One-pot synthesis of various xanthene derivatives using ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate as an efficient and reusable catalyst under solvent-free conditions

Article abstract of DOI:10.1016/j.cclet.2013.11.016

In this study, 1,3-disulfonic acid imidazolium hydrogen sulfate (DSIMHS) is used as an efficient and reusable ionic liquid for the green, mild, and efficient synthesis of xanthenes under solvent-free conditions. Simple and easy work-up, low cost, green pr

Full text of DOI:10.1016/j.cclet.2013.11.016

Chinese Chemical Letters 25 (2014) 341–347
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
One-pot synthesis of various xanthene derivatives using ionic liquid
1,3-disulfonic acid imidazolium hydrogen sulfate as an efficient and
reusable catalyst under solvent-free conditions
*
*
Farhad Shirini , Asieh Yahyazadeh , Kamal Mohammadi
Department of Chemistry, College of Science, University of Guilan, Rasht 41335, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, 1,3-disulfonic acid imidazolium hydrogen sulfate (DSIMHS) is used as an efficient and
reusable ionic liquid for the green, mild, and efficient synthesis of xanthenes under solvent-free
conditions. Simple and easy work-up, low cost, green process, short reaction times and excellent yields of
the products are the advantages of this procedure. Further, the catalyst can be recycled and reused at
least for four times without a noticeably decrease in its catalytic activity.
Received 8 August 2013
Received in revised form 10 October 2013
Accepted 23 October 2013
Available online 20 November 2013
ß 2013 Farhad Shirini. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Keywords:
Xanthene derivatives
One-pot reaction
1,3-Disulfonic acid imidazolium hydrogen
sulfate
Ionic liquid
Solvent-free conditions
1. Introduction
There has been a renewed and growing interest in MCRs as
notable synthetic tools for the synthesis of structurally diverse
The principles of green chemistry have been introduced to
eliminate or reduce the use or generation of hazardous materials in
chemical processes. One of the key areas of green chemistry is the
replacement of hazardous solvents with environmentally benign
ones or the elimination of solvents altogether [1–3]. One strategy is
the use of ionic liquids (ILs) that are defined as salts in liquid form
at or below 100 8C. ILs have gained much attention as ‘‘designer
solvents and/or catalysts’’ for a diversity of chemical applications
due to their special properties such as high polarity, good solvating
capability, wide liquid range, high thermal stability, negligible
vapour pressure, great selectivity, ease of isolation and reusability
[4–6]. Among the different kinds of ILs, Brønsted acidic ionic
liquids, which have a functional group such as –SO3H in their
framework, have been designed as green substitute for solid acids
and traditional mineral liquid acids such as sulfuric acid and
hydrochloric acid in chemical processes [7–11].
complex molecules, such as novel biologically and pharmaceuti-
cally active compounds. MCRs offer several attractive features to
the chemists, including access to a large number of novel and
diverse structures, low production and environmental costs due to
high convergence, atom economy, high selectivity and simple
purification [13–16].
Xanthenes and their derivatives have received special attention
due to their wide range of biological and pharmaceutical activities
such as antibacterial [17], antiviral [18] and antitumor effects [19].
In addition, they can be employed as dyes [20], pH sensitive
fluorescent materials for visualization of biomolecules [21] and
utilized in laser technology [22]. Thus, the synthesis of xanthene
derivatives currently is of much importance.
Various approaches have been reported for the synthesis of
xanthene derivatives in the literature [23–26]. One of the most
simple and general methods for the synthesis of this type of
In recent years, considerable efforts from both academic and
industrial researchers have been focused especially on the design
and development of multi-component reactions (MCRs) for the
generation of libraries of heterocyclic compounds [12].
compounds involves a one-pot MCR of aldehydes (1 equiv.) with b-
naphthol (1 or 2 equiv.) and dimedone (1 or 2 equiv.), in the
presence of an acidic or other type of catalysts. A variety of
catalysts have been applied for this reaction, such as ferric
hydrogen sulfate [27], proline triflate [28], NaHSO4ÁSiO2 [29],
strontium triflate [30], Zr(HSO4)4 [31], RuCl4 [32], P2O5/Al2O3 [33],
BF3ÁSiO2 [34], oxalic acid [35], (DBH)/kaolin [36], ZnO nanopar-
ticles [37], molecular iodine [38,39], heteropoly acid [40,41],
silica sulfuric acid [42,43], amberlyst-15 [44], nano-TiO2 [45],
*
Corresponding authors.
E-mail addresses: shirini@guilan.ac.ir (F. Shirini), yahyazadehphd@yahoo.com
(A. Yahyazadeh).
1001-8417/$ – see front matter ß 2013 Farhad Shirini. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.11.016

342
F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
KAl(SO4)2Á12H2O (alum) [46], NaHSO4/ionic liquid ([bmim]BF4)
[47], montmorillonite K-10 [48], p-TSA [49], cyanuric chloride [50],
H4[SiW12O40] [51], Yb(OTf)3 [52] and SuSA [53]. However, many of
these procedures suffer from one or more disadvantages including
the use of toxic metals and volatile organic solvents, low yields,
long reaction times, tedious work-up procedures, expensive
reagents, high catalyst loading, high temperature and hash
reaction conditions.
To avoid these drawbacks and develop useful synthetic
methodologies, herein, we wish to report a simple, green and
efficient method for the synthesis of xanthene derivatives using
DSIMHS as an eco-friendly catalyst with high catalytic activity
under solvent-free conditions. To the best of our knowledge, this
methodology has not been reported in the literature.
1110, 810, 740; 1H NMR (CDCl3, 300 MHz):
d 2.11 (s, 3H), 6.43 (s,
1H), 6.93 (d, 2H, J = 7.8 Hz), 7.22–7.36 (m, 8H), 7.42–7.81 (m, 4H),
8.37 (d, 2H, J = 8.4 Hz).
12-(4-Isoprpylphenyl)-9,9-dimethyl-8,9,10,12 tetrahydroben-
zo[a]xanthen-11-one(Table 4, entry 12): IR (KBr, cmÀ1):
n
3050,
2950, 2900, 2870, 1642, 1618, 1590, 1504, 1460, 1368, 1220, 1140,
1020, 830, 815, 740; 1H NMR (CDCl3, 400 MHz):
1.01 (s, 3H), 1.14
d
(s, 6H), 1.16 (s, 3H), 2.23 and 2.30 (AB system, 2H, J = 30 Hz), 2.48–
2.59 (d, 2H, J = 45 Hz), 2.77 (t, 1H, J = 6.8 Hz), 5.70 (s, 1H), 7.03 (d,
1H, J = 7.2 Hz), 7.19–7.46(m, 5H), 7.78 (t, 3H, J = 10 Hz), 8.05 (d, 1H,
J = 8 Hz).
12-(4-Cyanophenyl)-9,9-dimethyl-8,9,10,12-tetrahydroben-
zo[a]xanthen-11-one (Table 4, entry 13): IR (KBr, cmÀ1):
n
3070,
2956, 2931, 2869, 2220, 1651, 1618, 1594, 1515, 1460, 1370, 1220,
1175, 1140, 1020, 838, 802, 740; 1H NMR (CDCl3, 400 MHz):
0.97
d
2. Experimental
(s, 3H), 1.15 (s, 3H), 2.25 and 2.37 (AB system, 2H, J = 16 Hz), 2.61
(s, 2H), 5.78 (s, 1H), 7.37 (d, 1H, J = 8.8 Hz), 7.41–7.49 (m, 6H), 7.84
(t, 3H, J = 8.4 Hz).
All chemicals were purchased from Merck or Fluka Chemical
Companies. All yields refer to the isolated products. Products were
characterized by their physical constants and comparison with
authentic samples. Progress of the reactions was monitored by thin
layer chromatography (TLC) analyses using silica gel SIL G/UV 254
plates.
Melting points were recorded on an electrothermal digital
melting point apparatus model IA9100 in open capillary tubes. IR
spectra were recorded on a Perkin-Elmer model Spectrum One FT-
IR Spectrometer. The 1H NMR (300 or 400 MHz) was run on a
9-(2-Mehtoxyphenyl)-3,3,6,6-tetramethyl-1,8-dioxo-octahy-
droxanthene (Table 6, entry 12): IR (KBr, cmÀ1):
n
3010, 2950,
2870, 1650, 1620, 1590, 1490, 1460, 1360, 1250, 1020, 1200, 1000,
1160, 1140, 1118, 750; 1H NMR (CDCl3, 400 MHz):
0.97 (s, 6H),
d
1.11 (s, 6H), 2.14 and 2.23 (d, 2H, J = 6.4 Hz), 2.39 and 2.48 (d, 2H,
J = 17.4 Hz), 3.79 (s, 3H), 4.87 (s, 1H), 6.77 (d, 1H, J = 8 Hz), 6.89 (dt,
1H, J = 7.4 Hz, J = 1.2 Hz), 7.12 (m, 1H), 7.43 (dd, 1H, J = 7.4 Hz, J
1.6 Hz).
9-(4-Cyanophenyl)-3,3,6,6-tetramethyl-1,8-dioxo-octahydrox-
Bruker Avance DPX-250 FT-NMR spectrometer (
d
in ppm).
anthene (Table 6, entry 13): IR (KBr, cmÀ1):
n
3050, 2950, 2870,
2210, 1660, 1620, 1600, 1500, 1460, 1360, 1200, 1160, 1140, 1105,
850; 1H NMR (CDCl3, 400 MHz):
0.99 (s, 6H), 1.13 (s, 6H), 2.17 and
Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes: A mixture of
b
-naphthol (2 mmol), aldehyde (1 mmol) and DSIMHS
d
(0.25 mmol) was stirred and heated in an oil-bath at 90 8C for
an appropriate period of time. The progress of the reaction was
followed by TLC analyses. After the completion of the reaction, the
reaction mixture was cooled to room temperature, 10 mL of H2O
was added, stirred for 5 min and filtered to remove the catalyst.
DSIMHS is soluble in water and the product precipitated with high
purity. Then, 2 mL of hot EtOH was added to the resulting solid
product, stirred for 5 min, and filtered. Finally, the solid residue
was recrystallized from EtOH to give the pure product.
2.26 (d, 2H, J = 16.4 Hz), 2.5 (d, 2H, J = 1.2 Hz), 4.78 (s, 1H), 7.43 (dd,
2H, J = 9.6 Hz, J = 1.6 Hz), 7.54 (dd, 2H, J = 6.6 Hz, J = 1.6 Hz).
3. Results and discussion
In continuation of our ongoing research program on the
development of new applications of DSIMHS in organic synthesis
[54,55], we decided to study the preparation of 14-aryl-14H-
dibenzo[a,j]xanthenes from aldehydes and
presence of this catalyst (Scheme 1). For this purpose, the
condensation of -naphthol (2 mmol) with benzaldehyde
b-naphthol in the
Synthesis of 12-aryl-tetrahydrobenzo[a]xanthene-11-ones: A
mixture of
b-naphthol (1 mmol), dimedone (1 mmol), aldehyde
b
(1 mmol) and DSIMHS (0.25 mmol) was stirred and heated in an
oil-bath at 55 8C for an appropriate period of time. The reaction was
monitored by TLC analyses. After the completion of the reaction,
the reaction mixture was cooled to room temperature, 10 mL of
H2O was added, stirred for 5 min and filtered to remove the
catalyst. Then, 2 mL of hot EtOH was added to the resulting solid
product, stirred for 5 min, and filtered. Finally, the solid residue
was recrystallized from EtOH to give the pure product.
(1 mmol) was selected as a model reaction and the optimization
of the reaction conditions was performed using various amounts of
DSIMHS at different temperatures under solvent-free conditions.
The results are summarized in Table 1.
As it shown in Table 1, the best result was obtained by carrying
out the reaction using 0.25 mmol of DSIMHS at 90 8C (Table 1,
entry 5). Increasing the amount of the catalyst and the temperature
of the reaction did not improve the yields and reaction times. In
addition, results indicated that when the reaction proceeded at
room temperature in 240 min, the yield of the corresponding
product was low (Table 1, entry 1). The solvent-free reaction was
also tested at 90 8C in the absence of the catalyst and no significant
amount of product was observed after long reaction time (Table 1,
entry 2).
Synthesis of 1,8-dioxo-octahydroxanthenes:
A mixture of
dimedone (2 mmol), aldehyde (1 mmol) and DSIMHS (0.25 mmol)
was stirred and heated in an oil-bath at 55 8C for an appropriate
period of time. After the completion of the reaction, as monitored by
TLC analyses, the reaction mixture was cooled to room temperature,
10 mL of H2O was added, stirred for 5 min and filtered to remove the
catalyst. Then, 2 mL of hot EtOH was added to the resulting solid
product, stirred for 5 min, and filtered. Finally, the solid residue was
recrystallized from EtOH to give the pure product. Spectroscopic
data for the selected products are as follows:
In order to study the generality of this procedure, a variety of
aromatic aldehydes were applied under the optimal reaction
conditions. The results are shown in Table 2. The nature of the
14-(Phenyl)-14H-dibenzo[a,j]xanthene (Table 2, entry 1): IR
(KBr, cmÀ1):
n
3070, 3020, 1620, 1590, 1430, 1400, 1250, 1150,
R1
OH
DSIMHS
1075, 825, 740; 1H NMR (300 MHz, CDCl3):
d
6.46 (s, 1H), 6.96 (t,
+
2
RCHO
Solvent-free
90 o
1H, J = 7.2 Hz), 7.12 (t, 2H, J = 7.2 Hz), 7.36–7.58 (m, 8H), 7.74–7.81
(m, 4H), 8.37 (d, 2H, J = 8.4 Hz).
O
C
14-(4-Methylphenyl)-14H-dibenzo[a,j]xanthene (Table 2, en-
Scheme 1. Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes in the presence of
try 9): IR (KBr, cmÀ1):
n 3068, 3022, 1620, 1590, 1512, 1395, 1248,
DSIMHS under solvent-free conditions.

F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
343
Table 1
R2
O
O
O
OH
O
DSIMHS
The effect of different amounts of DSIMHS and temperature on the synthesis of 14-
(phenyl)-14H-dibenzo[a,j]xanthene.a
RCHO +
+
Solvent-free
55 oC
Entry
Catalyst (mmol)
Temperature (8C)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
0.25
–
25
90
240
360
10
3
25
18
80
92
94
85
94
Scheme 2. Three-component synthesis of 12-aryl-tetrahydrobenzo[a]xanthene-
11-ones in the presence of DSIMHS under solvent-free conditions.
0.15
0.25
0.25
0.25
0.35
90
100
90
3
O
R3
O
O
80
8
90
3
DSIMHS
+
2
a
RCHO
Solvent-free
55 oC
Reaction conditions:
b-naphthol (2 mmol) and benzaldehyde (1 mmol) under
O
solvent-free conditions.
b
O
Isolated yield.
Scheme 3. Ionic liquid catalyzed synthesis of 1,8-dioxo-octahydroxanthenes in the
presence of DSIMHS under solvent-free conditions.
functional group on the aromatic ring of the aldehyde exerted a
strong influence on the reaction time. It could be concluded that
the aldehydes substituted with electron-withdrawing groups
reacted very well and produced the corresponding xanthenes in
good to excellent yields in shorter times than aldehydes with
electron-donating groups.
After the successful synthesis of a series of 14-aryl-14H-
dibenzo[a,j]xanthenes, we turned our attention toward the
synthesis of 12-aryl-tetrahydrobenzo[a]xanthene-11-ones, in the
presence of DSIMHS (Scheme 2). In order to determine the optimal
Table 3
The effect of different amounts of DSIMHS and temperature on the synthesis of 12-
phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one.a
Entry
Catalyst (mmol)
Temperature (8C)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
0.25
–
25
55
55
65
55
45
55
240
360
35
22
14
87
92
93
84
93
0.15
0.25
0.25
0.25
0.35
20
20
40
conditions, we investigated the MCR of b-naphthol (1 mmol) with
20
dimedone (1 mmol) and benzaldehyde (1 mmol) in the presence of
various amounts of the catalyst, at different temperatures ranging
from 25 8C to 65 8C in the absence of solvent (Table 3).
According to Table 3, we found that 0.25 mmol of DSIMHS
was adequate to accomplish the reaction effectively at 55 8C
(Table 3, entry 4). Using smaller amounts of the catalyst resulted
in lower yields, while higher amounts of the catalyst did not
affect the reaction times and yields and in the absence of the
catalyst, no appreciable product could be detected. Moreover,
when the same reaction was performed at 25 8C, the corre-
sponding product was obtained in very low yield after prolonged
reaction time.
To generalize the suitability of this protocol, the reaction was
extended to different substituted aromatic aldehydes to prepare a
series of 12-aryl-tetrahydrobenzo[a]xanthene-11-ones. In all
cases, aldehydes bearing electron-donating groups needed slightly
longer time to complete the reaction as compared to aldehydes
containing halogens or other electron-withdrawing groups. The
results are summarized in Table 4.
a
Reaction conditions: b-naphthol (1 mmol), dimedone (1 mmol) and benzalde-
hyde (1 mmol) under solvent-free conditions.
b
Isolated yield.
reaction between dimedone (2 mmol) and benzaldehyde (1 mmol)
was chosen as a model reaction using different amounts of the
catalyst, at various temperatures under solvent-free conditions
(Scheme 3). As can be seen from Table 5, the shortest time and best
yield was achieved using 0.25 mmol of the reagent at 55 8C. It is
interesting to note that the yield of the reaction was not
significantly affected by increasing the amount of the catalyst
and temperature. Also, in the absence of the catalyst or at room
temperature, only a trace amount of the product was obtained
indicating that the catalyst and elevated temperature are
necessary for the reaction.
After the optimization of the reaction, we extended the
condensation of dimedone with various aromatic aldehydes to
further explore the scope and limitations of this reaction. The
results showed that the reaction proceeds very efficiently and in all
Finally, we evaluated the catalytic activity of DSIMHS in the
synthesis of 1,8-dioxo-octahydroxanthenes. In this regard, the
Table 2
DSIMHS catalyzed synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes.a
Entry
R
Time (min)
Yield (%)b
Mp (8C)
Ref.
Found
Reported
1
2
C6H5
3
9
8
4
2
5
3
2
8
5
4
6
2
94
91
90
88
95
93
95
92
88
91
94
90
94
184–185
204–205
133–135
213–215
290–292
192–194
297–298
239–240
227–228
213–215
312–314
154–156
292–294
183–185
204–207
135–136
211–213
289–291
190–192
298–300
241–243
226–228
211–213
312–314
153–155
293–295
[56]
[56]
[57]
[56]
[56]
[58]
[56]
[58]
[56]
[56]
[56]
[59]
[56]
4-OMeC6H4
4-OHC6H4
2-ClC6H4
3
4
5
4-ClC6H4
6
3-BrC6H4
4-BrC6H4
4-FC6H4
7
8
9
4-CH3C6H4
3-O2NC6H4
4-O2NC6H4
4-(CH3)2CHC6H4
4-CNC6H5
10
11
12
13
a
The products were characterized by the used spectroscopic data and melting point. These data closely matched with the reported data.
Isolated yield.
b

344
F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
Table 4
DSIMHS catalyzed synthesis of 12-aryl-tetrahydrobenzo[a]xanthene-11-ones.a
Entry
R
Time (min)
Yield (%)b
Mp (8C)
Ref.
Found
Reported
1
2
C6H5
20
35
35
20
12
22
16
14
30
15
14
35
10
93
90
91
91
93
90
93
91
87
89
92
90
92
148–150
202–204
222–223
181–183
176–178
161–163
184–186
193–195
171–173
168–170
174–176
152–154
196–198
148–151
201–203
223–225
180–181
178–179
161–164
185–187
191–193
170–172
167–170
175–178
154–155
196–199
[60]
[60]
[61]
[58]
[60]
[60]
[60]
[62]
[63]
[60]
[60]
[63]
[63]
4-OMeC6H4
4-OHC6H4
2-ClC6H4
3
4
5
4-ClC6H4
6
3-BrC6H4
4-BrC6H4
4-FC6H4
7
8
9
4-CH3C6H4
3-O2NC6H4
4-O2NC6H4
4-(CH3)2CHC6H4
4-CNC6H5
10
11
12
13
a
The products were characterized by the used spectroscopic data and melting point. These data closely matched with the reported data.
Isolated yield.
b
Table 5
The effect of different amounts of DSIMHS and temperature on the synthesis of 9-(phenyl)-3,3,6,6-tetramethyl-1,8-dioxo-octahydroxanthenes.a
Entry
Catalyst (mmol)
Temperature (8C)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
0.25
–
25
55
55
65
55
45
55
240
360
10
3
30
15
80
93
95
80
95
0.15
0.25
0.25
0.25
0.35
3
8
3
a
Reaction conditions: dimedone (2 mmol) and benzaldehyde (1 mmol) under solvent-free conditions.
Isolated yield.
b
Table 6
DSIMHS catalyzed synthesis of 1,8-dioxo-octahydroxanthenes.a
Entry
R
Time (min)
Yield (%)b
Mp (8C)
Ref.
Found
Reported
1
2
C6H5
4
10
10
6
95
93
90
89
94
91
92
93
89
90
93
86
91
206–208
239–241
245–247
227–228
231–233
189–191
240–242
221–223
211–213
165–167
222–224
205–207
226–228
205–206
241–243
246–248
226–228
230–232
190–192
240–241
223–224
216–218
167–168
221–223
199–200
220–222
[64]
[64]
[64]
[64]
[65]
[65]
[65]
[65]
[66]
[64]
[64]
[67]
[66]
4-OMeC6H4
4-OHC6H4
2-ClC6H4
4-ClC6H4
3-BrC6H4
4-BrC6H4
4-FC6H4
3
4
5
4
6
6
7
3
8
3
9
4-CH3C6H4
3-O2NC6H4
4-O2NC6H4
2-OMeC6H4
4-CNC6H5
12
8
10
11
12
13
5
14
5
a
The products were characterized by the used spectroscopic data and melting point. These data closely matched with the reported data.
Isolated yield.
b
Table 7
Comparison of the efficiency of various ILs in synthesis of 14-(phenyl)-14H-dibenzo[a,j]xanthene.
Entry
IL
Conditions
Time (min)
Yield (%)a
Ref.
1
2
3
4
5
6
7
DSIMHS (0.25 mmol)
[Dsim]Cl (10 mol%)
Solvent-free, 90 8C
Solvent-free, 110 8C
Solvent-free, 110 8C
Solvent-free, 110 8C
Water, 100 8C
3
5
94
90
85
88
93
96
85
This work
[68]
[Msim]PF6 (10 mol%)
[Msim]BF4 (10 mol%)
[MIMPS]HSO4 (0.25 mmol)
[Et3N-SO3H]Cl (15 mol%)
[Hmim]HSO4 (12 mol %)
7
[68]
8
[68]
7
[69]
Solvent-free, 120 8C
Solvent-free, 125 8C
30
90
[66]
[56]
a
Yields refer to isolated pure products.
cases, aromatic aldehydes substituted with either electron-
donating or electron-withdrawing groups underwent the reaction
successfully and gave the desired products in good to excellent
yields (Table 6).
In Scheme 4, a plausible mechanism has been offered for the
synthesis of xanthene derivatives in the presence of DSIMHS under
solvent-free conditions. On the basis of this mechanism, DSIMHS
donates the proton to the oxygen atom of the aldehyde and

F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
345
Fig. 1. Reusability of DSIMHS in the synthesis of (a) 14-(phenyl)-14H-dibenzo[a,j]xanthenes, (b) 12-phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one and
(c) 9-(phenyl)-3,3,6,6-tetramethyl-1,8-dioxo-octahydroxanthenes.
(A)
O
O
(B)
H
H
O
O
O
H
R
N
O
H
(I)
O
O
S
N
HSO
4
O
O
SO H
3
O
R
O
R
O
R
(A)
R
(A)
-H O
-H O
2
2
OH
O
OH
-H O
2
(B)
(B)
-H O
2
O
R
O
R
O
O
Scheme 4. Proposed mechanism for the synthesis of xanthene derivatives in the presence of DSIMHS under solvent-free conditions.
Table 8
Comparison of the efficiency of various ILs in synthesis of 12-(4-chlorophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one.
a
Entry
IL
Conditions
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
DSIMHS (0.25 mmol)
Solvent-free, 90 8C
MW oven at 240 W
Solvent-free, 80 8C
Solvent-free, 80 8C
Solvent-free, 100 8C
Solvent-free, 120 8C
12
10
93
84
98
85
87
89
This work
[70]
[BSMIM]Ts (5 mol%)
[(4-sulfobutyl)tris(4-sulfophenyl)phosphonium-hydrogen sulfate (10 mol%)
15
[71]
[bmim]BF4 (0.5 mL)
120
30
[72]
Py(HSO4)2 (10 mol%)
[Et3N-SO3H]Cl (25 mol%)
[73]
40
[74]
a
Yields refer to isolated pure products.
activates it. Then, nucleophilic
b
-naphthol (A) or dimedone (B)
cyclization with the participation of two hydroxyl groups to afford
the xanthene derivatives (Scheme 4).
It is noteworthy to mention that the catalyst is recyclable and
could be reused without significant loss of activity. Hence, we
decided to examine the reusability and recycling performance of
attacks the carbonyl group of the activated aldehyde and by
removing H2O, the Knoevenagel products (I or II) is generated. The
following addition of these intermediates to A or B, gives the
acyclic adduct intermediate, which undergoes an intramolecular
Table 9
Comparison of the efficiency of various ILs in synthesis of 9-(4-bromophenyl)-3,3,6,6-tetramethyl-1,8-dioxo-octahydroxanthenes under solvent-free conditions.
a
Entry
IL
Temperature (8C)
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
DSIMHS (0.25 mmol)
[Dsim]Cl (10 mol%)
90
70
3
4
92
96
91
95
90
91
94
This work
[68]
[Msim]PF6 (10 mol%)
[Msim]BF4 (10 mol%)
[bmim]HSO4 (100 mg)
[Hmim]TFA (100 mg)
[Et3NH][H2PO4] (400 mg)
70
6
[68]
70
8
[68]
80
210
180
5
[75]
80
[76]
100
[77]
a
Yields refer to isolated pure products.

346
F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
[15] A. Hasaninejad, A. Zare, M. Shekouhy, Highly efficient synthesis of triazolo[1, 2-
a]indazole-triones and novel spiro triazolo[1,2-a]indazole-tetraones under sol-
vent-free conditions, Tetrahedron 67 (2011) 390–400.
DSIMHS for the synthesis of xanthene derivatives. For this purpose,
all the water added for filtering and washing the product was
collected and washed with CH2Cl2 (2 Â 10 mL) to remove organic
impurities. Then, water was evaporated and the extracted catalyst
was dried. It was found that the recovered catalyst could be reused
directly in the next run without any loss of its activity even after
the fourth run (Fig. 1).
As shown in Tables 7–9 the performance of DSIMHS was
compared with the efficiency of other ILs reported in the literature.
It is clear from these data that DSIMHS can act as an efficient and
beneficial catalyst compared with the other mentioned reagents.
For example, in Table 7, amount of the catalyst, reaction times and
temperature in this work have been reduced relative to the other
ILs. In addition, Table 9 shows that the reaction can be carried out
using smaller amounts of DSIMHS in shorter reaction times
compared to the other catalysts.
[16] S. Stavber, Recent advances in the application of selectfluorTM F-TEDA-BF4 as a
versatile mediator or catalyst in organic synthesis, Molecules 16 (2011) 6432–
6464.
[17] J.P. Poupelin, G. Saint-Ruf, O. Foussard-Blanpin, et al., H4SiW12O40 catalyzed one-
pot synthesis of 12-aryl-8, 9,10,12-tetrahydrobenzo[a] xanthen-11-ones under
solvent-free conditions, Eur. J. Med. Chem. 13 (1978) 67–71.
[18] J.M. Jamison, K. Krabill, A. Hatwalkar, E. Jamison, C. Tsai, Potentiation of the
antiviral activity of poly r(A-U) by xanthene dyes, Cell. Biol. Int. Rep. 14 (1990)
1075–1084.
[19] G.W. Rewcastle, G.J. Atwell, L. Zhuang, B.C. Baguley, W.A. Denny, Potential
antitumor agents. 61. Structure-activity relationships for in vivo colon 38 activity
among disubstituted 9-oxo-9H-xanthene-4-acetic acids, J. Med. Chem. 34 (1991)
217–222.
[20] A.T. Peters, M.J. Bide, Amino derivatives of 1,8-naphthalic anhydride and derived
dyes for synthetic-polymer fibres, Dyes Pigm. 6 (1985) 349–375.
[21] C.G. Knight, T. Stephens, Xanthene-dye-labelled phosphatidylethanolamines as
probes of interfacial pH. Studies in phospholipid vesicles, Biochem. J. 258 (1989)
683–687.
[22] M. Ahmad, T.A. King, D.K. Ko, B.H. Cha, J. Lee, Performance and photostability of
xanthene and pyrromethene laser dyes in sol–gel phases, J. Phys. D: Appl. Phys. 35
(2002) 1473–1476.
4. Conclusion
[23] A. Bekaert, J. Andrieux, M. Plat, New total synthesis of bikaverin, Tetrahedron Lett.
33 (1992) 2805–2806.
[24] D.W. Knight, P.B. Little, The first efficient method for the intramolecular trapping
of benzynes by phenols: a new approach to xanthenes, J. Chem. Soc. Perkin Trans.
1 15 (2001) 1771–1777.
[25] C.W. Kuo, J.M. Fang, Synthesis of xanthenes, indanes, and tetrahydronaphthalenes
via intramolecular phenyl–carbonyl coupling reactions, Synth. Commun. 31
(2001) 877–892.
[26] J.Q. Wang, R.G. Harvey, Synthesis of polycyclic xanthenes and furans via palladi-
um-catalyzed cyclization of polycyclic aryltriflate esters, Tetrahedron 58 (2002)
5927–5931.
[27] H.R. Shaterian, A. Hosseinian, M. Ghashang, Ferric hydrogen sulfate as an efficient
heterogeneous catalyst for environmentally friendly greener synthesis of 1,8-
dioxo-octahydroxanthenes, Turk. J. Chem. 33 (2009) 233–240.
[28] J. Li, L. Lu, W. Su, A new strategy for the synthesis of benzoxanthenes catalyzed by
proline triflate in water, Tetrahedron Lett. 51 (2010) 2434–2437.
[29] B. Das, K. Laxminarayana, M. Krishnaiah, Y. Srinivas, An efficient and convenient
protocol for the synthesis of novel 12-aryl- or 12-alkyl-8,9,10,12-tetrahydroben-
zo[a]xanthen-11-one derivatives, Synlett (2007) 3107–3112.
In conclusion, herein we described DSIMHS as an inexpen-
sive, easily available, efficient, reusable and green catalyst
for the synthesis of xanthenes in a simple one-pot protocol
under solvent-free conditions with excellent yields. The
obtained results show that the catalytic activity of DSIMHS is
convincingly superior to other recently reported catalysts. In
addition, other advantages of this methodology such as short
reaction times, simplicity of operation and easy work-up, high
reaction rates, lack of side reactions, ease of preparation and
handling of the catalyst and simple experimental procedure
makes it attractive for organic chemists. Further work to explore
this novel catalyst in other organic transformations is in
progress.
[30] J. Li, W. Tang, L. Lu, W. Su, Strontium triflate catalyzed one-pot condensation of b-
naphthol, aldehydes and cyclic 1,3-dicarbonyl compounds, Tetrahedron Lett. 49
(2008) 7117–7120.
Acknowledgment
[31] N. Foroughifar, A. Mobinikhaledi, H. Moghanian, A catalytic and green procedure
for synthesis of 12-aryl- or 12-alkyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-
one derivatives under solvent-free conditions, Int. J. Green Nanotechnol. Phys.
Chem. 1 (2009) 57–63.
The authors are thankful to the University of Guilan Research
Council for the partial support of this work.
[32] K. Tabatabaeian, A. Khorshidi, M. Mamaghani, A. Dadashi, M.K. Jalali, One-pot
synthesis of tetrahydrobenzo[a]xanthen-11-one derivatives catalyzed by rutheni-
um chloride hydrate as a homogeneous catalyst, Can. J. Chem. 89 (2011) 623–627.
[33] A. Zarei, A.R. Hajipour, L. Khazdooz, The one-pot synthesis of 14-aryl or alkyl-14H-
dibenzo [a,j] xanthenes catalyzed by P2O5/Al2O3 under microwave irradiation,
Dyes Pigm. 85 (2010) 133–138.
[34] B.B. Mirjalili, A. Bamoniri, A. Akbari, BF3ÁSiO2: an efficient alternative for the
synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]xanthenes, Tetrahedron Lett. 49
(2008) 6454–6456.
References
[1] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice, Oxford Univer-
sity Press, Oxford, UK, 1998.
[2] M. Poliakoff, M.J. Fitzpatric, T.R. Farren, P.T. Anastas, Green chemistry: science and
politics of change, Science 297 (2002) 807–810.
[3] I.T. Horva´th, P.T. Anastas, Innovations and green chemistry, Chem. Rev. 107 (2007)
2169–2173.
[4] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis,
Chem. Rev. 99 (1999) 2071–2084.
[35] N.D. Kokare, J.N. Sangshetti, D.B. Shinde, Oxalic acid as a catalyst for efficient
synthesis of bis-(indolyl)methanes, and 14-aryl-14H-dibenzo[a,j]xanthenes in
water, Chin. Chem. Lett. 19 (2008) 1186–1189.
[5] T. Welton, Ionic liquids in catalysis, Coord. Chem. Rev. 248 (2004) 2459–
2477.
[36] F. Shirini, N.G. Khaligh, G.H. Imanzadeh, P.G. Ghasem-Abadi, 1,3-Dibromo-5,5-
dimethylhydantoin (DBH)/kaolin: an efficient reagent system for the synthesis of
14-aryl-14H-dibenzo[a,j]xanthenes under solvent-free conditions, Chin. Chem.
Lett. 23 (2012) 1145–1148.
[37] J. Safaei-Ghomi, M.A. Ghasemzadeh, Zinc oxide nanoparticles: a highly efficient
and readily recyclable catalyst for the synthesis of xanthenes, Chin. Chem. Lett. 23
(2012) 1225–1229.
[38] B. Das, B. Ravikanth, R. Ramu, K. Laxminarayana, B.V. Rao, Iodine catalyzed simple
and efficient synthesis of 14-aryl or alkyl-14-H-dibenzo[a,j]xanthenes, J. Mol.
Catal. A: Chem. 255 (2006) 74–77.
[39] A.P. Mohamed, P.J. Vaderapura, Molecular iodine catalyzed synthesis of aryl-14H-
dibenzo[a,j]xanthenes under solvent-free condition, Bioorg. Med. Chem. Lett. 17
(2007) 621–623.
[6] P. Wasserscheid, W. Keim, Ionic liquids-new ‘‘solutions’’ for transition metal
catalysis, Angew. Chem. Int. Ed. 39 (2000) 3772–3789.
[7] M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, 3-Methyl-1-sulfonic acid
imidazolium chloride as a new, efficient and recyclable catalyst and solvent for
the preparation of N-sulfonyl imines at room temperature, J. Iran. Chem. Soc. 7
(2010) 646–651.
[8] M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, Ionic liquid 3-methyl-1-
sulfonic acid imidazolium chloride as a novel and highly efficient catalyst for the
very rapid synthesis of bis(indolyl)methanes under solvent-free conditions, Org.
Prep. Proced. Int. 42 (2010) 95–102.
[9] A. Khazaei, M.A. Zolfigol, A.R. Moosavi-Zare, A. Zare, An efficient method for the
nitration of phenols with nano2 in the presence of 3-methyl-1-sulfonic acid
imidazolium, Sci. Iran. Trans. C: Chem. Chem. Eng. 17 (2010) 31–36.
[10] M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, V. Khakyzadeh, Rapid
synthesis of 1-amidoalkyl-2-naphthols over sulfonic acid functionalized imida-
zolium salts, Appl. Catal. A: Gen. 400 (2011) 70–81.
[40] M. Mohammadpour Amini, M. Seyyedhamzeh, A. Bazgir, Heteropolyacid: an
efficient and eco-friendly catalyst for the synthesis of 14-aryl-14H-dibenzo[a,j]-
xanthene, Appl. Catal. A: Gen. 323 (2007) 242–245.
[41] M.M. Heravi, Kh. Bakhtiari, Z. Daro@gheha, F.F. Bamoharram, Facile heteropolya-
cid-promoted synthesis of 14-substituted-14-H-dibenzo[a,j] xanthene deriva-
tives under solvent-free conditions, J. Mol. Catal. A: Chem. 273 (2007) 99–101.
[42] H.R. Shaterian, M. Ghashang, A. Hassankhani, One-pot synthesis of aryl 14H-
dibenzo[a,j]xanthene leuco-dye derivatives, Dyes Pigm. 76 (2008) 564–568.
[43] M. Seyyedhamzeh, P. Mirzaei, A. Bazgir, Solvent-free synthesis of aryl-14H-
dibenzo[a,j]xanthenes and 1,8-dioxo-octahydro-xanthenes using silica sulfuric
acid as catalyst, Dyes Pigm. 76 (2008) 836–839.
[11] S.L. Schreiber, Target-oriented and diversity-oriented organic synthesis in drug
discovery, Science 287 (2000) 1964–1969.
[12] A. Domling, Recent advances in isocyanide-based multicomponent chemistry,
Curr. Opin. Chem. Biol. 6 (2002) 306–313.
[13] J. Zhu, H. Bienayme, Multicomponent Reactions, Wiley-VCH, Weinheim,
Germany, 2005.
[14] V. Polshettiwar, R.S. Varma, Greener and expeditious synthesis of bioactive
heterocycles using microwave irradiation, Pure. Appl. Chem. 80 (2008) 777–790.

F. Shirini et al. / Chinese Chemical Letters 25 (2014) 341–347
347
[44] S. Ko, C.F. Yao, Heterogeneous catalyst: amberlyst-15 catalyzes the synthesis of
14-substituted-14H-dibenzo[a,j]xanthenes under solvent-free conditions, Tetra-
hedron Lett. 47 (2006) 8827–8829.
[45] B.F. Mirjalili, A. Bamoniri, A. Akbari, N. Taghavinia, Nano-TiO2: an eco-friendly and
re-usable catalyst for the synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]-
xanthenes, J. Iran. Chem. Soc. 8 (2011) S129–S134.
[46] M. Dabiri, M. Baghbanzadeh, M.S. Nikcheh, E. Arzroomchilar, Eco-friendly and
efficient one-pot synthesis of alkyl- or aryl-14H-dibenzo[a,j]xanthenes in water,
Bioorg. Med. Chem. Lett. 18 (2008) 436–438.
[47] H.J. Zang, Y. Zhang, B.W. Cheng, Efficient one-pot synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-one derivatives using NaHSO4 as catalyst in ionic
liquid, Adv. Mater. Res. 113 (2010) 1993–1996.
[61] G.C. Nandi, S. Samai, R. Kumar, M.S. Singh, An efficient one-pot synthesis of
tetrahydrobenzo[a]xanthene-11-one and diazabenzo[a]anthracene-9,11-dione
derivatives under solvent free condition, Tetrahedron 65 (2009) 7129–7134.
[62] M.M. Heravi, H. Alinejhad, K. Bakhtiari, H.A. Oskooie, Sulfamic acid catalyzed
solvent-free synthesis of 10-aryl-7,7-dimethyl-6,7,8,10-tetrahydro-9H-[1,3]-
dioxolo [4,5-b]xanthen-9-ones and 12-aryl-9,9-dimethyl-8,9,10,12-tetrahydro-
11H-benzo[a]xanthen-11-ones, Mol. Divers. 14 (2010) 621–626.
[63] F. Shirini, S. Akbari-Dadamahaleh, A. Mohammad-Khah, A.R. Aliakbar, Rice husk: a
mild, efficient, green and recyclable catalyst for the synthesis of 12-aryl-
8,9,10,12-tetrahydro[a]xanthene-11-ones and quinoxaline derivatives, C.R. Chim.
16 (2013) 207–216.
[64] K. Venkatesan, S.S. Pujari, R.J. Lahoti, K.V. Srinivasan, An efficient synthesis of 1,8-
dioxo-octahydro-xanthene derivatives promoted by a room temperature ionic
liquid at ambient conditions under ultrasound irradiation, Ultrason. Sonochem.
15 (2008) 548–553.
[48] A. Sharifi, M.S. Abaee, A. Tavakkoli, M. Mirzaei, A. Zolfaghari, Facile montmoril-
lonite k-10-supported synthesis of xanthene derivatives under microwave and
thermal conditions, Synth. Commun. 38 (2008) 2958–2966.
[49] A.R. Khosropour, M.M. Khodaei, H. Moghannian, A facile, simple and convenient
method for the synthesis of 14-alkyl or aryl-14-H-dibenzo[a,j]xanthenes cata-
lyzed by pTSA in solution and solvent-free conditions, Synlett (2005) 955–
958.
[50] N.A. Bigdeli, M.M. Heravi, G.H. Mahdavinia, Wet cyanuric chloride catalyzed
simple and efficient synthesis of 14-aryl or alkyl-14-H-dibenzo[a,j]xanthenes,
Catal Commun. 8 (2007) 1595–1598.
[51] S. Allameh, A. Davoodnia, A. Khojastehnezhad, An efficient and eco-friendly
synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using H4[SiW12O40] as a hetero-
geneous and reusable catalyst under solvent-free conditions, Chin. Chem. Lett. 23
(2012) 17–20.
[52] W. Su, D. Yang, C. Jin, B. Zhang, Yb(OTf)3 catalyzed condensation reaction of b-
naphthol and aldehyde in ionic liquids: a green synthesis of aryl-14H-diben-
zo[a,j]xanthenes, Tetrahedron Lett. 49 (2008) 3391–3394.
[53] F. Shirini, N.G. Khaligh, Succinimide-N-sulfonic acid: an efficient catalyst for the
synthesis of xanthene derivatives under solvent-free conditions, Dyes Pigm. 95
(2012) 789–794.
[54] F. Shirini, N.G. Khaligh, S. Akbari-Dadamahaleh, Preparation, characterization and
use of 1,3-disulfonic acid imidazolium hydrogen sulfate as an efficient, halogen-
free and reusable ionic liquid catalyst for the trimethylsilyl protection of hydroxyl
groups and deprotection of the obtained trimethylsilanes, J. Mol. Catal. A: Chem.
365 (2012) 15–23.
[55] F. Shirini, N.G. Khaligh, 1,3-Disulfonic acid imidazolium hydrogen sulfate as an
efficient and reusable ionic liquid catalyst for the N-Boc protection of amines, J.
Mol. Liq. 177 (2013) 386–393.
[56] L. Khazdooz, A. Zarei, A.R. Hajipour, N. Sheikhan, A study for the synthesis of
dibenzo [a,j] xanthenes and 1-amidoalkyl 2-naphthols catalyzed by
[Hmim][HSO4] as a green, efficient and reusable catalyst under solvent-free
conditions, Iran. J. Catal. 1 (2011) 1–10.
[57] G.H. Mahdavinia, S. Rostamizadeh, A.M. Amani, Z. Emdadi, Ultrasound-promoted
greener synthesis of aryl-14-H-dibenzo[a,j]xanthenes catalyzed by NH4H2PO4/
SiO2 in water, Ultrason. Sonochem. 16 (2009) 7–10.
[58] B. Maleki, M. Gholizadeh, Z. Sepehr, 1,3,5-Trichloro-2,4,6-triazinetrion: a versatile
heterocycle for the one-pot synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]-
xanthene, 1,8-dioxooctahydroxanthene and 12-aryl-8,9,10,12-tetrahydroben-
zo[a]xanthene-11-one derivatives under solvent-free conditions, Bull. Korean
Chem. Soc. 32 (2011) 1697–1702.
[59] H. Hashemi, A.R. Sardarian, 4-Dodecylbenzenesulfonic acid (DBSA) as an effi-
cient and recyclable catalyst for synthesis of 14-aryl- and 14-alkyl-14-H-
dibenzo[a,j]xanthenes under solvent-free conditions, Iran. J. Sci. Technol. A1
(2013) 75–82.
[60] A. Khazaei, M.A. Zolfigol, A.R. Moosavi-Zare, et al., Organocatalyst trityl chloride
efficiently promoted the solvent-free synthesis of 12-aryl-8,9,10,12-tetrahydro-
benzo[a]-xanthen-11-ones by in situ formation of carbocationic system in neutral
media, Catal. Commun. 20 (2012) 54–57.
[65] S. Kokkirala, N.M. Sabbavarapu, V.D.N. Yadavalli, b-Cyclodextrin mediated syn-
thesis of 1,8-dioxooctahydroxanthenes in water, Eur. J. Chem. 2 (2011) 272–275.
[66] A. Zare, A.R. Moosavi-Zare, M. Merajoddin, et al., Ionic liquid triethylamine-
bonded sulfonic acid {[Et3N–SO3H]Cl} as a novel, highly efficient and homoge-
neous catalyst for the synthesis of b-acetamido ketones, 1,8-dioxo-octahydrox-
anthenes and 14-aryl-14H-dibenzo[a,j]xanthenes, J. Mol. Liq. 167 (2012) 69–77.
[67] S. Rostamizadeh, A.M. Amani, G.H. Mahdavinia, G. Amiri, H. Sepehrian, Ultra-
sound promoted rapid and green synthesis of 1,8-dioxo-octahydroxanthenes
derivatives using nanosized MCM-41-SO3H as a nanoreactor, nanocatalyst in
aqueous media, Ultrason. Sonochem. 17 (2010) 306–309.
[68] M.A. Zolfigol, V. Khakyzadeh, A.R. Moosavi-Zare, et al., Preparation of various
xanthene derivatives over sulfonic acid functionalized imidazolium salts (SAFIS)
as novel, highly efficient and reusable catalysts, C.R. Chim. 15 (2012) 719–736.
[69] K. Gong, D. Fang, H.L. Wang, X.L. Zhou, Z.L. Liu, The one-pot synthesis of 14-alkyl-
or aryl-14H-dibenzo[a,j]xanthenes catalyzed by task-specific ionic liquid, Dyes
Pigm. 80 (2009) 30–33.
[70] D. Kundu, A. Majee, A. Hajra, Task-specific ionic liquid catalyzed efficient micro-
wave-assisted synthesis of 12-alkyl or aryl-8,9,10,12-tetrahydrobenzo[a]x-
anthen-11-ones under solvent-free conditions, Green Chem. Lett. Rev. 4 (2011)
205–209.
[71] B. Janardhan, S. Vijaya Laxmi, B. Rajitha, An efficient synthesis of 12-aryl-
8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones using (4-sulfobutyl)tris(4-sulfo-
phenyl)phosphonium hydrogen sulphate as catalyst under neat conditions, J.
Chem. Pharm. Res. 4 (2012) 519–525.
[72] J.M. Khurana, D. Magoo, pTSA-catalyzed one-pot synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-ones in ionic liquid and neat conditions, Tetra-
hedron Lett. 50 (2009) 4777–4780.
[73] A. Zare, R. Khanivar, M. Hatami, et al., Efficient synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]-xanthen-11-ones using ionic liquid pyrazinium di(hydrogen
sulfate) {Py(HSO4)2} as a novel, green and homogeneous catalyst, J. Mex. Chem.
Soc. 56 (2012) 389–394.
[74] A. Zare, R. Khanivar, M. Merajoddin, et al., Triethylamine-bonded sulfonic acid
{[Et3N-SO3H]Cl} as an efficient and homogeneous catalyst for the synthesis of 12-
aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones, Iran. J. Catal. 2 (2012) 107–
114.
[75] K. Niknam, M. Damya, 1-Butyl-3-methylimidazolium hydrogen sulfate
[bmim]HSO4: an efficient reusable acidic ionic liquid for the synthesis of 1,8-
dioxo-octahydroxanthenes, J. Chin. Chem. Soc. 56 (2009) 659–665.
[76] M. Dabiri, M. Baghbanzadeh, E. Arzroomchilar, 1-Methylimidazolium triflouroa-
cetate ([Hmim]TFA): an efficient reusable acidic ionic liquid for the synthesis of
1,8-dioxo-octahydroxanthenes and 1,8-dioxo-decahydroacridines, Catal. Com-
mun. 9 (2008) 939.
[77] M. Kalantari, Synthesis of 1,8-dioxo-octahydroxanthenes and bis(indolyl)-
methanes catalyzed by [Et3NH][H2PO4] as a cheap and mild acidic ionic liquid,
Arabian J. Chem. 5 (2012) 319–323.