Catalyst-free sonochemical synthesis of 1,8-dioxo-octahydroxanthene derivatives in carboxy functionalized ionic liquid

Article abstract of DOI:10.1016/j.crci.2012.01.006

An efficient methodology for the synthesis of 1,8-dioxo-octahydroxanthene derivatives under ultrasonic irradiation at room temperature using carboxy functionalized ionic liquid 1-carboxymethyl-3-methylimidazolium tetrafluoroborate, [cmmim][BF₄]

Full text of DOI:10.1016/j.crci.2012.01.006

C. R. Chimie 15 (2012) 378–383
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Preliminary communication/Communication
Catalyst-free sonochemical synthesis of 1,8-dioxo-octahydroxanthene
derivatives in carboxy functionalized ionic liquid
*
Abhishek N. Dadhania, Vaibhav K. Patel, Dipak K. Raval
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, 388 120 Gujarat, India
A R T I C L E I N F O
A B S T R A C T
Article history:
An efficient methodology for the synthesis of 1,8-dioxo-octahydroxanthene derivatives
under ultrasonic irradiation at room temperature using carboxy functionalized ionic liquid
1-carboxymethyl-3-methylimidazolium tetrafluoroborate, [cmmim][BF4], without any
added catalyst, is described. The reaction involves condensation between 5,5-dimethyl-
1,3-cyclohexanedione (dimedone) and structurally diversed aldehydes. The resultant
products were obtained in excellent isolated yield with high purity. Ionic liquid was
recycled and reused with prominent retention in its activity for at least six times.
Received 2 November 2011
Accepted after revision 16 January 2012
Available online 16 February 2012
Keywords:
Ultrasonic irradiations
1,8-dioxo-octahydroxanthenes
Heterocycles
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Aldehydes
Homogeneous catalysis
Ionic liquids
Cyclization
1. Introduction
formation and collapse of bubbles, which liberates
considerable energy in very short times. A literature
The growing concern for the environment demands
development of green and financially viable processes for
the synthesis of organic compounds. Synthetic chemists in
both academia and industry are constantly challenged to
consider more environmentally benign methods for
generation of the target molecules. In this sense, utilizing
‘‘safer solvents’’ and to ‘‘design for energy efficiency’’ can
be considered two key principles of green chemistry,
relevant to synthetic chemists. From this point of view,
non-classical methods, microwave-assisted synthesis,
ultrasonic irradiation, use of ionic liquids and supercritical
fluids find applications as tempting methods to achieve
these goals [1–3]. The use of ultrasound activation in
organic transformation is now well known to enhance
the reaction rate, selectivity and yield of the reaction [4,5].
The driving energy is provided through cavitation, the
survey reveals that, compared to traditional methods, this
approach is faster and facile and is more suitable as far as
consideration of green chemistry is concerned [6,7].
On the other hand, the use of non-aqueous room
temperature ionic liquids (ILs) as environmental benign
solvents in organic transformations has gained considerable
importance due to their negligible vapour pressure,
reusability and high thermal stability [8,9]. Apart from this,
someILspossessinherentLewis/Brønstedacidity, whichcan
promote and catalyze organic transformations of commer-
cial importanceinexcellentyields. Theuse ofionicliquidsas
reaction media may offer a convenient solution to both the
solvent emission and catalytic recycling problem with high
efficacy [10–12]. The use of ultrasound in combination with
ILs, which have no vapour pressure, may force the less
volatile substrates to undergo the cavitational activation by
changing the characteristics of cavitation and push the
reaction in the forward direction very quickly. The synergy
of ultrasound and ionic liquid in catalyst-free methodolo-
gies for the synthesis of heterocyclic compounds has
* Corresponding author.
E-mail address: dipanalka@yahoo.com (D.K. Raval).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2012.01.006

A.N. Dadhania et al. / C. R. Chimie 15 (2012) 378–383
379
Scheme 1. Catalyst-free one-pot synthesis of 1,8-dioxo-octahydroxanthene under ultrasonic irradiations in ionic liquid at ambient temperature.
attracted much interest because of their less hazardous
nature, reduced energy consumption and increased product
selectivity [13,14].
2. Results and discussion
To find out the optimum reaction condition, for the
sonochemical synthesis of titled compounds (3a–u), the
condensation reaction between benzaldehyde (1) and
dimedone (2) was chosen as a model reaction under
different reaction conditions (Scheme 1). The obtained
results are recorded in Table 1. It is clear from the results
that 1,8-dioxo-octahydroxanthene can be synthesized
with high yield and purity at ambient temperature using
200 mg IL in 1 mL methanol as the co-solvent without any
added catalyst, in 50 min. To examine the scope and
general applicability of the methodology, a broad range of
structurally diverse aromatic aldehydes (aromatic alde-
hydes having electron withdrawing and electron donating
groups as well as hetero aromatic aldehydes) was
condensed with dimedone under the found optimum
reaction condition. All the reactions were monitored by
thin layer chromatography (TLC) and taken to completion.
The time taken for the completion of each conversion,
aldehyde and the isolated yields are summarized in Table
2. It can be observed that all the aldehydes have reacted in
short reaction times (45–85 min) under these mild
conditions to afford the xanthenes in very good to
excellent isolated yields. The process was tolerated well
by all the aldehydes irrespective of their nature of
substituents. Methanol was added as co-solvent to
solubilize dimedone at ambient temperature. In absence
of MeOH, the reaction proceeded with poor yield (Entry 4,
Table 1), and the TLC of reaction mixture also indicated the
presence of unreacted dimedone. The reaction, in absence
of IL, did not result in the formation of product even after
ultrasonication for 180 min (Entry 1, Table 1). All the
synthesized xanthenes were homogeneous on TLC and
pure enough for the further practical purpose. However, all
the compounds were crystallized from hot ethanol. All the
In the families of heterocyclic compounds, xanthene
core and its derivatives serve as an important class of
compounds, as it is present in natural products with
broad biological activities [15–18]. Most notably among
them, xanthenedione constitutes a structural unit in a
number of natural products, and has a wide range of
therapeutic and pharmacological properties [19–21].
Several functionalized derivatives of xanthene possess
antiplasmodial [22], antiviral [23], antibacterial [24] and
anti-inflammatory [25] activities. Besides, these hetero-
cyclic molecules have been widely used as luminescent
dyes [26,27], sensitizers in photodynamic therapy [28–
31], in laser technology [32,33] as well as pH sensitive
fluorescent materials [34,35]. Attempts to synthesize
xanthenediones over various catalysts such as sulfuric
acid or hydrochloric acid [36], InCl3/ionic liquid [37], Fe+3
montmorilonite [38], amberlyst-15 [39], FeCl3/[bmim]
[BF4] [40], p-dodecylbenzenesulphonic acid [41], sulpha-
mic acid [42], HClO4ÁSiO2 [43], trimethylsilyl chloride
(TMSCl) [44] have been reported. Most of the reported
methods have used expensive reagents, hazardous
organic solvents, longer reaction time and tedious
workup. Hence, there is scope for further innovation
towards contemporary reaction with easy isolation of
product, reusability of catalyst, perhaps with minimal or
no waste.
As a part of our continuing studies in developing
efficient catalyst-free synthetic methodologies, using
ultrasound and ionic liquid in organic preparations [45],
herein, we report a general method for the synthesis of 1,8-
dioxo-octahydroxanthenes promoted by synergistic effect
of ultrasonic irradiation and ionic liquid at ambient
temperature without any added catalyst.
Table 1
Optimization of reaction condition for the synthesis of 1,8-dioxo-octahydroxanthene.
Entry
IL/co-solvent
Reaction condition
Time (min)a
% Yield
1
2
3
4
5
No IL/2 mL MeOH
RT,)))
180
50
0
[cmmim][BF4]/1 mL MeOH
[cmmim][BF4]/1 mL MeOH
[cmmim][BF4]/no co-solvent
[cmmim][BF4]/1 mL MeOH
RT,)))
87
50 8C,)))
RT,)))
50
89
28b
180
180
RT, stirring
Intermediate
a
All the reactions were run till completion as indicated by TLC.
Dimedone remains sparingly soluble under this condition.
b

380
A.N. Dadhania et al. / C. R. Chimie 15 (2012) 378–383
Table 2
Catalyst-free synthesis of 1,8-dioxo-octahydroxanthenes under ultrasonication at ambient temperature using variously substituted aromatic aldehydes.
Compound
R
Time (min)
Yield (%)a
M.P. (8C)
Found
Reported
3a
3b
3c
3d
3e
3f
C6H5
50
60
65
55
55
70
60
55
45
80
85
45
55
45
40
55
55
50
30
45
50
87
85
86
82
81
83
87
78
85
79
81
84
86
88
82
86
83
84
89
92
90
198–200
222–224
226–228
184–186
230–232
258–260
166–168
216–218
240–242
246–248
226–228
226–228
192–194
224–226
172–174
174–176
186–188
202–204
60–62
199–201 [37]
222–224 [37]
225–227 [37]
183–185 [47]
226–228 [47]
258–262 [48]
165–166 [47]
215–216 [47]
242–243 [47]
247–248 [47]
225–227 [47]
226–229 [47]
190–192 [50]
226–227 [47]
174–175 [47]
175–176 [47]
187–189 [50]
202–205 [49]
62–64 [50]
4-O2NC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-O2NC6H4
3-O2NC6H4
4-H3CC6H4
4-MeOC6H4
4-HOC6H4
4-HO,3-MeOC6H3
2-BrC6H4
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
3-BrC6H4
4-FC6H4
2,5-(MeO)2C6H3
3,4-(MeO)2C6H3
3,4,5-(MeO)3C6H2
2-HOC6H4
2-Furyl
2-Thienyl
164–166
204–206
164–165 [47]
203–205 [47]
3u
2-Pyridyl
a
Yield after crystallization.
synthesized compounds were characterized by melting
point, 1H NMR, 13C NMR and DEPT-135 spectral techni-
ques. Additional confirmation for the structures is also
obtained by IR and mass spectrometric studies for some
representative samples. All the data were in agreement
with the cited literature.
temperature (Entry 5, Table 1). No formation of product 3a
was observed even after 180 min of continuous stirring.
The absence of ultrasonic irradiation brings the reaction to
stop at the intermediate stage, the 1H NMR spectrum of
the intermediate is shown on Fig. 1 for the perusal. The
NMR spectrum corresponds well with the uncyclised
intermediate involving two enols and two ketons, since
the H on methylene bridge is a singlet. The integration for
To investigate the role of ultrasound, the model
reaction was carried out under simple stirring at ambient
Fig. 1. 1H NMR spectrum of uncyclised intermediate.

A.N. Dadhania et al. / C. R. Chimie 15 (2012) 378–383
381
Scheme 2. Plausible mechanistic pathway for the synthesis of 1,8-dioxo-octahydroxanthene.
hydroxyl proton is one instead of two; this may probably
be due to a broad signal and exchange with residual HDO/
D2O. The NMR data is in good agreement with an earlier
work by Singh et al. [46]. The increase in the rate and
selectivity of the reaction under ultrasonication appears
to be due to the occurrence of cavitation. Ultrasound
travelled through alternating compressions and rarefac-
tions in the transmitting medium along the direction of
wave propagation [47,48]. Small cavities or gas filled
micro bubbles are formed due to a large negative pressure
on liquid in the rarefaction section of the wave. An
acoustic cavitation, which includes nucleation, growth
and collapse of the cavity, is responsible for the chemical
and physical effects of ultrasound [49]. The energy
released involved with cavitation is best explained in
terms of the ‘‘hot spot’’ approach. Each cavitation bubble
acts as a localized microreactor and its violent collapse
generates the energy for chemical effects. Due to
negligible vapour pressure and strong cohesive forces, it
is difficult to induce cavitation in an ionic liquid.
Therefore, in such a medium, the cavitational bubble will
contain the vapour of reaction partners and co-solvent.
The ionic liquid shell surrounds it, forces it to go through
strong cavitational effects.
study [45], we demonstrated that hydrogen bonding can
be formed between carboxylic proton of IL and carbonyl
oxygen of aldehyde during the reaction. Accordingly, the
same hydrogen bonding can also be formed between the
carboxylic proton of the IL and the carbonyl oxygen of
dimedone. The formation of a hydrogen bond between the
IL and the substrate leads them to activation. Based on
this investigation, we suggest the plausible mechanistic
pathway for this reaction (Scheme 2).
To investigate the efficiency of the recycled ionic liquid,
we ran six successive cycles of the model reaction at
ambient temperature under the optimum reaction condi-
tions using recycled IL from the previous run. The ionic
liquid was found to be effective for at least six reaction
cycles with prominent retention in its activity. The
obtained results are plotted in Fig. 2.
3. Conclusion
The sonochemical reaction provided an improved
practical alternative to access functionalized xanthenes
in excellent yields. The beneficial features of the reaction,
such as mild conditions, absence of added catalyst and
reusability of the ionic liquid gives it an advantage over the
conventional acid/base catalyzed reaction.
The IL promotes the reactions due to its inherent
Brønsted acidity at ambient temperature. In our previous
4. Experimental
4.1. General
All chemicals were of research grade and were used as
obtained from Sigma-Aldrich, Alfa-Aesar or SDFCL. The IL
was prepared under ultrasonic irradiation, according to
the method reported earlier [45]. The melting points were
determined in capillary tubes using heavy paraffin liquid
in Thiele tube. Melting points are uncorrected and are
compared with the reported literature values. The
reaction progress and purity of products were determined
by TLC silica gel plates (Merck 60 F254). IR Spectra were
recorded on a Shimadzu FT-IR-S8401 and FT-IR-8400
spectrophotometer using KBr, mass spectra on AB
APPLIED BIOSYSTEMS IMDS SCIEX. API-2000 LC/MS/MS
Fig. 2. Recyclability of ionic liquid in the model reaction between
benzaldehyde and dimedone.

382
A.N. Dadhania et al. / C. R. Chimie 15 (2012) 378–383
spectrometer. The 1H NMR (400 MHz), 13C NMR
(100 MHz) and DEPT-135 spectra were recorded on
BRUKER AVANCE 400 MHz instrument using CDCl3 as
the solvent and TMS as an internal standard. All the
reactions were carried out in an ultrasonic cleaning bath
(D-compact ultrasonic cleaner) at 50 kHz frequency and
250 W power output. The reactions were carried out in a
round-bottomed flask of 25 mL capacity suspended at the
center of the cleaning bath, 5 cm below the surface of the
liquid. The temperature of water bath was controlled at
30 8C by addition or removal of water from the bath.
27.23, 29.24, 31.95, 32.18, 42.06, 50.09, 112.11, 125.23,
129.25, 135.26, 141.50, 162.54, 195.83. DEPT-135: up
peaks:
peaks:
d
d
21.20, 27.23, 29.24, 32.18, 125.23, 129.25. Down
42.06, 50.09. IR (KBr): 3135, 2954, 1720, 1592,
1378, 1191, 1081 cmÀ1
.
4.3.4. 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-(4-hydroxy-
3-methoxyphenyl)-2H-xanthene-1,8(5H,9H)-dione
(compound 3k)
1H NMR (400 MHz, CDCl3):
d
6.93 (m, 1H), 6.71 (m, 1H),
6.64 (m, 1H), 5.81 (s, 1H), 4.66 (s, 1H), 3.88 (s, 3H), 2.44 (s,
4H), 2.19 (dd, 4H, J = 16.3 Hz), 1.10 (s, 6H), 1.01 (s, 6H). 13
NMR: 27.31, 29.15, 31.98, 32.08, 42.36, 51.13, 55.92,
111.13, 113.16, 117.12, 120.54, 132.31, 145.56, 148.23,
162.21, 196.12. DEPT-135: up peaks: 27.31, 29.15, 32.08,
55.92, 111.13, 117.12, 120.54. Down peaks: 42.36, 51.13.
IR (KBr): 3692, 3581, 3155, 2964, 2228, 1645, 1509, 1362,
1164 cmÀ1
C
4.2. General procedure for the synthesis of 1,8-dioxo-
octahydroxanthenes
d
d
A
mixture of 5,5-dimethyl-1,3-cyclohexanedione
d
(2 mmol), aldehyde (1 mmol) and [cmmim][BF4]
(200 mg) was charged into a 25 mL flask. The mixture
was stirred gently with a spatula for a few seconds to
ensure homogeneous mixing of reactants with ionic liquid.
One milliliter of methanol was added to it. Then, the
reaction mixture was subjected to ultrasonic irradiation at
30 8C. After completion of reaction (as indicated by TLC),
the reaction mixture was poured onto crushed ice (ꢀ20
gm) and stirred well. The solid separated was washed with
ice-cold water (ꢀ 4 Â 5 mL), and then crystallized from hot
ethanol to afford pure 1,8-dioxo-octahydroxanthenes. The
aqueous filtrate was subjected to vacuo at 80 8C under
reduced pressure (10 mmHg) for 4 h to leave behind the IL
in near complete recovery, pure enough for the next run.
.
4.3.5. 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-(3-
bromophenyl)-2H-xanthene-1,8(5H,9H)-dione (compound
3m)
1H NMR (400 MHz, CDCl3):
d
7.42-6.94 (m, 4H), 4.86 (s,
1H), 2.43 (s, 4H), 2.23 (dd, 4H, J = 16.0 Hz), 1.21 (s, 6H), 1.10
(s, 6H). 13C NMR:
27.31, 29.51, 32.06, 32.51, 42.13, 51.24,
112.65, 122.89, 125.23, 127.21, 130.26, 131.86, 141.92,
162.23, 196.46. DEPT-135: up peaks: 27.31, 29.51, 32.51,
42.13,
d
d
125.23, 127.21, 130.26, 131.86. Down peaks:
51.24. IR (KBr): 3020, 2965, 2886, 1594, 1364, 1215 cmÀ1
d
.
4.3.6. 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-(2-thienyl)-
4.3. Spectral data of selected compounds
2H-xanthene-1,8(5H,9H)-dione (compound 3t)
1H NMR (400 MHz, CDCl3):
d
7.82-7.98 (m, 2H), 6.78 (m,
1H), 4.92 (s, 1H), 2.53 (s, 4H), 2.26 (dd, 4H, J = 16.0 Hz), 1.13
(s, 6H), 1.01 (s, 6H). 13C NMR:
27.18, 29.33, 31.02, 31.89,
41.06, 49.63, 113.69, 119.39, 124.01, 132.26, 136.25,
161.84, 196.33. DEPT-135: up peaks: 27.18, 29.33,
31.89, 119.39, 124.01, 132.26. Down peaks: 41.06,
49.63. IR (KBr): 2955, 2896, 2871, 1659, 1624, 1371,
1360, 1201 cmÀ1
4.3.1. 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-phenyl-2H-
xanthene-1,8(5H,9H)-dione (compound 3a)
d
1H NMR (400 MHz, CDCl3):
d
7.29 (m, 2H), 7.23 (m, 2H),
7.11 (m, 1H), 4.77 (s, 1H), 2.48 (s, 4H), 2.21 (dd, 4H,
J = 16.4 Hz), 1.12 (s, 6H), 1.01 (s, 6H). 13C NMR:
27.34,
29.26, 31.84, 32.19, 40.90, 50.76, 115.70, 126.36, 128.04,
d
d
d
128.38, 144.10, 162.22, 196.32. DEPT-135: up peaks:
27.34, 29.26, 31.84, 126.36, 128.04, 128.38. Down peaks:
d
d
.
40.89, 50.75. IR (KBr): 3030, 2955, 1662, 1624, 1361, 1197,
1163 cmÀ1.m/z (ESI): 351.0 [M + H+].
Acknowledgements
Authors thank the Head, Department of Chemistry,
Sardar Patel University for providing necessary research
facilities. A.N. Dadhania also thanks University Grant
Commission – New Delhi, India for the meritorious
fellowship.
4.3.2. 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-(4-
nitrophenyl)-2H-xanthene-1,8(5H,9H)-dione (compound 3b)
1H NMR (400 MHz, CDCl3):
d
8.10 (m, 2H), 7.49 (m, 2H),
4.84 (s, 1H), 2.51 (s, 4H), 2.22 (dd, 4H, J = 16.4 Hz), 1.13 (s,
6H), 1.00 (s, 6H). 13C NMR:
27.28, 29.23, 32.22, 32.37,
40.58, 50.61, 114.54, 123.42, 129.36, 146.50, 151.51, 162.95,
196.23. DEPT-135: up peaks: 27.27, 29.23, 32.37, 123.42,
129.36. Down peaks: 40.84, 50.60. IR (KBr): 3020, 2956,
d
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