An efficient synthesis of novel 3-hydroxy-12-arylbenzo[ A ]xanthen-11-ones and 5,12-diarylxantheno[2,1- A ]xanthene-4,12-diones using p TSA in [bmim]BF4

Article abstract of DOI:10.1139/cjc-2012-0494

We introduce a simple, efficient, and environmentally benign synthesis of a series of novel xanthenes via one-pot condensation of 2,6-dihydroxynaphthalene, aldehydes, and dimedone in [bmim]BF₄ with pTSA as catalyst in high yields.

Full text of DOI:10.1139/cjc-2012-0494

698
ARTICLE
An efficient synthesis of novel 3-hydroxy-12-arylbenzo[a]xanthen-11-ones
and 5,12-diarylxantheno[2,1-a]xanthene-4,12-diones using pTSA in
[bmim]BF4
Sneha Yadav, Bhaskara Nand, and Jitender M. Khurana
Abstract: We introduce a simple, efficient, and environmentally benign synthesis of a series of novel xanthenes via one-pot
condensation of 2,6-dihydroxynaphthalene, aldehydes, and dimedone in [bmim]BF4 with pTSA as catalyst in high yields.
Key words: mono- and bis(tetrahydrobenzo[a]xanthen-11-ones), ionic liquid, one-pot condensation, pTSA.
Résumé : On présente une synthèse simple, efficace et respectueuse de l'environnement d'une série de nouveaux xanthènes au
moyen d'une condensation monotope du 2,6-dihydroxynaphthalène, d'aldéhydes et de dimédone en solution dans [bmim]BF4 en
présence d'APTS comme catalyseur qui donne de hauts rendements. [Traduit par la Rédaction]
Mots-clés : mono- et bis(tétrahydrobenzo[3 a]xanthén-11-ones), liquide ionique, condensation monotope, APTS.
thetic routes for scaffold manipulation of xanthene derivatives.
Introduction
The synthesis of tetrahydrobenzo[a]xanthen-11-ones has been
Ionic liquids are rapidly substituting the classical organic sol-
reported in the presence of strontium triflate,25 NH2SO3H,26
vents on account of the interesting advantages that they offer
NaHSO4.SiO2 under reflux in halogenated solvents for long hours,24
such as extremely low vapor pressure, excellent thermal stability,
TBAF,27 and manganese perchlorate.28 As part of our ongoing re-
reusability, and ability to dissolve many organic and inorganic
search29,30 on the synthesis of xanthenes, we report a facile route for
the preparation of novel xanthenes from a one-pot reaction of alde-
hydes, 2,6-dihydoxynaphthalene, and 5,5-dimethylcyclohexane-1,3-
dione (dimedone) in ionic liquid [bmim]BF4 in the presence of a
catalytic amount of pTSA.
substrates.1 The use of ionic liquids as reaction medium may
offer a convenient solution to both the solvent emission prob-
lem and the catalytic recycling problem.2 Room temperature
ionic liquids,3 especially those based on the 1-N-alkyl–3-methyl
imidazolium cation, have attracted much attention in recent
years as alternative green reaction media and have found growing
applications in organic reactions. Several examples of these reac-
tions include hydrogenations,4 Friedel–Crafts reactions,5 Heck reac-
tions,6 Bishler–Napieralski reactions,7 olefin hydrodimerizations,8
and olefin dimerizations.9
Xanthenes and related compounds have been of significant in-
terest because of their broad spectrum of pharmaceutical impor-
tance, such as analgesic,10 anti-bacterial,11 anti-inflammatory,12
and anti-viral activities.13 These compounds are being used as
antagonists for the paralyzing action of zoxazolamine14 and in
photodynamic therapy.15
Some xanthene derivatives have been investigated for agricul-
tural bactericide activity.16 Some other derivatives have commer-
cial application as dyes in laser technology17 and fluorescent
materials for visualization of biomolecules.18 Many procedures
have been reported for the synthesis of xanthene derivatives, in-
cluding cyclodehydrations,19 ␥-alkylations to the heteroatoms,20
trapping of benzynes by phenols,21 cyclocondensation between
2-hydroxyaromatic aldehydes and 2-tetralone,22 the reaction of
␤-naphthol with aldehydes or acetals under acidic conditions, and
intramolecular phenyl carbonyl coupling reactions of benzaldehydes
and acetophenones.23 A variety of 12-aryl-9,9-dimethyl-8,9,10,12-
tetrahydro-11H-benzo[a]xanthen-11-ones have been synthesized
from the condensation of ␤-naphthol, aldehydes, and cyclic 1,3-
dicarbonyl compounds.16,24
Experimental
All of the chemicals used in the synthesis were purchased from
Sigma-Aldrich and used as received. TLC was used to monitor
reaction progress. Melting points were determined on a melting
point apparatus and are uncorrected. 1H NMR and 13C NMR spec-
tra were recorded on a JEOL JNM ECX-400P (400 MHz) with CDCl3
or DMSO-d6 as solvent and TMS as the internal standard. IR spec-
tra were recorded on a Perkin-Elmer FT-IR SPECTRUM-2000. The
chemical shift values are recorded on the ␦ scale and the coupling
constants (J) are in hertz. Mass spectra were recorded on a JEOL-
AccuTOF JMS-T100 mass spectrometer having a DART source.
General procedure for the synthesis of 3-hydroxy-12-aryl-9,
9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one
(IIa–IIk)
A mixture of aldehyde (1 mmol), 2,6-dihydroxynaphthalene
(1 mmol), dimedone (1.1 mmol), and pTSA (2 mol %) was placed into a
50 mL round-bottomed flask containing 0.5 mL of [bmim]BF4. The
mixture was stirred at 60 °C for an appropriate time as mentioned in
Table 2. After completion of the reaction as monitored by TLC, the
mixture was allowed to cool to room temperature and quenched
with water (ϳ10 mL). The precipitate formed was collected by filtra-
tion pump, washed with ethanol, and dried to obtain pure xanthen-
11-one derivatives. The products were characterized by IR, 1H NMR,
13C NMR, and mass spectroscopy.
Thus, a broad utility range has made xanthenes prime synthetic
candidates thereby accentuating the need to develop newer syn-
Received 25 December 2012. Accepted 12 March 2013.
S. Yadav, B. Nand, and J.M. Khurana. Department of Chemistry, University of Delhi, Delhi 110007, India.
Corresponding author: Jitender M. Khurana (e-mail: jmkhurana1@yahoo.co.in).
Can. J. Chem. 91: 698–703 (2013) dx.doi.org/10.1139/cjc-2012-0494
Published at www.nrcresearchpress.com/cjc on 10 April 2013.

Yadav et al.
699
Spectral data
12-(2,4-Dichlorophenyl)-3-hydroxy-9,9-dimethyl-8,9,10,12-
tetrahydro-benzo[a]xanthen-11-one (IIe)
12-(4-Chlorophenyl)-3-hydroxy-9,9-dimethyl-8,9,10,12-tetrahydro-
benzo[a]xanthen-11-one (IIa)
Yellow solid; yield 90%; mp 175 °C. IR (KBr, cm−1) ␷max: 3399,
2929, 1645, 1595, 1468, 1378, 1233, 1214, 1159. ␦H (400 MHz, DMSO-
d6): 9.77 (s, 1H, OH), 7.85 (d, J = 8.8 Hz, 1H, Ar-H), 7.65 (d, J = 8.8 Hz,
1H, Ar-H), 7.41 (s, 1H, Ar-H), 7.28–7.22 (m, 3H, Ar-H), 7.10–7.09 (m,
1H, Ar-H), 7.03–6.99 (m, 1H, Ar-H), 5.68 (s, 1H, CH), 2.66 and 2.51 (AB
system, J = 17.5 Hz, 2H, CHa.HbCMe2), 2.46–2.45 (m, 1H, CHa.HbCO),
2.28 (d, J = 16.0 Hz, 1H, CHa.HbCO), 1.02 (s, 3H, CH3), 0.85 (s, 3H,
CH3). ␦C (100 MHz, DMSO-d6): 206.82, 195.95, 164.59, 154.76, 145.38,
141.21, 132.89, 132.79, 131.70, 128.96, 127.91, 127.59, 124.88, 124.51,
119.39, 117.45, 110.29, 79.24, 50.20, 40.43, 31.87, 30.78, 28.95, 26.25.
MS (ESI) m/z calcd.: 438.08; found: 439.15 [M++H], 441.15 [(M++H)+2].
Anal. calcd. for C25H20Cl2O3: C, 68.35; H, 4.59; Cl, 16.14%; found: C,
68.32; H, 4.53; Cl, 16.08%.
Colorless solid; yield 93%; mp 250–252 °C. IR (KBr, cm−1) ␷
:
max
3101, 2963, 1617, 1588, 1489, 1388, 1223, 1212, 1162. ␦H (400 MHz,
DMSO-d6): 9.65 (s, 1H, OH), 7.76 (d, J = 8.0 Hz, 1H, Ar-H), 7.60 (d, J =
8.0 Hz, 1H, Ar-H), 7.26–7.24 (m, 1H, Ar-H), 7.21–7.14 (m, 4H, Ar-H),
7.06–7.05 (m, 1H, Ar-H), 6.99–6.96 (m, 1H, Ar-H), 5.43 (s, 2H, CH),
2.58 (d, J = 17.4 Hz, 1H, CHa.HbCMe2), 2.43 (d, J = 11.9 Hz, 1H,
CHa.HbCMe2), 2.24 (d, J = 16.3 Hz, 1H, CHa.HbCO), 2.03 (d, J = 20.0 Hz,
1H, CHa.HbCO), 0.96 (s, 3H, CH3), 0.78 (s, 3H, CH3). ␦C (100 MHz,
DMSO-d6): 206.68, 196.03, 164.21, 154.72, 145.13, 144.04, 132.83,
130.73, 129.97, 128.14, 127.50, 124.82, 124.55, 119.43, 117.34, 116.83,
112.63, 110.04, 50.12, 33.72, 31.93, 30.73, 28.85, 26.24. MS (ESI) m/z
calcd.: 404.12; found: 405.21 [M++H]. Anal. calcd. for C25H21ClO3: C,
74.16; H, 5.23; Cl, 8.76%; found: C, 74.10; H, 5.18; Cl, 8.72%.
3-Hydroxy-9,9-dimethyl-12-(4-methoxyphenyl)-8,9,10,
12-tetrahydro-benzo[a]xanthen-11-one (IIf)
12-(4-Bromophenyl)-3-hydroxy-9,9-dimethyl-8,9,10,12-tetrahydro-
Colorless solid; yield 88%; mp 238–240 °C. IR (KBr, cm−1) ␷
:
benzo[a]xanthen-11-one (IIb)
max
Colorless solid; yield 90%; mp 208–210 °C. IR (KBr, cm−1) ␷
:
3062, 2962, 1621, 1589, 1508, 1391, 1249, 1222, 1034. ␦H (400 MHz,
DMSO-d6): 9.61 (s, 1H, OH), 7.79 (d, J = 9.5 Hz, 1H, Ar-H), 7.58 (d, J =
8.8 Hz, 1H, Ar-H), 7.24 (d, J = 8.8 Hz, 1H, Ar-H), 7.07–7.03 (m, 3H,
Ar-H), 6.98–6.95 (m, 1H, Ar-H), 6.89 (d, J = 8.0 Hz, 2H, Ar-H), 5.36
(s, 1H, CH), 3.26 (s, 3H, OCH3), 2.58 (d, J = 16.4 Hz, 1H, CHa.HbCMe2),
2.44–2.42 (m, 2H, CH2), 2.23 (d, J = 16.0 Hz, 1H, CH2CO), 0.97
(s, 3H, CH3), 0.79 (s, 3H, CH3). ␦C (100 MHz, DMSO-d6): 195.87, 163.80,
154.58, 145.02, 142.18, 135.12, 132.71, 128.66, 127.96, 127.08, 124.86,
124.65, 119.21, 117.55, 117.26, 113.08, 109.89, 50.12, 40.28, 33.76,
31.89, 28.88, 26.23, 20.49. MS (ESI) m/z calcd.: 400.17; found: 401.21
[M++H]. Anal. calcd for C26H24O4: C, 77.98; H, 6.04; found: C, 77.92;
H, 5.99%.
max
3093, 2962, 2771, 1589, 1484, 1391, 1234, 1221, 1163, 1010. ␦H (400
MHz, DMSO-d6): 9.73 (s, 1H, OH), 7.82 (d, J = 9.5 Hz, 1H, Ar-H), 7.67
(d, J = 9.5 Hz, 1H, Ar-H), 7.37–7.33 (m, 3H, Ar-H), 7.20 (d, J = 8.04 Hz,
2H, Ar-H), 7.13–7.10 (m, 1H, Ar-H), 7.05–7.02 (m, 1H, Ar-H), 5.48 (s,
1H, CH), 2.64 (d, J = 18.3 Hz, 1H, CHa.HbCMe2), 2.55–2.51 (m, 1H,
CHa.HbCMe2), 2.31 (d, J = 16.0 Hz, 1H, CHa.HbCO), 2.12–2.07 (m, 1H,
CHa.HbCO), 1.03 (s, 3H, CH3), 0.85 (s, 3H, CH3). ␦C (100 MHz, DMSO-
d6): 195.96, 164.14, 154.70, 145.07, 144.43, 132.79, 131.02, 130.77,
130.34, 127.48, 124.78, 124.51, 119.40, 119.23, 117.30, 116.73, 112.53,
110.01, 50.08, 33.78, 31.91, 28.83, 28.64, 26.48, 26.23. MS (ESI) m/z
calcd.: 448.07; found: 449.33 [M++H], 451.14 [(M++H)+2]. Anal. calcd.
for C25H21BrO3: C, 66.82; H, 4.71; Br, 17.78%; found: C, 66.74; H,
4.66; Br, 17.70%.
12-(3,4-Dimethoxyphenyl)-3-hydroxy–9,9-dimethyl-8,9,10,
12-tetrahydro-benzo[a]xanthen-11-one (IIg)
3-Hydroxy-9,9-dimethyl-12-(4-trifluoromethyl-phenyl)-8,9,10,
Yellow solid; yield 91%; mp 260–265 °C. IR (KBr, cm−1) ␷max: 3296,
2956, 1628, 1589, 1520, 1381, 1265, 1225, 1140, 1028. ␦H (400 MHz,
DMSO-d6): 9.67 (s, 1H, OH), 7.91 (d, J = 8.8 Hz, 1H, Ar-H), 7.62 (d, J =
8.8 Hz, 1H, Ar-H), 7.28 (d, J = 8.8 Hz, 1H, Ar-H), 7.09–7.08 (m, 1H,
Ar-H), 7.04–7.01 (m, 1H, Ar-H), 6.90–6.89 (m, 1H, Ar-H), 6.69–6.67
(m, 1H, Ar-H), 6.63–6.61 (m, 1H, Ar-H), 5.40 (s, 1H, CH), 3.61 (s, 3H,
OCH3), 3.57 (s, 3H, OCH3), 2.62 (d, J = 17.6 Hz, 1H, CHa.HbCMe2), 2.51
(d, J = 16.8 Hz, 1H, CHa.HbCMe2), 2.28 (d, J = 16.1 Hz, 1H, CHa.HbCO),
2.08 (d, J = 16.0 Hz, 1H, CHa.HbCO), 1.01 (s, 3H, CH3), 0.85 (s, 3H, CH3).
12-tetrahydro-benzo[a]xanthen-11-one (IIc)
Colorless solid; yield 90%; mp 245–247 °C. IR (KBr, cm−1) ␷
:
max
3129, 3073, 2960, 1622, 1591, 1419, 1388, 1320, 1216, 1136. ␦H (400
MHz, DMSO-d6): 9.74 (s, 1H, OH), 7.75 (d, J = 8.8 Hz, 1H, Ar-H), 7.61
(d, J = 8.0 Hz, 1H, Ar-H), 7.45–7.39 (m, 4H, Ar-H), 7.27 (d, J = 9.5 Hz,
1H, Ar-H), 7.05–7.04 (m, 1H, Ar-H), 6.98–6.96 (m, 1H, Ar-H), 5.51 (s,
1H, CH), 2.57 (d, J = 16.0 Hz, 1H, CHa.HbCMe2), 2.44–2.41 (m, 1H,
CHa.HbCMe2), 2.25–2.18 (m, 1H, CHa.HbCO), 2.03–1.98 (m, 1H,
CHa.HbCO), 0.94 (s, 3H, CH3), 0.75 (s, 3H, CH3). ␦C (100 MHz, DMSO-
d6): 196.27, 164.66, 154.86, 149.65, 145.24, 132.96, 129.10, 128.30,
127.80, 126.83, 125.66, 125.21, 124.85, 124.63, 119.64, 117.53, 116.58,
112.43, 110.22, 50.17, 40.12, 34.42, 32.05, 28.91, 26.36. MS (ESI) m/z
calcd.: 438.14; found: 439.21 [M++H]. Anal. calcd. for C26H21F3O3: C,
71.23; H, 4.83; F, 13.00%; found: C, 71.19; H, 4.78; F, 12.97%.
␦
(100 MHz, DMSO-d6): 196.05, 163.85, 154.60, 148.13, 147.05,
C
145.04, 137.71, 132.73, 127.05, 125.06, 124.76, 120.09, 119.22, 117.67,
117.27, 113.18, 112.22, 111.46, 109.89, 55.46, 55.30, 50.15, 33.58, 31.91,
28.92, 26.18. MS (ESI) m/z calcd.: 430.18; found: 431.25 [M++H]. Anal.
calcd. for C27H26O5: C, 75.33; H, 6.09%; found: C, 75.30; H, 6.02%.
3-Hydroxy-9,9-dimethyl-12-(2-methylphenyl)-8,9,10,
12-tetrahydrobenzo[a]xanthen-11-one (IIh)
Colorless solid; yield 90%; mp 245–248 °C. IR (KBr, cm−1) ␷
12-(3-Bromophenyl)-3-hydroxy-9,9-dimethyl-8,9,10,12-tetrahydro-
benzo[a]xanthen-11-one (IId)
Colorless solid; yield 89%; mp 203–205 °C. IR (KBr, cm−1) ␷
:
:
max
max
3183, 2955, 1648, 1617, 1592, 1458, 1382, 1229, 1213, 1148. ␦H
(400 MHz, DMSO-d6): 9.63 (s, 1H, OH), 7.58–7.56 (m, 2H, Ar-H), 7.23
(d, J = 8.0 Hz, 1H, Ar), 7.03–6.96 (m, 3H, Ar-H), 6.86–6.82 (m, 3H,
Ar-H), 5.44 (s, 1H, CH), 2.73 (s, 3H, CH3), 2.57 (d, J = 16.8 Hz, 1H,
CHa.HbCMe2), 2.44–2.41 (m, 1H, CHa.HbCMe2), 2.21 (d, J = 16.1 Hz,
1H, CHa.HbCO), 2.00 (d, J = 15.4 Hz, 1H, CHa.HbCO), 0.95 (s, 3H, CH3),
0.76 (s, 3H, CH3). ␦C (100 MHz, DMSO-d6): 196.21, 163.91, 154.60,
145.20, 143.73, 134.59, 132.66, 130.45, 129.24, 127.17, 126.11, 126.00,
124.82, 124.35, 119.20, 118.38, 117.34, 113.63, 110.06, 50.24, 40.43,
31.74, 31.15, 28.83, 26.12, 19.43. MS (ESI) m/z calcd.: 384.17; found:
385.23 [M++H]. Anal. calcd. for C26H24O3: C, 81.22; H, 6.29%; found:
C, 81.15; H, 6.27%.
3344, 2926, 1654, 1623, 1517, 1376, 1289, 1226. ␦H (400 MHz, DMSO-
d6): 9.76 (s, 1H, OH), 7.84 (d, J = 9.5 Hz, 1H, Ar-H), 7.66 (d, J = 9.5 Hz,
1H, Ar-H), 7.43–7.41 (m, 1H, Ar-H), 7.31 (d, J = 10.2 Hz, 1H, Ar-H),
7.23–7.17 (m, 2H, Ar-H), 7.12–7.08 (m, 2H, Ar-H), 7.04–7.01 (m, 1H,
Ar-H), 5.47 (s, 1H, CH), 2.66–2.51 (m, 2H, CH2CMe2), 2.29 (d, J =
16.1 Hz, 1H, CHa.HbCO), 2.08 (d, J = 15.3 Hz, 1H, CHa.HbCO), 1.01 (s, 3H,
CH3), 0.82 (s, 3H, CH3). ␦C (100 MHz, DMSO-d6): 195.99, 164.37,
154.76, 147.65, 145.14, 132.81, 130.72, 130.41, 129.18, 127.59, 127.19,
124.83, 124.50, 121.41, 119.49, 117.32, 116.57, 112.49, 110.04, 50.07,
33.94, 31.96, 30.72, 28.80, 26.16. MS (ESI) m/z calcd.: 448.07; found:
449.14 [M++H], 451.13 [(M++H)+2]. Anal. calcd. for C25H21BrO3: C,
66.82; H, 4.71; Br, 17.78%; found: C, 66.76; H, 4.65; Br, 17.72%.
Published by NRC Research Press

700
Can. J. Chem. Vol. 91, 2013
3-Hydroxy-12-(4-isopropylphenyl)-9,9-dimethyl-8,9,10,
12-tetrahydro-benzo[a]xanthen-11-one (IIi)
C
40H34Br2O4: C, 65.05; H, 4.64; Br, 21.64%; found: C, 64.98; H, 4.61; Br,
21.58%.
Colorless solid; yield 92%; mp 240–242 °C. IR (KBr, cm−1) ␷max: 3163,
2957, 1624, 1591, 1382, 1225, 1152, 1031. ␦H (400 MHz, DMSO-d6): 9.59 (s,
1H, OH), 7.79 (d, J = 9.5 Hz, 1H, Ar-H), 7.53 (d, J = 8.8 Hz, 1H, Ar-H), 7.20
(d, J = 8.8 Hz, 1H, Ar-H), 7.06–7.01 (m, 3H, Ar-H), 6.95–6.89 (m, 3H,
Ar-H), 5.34 (s, 1H, CH), 2.60–2.55 (m, 1H, CHMe2), 2.46–2.37 (m, 2H,
CH2CMe2), 2.17 (d, J = 16.0 Hz, 1H, CHa.HbCO), 1.99 (d, J = 16.1 Hz, 1H,
CHa.HbCO), 0.94 (d, J = 6.6 Hz, 6H, CHMe2), 0.91 (s, 3H, CH3), 0.76 (s,
3H, CH3). ␦C (100 MHz, DMSO-d6): 196.53, 164.56, 155.23, 146.59,
145.66, 143.12, 133.34, 128.55, 127.64, 126.62, 125.45, 125.28, 119.90,
118.29, 117.92, 113.77, 110.55, 50.76, 40.92, 34.32, 33.47, 32.53, 29.37,
27.06, 24.44, 24.25. MS (ESI) m/z calcd.: 412.20, found: 413.26 [M++H].
Anal. calcd. for C28H28O3: C, 81.52; H, 6.84%; found: C, 81.44; H, 6.80%.
2,2,10,10-Tetramethyl-5,13-bis(4-nitrophenyl)-2,3,9,10,11,
13-hexahydroxantheno [2,1-a]xanthene-4,12(1H,5H)-dione (IIIc)
Colorless solid; yield 89%; mp >300 °C. IR (KBr, cm−1) ␷max: 3435,
2960, 1711, 1654, 1596, 1518, 1378, 1346, 1241, 1221. ␦H (400 MHz,
CDCl3): 8.0 (d, J = 8.7 Hz, 4H, Ar-H), 7.81 (d, J = 9.1 Hz, 2H, Ar-H),
7.46–7.42 (m, 4H, Ar-H), 7.30 (d, J = 9.1 Hz, 2H, Ar-H), 5.76 (s, 2H,
CH), 2.54–2.45 (m, 4H, CH2CMe2), 2.28–2.17 (m, 4H, CH2CO), 1.06 (s,
6H, CH3), 0.89 (s, 6H, CH3). ␦C (100 MHz, CDCl3): 207.07, 196.70,
164.54, 151.52, 147.29, 146.32, 129.24, 124.47, 123.67, 118.46, 117.09,
112.60, 50.65, 41.24, 34.85, 32.25, 30.91, 29.01, 27.13. MS (ESI) m/z
calcd.: 670.23; found: 671.33 [M++H]. Anal. calcd. for C40H34N2O8:
C, 71.63; H, 5.11; N, 4.18%; found: C, 71.57; H, 5.06; N, 4.13%.
3-Hydroxy-9,9-dimethyl-12-(1-naphthyl)-8,9,10,
12-tetrahydrobenzo[a]xanthen-11-one (IIj)
2,2,10,10-Tetramethyl-5,13-bis(4-(trifluoromethyl)phenyl)-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIId)
Colorless solid; yield 93%; mp >300 °C. IR (KBr, cm−1) ␷max: 2927,
1647, 1378, 1325, 1219, 1121, 1067. ␦H (400 MHz, CDCl3): 7.87 (d, J =
9.1 Hz, 2H, Ar-H), 7.38–7.43 (m, 8H, Ar-H), 7.26 (d, J = 9.1 Hz, 2H, Ar-H),
Yellow solid; yield 87%; mp 285 °C. IR (KBr, cm−1) ␷max: 3448,
2935, 1623, 1516, 1374, 1287, 1238, 1225, 1155, 1024. ␦H (400 MHz,
DMSO-d6): 9.70 (s, 1H, OH), 7.81 (d, J = 8.0 Hz, 1H, Ar-H), 7.67–7.59
(m, 5H, Ar-H), 7.51–7.47 (m, 1H, Ar-H), 7.34 (d, J = 8.8 Hz, 1H, Ar-H),
7.26–7.23 (m, 1H, Ar-H), 7.15–7.13 (m, 1H, Ar-H), 7.05–7.04 (m, 1H,
Ar-H), 6.74–6.71 (m, 1H, Ar-H), 6.20 (s, 1H, CH), 2.66 and 2.54 (AB
system, J = 17.6 Hz, 2H, CHa.HbCMe2), 2.24 (d, J = 16.1 Hz, 1H,
CHa.HbCO), 1.97 (d, J = 16.1 Hz, 1H, CHa.HbCO), 1.00 (s, 3H, CH3), 0.76
(s, 3H, CH3). ␦C (100 MHz, DMSO-d6): 196.19, 163.77, 154.50, 145.13,
132.71, 128.40, 127.11, 126.90, 126.11, 125.57, 125.04, 124.66, 118.99,
117.50, 110.01, 50.15, 48.62, 40.11, 31.76, 28.84, 26.15. MS (ESI) m/z
calcd.: 420.17; found: 421.23 [M++H]. Anal. calcd. for C29H24O3: C,
82.83; H, 5.75%; found: C, 82.78; H, 5.70%.
5.74 (s, 2H, CH), 2.55 and 2.48 (AB system, J = 16.9 Hz, 4H, CHa.Hb
-
CMe2), 2.28 and 2.22 (AB system, J = 16.4 Hz, 4H, CHa.HbCO), 1.08 (s,
6H, CH3), 0.94 (s, 6H, CH3). ␦C (100 MHz, CDCl3): 196.81, 164.27,
148.34, 147.26, 129.28, 128.65, 128.21, 125.30, 124.41, 122.58, 118.28,
117.63, 113.19, 50.75, 41.29, 34.74, 32.28, 28.99, 27.29. MS (ESI) m/z
calcd.: 716.24; found: 717.38 [M++H]. Anal. calcd. for C42H34F6O4: C,
70.38; H, 4.78; F, 15.90%; found: C, 70.33; H, 4.72; F, 15.86%.
2,2,10,10-Tetramethyl-5,13-bis(3-nitrophenyl)-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIe)
Colorless solid; yield 90%; mp >300 °C. IR (KBr, cm−1) ␷max: 3425,
3097, 2960, 1642, 1594, 1528, 1409, 1349, 1220, 1165, 1011. ␦H
(400 MHz, CDCl3): 7.90–7.86 (m, 2H, Ar-H), 7.71–7.64 (m, 4H, Ar-H), 7.47–
7.41 (m, 2H, Ar-H), 7.17–7.12 (m, 4H, Ar-H), 5.56 (s, 2H, CH), 2.35–
2.27 (m, 4H, CH2CMe2), 2.10–1.93 (m, 4H, CH2CO), 0.85 (s, 6H, CH3),
0.69 (s, 6H, CH3). ␦C (100 MHz, CDCl3): 196.70, 164.37, 148.30,
147.38, 146.79, 134.68, 129.18, 124.52, 123.10, 121.67, 118.57, 112.83,
50.69, 41.26, 34.76, 32.31, 28.82, 27.12. MS (ESI) m/z calcd.: 670.23;
found: 671.35 [M++H]. Anal. calcd. for C40H34N2O8: C, 71.63; H, 5.11;
N, 4.18%; found: C, 71.55; H, 5.08; N, 4.11%.
General procedure for the synthesis of 5,13-diaryl-2,2,10,
10-tetramethyl-2,3,9,10,11,13-hexahydroxantheno[2,1-a]
xanthene-4,12(1H,5H)-dione (IIIa–IIIk)
A mixture of aldehyde (2.0 mmol), 2,6-dihydroxynaphthalene
(1.0 mmol), dimedone (2.2 mmol), and pTSA (5 mol%) was placed
into a 50 mL round-bottomed flask containing 1 mL of [bmim]BF4.
The mixture was stirred at 90 °C for an appropriate time. After
completion of the reaction as monitored by TLC, the mixture was
allowed to cool to room temperature and quenched with water
(ϳ10 mL). The precipitate formed was collected by filtration at
pump, recrystallized with chloroform, and dried to obtain pure
xanthene-4,12-dione derivatives.
5,13-Bis(2-chlorophenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIf)
Colorless solid; yield 91%; mp >300 °C. IR (KBr, cm−1) ␷max: 3168,
2954, 1651, 1629, 1599, 1471, 1371, 1236, 1166, 1010. ␦H (400 MHz,
CDCl3): 8.12 (d, J = 8.7 Hz, 1H, Ar-H), 7.58 (d, J = 9.0 Hz, 1H, Ar-H),
7.31–7.28 (m, 1H, Ar-H), 7.23–7.18 (m, 2H, Ar-H), 7.10–7.09 (m, 2H,
Ar-H), 7.08–6.97 (m, 5H, Ar-H), 5.94 (s, 2H, CH), 2.59–2.53 (m, 4H,
CH2CMe2), 2.33–2.20 (m, 4H, CH2CO), 1.13 (s, 6H, CH3), 0.99 (s, 6H,
CH3). ␦C (100 MHz, CDCl3): 212.57, 196.50, 153.01, 132.60, 130.89,
129.76, 127.49, 126.55, 125.60, 118.23, 117.10, 110.29, 50.55, 41.48, 31.83,
29.00, 26.54. MS (ESI) m/z calcd.: 648.18; found: 649.31 [M++H], 651.31
[(M++H)+2], 653.32 [(M++H)+4]. Anal. calcd. for C40H34Cl2O4: C, 73.96;
H, 5.28; Cl, 10.92%; found: C, 73.89; H, 5.25; Cl, 10.87%.
5,13-Bis(4-chlorophenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno [2,1-a]xanthene-4,12(1H,5H)-dione (IIIa)
Colorless solid; yield 92%; mp >300 °C. IR (KBr, cm−1) ␷max: 2726,
1647, 1595, 1377, 1218, 1165, 1010. ␦H (400 MHz, CDCl3): 7.84 (d, J =
8.8 Hz, 2H, Ar-H), 7.27 (s, 1H, Ar-H), 7.20–7.18 (m, 5H, Ar-H), 7.10 (d, J =
8.0 Hz, 4H, Ar-H), 5.63 (s, 2H, CH), 2.54–2.43 (m, 4H, CH2CMe2), 2.27–
2.16 (m, 4H, CH2CO), 1.05 (s, 6H, CH3), 0.92 (s, 6H, CH3). ␦C (100 MHz,
DMSO-d6): 196.84, 164.02, 146.98, 142.63, 139.42, 132.02, 129.67, 129.31,
128.44, 124.34, 118.13, 113.49, 50.79, 41.30, 34.21, 32.26, 29.09, 27.24.
MS (ESI) m/z calcd.: 648.18; found: 649.12 [M++H], 651.14 [(M++H)+2],
653.17 [(M++H)+4]. Anal. calcd. for C40H34Cl2O4: C, 73.96; H, 5.28; Cl,
10.92%; found: C, 73.91; H, 5.24; Cl, 10.85%.
5,13-Bis(4-methoxyphenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIg)
Colorless solid; yield 85%. mp >300 °C. IR (KBr, cm−1) ␷max: 2954,
1651, 1609, 1510, 1405, 1374, 1226, 1177. ␦H (400 MHz, CDCl3): 7.91 (d, J =
9.1 Hz, 2H, Ar-H), 7.27–7.25 (m, 3H, Ar-H), 7.19–7.17 (m, 3H, Ar-H),
6.67–6.70 (m, 4H, Ar-H), 5.62 (s, 2H, CH), 3.68 (s, 6H, OCH3), 2.52 and
2.45 (AB system, J = 17.4 Hz, 4H, CHa.HbCMe2), 2.26 and 2.21 (AB
system, J = 16.0 Hz, 4H, CHa.HbCO), 1.06 (s, 6H, CH3), 0.94 (s, 6H, CH3).
5,13-Bis(4-bromophenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno [2,1-a]xanthene-4,12(1H,5H)-dione (IIIb)
Colorless solid; yield 91%; mp >300 °C. IR (KBr, cm−1) ␷max: 2726,
1647, 1595, 1377, 1218, 1165, 1010. ␦H (400 MHz, CDCl3): 7.84 (d, J =
10.5 Hz, 2H, Ar-H), 7.27–7.25 (m, 6 H, Ar-H), 7.13 (d, J = 8.6 Hz, 4H, Ar-H),
5.61 (s, 2H, CH), 2.51 and 2.45 (AB system, J = 17.7 Hz, 4H, CHa.HbCMe2),
2.25 and 2.19 (AB system, J = 16.4 Hz, 4H, CHa.HbCO), 1.05 (s, 6H, CH3),
0.92 (s, 6H, CH3). ␦C (100 MHz, CDCl3): 196.85, 164.04, 147.17, 143.58,
131.38, 130.05, 129.28, 124.33, 120.19, 118.14, 117.83, 113.38, 50.77, 41.27,
34.29, 32.26, 29.03, 27.25. MS (ESI) m/z calcd.: 738.08; found: 739.23
[M++H], 741.22 [(M++H)+2], 743.27 [(M++H)+4]. Anal. calcd. for
␦
(100 MHz, CDCl3): 27.31, 29.01, 32.27, 33.85, 41.31, 50.87, 55.07,
C
113.61, 114.10, 117.92, 118.59, 124.23, 129.24, 129.42, 137.10, 147.07,
157.77, 163.68, 197.02. MS (ESI) m/z calcd.: 640.28; found: 641.40
[M++H]. Anal. calcd. for C42H40O6: C, 78.73; H, 6.29%; found: C, 78.67;
H, 6.26%.
Published by NRC Research Press

Yadav et al.
701
Table 1. Effect of different ionic liquids and catalyst amount on the condensation of
4-chlorobenzaldehyde, 2,6-dihydroxynaphthalene, and dimedone (molar ratio 1:1:1.1).
Entry
Solvent
Temperature (°C)
Catalyst
mol%
Time (h)
Yield (IIa) (%)
1
[bmim]Br
[bmim]Br
[bmim]Cl
[bmim]Cl
60
120
60
120
120
60
60
60
60
60
pTSA
pTSA
pTSA
pTSA
pTSA
pTSA
2
2
2
2
2
2
10
10
10
10
5
2
5
6
5
35a
42a
45a
37a
54a
93
2
3
4
5
6
7
8
9
10
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
55a
72
pTSA
pTSA
pTSA
0.5
1
5
80
92
2
aIncomplete reaction.
Table 2. Synthesis of 3-hydroxy-12-aryl-9,9-dimethyl-9,10-dihydro-8H-
benzo[a]xanthen-11(12H)-ones IIa–IIj.
Table 3. Synthesis of 5,13-diaryl-2,2,10,10-tetramethyl-2,3,9,10,11,13-
hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-diones IIIa–IIIk.
Entry
Ar (ArCHO)
Product
Time (h)
Yield (%)a
Entry
Ar (ArCHO)
Product
Time (h)
Yield (%)
1
4-ClC6H4 (Ia)
4-BrC6H4 (Ib)
4-F3CC6H4 (Ic)
3-BrC6H4 (Id)
2,4-Cl2C6H3 (Ie)
4-CH3OC6H4 (If)
3,4-(CH3O)2C6H3 (Ig)
2-H3CC6H4 (Ih)
4-Me2CHC6H4 (Ii)
1-Naphthyl (Ij)
IIa
IIb
IIc
IId
IIe
IIf
IIg
IIh
IIi
2
2
1.5
2
3
2
3
2
1.5
2.5
93
90
90
89
90
88
91
90
92
87
1
2
4-ClC6H4 (Ia)
4-BrC6H4 (Ib)
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
IIIg
IIIh
IIIi
2
2.5
3
1.5
2.5
2
3.5
3
92a
91a
89a
93a
90a
91a
85b
89b
91b
88b
90a
2
3
4
5
6
7
8
9
10
3
4
5
6
7
8
9
10
11
4-O2NC6H4 (Ik)
4-F3CC6H4 (Ic)
3-O2NC6H4 (Il)
2-ClC6H4 (Im)
4-CH3OC6H4 (If)
3,4-(CH3O)2C6H3 (Ig)
3-H3CC6H4 (In)
4-Me2CHC6H4 (Ii)
2-Naphthyl (Ij)
2
1.5
1.5
IIj
IIIj
IIIk
Note: Reaction conditions: aldehyde (1.0 mmol), 2,6-dihydroxynaphthalene
(1.0 mmol), dimedone (1.1 mmol), pTSA (2 mol%), 60 °C.
aIsolated yields.
aReaction of aldehyde, 2,6-dihydroxynaphthalene, and dimedone (2:1:2.2 molar
ratio) at 90 °C.
bReaction of aldehyde, 2,6-dihydroxynaphthalene, and dimedone (3:1:3.3 molar
ratio) at 90 °C.
5,13-Bis(3,4-dimethoxyphenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIh)
Yellow solid; yield 89%; mp >300 °C. IR (KBr, cm−1) ␷max: 2957,
1647, 1593, 1513, 1464, 1377, 1266, 1219, 1141, 1027. ␦H (400 MHz,
CDCl3): 7.96–7.93 (m, 2H, Ar-H), 7.27 (d, J = 9.1 Hz, 2H, Ar-H), 6.92–
6.89 (m, 2H, Ar-H), 6.72–6.61 (m, 4H, Ar-H), 5.64 (s, 2H, CH), 3.79 (s,
6H, OCH3), 3.73 (s, 6H, OCH3), 2.53–2.49 (m, 4H, CH2CMe2), 2.29–
2.23 (m, 4H, CH2CO), 1.08 (s, 6H, CH3), 0.95 (s, 6H, CH3). ␦C
(100 MHz, CDCl3): 197.12, 163.84, 148.54, 147.21, 137.45, 129.47, 124.29,
124.19, 120.47, 120.28, 118.44, 117.89, 114.06, 111.69, 110.75, 55.80,
55.64, 50.84, 41.32, 34.27, 34.13, 32.25, 29.36, 29.08, 27.17, 26.95. MS
(ESI) m/z calcd.: 700.30; found: 701.45 [M++H]. Anal. calcd. for
C44H44O8: C, 75.41; H, 6.33%; found: C, 75.35; H, 6.26%.
CHa.HbCMe2), 2.26–2.17 (m, 4H, CH2CO), 1.09 (d, J = 7.3 Hz, 12H,
CHMe2), 1.04 (s, 6H, CH3), 0.93 (s, 6H, CH3). ␦C (100 MHz, CDCl3):
196.98, 163.79, 147.08, 146.09, 141.99, 129.42, 128.04, 126.29, 124.27,
118.65, 117.93, 114.15, 50.87, 41.33, 34.28, 33.51, 32.32, 28.88, 27.54,
23.85, 23.77. MS (ESI) m/z calcd.: 664.36; found: 665.46 [M++H].
Anal. calcd. for C46H48O4: C, 83.10; H, 7.28%; found: C, 83.05; H,
7.25%.
2,2,10,10-Tetramethyl-5,13-di(naphthalen-2-yl)-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIk)
Colorless solid; yield 90%; mp >300 °C. IR (KBr, cm−1) ␷max: 3057,
2959, 1651, 1597, 1400, 1378, 1241, 1225, 1166, 1010. ␦H (400 MHz,
CDCl3): 8.01 (d, J = 8.0 Hz, 2H, Ar-H), 7.75–7.72 (m, 4H, Ar-H), 7.69–
7.62 (m, 4H, Ar-H), 7.41–7.38 (m, 3H, Ar-H), 7.36–7.34 (m, 3H, Ar-H),
7.29–7.26 (m, 2H, Ar-H), 5.84 (s, 2H, CH), 2.53 and 2.45 (AB system,
J = 17.4 Hz, 4H, CHa.HbCMe2), 2.24 and 2.17 (AB system, J = 16 Hz,
4H, CHa.HbCO), 1.04 (s, 6H, CH3), 0.89 (s, 6H, CH3). ␦C (100 MHz,
CDCl3): 196.88, 163.94, 147.23, 141.99, 133.23, 132.08, 129.52, 127.99,
127.40, 127.01, 126.54, 125.75, 125.45, 124.47, 118.25, 118.03, 113.60,
50.83, 41.32, 34.91, 32.23, 29.01, 27.29. MS (ESI) m/z calcd.: 680.29;
found: 681.42 [M++H]. Anal. calcd. for C48H40O4: C, 84.68; H, 5.92%;
found: C, 84.63; H, 5.87%.
2,2,10,10-Tetramethyl-5,13-di(m-tolyl)-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIi)
Colorless solid; yield 91%; mp >300 °C. IR (KBr, cm−1) ␷max: 2925,
1645, 1599, 1406, 1377, 1242, 1223, 1171, 1011. ␦H (400 MHz, CDCl3):
7.95 (d, J = 9.2 Hz, 1H, Ar-H), 7.29–7.26 (m, 3H, Ar-H), 7.07–7.01 (m,
6H, Ar-H), 6.85–6.84 (m, 2H, Ar-H), 5.63 (s, 2H, CH), 2.54 and 2.45
(AB system, J = 17.4 Hz, 4H, CHa.HbCMe2), 2.24–2.17 (m, 4H,
CH2CO), 1.06 (s, 6H, CH3), 0.95 (s, 6H, CH3). ␦C (100 MHz, CDCl3):
196.82, 163.84, 147.13, 144.59, 137.71, 129.47, 128.95, 128.08, 127.09,
125.43, 124.25, 118.55, 117.97, 114.06, 50.88, 41.35, 34.66, 32.32,
28.95, 27.39, 21.48. MS (ESI) m/z calcd.: 608.29; found: 609.38
[M++H]. Anal. calcd. for C42H40O4: C, 82.86; H, 6.62%; found: C,
82.83; H, 6.58%.
Recyclability of [bmim]BF4
5,13-Bis(4-isopropylphenyl)-2,2,10,10-tetramethyl-2,3,9,10,11,
13-hexahydroxantheno[2,1-a]xanthene-4,12(1H,5H)-dione (IIIj)
Colorless solid; yield 88%; mp >300 °C. IR (KBr, cm−1) ␷max: 3423,
2960, 1669, 1652, 1598, 1405, 1377, 1243, 1221, 1165. ␦H (400 MHz,
CDCl3): 7.93 (d, J = 9.5 Hz, 2H, Ar-H), 7.26 (s, 1H, Ar-H), 7.15 (d, J =
8.0 Hz, 4H, Ar-H), 6.99–6.94 (m, 5H, Ar-H), 5.61 (s, 2H, CH), 2.75–
2.68 (m, 2H, CHMe2), 2.52 and 2.42 (AB system, J = 17.6 Hz, 4H,
A reaction of 4-chlorobenzaldehyde (1 mmol), 2,6-dihydro-
xynaphthalene (1 mmol), and dimedone (1.1 mmol) was carried
out in 2 mol% pTSA at 60 °C. The precipitate formed was col-
lected by filtration pump, washed with water, and dried. The
filtrate was concentrated under reduced pressure and dried at
100 °C to recover the ionic liquid for subsequent use.
Published by NRC Research Press

702
Can. J. Chem. Vol. 91, 2013
Fig. 1. Reusability of [bmim]BF4 and the effect on isolated yields of IIa.
Results and discussion
We herein report the synthesis of novel 3-hydroxy-12-aryl-9,
9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one (IIa–IIj) and
5,13-diaryl-2,2,10,10-tetramethyl-2,3,9,10,11,13-hexahydroxantheno
[2,1-a]xanthene-4,12(1H,5H)-diones (IIIa–IIIk) by cyclocondensation of
aldehydes, 2,6-dihydroxynaphthalene, and 5,5-dimethylcyclohexane-
1,3-dione (dimedone) in [bmim]BF4 in the presence of pTSA as catalyst.
The initial reactions of 4-chlorobenzaldehyde (1.0 mmol) (Ia), 2,6-
dihydroxynaphthalene (1.0 mmol), and dimedone (1.1 mmol) were
attempted in different ionic liquids in the presence of pTSA to
identify the most suitable ionic liquid. Reactions carried out in
[bmim]Br and [bmim]Cl using 2 mol% pTSA at varied temperatures
were incomplete even after 10 h but yielded a new product, 12-(4-
chlorophenyl)-2-hydroxy-9,9-dimethyl-8,9,10,12-tetrahydrobenzo
[a]xanthen-11-one (IIa). The reaction was then attempted under
solvent-free conditions by heating the components at 120 °C. The
reaction was incomplete after 5 h and yielded only 54% of IIa after
workup. The reaction performed in [bmim]BF4 as reaction me-
dium in the presence of 2 mol% pTSA at 60 °C resulted in comple-
tion of the reaction in 2 h with the formation of IIa in 93% yield.
The optimum amount of pTSA was observed to be 2 mol%. A lower
amount of catalyst affected the yield of the product, while a
Reusability of ionic liquid
100
90
80
70
60
50
40
0
1
2
3
4
5
6
7
Number of cycles
higher concentration had no effect on the isolated yield. All of
these results are summarized in Table 1.
Having optimized the reaction conditions, a whole range of
electronically divergent aromatic aldehydes were then exploited
to successfully synthesize a series of xanthene derivatives in quan-
titative yields (eq. 1; Table 2):
O
O
OH
+
(1)
[bmim]BF4
+
HO
ArCHO
I
pTSA, 60oC
Ar
O
O
HO
IIa-j
O
O
O
Ar
O
OH
+
(2)
[bmim]BF4
pTSA, 90oC
+
2 ArCHO
2
O
Ar
O
HO
IIIa-k
Continuing our endeavours to form bis-condensation products
harnessing the other phenolic –OH,30 we focused our efforts on
the formation of novel bis-condensation products by executing
the reaction of 2,6-dihydroxynaphthalene with double molar ra-
tios of aldehydes and dimedone under varied reaction conditions.
The reaction between 4-chlorobenzaldehyde (2.0 mmol), 2,6-
dihydroxynaphthalene (1.0 mmol), and dimedone (2.2 mmol) resulted
in the formation of bis-condensed product 5,13-bis(4-chlorophenyl)-
2,2,10,10-tetramethyl-2,3,9,10,11,13-hexahydroxantheno[2,
1-a]xanthene-4,12(1H,5H)-dione IIIa in [bmim]BF4 in the pres-
ence of 5 mol% pTSA at 90 °C. A variety of novel bis-condensed
products were synthesized subsequently in high yields under
the above conditions by changing the aromatic aldehydes
(eq. 2; Table 3):
The reactions were complete in 1.5–3.5 h. It was noticed that the
reactions with electron-deficient aldehydes gave the correspond-
ing bis-products when the molar concentrations of aldehyde and
dimedone were doubled but electron-rich aldehydes demanded
higher concentrations (1:3:3) to afford the desired products.
The recyclability of the ionic liquid was also investigated for the
synthesis of product IIa. The recovered ionic liquid gave compa-
rable yields of 90%, 88%, and 85% in the second, third, and fourth
cycles, respectively. However, the yields went down to 79% and
72% in the fifth and sixth cycles, respectively, as depicted in Fig. 1.
diaryl-2,2,10,10-tetramethyl-2,3,9,10,11,13-hexahydroxantheno [2,
1-a]xanthene-4,12(1H,5H)-diones synthesized via condensation of
2,6-dihydroxynaphthalene, aromatic aldehydes, and dimedone in
high yields using pTSA as a catalyst and [bmim]BF4 as the reaction
medium. Our method avoids the use of expensive reagents and
harsh reaction conditions and is eco-friendly.
Acknowledgements
S.Y. and B.N. are thankful to CSIR, New Delhi, India, for the
award of Junior and Senior Research Fellowships.
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