From enolates to anthraquinones

Article abstract of DOI:10.1139/V06-054

A series of highly reactive cyclopentadienones were prepared in situ from the corresponding hydroxycyclopent-2-enones and trapped with a variety of quinones. Reaction of 1,4-naphthoquinone with 4-hydroxy-3,4-diphenyl-cyclopent- 2-enone afforded 2,3-diphenylanthraquinone, whereas reaction of benzoquinone with this same cyclopentadienone precursor yielded a mixture of 6,7-dipheny 1-1,4-naphthoquinone and 2,3,6,7-tetraphenylanthraquinone. A number of other 2,3-diarylanthraquinones were likewise prepared in moderate yields from the reaction of 1,4-naphthoquinone with the appropriate 4-hydroxy-3,4- diarylcyclopent-2-enones. This method appears to be generally applicable to the synthesis of anthraquinone derivatives substituted at the 2- and 3-positions from inexpensive starting materials.

Full text of DOI:10.1139/V06-054

659
From enolates to anthraquinones1
David Bailey, Jeffrey N. Murphy, and Vance E. Williams
Abstract: A series of highly reactive cyclopentadienones were prepared in situ from the corresponding hydroxy-
cyclopent-2-enones and trapped with a variety of quinones. Reaction of 1,4-naphthoquinone with 4-hydroxy-3,4-
diphenyl-cyclopent-2-enone afforded 2,3-diphenylanthraquinone, whereas reaction of benzoquinone with this same
cyclopentadienone precursor yielded a mixture of 6,7-diphenyl-1,4-naphthoquinone and 2,3,6,7-tetraphenylanthra-
quinone. A number of other 2,3-diarylanthraquinones were likewise prepared in moderate yields from the reaction of
1,4-naphthoquinone with the appropriate 4-hydroxy-3,4-diarylcyclopent-2-enones. This method appears to be generally
applicable to the synthesis of anthraquinone derivatives substituted at the 2- and 3-positions from inexpensive starting
materials.
Key words: anthraquinone, Diels–Alder, cyclopentadienone, in situ.
Résumé : On a préparé in situ une série de cyclopentadiénones très réactives à partir des hydroxycyclopent-2-énones
correspondantes et on les a piégées à l’aide d’une variété de quinones. La réaction de la 1,4-naphtoquinone avec la 4-
hydroxy-3,4-diphénylcyclopent-2-énone conduit à la formation de 2,3-diphénylanthraquinone alors que la réaction de la
benzoquinone avec la même cyclopentadiénone conduit à un mélange de 6,7-diphényl-1,4-naphtoquinone et de 2,3,6,7-
tétraphénylanthraquinone. On a préparé un certain nombre d’autres 2,3-diarylanthraquinones avec des rendements mo-
dérés par réaction de la 1,4-naphtoquinone avec des 4-hydroxy-3,4-diarylcyclopent-2-énones appropriées. Le domaine
d’application de cette méthode semble général pour la synthèse de dérivés de l’anthraquinone substitués dans les posi-
tions 2 et 3, à partir de produits de départs peu coûteux.
Mots clés : anthraquinone, Diels–Alder, cyclopentadiénone, in situ.
[Traduit par la Rédaction] Bailey et al. 666
Introduction
the use of in situ generated cyclopentadienones as synthetic
equivalents of sulfones in the construction of anthraquin-
ones.
If you learn nothing else from this class, I want you to re-
member: “Enolates are nucleophiles.” — Walter Szarek,
speaking to his 3rd year organic chemistry class in 1991.
Cyclopentadienones condense with alkynes to form Diels–
Alder adducts, which, upon loss of carbon monoxide, afford
highly substituted benzene derivatives (13–16). These cy-
clones therefore behave in a similar manner to thiophene-
1,1-dioxides, although the former tend to be much more re-
active than the latter. Unlike sulfones, cyclopentadienones
are highly unstable unless substituted at the 2- and 5-
positions, and derivatives that lack bulky groups at these
sites generally must be generated in situ from precursors
such as the corresponding hydroxycyclopent-2-enones (15,
17, 18). Perhaps for this reason, cyclopentadienones have
rarely been employed in the synthesis of anthraquinones (15,
19, 20), and only relatively stable derivatives of this type
have been used for this purpose. Indeed, while there is some
precedent for in situ generated cyclones being used in Diels–
Alder reactions with alkynes (15, 19), we are aware of no
examples of these compounds being trapped with quinone
dienophiles to afford either anthraquinones or naphthoquin-
ones. We therefore wished to explore whether the use of
Anthraquinones are a versatile class of compounds that
have been exploited as dyes (1), chemical sensors (2), organo-
gelators (3), liquid crystals (4–8), anticancer agents (9),
DNA photooxidants (10), and as precursors to substituted
anthracenes. Our own interest in the synthesis of anthra-
quinone derivatives arose out of investigations into the
[4+4]-photocycloaddition reactions of peripherally substituted
chromophores such as 2,3,6,7-tetraphenylanthracene (11),
the logical precursor of which is the anthraquinone (1). This
quinone could be prepared in a one-pot reaction between
3,4-diphenylthiophene-1,1-dioxide (2) and benzoquinone
(Scheme 1). Likewise, treatment of compound 2 with 1,4-
naphthoquinone afforded 2,3-diphenylanthraquinone (3) in
70% yield (12). Although these sulfones are relatively stable
and can be prepared from readily accessible thiophenes, the
large-scale synthesis of these molecules has, in our hands,
proven unreliable. For this reason, we decided to investigate
Received 31 August 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 12 May 2006.
D. Bailey, J.N. Murphy, and V.E. Williams.2 Department of Chemistry, Simon Fraser University, 8888 University Dr., Burnaby,
BC V5A 1S6, Canada.
1This article is part of a Special Issue dedicated to Professor Walter A. Szarek.
2Corresponding author (e-mail: vancew@sfu.ca).
Can. J. Chem. 84: 659–666 (2006)
doi:10.1139/V06-054
© 2006 NRC Canada

660
Can. J. Chem. Vol. 84, 2006
Scheme 1. Diels–Alder synthesis of compounds 1 and 3. See ref. 12.
O
O
O
O
O
O
S
O
O
O
O
AcOH
AcOH
1
3
2
70%
52%
hydroxycyclopent-2-enones in the synthesis of anthraquin-
ones was a viable strategy or would merely result in a fiasco.
180 mL acetic acid was added 0.84 g (3.3 mmol) of 4-
hydroxy-3,4-diphenylcyclopent-2-enone (4) and 0.065 g
(0.38 mmol) of p-toluenesulfonic acid. This mixture was
refluxed for 49 h, then cooled to room temperature, poured
over water (150 mL), and extracted with CH2Cl2 (3 ×
100 mL). The combined organic extracts were dried over
MgSO4 and the solvent was removed in vacuo. The crude
product was purified by column chromatography using silica
gel with toluene as the eluent. 2,3,6,7-Tetraphenylanthra-
quinone (1) was eluted from the column first, followed by
6,7-diphenylnaphthoquinone (7).
Experimental
Materials and methods
1H NMR (400 MHz) and 13C NMR (100 MHz) were car-
ried out on a Bruker AMX-400 400 MHz spectrometer; FT-
IR spectroscopy was performed on a Thermo Nicolet Nexus
670 FT-IR ESP spectrometer, using KBr pellets. Micro-
analyses (C, H, N) were performed at Simon Fraser Univer-
sity, Burnaby, British Columbia, by Mr. Miki Yang. Low-
resolution EI (70 eV) mass spectrometry was carried out on
a Hewlett Packard 5985 mass spectrometer. Melting points
were performed on either an Olympus BX50 polarizing mi-
croscope with a Linkam 94 heating stage or a MelTempII
melting point apparatus and are uncorrected. MALDI-TOF
mass spectra were obtained on a Perspective Voyager-DE
STR from PE Applied Biosystems with a nitrogen laser
(337 nm) and using 2,5-dihydroxybenzoic acid as the matrix.
1,4-Benzoquinone and p-toluenesulfonic acid were pur-
chased from the Fisher Scientific Company. 1,4-Naptho-
quinone was obtained from Eastman Organic Chemicals.
Copper sulfate, tetrakis(triphenylphosphine)palladium, cop-
per(I) iodide, diisopropylamine, potassium iodide, benzal-
dehyde, p-anisaldehyde, 2-naphthaldehyde, 4-bromobenzalde-
hyde, 4-iodoanisole, and phenylacetylene were purchased
from the Sigma-Aldrich Chemical Company Inc. The o-
tolualdehyde was purchased from Matheson, Coleman, and
Bell. Silica gel (230–400 mesh ASTM) was purchased from
EMD Chemicals Inc. 1,4-Benzoquinone and 1,4-naphtho-
quinone were sublimed prior to use. Tetrahydrofuran was
dried over sodium using benzophenone as an indicator.
Benzaldehyde was distilled at reduced pressure. All other re-
agents were used as received.
2,3,6,7-Tetraphenylanthraquinone (1)
2,3,6,7-Tetraphenylanthraquinone (1) was recrystallized
from ethanol to afford 0.22 g (0.46 mmol) of a yellow solid;
1
mp: 323 to 324 °C. IR (cm–1): 1672 (C=O stretch). H NMR
(ppm, acetone-d6) δ: 8.39 (s, 4H), 7.22–7.29 (m, 20H). 13C
NMR (ppm, CDCl3) δ: 182.8, 146.4, 139.6, 132.4, 129.7,
128.2, 127.7 (eight peaks predicted, seven observed). MS
(EI) m/z: 512. HRMS calcd. for C38H24O2: 512.1776; found:
512.1774.
6,7-Diphenyl-1,4-naphthoquinone (7)
6,7-Diphenyl-1,4-naphthoquinone (7) was recrystallized
from ethanol to afford 150 mg (0.48 mmol) of a yellow
1
solid; mp: 123–125 °C. IR (cm–1): 1668 (C=O stretch). H
NMR (ppm, CD2Cl2) δ: 8.11 (s, 2H), 7.17–7.29 (m, 10H),
7.00 (s, 2H). 13C NMR (ppm, CDCl3) δ: 184.9, 146.2, 139.5,
138.8, 130.6, 129.6 128.8, 128.2, 127.7. MS (EI) m/z: 310
[M+]. HRMS calcd. for C22H14O2: 310.0994; found:
310.0996.
2,3-Diphenylanthraquinone (3)
To a solution of 1,4-naphthoquinone (0.81 g, 5.2 mmol) in
130 mL acetic acid was added 0.64 g (2.6 mmol) of 4-
hydroxy-3,4-diphenylcyclopent-2-enone and 0.090
g
Note that in the case of compounds 1, 3, 8, 9, 11, 12, and
24, fewer peaks were observed in the 13C NMR spectra than
were predicted. This is probably due to inadequate peak res-
olution arising from the similarity in the chemical environ-
ments of many of the carbon nuclei. Attempts to resolve
these peaks using d6-benzene as the solvent were unsuccessful.
(0.52 mmol) of p-toluenesulfonic acid. This mixture was
then refluxed for 39 h. The solution was then cooled to room
temperature, poured over water (150 mL), and then extracted
with CH2Cl2 (3 × 100 mL). The combined organic extracts
were dried over MgSO4 and the solvent removed in vacuo.
The crude product was purified by column chromatography
on silica gel with toluene as the eluent and then
recrystallized from ethanol to afford 0.39 g (1.1 mmol, 42%)
of 3; mp: 203–206 °C (lit. value (21) mp 211 to 212 °C). IR
Synthesis
2,3,6,7-Tetraphenylanthraquinone (1) and 6,7-diphenyl-
1,4-naphthaquinone (7)
To a solution of 1,4-benzoquinone (0.71 g, 6.6 mmol) in
1
(cm–1): 1676 (C=O stretch). H NMR (ppm, CDCl3) δ: 8.36
(s, 2H), 8.35 (dd, 2H, J = 3.36, 5.80 Hz), 7.82 (dd, 2H, J =
© 2006 NRC Canada

Bailey et al.
661
3.36, 5.80 Hz), 7.30–7.20 (m, 10H). 13C NMR (ppm,
CDCl3) δ: 183.0, 146.4, 139.6, 134.1, 132.2, 129.7, 128.3,
127.7, 127.3 (11 peaks predicted, 9 observed). MS (EI) m/z:
360 [M+].
8.34 (m, 4H), 7.82 (dd, 2H, J = 3.3, 5.8 Hz), 7.30–7.29 (m,
3H), 7.23 (dd, 2H, J = 3.0, 6.7 Hz), 7.14 (d, 2H, J = 8.7 Hz),
6.80 (d, 2H, J = 8.7 Hz), 3.80 (s, 3H). 13C NMR (ppm,
CDCl3) δ: 183.3, 183.2, 159.5, 146.5, 146.3, 140.1, 134.3,
134.3, 134.0, 133.9, 132.5, 132.1, 131.1, 130.0, 129.8,
129.7, 128.5, 127.8, 127.5, 113.9, 55.5 (23 peaks predicted,
21 observed). MS (EI) m/z: 390.4 [C27H18O3 , 100]. Ele-
mental anal. calcd. for C27H18O3: C 83.06, H 4.65; found: C
82.80, H 4.89.
2,3-Di(naphthalen-2-yl)anthraquinone (8)
+
2,3-Di(naphthalen-2-yl)anthraquinone (8) was prepared
from 4-hydroxy-3,4-di(naphthalen-2-yl)cyclopent-2-enone
(16) according to the method described previously. The
crude product was eluted through silica gel with a 1:1 mix-
ture of hexanes–toluene, and then recrystallized from etha-
nol to afford 0.055 g (0.13 mmol, 18%) of 8 as a yellow
2-(4-Bromophenyl)-3-(4-methoxyphenyl)anthraquinone (12)
2-(4-Bromophenyl)-3-(4-methoxyphenyl)anthraquinone (12)
was prepared from 3-(4-bromophenyl)-4-hydroxy-4-(4-
methoxyphenyl)cyclopent-2-enone/4-(4-bromophenyl)-4-
hydroxy-3-(4-methoxyphenyl)cyclopent-2-enone (23) ac-
cording to the method described previously. The product was
eluted through silica gel with toluene, then recrystallized
from ethanol to afford 0.120 g (0.26 mmol, 21%) of 12 as an
1
solid; mp: 238 °C. H NMR (ppm, CDCl3) δ: 8.53 (s, 2H),
8.38 (dd, 2H, J = 3.3, 5.8 Hz), 7.96 (s, 2H), 7.85 (dd, 2H,
J = 3.3, 5.8 Hz), 7.81 (dd, 2H, J = 3.4, 6.0 Hz), 7.76 (dd,
2H, J = 3.4, 6.0 Hz), 7.59 (d, 2H, J = 8.6 Hz), 7.48 (dd, 4H,
J = 3.2, 6.2 Hz), 7.17 (dd, 4H, J = 1.7, 8.5 Hz). 13C NMR
(ppm, CDCl3) δ: 183.0, 146.4, 137.3, 134.2, 133.8, 133.3,
132.6, 132.4, 130.1, 128.9, 128.3, 127.7, 127.4, 127.3,
126.5, 126.3 (17 peaks predicted, 16 observed). MALDI-
TOF m/z: 462 [M + 2, 100]. Elemental anal. calcd. for
C34H20O2: C 88.67, H 4.38; found: C 88.51, H 4.58.
1
orange solid; mp 215 to 216 °C. H NMR (ppm, CDCl3) δ:
8.34 (dd, 2H, J = 3.1, 5.4 Hz), 8.33 (s, 1H), 8.30 (s, 1H),
7.83 (dd, 2H, J = 3.3, 5.8 Hz), 7.43 (d, 2H, J = 8.4 Hz),
7.14–7.09 (m, 4H), 6.83 (d, 2H, J = 8.7 Hz), 3.82 (s, 3H).
13C NMR (ppm, CDCl3) δ: 183.2, 183.1, 159.6, 146.3,
145.1, 139.0, 134.4, 134.4, 133.9, 132.7, 132.2, 131.8,
131.7, 131.4, 131.1, 129.8, 129.7, 127.5, 122.3, 114.1, 55.5
(23 peaks predicted, 21 observed). MS (CI) m/z (relative in-
tensity): 468.2 (80%), 469.2 (60%), 470.2 (100%), 471.2
(58%), 472 (13%). Elemental anal. calcd. for C27H17BrO3: C
69.10, H 3.65; found: C 69.27, H 3.72.
2,3-Di(o-tolyl)anthraquinone (9)
2,3-Di(o-tolyl)anthraquinone (9) was prepared from 4-
hydroxy-3,4-di(o-tolyl)cyclopent-2-enone (17) according to
the method described previously. The crude product was
eluted through silica gel with toluene and recrystallized from
ethanol to yield 0.090 g (0.23 mmol, 17%) of 9 as an off-
1
white solid; mp 247 °C. H NMR (ppm, CDCl3) δ: 8.34 (dd,
2H, J = 3.3, 5.7 Hz), 8.28 (s, 2H), 7.82 (dd, 2H, J = 3.3,
5.7 Hz), 7.13–6.91 (broad m, 8H), 2.11 (s, 6H). 13C NMR
(ppm, CDCl3) δ: 183.1, 147.3 (broad), 139.1 (broad), 135.3,
134.1, 133.8, 131.9, 130.2, 129.8, 127.8, 127.2, 125.2, 20.2
(broad) (14 peaks predicted, 13 observed). MS (EI) m/z:
388.1 [M+, 100]. Elemental anal. calcd. for C28H20O2: C
86.57, H 5.19; found: C 86.38, H 5.27.
2-(4-Methoxyphenyl)-3,6,7-triphenylanthraquinone (24)
2-(4-Methoxyphenyl)-3,6,7-triphenylanthraquinone (24)
was prepared from 6,7-diphenyl-1,4-naphthoquinone and a
mixture of 21a and 21b. The crude product was purified
through silica gel with toluene as the eluent. Recrystalliza-
tion of this product from ethanol afforded 0.14
g
1
(0.26 mmol, 26%) of 24 as an orange solid; mp 230 °C. H
NMR (ppm, CDCl3) δ: 8.40 (s, 2H), 8.38 (d, 2H, J =
2.9 Hz), 7.31–7.23 (m, 15H), 7.17 (d, 2H, J = 8.8 Hz), 6.82
(d, 2H, J = 8.8 Hz), 3.81 (s, 3H). 13C NMR (ppm, CDCl3) δ:
183.1, 183.0, 159.5, 146.6, 146.6, 146.4, 146.3, 140.1,
139.9, 132.7, 136.7, 132.3, 132.1, 131.2, 130.0, 129.9,
129.8, 129.7, 128.5, 128.4, 127.9, 127.8, 113.9, 55.5 (31
peaks predicted, 24 observed). MALDI-TOF m/z: 543 [M+,
100]. Elemental anal. calcd. for C39H26O3: C 86.32, H 4.83;
found: C 85.97, H 5.03.
2,3-Bis(4-methoxyphenyl)anthraquinone (10)
2,3-Bis(4-methoxyphenyl)anthraquinone (10) was pre-
pared from 4-hydroxy-3,4-bis(4-methoxyphenyl)cyclopent-
2-enone (18) according to the method described previously.
The crude product was eluted through silica gel with a 4:1
mixture of hexanes – ethyl acetate as the eluent. The result-
ing product was recrystallized from ethanol to afford 0.04 g
1
(0.09 mmol, 7%) of 10 as a brown solid; mp 168 °C. H
NMR (ppm, CDCl3) δ: 8.34 (dd, 2H, J = 3.3, 5.8 Hz), 8.31
(s, 2H), 7.81 (dd, 2H, J = 3.3, 5.8 Hz), 7.16 (d, 4H, J =
8.7 Hz), 6.82 (d, 4H, J = 8.7 Hz), 3.82 (s, 6H). 13C NMR
(ppm, CDCl3) δ: 183.0, 159.2, 145.9, 134.0, 133.8, 132.1,
131.9, 130.9, 129.5, 127.2, 113.8, 55.2. MS (CI) m/z: 421.2,
422.1 [M + 1, 100] (29%). Elemental anal. calcd. for
C28H20O4: C 79.98, H 4.97; found: C 80.13, H 4.90.
2,3-Bis(4-methoxyphenyl)-6,7-diphenylanthraquinone (25)
2,3-Bis(4-methoxyphenyl)-6,7-diphenylanthraquinone (25)
was prepared from 6,7-diphenyl-1,4-napthoquinone (7) and
4-hydroxy-3,4-bis(4-methoxyphenyl)cyclopent-2-enone (18)
according to the method described previously. This mixture
of products was then purified through silica gel with a 4:1
mixture of hexanes – ethyl acetate as the eluent. The result-
ing solid was recrystallized from ethanol to afford 0.050 g
2-(4-Methoxyphenyl)-3-phenylanthraquinone (11)
1
2-(4-Methoxyphenyl)-3-phenylanthraquinone (11) was
prepared from a mixture of 21a and 21b according to the
method described previously. The crude product was eluted
through a silica gel column using toluene. Recrystallization
from ethanol afforded 0.087 g (0.22 mmol, 18%) of 11 as a
yellow solid; mp 89–91 °C. 1H NMR (ppm, CDCl3) δ: 8.36–
(0.09 mmol, 7%) of 25 as an orange solid; mp 235 °C. H
NMR (ppm, CDCl3) δ: 8.39 (s, 2H), 8.35 (s, 2H), 7.30–7.22
(m, 10H), 7.18 (d, 4H, J = 8.8 Hz), 6.83 (d, 4H, J = 8.7 Hz),
3.82 (s, 6H). 13C NMR (ppm, CDCl3) δ: 183.1, 159.4, 146.6,
146.1, 139.9, 132.7, 132.4, 132.3, 131.1, 129.9, 129.9,
129.8, 128.4, 127.9, 114.0, 55.5. MALDI-TOF m/z: 584
© 2006 NRC Canada

662
Can. J. Chem. Vol. 84, 2006
[M + 2, 100]. Elemental anal. calcd. for C40H28O4: C 83.90,
H 4.93; found: C 84.20, H 5.16.
Alternate synthesis of 1-(4-methoxyphenyl)-2-phenyl-
ethane-1,2-dione (20)
To a dried Schlenk flask was added 4-iodoanisole (3.2 g,
13.8 mmol), tetrakis(triphenylphosphine)palladium (0.059 g,
0.05 mmol), and copper(I) iodide (0.815 g, 4.3 mmol) under
nitrogen. To this mixture was added via syringe phenyl-
acetylene (1.75 mL, 16 mmol), 50 mL of dry tetrahydro-
furan, and 10 mL of dry diisopropylamine. The resulting
mixture was refluxed under nitrogen for 45 h, at which time
it was cooled to room temperature and eluted through a short
plug of silica gel with dichloromethane. After removing the
solvent in vacuo, the crude product was oxidized by treating
it with iodine (3.94 g, 15.6 mmol) in dimethylsulfoxide
(50 mL) at 145 °C for 44 h. This solution was cooled to
room temperature, poured into water, and the resulting mix-
ture was extracted with dichloromethane. The combined or-
ganic extracts were washed with brine, then with a saturated
aqueous potassium iodide solution, dried (MgSO4), and the
solvent removed under reduced pressure. The product was
then recrystallized from a methanol–water mixture to 20 as a
pale yellow solid (1.7 g, 8.2 mmol, 59% yield).
General method for the preparation of ethane diones
The aldehyde (32 mmol) and potassium cyanide (1.3 g,
20 mmol) were dissolved in 20 mL of ethanol and 10 mL of
water. After refluxing for 24 h, the solution was poured over
water and extracted with dichloromethane, yielding the cou-
pled α-hydroxyketone. This product was then dissolved in
25 mL pyridine and 15 mL water, and 5.0 g CuSO4
(35 mmol) was added. This solution was refluxed for 16 h,
then cooled to room temperature and extracted with di-
chloromethane. The combined organic extracts were washed
water and the solvent removed to afford the 1,2-dione.
1,2-Di(o-tolyl)ethane-1,2-dione (14)
1,2-Di(o-tolyl)ethane-1,2-dione (14) was prepared in the
manner described in the previous paragraph and was isolated
as a pale yellow solid (66% yield); mp 88 to 89 °C (lit. value
1
(22) mp 92 °C). H NMR (ppm, CDCl3) δ: 7.66 (d, 2H, J =
7.8 Hz), 7.49 (t, 2H, J = 7.5 Hz), 7.35 (d, 2H, J = 7.6 Hz),
7.28 (m, 2H), 2.71 (s, 6H).
General method for the preparation of 4-hydroxy-3,4-
diarylcyclopent-2-enones
1,2-Bis(4-methoxyphenyl)ethane-1,2-dione (15)
Ethane-1,2-dione (10 mmol) was stirred for 1 week at
room temperature in an ethanolic solution of potassium hy-
droxide (0.56 g, 10 mmol) and acetone (12 mL, 160 mmol).
This solution was then poured over water and extracted with
dichloromethane, yielding the desired product. Since these
compounds proved difficult to purify, they were used di-
rectly in the condensation reactions with quinones.
1,2-Bis(4-methoxyphenyl)ethane-1,2-dione (15) was pre-
pared in the manner described previously and was isolated
as a yellow solid (32% yield); mp obs. 131 °C (lit. value
(23) mp 133 °C). H NMR (ppm, DCl3) δ: 7.95 (d, 4H, J =
8.8 Hz), 6.97 (d, 4H, J = 8.9 Hz), 3.89 (s, 6H).
1
1,2-Di(naphthalen-2-yl)ethane-1,2-dione (16)
1,2-Di(naphthalen-2-yl)ethane-1,2-dione (16) was pre-
pared in the manner described previously, except NaCN was
used in place of KCN, and the benzoin condensation was
only allowed to proceed for 1.5 h instead of 24 h. Com-
pound 16 was isolated in 64% yield as an off-white solid;
4-Hydroxy-3,4-diphenylcyclopent-2-enone (4)
4-Hydroxy-3,4-diphenylcyclopent-2-enone (4) was pre-
pared in the manner described in the previous paragraph as a
1
pale yellow solid (96% yield). H NMR (ppm, CDCl3) δ:
1
7.52–7.50 (m, 2H), 7.47–7.44 (m, 2H), 7.37–7.27 (m, 6H),
6.71 (s, 1H), 3.02 (d, 1H, J = 18.5 Hz), 2.90 (d, 1H, J =
18.5 Hz).
mp 155 °C (lit. value (24) mp 155 to 156 °C). H NMR
(ppm, CDCl3) δ: 8.45 (s, 2H), 8.15 (dd, 2H, J = 1.6 8.6 Hz),
7.99 (d, 2H, J = 8.7 Hz), 7.90 (d, 4H, J = 8.8 Hz), 7.64 (t,
2H, J = 7.6 Hz), 7.55 (t, 2H, J = 7.6 Hz). 13C NMR (ppm,
CDCl3) δ: 194.8, 136.4, 133.6, 132.4, 130.5, 130.0, 129.6,
129.2, 128.0, 127.2, 123.8.
4-Hydroxy-3,4-di(naphthalene-2-yl)cyclopent-2-enone (16)
4-Hydroxy-3,4-di(naphthalene-2-yl)cyclopent-2-enone (16)
was prepared in the manner described previously as a brown
1
solid. H NMR (ppm, CDCl3) δ: 8.19 (d, 2H, J = 11.5 Hz),
1-(4-Methoxyphenyl)-2-phenylethane-1,2-dione (20)
7.85–7.40 (m, 12H), 6.90 (s, 1H), 3.13 (d, 1H, J = 18.6 Hz),
3.02 (d, 1H, J = 18.6 Hz).
1-(4-Methoxyphenyl)-2-phenylethane-1,2-dione (20) was
prepared in the manner described previously and was puri-
fied by column chromatography (silica gel, toluene) as a
pale yellow solid (22% yield); mp 58.5–60 °C (lit. value
4-Hydroxy-3,4-di(o-tolyl)cyclopent-2-enone (17)
4-Hydroxy-3,4-di(o-tolyl)cyclopent-2-enone (17) was pre-
pared in the manner described previously as a brown solid.
1H NMR (ppm, CDCl3) δ: 7.55–7.53 (m, 2H), 7.20–7.14 (m,
4H). 6.99–6.97 (m, 2H), 6.40 (s, 1H), 3.07 (d, 2H, J =
18.6 Hz), 2.98 (d, 2H, J = 18.6 Hz), 2.41 (s, 3H), 2.31 (s,
3H).
1
(25) mp 61 °C). H NMR (ppm, CDCl3) δ: 7.96 (m, 4H),
7.65 (t, 1H, J = 7.4 Hz), 7.51 (t, 2H, J = 7.8 Hz), 6.98 (d,
2H, J = 8.9 Hz), 3.89 (s, 1H).
1-(4-Bromophenyl)-2-(4-methoxyphenyl)ethane-1,2-dione
(22)
1-(4-Bromophenyl)-2-(4-methoxyphenyl)ethane-1,2-dione
(22) was prepared in the manner described previously and
was isolated as a yellow solid (46% yield); mp 135 °C (lit.
value (26) mp 136 to 137 °C). H NMR (ppm, CDCl3) δ:
7.93 (d, 2H, J = 8.9 Hz), 7.84 (d, 2H, J = 8.6 Hz), 7.65 (d,
2H, J = 8.6 Hz), 6.98 (d, 2H, J = 8.9 Hz), 3.89 (s, 3H).
4-Hydroxy-3,4-bis(4-methoxyphenyl)cyclopent-2-enone (18)
4-Hydroxy-3,4-bis(4-methoxyphenyl)cyclopent-2-enone (18)
was prepared in the manner described previously as a brown
solid. H NMR (ppm, CDCl3) δ: 7.52 (d, 2H, J = 9.0 Hz),
7.34 (d, 2H, J = 8.8 Hz), 6.83 (dd, 4H, J = 8.9, 20.5 Hz),
1
1
© 2006 NRC Canada

Bailey et al.
663
Scheme 2. Reagents and conditions: (a) acetone, KOH, 1 week; (b) AcOH, TsOH, reflux 48 h.
O
O
O
O
Ph
Ph
O
a
b
OH
O
Ph
Ph
4
5
6
Scheme 3. Condensation of compound 4 with benzoquinone.
6.60 (s, 1H), 3.79 (s, 3H), 3.78 (s, 3H), 2.98 (d, 2H, J =
18.5 Hz), 2.85 (d, 2H, J = 18.5 Hz).
O
O
TsOH
AcOH
4-Hydroxy-3-(4-methoxyphenyl)-4-phenylcyclopent-2-enone
(21a)
+
4
+
1
reflux
4-Hydroxy-3-(4-methoxyphenyl)-4-phenylcyclopent-2-enone
(21a) was prepared in the manner described previously and
isolated via column chromatography (silica, toluene–EtOAc,
7
O
O
1
9:1 as eluent) as a white solid. H NMR (ppm, CDCl3) δ:
7.51 (dt, 2H, J = 9.0, 2.1 Hz), 7.45 (dt, 2H, J = 7.3, 1.6 Hz),
7.35 (td, 2H, J = 7.6, 1.8 Hz), 7.28–7.25 (m, 1H), 2.88 (d,
1H, J = 18.4 Hz), 2.53 (br, 1H).
termediate 5 (17, 18), demonstrates that this cyclone was
indeed being generated under these conditions (Scheme 2).
We next turned our attention to trapping this in situ gener-
ated cyclone with 1,4-naphthoquinone. When cyclopenta-
dienone 5 was generated in the presence of excess quinone,
compound 3 was formed as the major product in 42% yield.
Significantly, only trace quantities (<5%) of the dimer 6
were observed under these conditions. More generally, this
side product did not arise to an appreciable extent in any
subsequent reactions of 4 with quinones, indicating that the
dimerization of 5 does not pose a significant problem in
these syntheses.
The condensation of 4 with 1,4-benzoquinone was next in-
vestigated. This reaction affords two major products: the
anthraquinone derivative 1 and the naphthoquinone 7 in an
approximately 1:1 ratio (Scheme 3). That significant quanti-
ties of the anthraquinone were formed even in the presence
of a twofold excess of benzoquinone — conditions that should
strongly promote formation of the naphthoquinone 7 —
suggests that compound 7 or one of its precursors is a much
more reactive dienophile than benzoquinone itself, thus fa-
vouring the rapid reaction with a second equivalent of the
cyclone. This tendency for benzoquinone to form bisadducts
in acetic acid has previously been observed in its reaction
with anthracene (28) and 1,1-thiophene dioxides (12).
Far from being an unwanted side product, the naphtho-
quinone 7 is desirable as a potential dienophile for the sub-
sequent preparation of unsymmetrical anthraquinones. In an
attempt to bias the reaction towards the formation of this
compound, the condensation of 4 with benzoquinone was
carried out in refluxing toluene, which has previously been
found to favor formation of monoadducts (12, 28). Once
again, catalytic p-toluenesulfonic acid was added to activate
4 towards dehydration. Under these conditions, the forma-
tion of the cyclopentadienone intermediate 5 is considerably
slower than when the reaction was carried out in acetic acid.
After 2 days, no appreciable reaction of the starting materi-
als was observed. Allowing this reaction to proceed for
6 days led to only 25% of compound 4 being consumed, al-
though it was gratifying to note that the ratio of naphtho-
quinone to anthraquinone obtained had increased to 20:1.
Addition of a small amount of acetic acid to this reaction
4-Hydroxy-4-(p-methoxyphenyl)-3-phenylcyclopent-2-enone
(21b)
4-Hydroxy-4-(p-methoxyphenyl)-3-phenylcyclopent-2-enone
(21b) was prepared in the manner described previously and
isolated via column chromatography (silica, toluene–EtOAc,
1
7:3 as eluent) as a yellow solid. H NMR (ppm, CDCl3) δ:
7.52 (dt, 2H, J = 7.1, 1.3 Hz), 7.37–7.35 (m, 3H), 7.30 (td,
2H, J = 7.4, 1.5 Hz), 6.87 (dt, 2H J = 8.8, 2.5 Hz), 6.66 (s,
1H), 3.79 (s, 3H), 3.00 (d, 1H, J = 18.5 Hz), 2.88 (d, 1H, J =
18.5 Hz), 2.65 (br, 1H).
3-(4-Bromophenyl)-4-hydroxy-4-(4-methoxyphenyl)cyclo-
pent-2-enone and 4-(4-bromophenyl)-4-hydroxy-3-(4-
methoxyphenyl)cyclopent-2-enone (23)
3-(4-Bromophenyl)-4-hydroxy-4-(4-methoxyphenyl)cyclo-
pent-2-enone and 4-(4-bromophenyl)-4-hydroxy-3-(4-meth-
oxyphenyl)cyclopent-2-enone (23) were prepared in the
manner described previously as a yellow solid. No attempt
1
was made to separate these isomers. H NMR (ppm, CDCl3)
δ: 7.52–7.43 (m, 4H), 7.34–7.32 (m, 2H), 6.86 (d, 2H, J =
9.0 Hz), 6.82 (d, 2H, J = 8.9 Hz), 6.66 (s, 1H), 6.64 (s, 1H),
3.79 (s, 3H), 2.97 (d, 1H, J = 18.6 Hz), 2.83 (d, 1H, J =
18.5 Hz).
Results and discussion
Our initial studies focused on the construction of 2,3-
diphenylanthraquinone (3) starting from benzil, which has
the advantage of being an inexpensive, commercially avail-
able starting material. 4-Hydroxy-3,4-diphenylcyclopent-2-
enone (4) was synthesized in 96% yield by the condensation
of benzil and acetone in the presence of catalytic potassium
hydroxide (27). Heating 4 in refluxing acetic acid for 48 h
caused no appreciable decomposition of this compound.
When this reaction was carried out in the presence of cata-
lytic p-toluenesulfonic acid, however, compound 6 was ob-
tained almost quantitatively. The formation of this product,
which arises from the dimerization of the highly reactive in-
© 2006 NRC Canada

664
Can. J. Chem. Vol. 84, 2006
Scheme 4. Reagents and conditions: (a) NaCN or KCN, EtOH–H2O, reflux; (b) acetone, KOH, RT, 1 week; (c) 1,4-naphthoquinone,
AcOH, TsOH, reflux 48 h.
O
O
O
a
c
b
Ar
8-10
Ar
Ar
H
OH
Ar
16 Ar = 2-naphthyl
17 Ar = o-CH3C6H4
18
O
Ar
13 Ar = 2-naphthyl
14 Ar = o-CH3C6H4
15
Ar = p-CH3OC6H4
-
-
Ar = p-CH3OC6H4
Fig. 1. syn–anti Isomerization of compound 10.
O
O
O
O
O
O
mixture greatly increased the rate; under these conditions,
40% of 4 had reacted after 2 days, but the ratio of
naphthoquinone–anthraquinone dropped to 5:1.
for 4, although both electronic and steric effects may have
been in part responsible.
In the course of characterizing the tolyl derivative 9, it
1
The success of the approach described above prompted us
to attempt the preparation of other 2,3-disubstituted anthra-
quinones. Since electronic and steric effects play important
roles in the photochemistry of anthracene derivatives (11,
29), we were interested in synthesizing a range of anthra-
quinones with different pendant aromatic rings. The
naphthyl- and o-tolyl-substituted derivatives 8 and 9 were
therefore targeted as more sterically congested analogs of 3.
On the other hand, the introduction of para-substituents onto
the peripheral aromatic rings in compounds 10–12 should
not appreciably alter lateral bulk of the phenyl rings, but will
change electronic properties of these groups. More specifi-
cally, methoxy- and bromo-substituents were chosen because
they also provide useful points of attachment for the even-
tual incorporation of these compounds into polymers or
other structures.
The symmetrical 1,2-diaryl-1,2-ethanediones (13–15)
were synthesized from 2-naphthaldehyde, o-tolualdehyde,
and p-anisaldehyde, respectively, using slightly modified lit-
erature procedures (22–24). Like benzil, these compounds
are readily converted to the corresponding hydroxycyclo-
pent-2-enones (16–18). These hydroxy compounds in gen-
eral proved difficult to purify and were therefore used as
obtained in subsequent reactions. Condensation of these
compounds with 1,4-naphthoquinone afforded the desired
products 8–10 in isolated yields that varied from 7%–18%
(Scheme 4). It is unclear at this time why the reactions in-
volving these substituted derivatives were much lower than
was noted that several peaks in both the H and 13C NMR
spectra of this compound were appreciably broadened. The
affected peaks were identified as those associated with the
pendant tolyl groups, suggesting that these rings are under-
going conformational syn-/anti-isomerism on the NMR
timescale (Fig. 1). This is consistent with the observation
that a similar molecule (2,2′-dimethyl-o-terphenyl) has a
barrier to isomerization of 62 kJ/mol, with an NMR coales-
cence temperature of 9 °C (30). Although quantitative vari-
able-temperature NMR experiments have not yet been
carried out on 9, the tolyl peaks were observed to sharpen at
elevated temperatures, as expected.
This general synthetic approach can also be employed in
the preparation of lower symmetry anthraquinones such as
11 and 12, which were derived from the corresponding un-
symmetrical benzils 20 and 22, respectively (Schemes 5 and
6). Two routes were explored for the preparation of benzil
20. This compound can be obtained via the cross-benzoin
condensation of benzaldehyde and anisaldehyde (31). Al-
though this synthesis has the advantage of employing rela-
tively inexpensive starting materials, it does yield a mixture
of benzil and compound 20 in an approximately 1:3 ratio,
with 20 being isolated in only a 22% yield. A more system-
atic synthesis of this compound proceeds via the oxidation
of the diphenylacetylene derivative (19), which is easily pre-
pared from phenylacetylene and p-iodoanisole using stan-
dard Hagihara–Sonagashira conditions. Conversion of this
compound to 20 has previously been reported using catalytic
OCH3
O
O
O
O
O
O
10 X = OCH3
X
8
9
11
12
X = H
X = Br
© 2006 NRC Canada

Bailey et al.
665
Scheme 5. Reagents and conditions: (a) NaCN or KCN, EtOH–H2O, reflux; (b) CuSO4, pyridine–H2O; (c) Pd(PPh3)4, CuI, (i-Pr)2NH,
THF, reflux 48 h; (d) I2, DMSO, 145 °C; (e) acetone, KOH, RT, 1 week; (f) 1,4-naphthoquinone, AcOH, TsOH, reflux 48 h.
OCH3
O
OCH3
H
OCH3
O
a, b
d
c
H3CO
+
+
O
O
20
19
I
H
e
O
O
f
+
11
OH
OH
H3CO
OCH3
21a
21b
Scheme 6. Reagents and conditions: (a) NaCN or KCN, EtOH–
H2O, reflux; (b) CuSO4, pyridine–H2O; (c) I2, DMSO, 145 °C;
(d) acetone, KOH, RT, 1 week; (e) 1,4-naphthoquinone, AcOH,
TsOH, reflux 48 h.
tify the two isomers, NOESY spectra were obtained for both
compounds. The spectrum of the major product showed a
strong NOE between the protons on the anisole ring and the
vinylic proton on the cyclopentene ring, which led to the
identification of this compound as 21a. Conversely, a signif-
icant cross peak was observed between the phenyl ring and
the vinylic proton in the NOESY spectrum of the minor
product, confirming the identity of this compound as the
second regioisomer 21b. The formation of 21a as the major
product is consistent with the stabilization of the incipient
positive charge on the cyclopentane ring by the electron-
donating p-methoxy group during the elimination of water.
From a practical standpoint, the formation of two isomers is
not problematic, since both 21a and 21b yield the same
cyclopentiadienone upon loss of water. These isomeric
hydroxycyclopent-2-enones were therefore not separated
prior to carrying out Diels–Alder reactions with naphtho-
quinone derivatives in subsequent experiments. Condensa-
O
OCH3
H
O
a, b
H3CO
+
O
O
22
Br
H
Br
c
O
d
12
OH
tion of
a mixture of these intermediates with 1,4-
naphthoquinone afforded 11 in 18% yield.
X
In a similar manner, the brominated anthraquinone (12)
was prepared from p-bromobenzaldehyde and anisaldehyde
(Scheme 6). The two isomers of the cyclone precursor, col-
lectively denoted as 23, were not separated from one another
prior to condensation with 1,4-naphthoquinone.
Y
23
X=Br ;Y=OCH3
and X=OCH3 ,Y=Br
It is also possible to use these hydroxycyclopent-2-enone
intermediates in the synthesis of 2,3,6,7-tetraarylanthra-
quinones bearing different aromatic rings at one or more of
the sites. To demonstrate the feasibility of this approach, we
condensed 6,7-diphenyl-1,4-naphthoquione (7) with both
21a, 21b, and 18. In this manner, we were able to prepare
compounds 24 and 25 in yields of 26% and 7%, respectively.
In conclusion, we have developed a new strategy for syn-
thesizing highly substituted anthraquinone derivatives from
in situ generated cyclopentadienones. This approach is com-
plementary to other methods that have commonly been used
to prepare anthraquinones, such as the Friedel–Crafts con-
palladium chloride in DMSO (32). We have found that, con-
trary to this previous report, 19 was also cleanly oxidized to
20 in the presence of I2/DMSO. The overall yields for this
alkyne route are considerably higher (59% vs. 22%) than for
the cross-benzoin method, although the use of more costly
reagents is required.
Condensation of the unsymmetrical benzil 20 with ace-
tone yields two products that were formed in a 5:2 ratio.
These products were readily separated by column chroma-
tography and were identified as the two regioisomers of the
hydroxycyclopent-2-enones 21a and 21b. To positively iden-
© 2006 NRC Canada

666
Can. J. Chem. Vol. 84, 2006
OCH3
4. V. Prasad, K. Krishnan, and V.S.K. Balagurusamy. Liq. Cryst.
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24
X = H
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9. M.A. Tius, J. Gomez-Galeno, X.Q. Gu, and J.H. Zaidi. J. Am.
densation of phthalic anhydrides with benzene derivatives,
an approach that is incompatible with functional groups such
as pendant aromatic rings. Although the yields of the con-
densation reactions reported herein were generally low, our
approach nonetheless offers several advantages. These com-
pounds were prepared in only four steps from inexpensive
starting materials such as acetone, 1,4-naphthoquinone, and
commercially available aldehydes. Significantly, these syn-
theses are also amenable to working at larger scales. The
hydroxycyclopent-2-enone precursors reported herein are
readily prepared in multigram quantities, and gram-scale re-
actions of naphthoquinone with these compounds have been
routinely carried out in our laboratory. In contrast, the
sulfone 2 is synthesized using much more expensive starting
materials and catalysts and could not, in our hands, be pre-
pared on large scales. Moreover, the use of cyclopenta-
dienones lends itself to the formation of lower-symmetry
derivatives, such as 10–12, 24, and 25, which would be
more difficult to prepare from the corresponding sulfones.
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scale preparation of anthraquinones that are not otherwise
readily accessible. Further work is being carried out in our
laboratory in an ongoing attempt to optimize these reactions.
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