Efficient and green syntheses of 12-aryl-2,3,4,12-tetrahydrobenzo[b] xanthene-1,6,11-triones in water and task-specific ionic liquid

Article abstract of DOI:10.1080/00397911.2012.688230

Facile and convenient one-pot cascade/tandem approaches for the syntheses of privileged medicinal scaffolds, 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene- 1,6,11-trione derivatives, have been reported under extremely mild reaction conditions using a catalytic amount of H₂SO₄ in water or in the presence of the acidic ionic liquid bmim[HSO₄], which could be recycled. Copyright

Full text of DOI:10.1080/00397911.2012.688230

This article was downloaded by: [Brown University]
On: 16 June 2013, At: 22:55
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lsyc20
Efficient and Green Syntheses
of 12-Aryl-2,3,4,12-
tetrahydrobenzo[b]xanthene-1,6,11-
triones in Water and Task-Specific Ionic
Liquid
Jitender M. Khurana a , Anshika Lumb a , Ankita Chaudhary a
Bhaskara Nand a
&
a Department of Chemistry , University of Delhi , Delhi , India
Published online: 03 Jun 2013.
To cite this article: Jitender M. Khurana , Anshika Lumb , Ankita Chaudhary & Bhaskara Nand (2013):
Efficient and Green Syntheses of 12-Aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-triones in
Water and Task-Specific Ionic Liquid, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 43:16, 2147-2154
To link to this article: http://dx.doi.org/10.1080/00397911.2012.688230
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae, and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

Synthetic Communications1, 43: 2147–2154, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.688230
EFFICIENT AND GREEN SYNTHESES
OF 12-ARYL-2,3,4,12-TETRAHYDROBENZO[b]
XANTHENE-1,6,11-TRIONES IN WATER AND
TASK-SPECIFIC IONIC LIQUID
Jitender M. Khurana, Anshika Lumb, Ankita Chaudhary, and
Bhaskara Nand
Department of Chemistry, University of Delhi, Delhi, India
GRAPHICAL ABSTRACT
Abstract Facile and convenient one-pot cascade/tandem approaches for the syntheses of
privileged medicinal scaffolds, 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-trione
derivatives, have been reported under extremely mild reaction conditions using a catalytic
amount of H2SO4 in water or in the presence of the acidic ionic liquid bmim[HSO4], which
could be recycled.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Aqueous media; benzoxanthenes; bmim[HSO4],2-hydroxy-1,4-naphthoqui-
none; cyclic-1,3-dicarbonyl compounds
INTRODUCTION
The concept of privileged medicinal structures or scaffolds[1] has emerged as
one of the guiding principles of the drug discovery process. These privileged
scaffolds, such as substituted benzoxanthenes and naphthoquinones, commonly
consist of a rigid heterocyclic system with well-defined orientation of appended
functionalities for target recognition.[2] Benzoxanthenes show therapeutic and
biological properties,[3] and they are used in photodynamic therapy,[4] polymer
photoimaging systems,[5] and laser technologies.[6]
2-Hydroxy-1,4-napthoquinone (HNQ) is a principal natural dye in the leaves
of henna, Lawsonia inermis. Today, semipermanent hairdyes containing henna as
Received March 14, 2012.
Address correspondence to Jitender M. Khurana, Department of Chemistry, University of Delhi,
Delhi 110007, India. E-mail: jmkhurana1@yahoo.co.in
2147

2148
J. M. KHURANA ET AL.
well as its pure dye ingredient HNQ are extensively used and have become increas-
ingly popular because of their natural origin.[7] An array of naphthoquinone
pigments (streptocarpone, a-dunnione, dunniol, and dunnione) have been isolated
and characterized from Streptocarpus dunni.[8] Moreover, molecules containing
naphthoquinone structure constitute an important class in organic chemistry
because of their various biological activities, industrial applications, and potential
as intermediates in the synthesis of heterocycles.[9]
The development of new and simple synthetic methods for xanthene containing
a naphthoqinone framework is therefore an interesting challenge, and only few
methods have been reported for the synthesis.[10–12] These methods have their own
merits and demerits. An application of water as a green solvent for organic synthesis
has received considerable attention.[13] Ionic liquids (ILs) also offer a unique
environment because of their interesting properties such as chemical and thermal
stability, nonvolatility, noncoordinating nature, good solvating capability, and ease
of recycling.[14] Recently, task-specific acidic ionic liquids which can act as solvent
and=or catalyst have been successfully used in different reactions.[15,16]
In continuation of our focus on the environmentally benign synthesis of diverse
heterocyclic compounds of biological significance,[17] we disclose herein new and
efficient protocols for the syntheses of 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-
1,6,11-triones.
RESULTS AND DISCUSSION
In the present study, we report ‘‘one-pot’’ syntheses of 12-aryl-2,3,4,12-
tetrahydrobenzo[b]xanthenes-1,6,11-triones (1a–x) by condensation of 2-hydroxy
naphthalene-1,4-dione, aromatic aldehydes, and cyclic 1,3-dicarbonyl compounds
catalyzed by sulfuric acid or the task-specific ionic liquid 1-butyl-3-methylimidazo-
lium hydrogen sulfate (bmim[HSO4]).
The reaction conditions were optimized by attempting the condensation
of 4-nitrobenzaldehyde (1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol), and
5,5-dimethylcyclohexane-1,3-dione (dimedone) (1.2 mmol) in the presence or absence
of acids such as HCl, H2SO4, HNO3, and AcOH in various solvents such as water,
acetonitrile, dimethylformanide (DMF), EtOH, tetrahydrofurane (THF), and
bmim[HSO4]. The standardization results are summarized in Table 1.
From Table 1 it is notable that 92% and 91% of 3,3-dimethyl-12-
(4-nitrophenyl)-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-trione (1a) was obtained
when the reaction was carried out in H2O=H2SO4 under reflux (Table 1, entry 5) and
bmim[HSO4] at 70 ꢀC (Table 1, entry 9) respectively. Increasing the amount of cata-
lyst (H2SO4) did not affect the rate of reaction or yield while decreasing the amount
of catalyst (H2SO4) decreases the rate of reaction and yields significantly. Hence, the
reaction of 4-nitrobenzaldehyde, 2-hydroxy-1,4-naphthoquinone and dimedone
(1.2 mmol) gives best and comparable results in water=H2SO4 (method A) and
bmim[HSO4] (method B). Therefore, we decided to pursue both protocols for the
synthesis of 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthenes-1,6,11-triones (Scheme 1).
Then, the generality of this protocol was applied to the synthesis of a variety of
tetrahydrobenzo[b]xanthene-1,6,11-triones (Table 2, 1a–m) using diversely subsi-
tuted aldehydes, 2-hydroxy-1,4-naphthpquinone, and dimedone. The scheme was

TETRAHYDROBENZO[b]XANTHENE-TRIONES
2149
Table 1. Effect of different media and catalyst on the condensation of 4-nitrobenzaldehyde,
2-hydroxy-1,4-naphthoquinone, and dimedone (molar ratio 1:1:1.2)
Entry
Reaction media
Temperature (ꢀC)
Catalyst
Amount
Time
Yield (%)a
1
2
3
4
5
6
7
8
9
DMF
CH3CN
100
80–85
65–70
70–80
100
H2SO4
20
20
20
20
20
20
20
20
—
5.0 h
5.0 h
40
45
25
78
92
H2SO4
H2SO4
H2SO4
H2SO4
CH3COOH
HCl
THF
EtOH
5.0 h
2.0 h
H2O
H2O
25 min
5.0 h
5.0 h
b
100
100
—
b
H2O
H2O
—
—
c
100
70
HNO3
—
5.0 h
30 min
bmim[HSO4] (1 mL)
91
aIsolated yield (%) of 1a.
bSluggish and incomplete reactions.
cNumber of products on TLC.
extended to another cyclic b-diketones viz. 5-methyl-cyclohexane-1,3-dione and
cyclohexane-1,3-dione to give the corresponding novel tetrahydrobenzo[b]xanthene-1,6,
11-triones in excellent yields (Table 2, 1n–v). Reaction of aromatic aldehydes and
2-hydroxy-1,4-naphthoquinone with cyclopentane-1,3-dione also underwent successful
condensation under similar conditions to afford novel dihydro-2H-4-oxa-cyclopenta
[b]anthracene-1,5,10-trione derivatives (Table 2, 1w–x).
The ionic liquid employed in method B could be recycled for three cycles before
losing activity. A plausible mechanism for the formation of 12-aryl-2,3,4,12-
tetrahydrobenzo[b]xanthene-1,6,11-trione in the presence of acid catalyst is proposed
in Scheme 2, which is confirmed by the initial condensation of 4-chlorobenzaldehyde
and dimedone to form 2-[(4-chlorophenyl)methylene]-5,5-dimethylcyclohexane-1,3-
dione[18] (A), which is then condensed with 2-hydroxy-1,4-naphthoquinone using
H2SO4 in aqueous media and bmim[HSO4] to give desired product 1c.
The electronic absorption spectra of 2 Â 10À5 M solutions of 1a–x in CHCl3
were measured (Table 3). The longest wavelength maximum absorption (kmax) of
all the compounds was located between 335 and 341 nm. The color of the
compounds varied from yellow to orange.
Scheme 1. Synthesis of 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-triones.

2150
J. M. KHURANA ET AL.
Table 2. Synthesis of 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-trione derivatives by conden-
sation of aldehydes, 2-hydroxy-1,4-naphthoquinone, and cyclic 1,3-dicarbonyl compounds
Method A
Method B
Cyclic-1,3-
Time Yield Time Yield
Mp
Mp
(ꢀC) lit.
Entry
1a
1b
1c
Ar
dicarbonyl compounds (min) (%) (min) (%) (ꢀC) obs.
4-O2NC6H4
3-O2NC6H4
4-ClC6H4
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
25
25
30
40
25
35
25
35
25
20
45
40
30
92
92
90
88
90
89
88
90
90
89
85
88
90
30
30
25
30
30
25
25
30
25
30
30
25
25
91
89
89
88
89
90
91
86
88
85
87
88
87
256–258
236–238
—
—
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
275–278 282–284[12]
268–270
260–262 263–265[12]
1d
1e
4-CH3OC6H4 5,5-Dimethyl-cyclohexa-
ne-1,3-dione
—
C6H5
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
1f
4-CH3C6H4
2-ClC6H4
3-ClC6H4
2,4-ClC6H3
2-Naphthyl
4-OH C6H4
3-OH C6H4
Thien-2-yl
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
178–180
242–244
210–212
256–258
268–270
198–200
194–196
178–180
—
—
—
—
—
—
—
—
1g
1h
1i
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
1j
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
1k
1l
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
1m
5,5-Dimethyl-cyclohexa-
ne-1,3-dione
1n
1o
1p
1q
1r
1s
1t
4- O2NC6H4 Cyclohexane-1,3-dione
4-ClC6H4 Cyclohexane-1,3-dione
4-CH3OC6H4 Cyclohexane-1,3-dione
15
20
40
30
35
25
30
20
94
90
89
91
85
91
92
90
25
30
30
30
30
30
25
30
91
92
89
88
87
86
89
88
218–220
188–190
190–192
220–222
260–262
252–254
212–214
180–184
—
—
—
—
—
—
—
—
C6H5
Cyclohexane-1,3-dione
Cyclohexane-1,3-dione
Cyclohexane-1,3-dione
Cyclohexane-1,3-dione
5-Methyl-cyclohexane-
1,3-dione
4-CH3C6H4
2,4-ClC6H3
Thien-2-yl
4-O2NC6H4
1u
1v
4-CH3OC6H4 5-Methyl-cyclohexane-
1,3-dione
35
89
30
90
182–184
—
1w
1x
4-O2NC6H4 Cyclopentane-1,3-dione
4-CH3OC6H4 Cyclopentane-1,3-dione
40
35
88
90
25
25
91
91
198–200
256–258
—
—
Notes. Method A: H2SO4, water, reflux.
Method B: bmim[HSO4], 70 ꢀC.
EXPERIMENTAL
All the chemicals used were purchased from Sigma-Aldrich and used as
received. Silica gel 60 F254 (precoated aluminium plates) from Merck was used to
monitor reaction progress. Melting points were determined on a Tropical Labequip

TETRAHYDROBENZO[b]XANTHENE-TRIONES
2151
Scheme 2. Proposed pathway for the synthesis of 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-
triones.
apparatus and are uncorrected. Infrared (IR) (KBr) spectra were recorded on
Perkin-Elmer Fourier transform (FT–IR) spectrophotometer, and the values are
expressed as nmax cmÀ1. Absorbance measurements were made using an Analytikjena
Specord 250 spectrophotometer. Mass spectral data were recorded on
a
Table 3. UV=visible data for compounds 1 in CHCl3
Compound
kmax (nm)
e (104 L molÀ1 cmÀ1
)
1a
1b
1c
1d
1e
1f
343
336
335
336
338
337
333
338
333
339
340
338
338
343
336
338
335
338
336
339
338
340
340
339
6.00
7.50
5.50
3.30
5.80
3.50
5.00
5.50
5.50
6.00
3.50
6.50
4.00
4.50
7.50
3.50
6.00
6.00
4.00
6.50
5.50
5.00
5.00
5.50
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
1s
1t
1u
1v
1w
1x

2152
J. M. KHURANA ET AL.
Jeol-AccuTOF mass spectrometer having a direct analysis in real time (DART)
source. The 1H NMR and 13C NMR spectra were recorded on Jeol JNM ECX-400P
at 400 MHz, using tetramethylsilane (TMS) as an internal standard. The chemical
shift values are recorded on d scale and the coupling constants (J) are in hertz
(Hz). Ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate bmim[HSO4] was
synthesized according to a reported procedure.[19]
General Procedure for the Synthesis of
12-Aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-1,6,11-triones (1a–x)
Method A. In
a
50-mL round-bottom flask, aldehyde (1.0 mmol),
2-hydroxy-1,4-naphthoquinone (1.0 mmol), 5,5-dimethylcyclohexane-1,3-dione or
5-methyl-cyclohexane-1,3-dione or cyclohexane-1,3-dione or cyclopentane-1,3-dione
(1.2 mmol), H2SO4 (0.20 mmol), and 10 mL water were taken. The reaction mixture
was refluxed for appropriate time as mentioned in Table 2, Method A. The progress
of the reaction was monitored by thin-layer chromatography (TLC) using ethyl
acetate–petroleum ether (30:70, v=v) as eluent. After completion, the reaction
mixture was filtered and the crude product was purified on silica gel (100–200 mesh)
column chromatography to yield pure 12-aryl-2,3,4,12-tetrahydrobenzo[b]xanthene-
1,6,11-trione derivatives.
Method B. A mixture of aldehyde (1.0 mmol), 2-hydroxy-1,4-
naphthoquinone (1.0 mmol), and cyclic-1,3-dicarbonyl (1.2 mmol) was placed in a
50-mL, round-bottomed flask and 1 mL bmim[HSO4] was added. The mixture was
heated at 70 ꢀC. After completion of the reaction as monitored by TLC, water
(ꢁ5–7 mL) was added to the mixture. The product was filtered at the pump and
purified through column chromatography (silica gel 100–200 mesh) to yield pure
products 1a–x.
After separation of the product by filtration, the filtrate containing the ionic
liquid was rinsed with ether and further vacuumed to dryness at 100 ꢀC to eliminate
any trapped moisture. It afforded bmim[HSO4], which was reused directly for the
next run. Marginal loss in the yield of the products were observed in first three runs
(90%, 88%, and 86%), while in fourth and fifth runs the yields were quite poor.
The spectral data of the synthesized compounds have been provided in the
Supplementary Information, available online.
CONCLUSION
In conclusion, we have reported efficient syntheses of 12-aryl-2,3,4,12-
tetrahydrobenzo[b]xanthene-1,6,11-trione derivatives via an acid-catalyzed conden-
sation reaction in aqueous media as well as with the task-specific ionic liquid
bmim[HSO4].
ACKNOWLEDGMENTS
A. L. thanks the University of Delhi for the award of a university teaching
assistantship. A. C. and B. N. thank the Council for Scientific and Industrial
Reserch, New Delhi, for the grant of a junior=senior research fellowship.

TETRAHYDROBENZO[b]XANTHENE-TRIONES
2153
REFERENCES
1. (a) Evans, B. E.; Rittle, K. E.; Bock, G.; DiPardo, R. M.; Freidinger, R. M.; Whitter, W. L.;
Lundell, G. F.; Veber, D. F.; Anderson, P. S.; Chang, R. S. L.; Lotti, V. G.; Cerino, D. J.;
Chen, T. B.; Kling, P. J.; Kunkel, K. A.; Springer, J. P.; Hirshfield, J. Methods for drug
discovery: Development of potent, selective, orally effective cholecystokinin antagonists.
J. Med. Chem. 1988, 31, 2235–2246; (b) Patchett, A. A.; Nargund, R. P. Privileged struc-
tures: An update. Ann. Rep. Med. Chem. 2000, 35, 289–298.
2. Poupaert, J.; Carato, P.; Colacino, E. 2(3H)-Benzoxazolone and bioisosters as ‘‘privi-
leged scaffold’’ in the design of pharmacological probes. Curr. Med. Chem. 2005, 12,
877–885.
3. (a) Chatterjee, S.; Iqbal, M.; Kauer, J. C.; Mallamo, J. P.; Senadhi, S.; Mallaya, S.;
Bozyczko-Coyne, D.; Siman, R. Xanthene-derived potent nonpeptidic inhibitors of
recombinant human calpain I. Bioorg. Med. Chem. Lett. 1996, 6, 1619–1622; (b) Naya,
A.; Ishikawa, M.; Matsuda, K.; Ohwaki, K.; Saeki, T.; Noguchi, K.; Ohtake, N. Struc-
ture–activity relationships of xanthene carboxamides, novel CCR1 receptors antagonists.
Bioorg. Med. Chem. 2003, 11, 875–884.
4. Saint-Ruf, G.; Hieu, H. T.; Poupelin, J. P. The effect of dibenzoxanthenes on the paralyzing
action of zoxazolamine. Naturwissenschaften 1975, 62, 584–585.
5. Monroe, B. M.; Weed, G. C. Photoinitiators for free-radical-initiated photoimaging
systems. Chem. Rev. 1993, 93, 435–448.
6. Green, G. R.; Evans, J. M.; Vong, A. K.; Katrizky, A. R.; Rees, C. W.; Scriven, E. F. V.
Comprehensive Heterocyclic Chemistry II; Pergamon Press: Oxford, 1995; vol. 5, p. 469.
7. Siddiqui, B. S.; Kardar, M. N.; Ali, T.; Khan, S. Two new and a known compound from
Lawsonia inermis. Helv. Chim. Acta 2003, 86, 2164–2169.
8. Perez, A. L.; Lamoureux, G.; Sanchez-Kopper, A. Efficient syntheses of streptocarpone
and (Æ)-a-dunnione. Tetrahedron Lett. 2007, 48, 3735.
9. Thomson, R. H. In Naturally Occurring Quinones, 4th ed.; Chapman & Hall: London,
1997.
10. Yuling, L.; Baixiang, D.; Xiaoping, X.; Daqing, S.; Shunjun, J. A green and efficient syn-
thesis of 13-aryl-5,7,12,14-tetrahydrodibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone deriva-
tives in ionic liquid. Chin. J. Chem. 2009, 27, 1563–1568.
11. Tisseh, Z. N.; Azimi, S. C.; Mirzaei, P.; Bazgir, A. The efficient synthesis of aryl-5H-
dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone leuco-dye derivatives. Dyes Pigm. 2008,
79, 273–275.
12. (a) Dabiri, M.; Tisseh, Z. N.; Bazgir, A. An efficient three-component synthesis of benzox-
anthenes in water. J. Heterocycl. Chem. 2010, 47, 1062–1065; (b) Shaterian, H. R.; Ranjbar,
M.; Azizi, R. Synthesis of benzoxanthene derivatives using Brønsted acidic ionic liquids
(BAILs), 2-pyrrolidonium hydrogen sulfate and (4-sulfobutyl)tris(4-sulfophenyl)
phosphonium hydrogen sulfate. J. Mol. Liq. 2011, 162, 95–99.
13. (a) Lindstrom, U. M. Stereoselective organic reactions in water. Chem. Rev. 2002, 102,
2751; (b) Li, C. J.; Chen, L. Organic chemistry in water. Chem. Soc. Rev. 2006, 35, 68–82.
14. Welton, T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. Chem.
Rev. 1999, 99, 2071–2084.
15. Wasserschield, P.; Keim, W. Ionic liquids—New ‘‘solutions’’ for transition metal catalysis.
Angew. Chem. Int. Ed. 2000, 39, 3772–3789.
16. Gupta, N.; Sonu, Kad, G. L.; Singh, J. Acidic ionic liquid [bmim]HSO4: An efficient cata-
lyst for acetalization and thioacetalization of carbonyl compounds and their subsequent
deprotection. Catal. Commun. 2007, 8, 1323–1328.
17. (a) Khurana, J. M.; Lumb, A.; Pandey, A.; Magoo, D. Green approaches for the synthesis
of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones in aqueous media and microwave

2154
J. M. KHURANA ET AL.
irradiation in solventless condition. Synth. Commun. 2012, 42, 1796–1803; (b) Khurana,
J. M.; Nand, B.; Kumar, S. Rapid synthesis of polyfunctionalized pyrano[2,3-c]pyrazoles
via multicomponent condensation in room-temperature ionic liquids. Synth. Commun.
2011, 41, 405–410; (c) Khurana, J. M.; Magoo, D. pTSA-catalyzed one-pot synthesis of
12-aryl-8,9,10,12-tetrahydrobenzo[a] xanthen-11-ones in ionic liquid and neat conditions.
Tetrahedron Lett. 2009, 50, 4777–4780; (d) Khurana, J. M.; Nand, B.; Saluja, P. DBU: A
highly efficient catalyst for one-pot synthesis of substituted 3,4-dihydropyrano[3,2-c]chro-
menes, dihydropyrano[4,3-b]pyranes, 2-amino-4H-benzo[h]chromenes, and 2-amino-4H
benzo[g]chromenes in aqueous medium. Tetrahedron 2010, 66, 5637–5641.
18. Dandia, A.; Arya, K.; Khaturia, S.; Jain, A. K. Microwave-induced preparation of
biologically important benzothiazolo[2,3-b]quinazolines, and comparison with ultrasonic
and classical heating. Monatsh. Chem. 2010, 141, 979–985.
19. Wang, W.; Shao, L.; Cheng, W.; Yang, J.; He, M. Brønsted acidic ionic liquids as novel
catalysts for Prins reaction. Catal. Commun. 2008, 9, 337–341.