10-anthraquinone (1b) into 1,4,5,8-tetramethoxyanthracene (2b).
Conditions: see Ref. [28], and (iv) Oxidation to 1,4,5,8-anthracenetetrone 3b. Conditions: silica-
supported ceric ammonium nitrate (CAN), CH2Cl2, RT, 1 h.
routes to these tetrones, starting from dihydroxy-9,10-anthraquinones (as for the synthe-
sis of 1,4,9,10-anthracenetetrone). The syntheses should be optimized regarding work-
load and efficiency.
Results and discussion
For the synthesis of 1,2,5,6-anthracenetetrone 3a, oxidation of 2,6-dihydroxyanthracene
2a (Scheme 2, top) using benzeneseleninic acid anhydride was reported to be superior
to other reactions.[10] Fortunately, the required precursor 2a can be efficiently prepared
from 2,6-dihydroxy-9,10-anthraquinone 1a. Screening the literature for reliable protocols
to minimize the workload and maximize the yields, we discovered a protocol reported
by Guo et al.[27] Following this protocol, we immediately achieved 86% yield (just 2%
less than reported). There is no need for column chromatography or recrystallization to
achieve high purity, which significantly reduces the workload.
In contrast to the synthesis of 2a, the subsequent oxidation to 3a required consider-
able optimization. Initially, we followed a protocol reported by Boldt et al.,[10]
which relies on crystallization of 3a from the reaction medium at room temperature and
additionally requires extraction and crystallization steps to improve the yield to 71%.
Unfortunately, we did not observe any crystallization of 3a at room temperature (also
storing at ꢀ18 ꢁC overnight just afforded small amounts of precipitate). As we wanted
to simplify the protocol anyways, we decided to directly evaporate the reaction solvent
as well as volatile by-products instead of storing the reaction for crystallization. The
resulting solid was then recrystallized from 1,4-dioxane affording tetrone 3a as red nee-
dles. Rapid weathering prevented analysis of the crystal structure, but NMR spectroscopy
confirmed the purity, except for some 1,4-dioxane, which disappears during storage or
when heating the solid, as was observed before.[9] Very good yields of 85%
were obtained.

€
4
F. GLOCKLHOFER ET AL.
Our other target compound, 1,4,5,8-anthracenetetrone 3b, can be prepared by oxida-
tion of 1,4,5,8-tetrahydroxyanthracene using freshly prepared silver oxide,[29] but the
yields of this reaction (and the five reactions needed to prepare the tetrahydroxy anthra-
cene) were reported to be unsatisfactory by Miller et al.[11] Instead, they suggest to oxi-
dize 5,8-dimethoxy-1,4-anthraquinone, which they prepared in a sequence of seven
reactions. Obviously, both routes do not comply with our aims of high yields and
low workload.
Fortunately, tetrone 3b can also be prepared by oxidation of 1,4,5,8-tetramethoxyan-
thracene 2b (Scheme 2, bottom) using ceric ammonium nitrate (CAN),[12] a reaction
that seemed more promising to us despite the reported moderate yield of 45% and the
not reported experimental details and characterization results. The required tetrame-
thoxy anthracene 2b can be prepared from 1-bromo-2,5-dimethoxybenzene.[30] The easy
accessibility of this precursor certainly is an advantage of this reaction, but the low yield
of 32% as well as the need for lithium 2,2,6,6-tetramethylpiperidide (LTMP) as the
reagent are disadvantages that have to be considered. We decided to follow a different
approach starting from low-cost 1,8-dihydroxy-9,10-anthraquinone 1b (Scheme 2,
bottom), as this approach was reported to be useful also on a larger scale:
Starting from 1b, an overall yield of 68% was reported for the synthesis of 2b by
methylation of the hydroxyl groups, subsequent reduction to 1,8-dimethyoxy anthracene,
2-fold bromination to 1,8-dibromo-4,5-dimethoxy anthracene, and final replacement of
the bromo substituents by methoxy groups.[28] Despite carrying out this reaction
sequence on a 50% larger scale, we achieved exactly the same overall yield at the first
try (lower yield for the methylation, higher yield for the reduction, similar yields for the
two other steps). We followed the published protocol with slight adaptions: the crude
methylation product was concentrated in vacuo and redissolved in CH2Cl2 before pass-
ing through a pad of silica, 2 N NaOH was used for washing the crude bromination
product instead of 5% HCl, and 2b was triturated in boiling CH2Cl2 instead of room
temperature. In contrast to the alternative synthesis of 2b, which uses 1-bromo-2,5-
dimethoxybenzene as a precursor, none of the reactions required purification by column
chromatography. Nevertheless, the synthesis using 1-bromo-2,5-dimethoxybenzene may
be more convenient, if only small amounts of 2b are required.
For the final oxidation to target compound 3b, we developed a protocol based on a
work using silica-supported CAN for the synthesis of benzoquinones.[31] Besides other
advantages, silica-supported CAN can be easily removed by filtration after the reaction.
Furthermore, it allows for using CH2Cl2 as the solvent, which dissolves 2b better than
the usual aqueous acetonitrile. The reaction was carried out simply by adding CH2Cl2
and 2b to freshly prepared silica-supported CAN and stirring for 1 h at room tempera-
ture. Facile work-up afforded crude 3b in quantitative yields. For removing impurities,
the solid was recrystallized from 1,4-dioxane, which reduced the yield to 64%. Despite
the loss during recrystallization, this is still significantly higher than reported before
without experimental details.
In contrast to 3a, needles of 3b were suitable for structural characterization (Figure 1
and Supplementary Material). In the solid state, 3b is essentially flat (largest distances
from least-squares plane defined by the C-atoms: O2, 0.0385(9) Å and C3, 0.0183(10)
ꢀ
Å). It crystallizes with one molecule of 1,4-dioxane in the space group P1. The asymmet-
ric unit contains half of a formula unit (Z0 ¼ 1/2), whereby 3b and 1,4-dioxane

R
SYNTHETIC COMMUNICATIONSV
5
Figure 1. Crystal structure of 3bꢂ1,4-dioxane viewed down (a) [100] and (b) [010]. (c) The molecular
structure of 3b. C (grey) and O (red) atoms are represented by ellipsoids drawn at the 50% probability
levels; H atoms by white spheres of arbitrary radius.
are located each on a center of inversion. No significant p–p stacking is observed.
In addition to these results, we were also able to determine the crystal structure of the
precursor 1,8-dibromo-4,5-dimethoxyanthracene, which has not yet been reported
(in contrast to the crystal structure of the direct precursor 2b) and is provided as
Supplementary Material.
Conclusions
1,2,5,6- and 1,4,5,8-Anthracenetetrone 3a and 3b can be prepared in efficient reactions
starting from dihydroxy-9,10-anthraquinones by following the presented synthetic
routes. None of the reactions requires column chromatography for purification and
the workload is generally low. For the synthesis of the precursor 2b, an alternative
procedure is available, which may be preferred on a smaller scale. Both 3a and 3b form
solvates when recrystallized from 1,4-dioxane, but the solvent can be removed easily.
The developed synthetic routes greatly facilitate the exploration of tetrones 3a and 3b in
devices and in the synthesis of p-conjugated small molecules and polymers.
Experimental
Reagents and solvents were purchased from commercial suppliers and used without
further purification. Anhydrous solvents were prepared by filtration through drying
columns (methanol, THF) or used as received from commercial suppliers (acetone).
Reactions were monitored by thin-layer chromatography (Merck, silica gel 60 F254).

€
6
F. GLOCKLHOFER ET AL.
2,6-Dihydroxyanthracene 2a was prepared from 2,6-dihydroxy-9,10-anthraquinone 1a
following a published protocol.[27] The synthesis of 1,4,5,8-tetramethoxyanthracene 2b
from 1,8-dihydroxy-9,10-anthraquinone 1b was also achieved following a published pro-
cedure, but with minor changes described above.[28]
1
NMR spectra were recorded at 600 MHz for H and 151 MHz for 13C on a Bruker
Avance III HD spectrometer. 1H NMR signals were assigned as suggested by
MestReNova 12.0.1-20560 (Supplementary Material). For 13C NMR signals, the number
of attached protons (C, CH) was assigned by attached proton test (APT) experiments.
Synthesis of 1,2,5,6-anthracenetetrone 3a
2,6-Dihydroxyanthracene 2a (263 mg, 1.25 mmol, 1.00 equiv.) was dissolved in 25 mL
dry THF and purged with argon. The solution was then added in portions to a stirred
solution of benzeneseleninic acid anhydride (900 mg, 2.50 mmol, 2.00 equiv.) in 50 mL
dry THF under argon (in a three-necked round-bottom flask equipped with a reflux
condenser with argon balloon and a thermometer). The reaction was heated to 50 ꢁC for
3 h and was then allowed to cool to room temperature. After storing overnight, the solv-
ent and volatile by-products were removed in vacuo using a rotary evaporator, which
was placed in a fume hood to prevent inhalation of volatile selenium by-products.
Recrystallization of the resulting solid from 1,4-dioxane afforded red needles (1,4-
dioxane solvates), which were dried to afford pure tetrone 3a (252 mg, 1.06 mmol) in
1
yields of 85%. H NMR (600 MHz, THF-d8): d ¼ 8.10 (s, 2H), 7.64 (d, J ¼ 10.2 Hz, 2H),
6.44 (d, J ¼ 10.2 Hz, 2H) ppm (in accordance with the literature[18]). 1H NMR
(600 MHz, CDCl3): d ¼ 8.13 (s, 2H), 7.57 (d, J ¼ 10.2 Hz, 2H), 6.62 (d, J ¼ 10.2 Hz, 2H)
ppm. 13C NMR (151 MHz, CDCl3): d ¼ 179.5 (C), 177.9 (C), 143.3 (CH), 136.4 (C),
135.8 (C), 130.9 (CH), 130.1 (CH) ppm.
Synthesis of 1,4,5,8-anthracenetetrone 3b
Ceric ammonium nitrate (CAN) (6.03 g, 11.0 mmol, 5.50 equiv.) was dissolved in 6 mL
water and added dropwise to 13 g silica (in a stirred 100 mL round-bottom flask
equipped with a septum). The solid was stirred until a free-flowing yellow solid was
obtained (about 30 min). 25 mL of CH2Cl2 was then added, followed by the addition of
1,4,5,8-tetramethoxyanthracene 2b (597 mg, 2.00 mmol, 1.00 equiv.) suspended in 35 mL
CH2Cl2. Another 10 mL CH2Cl2 was used to transfer residues of 2b. The reaction stirred
at room temperature for 1 h and was then filtered through a sintered glass funnel to
remove the silica. The silica was washed with 200 mL CH2Cl2 and the combined solu-
tions were washed with water and dried over sodium sulfate. The solvent was removed
in vacuo and the solid residue was recrystallized from 1,4-dioxane, which afforded red
needles (1,4-dioxane solvates). After drying, tetrone 3b (303 mg, 1.27 mmol) was
1
obtained in yields of 64%. H NMR (600 MHz, CDCl3): d ¼ 8.82 (s, 2H), 7.14 (s, 4H)
ppm (in accordance with the literature apart from reversed integrals[11]). 13C NMR
(151 MHz, CDCl3): d ¼ 183.3 (C), 139.4 (CH), 135.1 (C), 125.8 (CH) ppm.

R
SYNTHETIC COMMUNICATIONSV
7
Acknowledgments
We thank Markus Lunzer and Andreas J. Morawietz for contributing to synthetic experiments.
Brigitte Holzer and Christian Hametner are acknowledged for NMR measurements. The authors
acknowledge the TU Wien University Library for financial support through its Open Access
Funding Program.
Funding
€
This study was supported by the Technische Universitat Wien.
ORCID
€
Florian Glocklhofer
http://orcid.org/0000-0002-6911-8563
http://orcid.org/0000-0002-0087-474X
http://orcid.org/0000-0003-0840-2314
€
Berthold Stoger
€
Johannes Frohlich
References
[1] Isikli, S.; Díaz, R. J. Power Sources 2012, 206, 53–58. DOI:10.1016/j.jpowsour.2012.01.088.
[2] Kaul, B. B.; Yee, G. T. Polyhedron 2001, 20, 1757–1759. DOI:10.1016/S0277-
5387(01)00685-4.
[3] Asahi, H.; Inabe, T. Synth. Met. 1995, 70, 1117–1118. DOI:10.1016/0379-6779(94)02780-3.
[4] Asahi, H.; Inabe, T. Chem. Mater. 1994, 6, 1875–1879. DOI:10.1021/cm00046a050.
[5] Schenck, L. W.; Kuna, K.; Frank, W.; Albert, A.; Asche, C.; Kucklaender, U. Bioorg. Med.
Chem. 2006, 14, 3599–3614. DOI:10.1016/j.bmc.2006.01.026.
[6] Dengiz, C.; Swager, T. M. Synlett 2017, 28, 1427–1431. DOI:10.1055/s-0036-1588771.
[7] Cassis, R.; Valderrama, J. A. Synth. Commun. 1983, 13, 347–356. DOI:10.1080/
00397918308066988.
[8] Kitamura, C.; Kawase, T. Acta Crystallogr., Sect. E 2013, 69, o1597. DOI:10.1107/
S1600536813026342.
[9] Boldt, P. Chem. Ber. 1966, 99, 2322–2336. DOI:10.1002/cber.19660990731.
[10] Zippel, S.; Boldt, P. Synthesis 1997, 1997, 173–175. DOI:10.1055/s-1997-1171.
[11] Almlöf, J. E.; Feyereisen, M. W.; Jozefiak, T. H.; Miller, L. L. J. Am. Chem. Soc. 1990, 112,
1206–1214. DOI:10.1021/ja00159a049.
[12] Cory, R. M.; McPhail, C. L.; Dikmans, A. J. Tetrahedron Lett. 1993, 34, 7533–7536.
DOI:10.1016/S0040-4039(00)60392-1.
[13] Glöcklhofer, F.; Lunzer, M.; Stöger, B.; Fröhlich, J. Chem. - Eur. J. 2016, 22, 5173–5180.
DOI:10.1002/chem.201600004.
[14] Glöcklhofer, F.; Kautny, P.; Fritz, P.; Stöger, B.; Fröhlich, J. ChemPhotoChem 2017, 1,
51–55. DOI:10.1002/cptc.201600018.
[15] Glöcklhofer, F.; Morawietz, A. J.; Stöger, B.; Unterlass, M. M.; Fröhlich, J. ACS Omega
2017, 2, 1594–1600. DOI:10.1021/acsomega.7b00245.
[16] Glöcklhofer, F.; Petritz, A.; Karner, E.; Bojdys, M. J.; Stadlober, B.; Fröhlich, J.; Unterlass,
M. M. J. Mater. Chem. C 2017, 5, 2603–2610. DOI:10.1039/c7tc00143f.
[17] Imai, K.; Kurihara, M.; Mathias, L.; Wittmann, J.; Alston, W. B.; Stille, J. K.
Macromolecules 1973, 6, 158–162. DOI:10.1021/ma60032a002.
[18] Hahn, S.; Koser, S.; Hodecker, M.; Tverskoy, O.; Rominger, F.; Dreuw, A.; Bunz, U. H. F.
Chem. - Eur. J. 2017, 23, 8148–8151. DOI:10.1002/chem.201701304.
[19] Baumgartner, B.; Bojdys, M. J.; Unterlass, M. M. Polym. Chem. 2014, 5, 3771–3776.
DOI:10.1039/C4PY00263F.

€
F. GLOCKLHOFER ET AL.
8
[20] Baumgartner, B.; Svirkova, A.; Bintinger, J.; Hametner, C.; Marchetti-Deschmann, M.;
Unterlass, M. M. Chem. Commun. 2017, 53, 1229–1232. DOI:10.1039/c6cc06567h.
[21] Cory, R. M.; McPhail, C. L.; Dikmans, A. J.; Vittal, J. J. Tetrahedron Lett. 1996, 37,
1983–1986. DOI:10.1016/0040-4039(96)00263-8.
[22] Bénard, C. P.; Geng, Z.; Heuft, M. A.; VanCrey, K.; Fallis, A. G. J. Org. Chem. 2007, 72,
7229–7236. DOI:10.1021/jo0709807.
[23] Chow, T. J.; Wu, C.-C.; Chuang, T.-H.; Hsieh, H.-H.; Huang, H.-H.; Synthesis and applica-
tions of soluble pentacene precursors and related compounds. U.S. Patent 8,277,903 B2,
2012.
[24] Nourmohammadian, F.; Salami, S. J. Mol. Struct.: THEOCHEM 2008, 861, 147–148.
DOI:10.1016/j.theochem.2008.04.026.
[25] Fukuda, M.; Tajiri, A.; Oda, M.; Hatano, M. Bull. Chem. Soc. Jpn. 1983, 56, 592–596.
DOI:10.1246/bcsj.56.592.
[26] Yu, Y.-X. ACS Appl. Mater. Interfaces 2014, 6, 16267–16275. DOI:10.1021/am504452a.
[27] Weidman, J. R.; Luo, S.; Zhang, Q.; Guo, R. Ind. Eng. Chem. Res. 2017, 56, 1868–1879.
DOI:10.1021/acs.iecr.6b04946.
[28] Navale, T. S.; Rathore, R. Synthesis 2012, 44, 805–809. DOI:10.1055/s-0031-1289695.
[29] Boldt, P.; Vardakis, F. Angew. Chem., Int. Ed. 1965, 4, 1078–1078. DOI:10.1002/
anie.196510782.
[30] Fitzgerald, J. J.; Drysdale, N. E.; Olofson, R. A. Synth. Commun. 1992, 22, 1807–1812.
DOI:10.1080/00397919208020501.
[31] Hashmat Ali, M.; Niedbalski, M.; Bohnert, G.; Bryant, D. Synth. Commun. 2006, 36,
1751–1759. DOI:10.1080/00397910600616859.