A new green approach for the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one derivatives using task specific acidic ionic liquid [NMP]H2PO4

Article abstract of DOI:10.1016/j.cclet.2014.05.014

12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one derivatives were synthesized by a convenient and environmentally benign procedure involving multicomponent condensation reactions of substituted aromatic aldehydes with 2,7-dihydroxynaphthalene/2-naphtho

Full text of DOI:10.1016/j.cclet.2014.05.014

G Model
CCLET 2992 1–5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
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1
2
Original article
3
4
5
A new green approach for the synthesis of 12-aryl-8,9,10,
12-tetrahydrobenzo[a]xanthene-11-one derivatives using task
specific acidic ionic liquid [NMP]H2PO4
*
6
7
Q1 Harjinder Singh, Sudesh Kumari, Jitender M. Khurana
Department of Chemistry, University of Delhi, Delhi 110007, India
A R T I C L E I N F O
A B S T R A C T
Article history:
12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one derivatives were synthesized by a convenient
and environmentally benign procedure involving multicomponent condensation reactions of
substituted aromatic aldehydes with 2,7-dihydroxynaphthalene/2-naphthols/2,6-dihydroxynaphthlene
and cyclic 1,3-dicarbonyl compounds in task specific acidic ionic liquid [NMP]H2PO4 at 80 8C. This
protocol has proved to be efficient in terms of good yields, operational simplicity, easy workup,
recyclability of reaction medium/catalyst, and short reaction time.
Received 16 January 2014
Received in revised form 8 April 2014
Accepted 28 April 2014
Available online xxx
Keywords:
ß 2014 Jitender M. Khurana. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Benzo[a]xanthenes
Ionic liquids
Multicomponent reactions
Green synthesis
8
9
1. Introduction
Tetrahydrobenzo[a]xanthene derivatives act as anti-microbial [8], 31
anti-malarial [9], and cytotoxic agents [10]. Furthermore, some of 32
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
The efficient and high-throughput synthesis of complex organic
molecules with lower impact on the environment is one of the most
important objectives in modern synthetic organic and green
chemistry. There has been increasing concern for the development
of new greener synthetic pathways which avoid the use of volatile
organic solvents and replace them with non-flammable, non-
volatile, non-toxic, and inexpensive green solvents [1–3]. In this
context task specific ionic liquids have shown great promise as
attractive alternatives to conventional solvents owing to their
negligiblevapour pressure, recyclability, solvophobic properties, and
ability to promote association of reactants in their solvent cavity
duringtheactivation process[4]. However thereare certain concerns
over the use of ionic liquids as green solvents because large amounts
of organic solvent and energy are used in the preparations of these
ionic liquids [5]. Task specific acidic ionic liquid [NMP]H2PO4 can be
considered environmentally benign, as it can be prepared in one step
without the use of any organic solvent [6].
these compounds have been used in photodynamic therapy [11], 33
dyes [12], pH-sensitive fluorescent materials for visualization of 34
biomolecules [13], and laser technology [14]. 12-Aryl-8,9,10,12- 35
tetrahydrobenzo[a]xanthenes can be prepared by multicomponent 36
condensation of 2-naphthols, aldehydes, and cyclic 1,3-dicarbonyl 37
compounds in the presence of various acid catalysts [15]. Some of 38
the reported methods have limitations such as longer reaction time, 39
harsher reaction conditions, expensive catalysts, and generation of 40
large amounts of side products. In view of our interest in the 41
development of new syntheses of various heterocycles [16], we 42
investigated the synthesis of 12-aryl-8,9,10,12-tetrahydroben- 43
zo[a]xanthene-11-one derivatives by one pot three-component 44
condensation of naphthols, aldehydes, and cyclic 1,3-dicarbonyl 45
compounds in task specific acidic ionic liquid [NMP]H2PO4.
46
2. Experimental
47
Multicomponent reactions (MCR) combined with the applica-
tion of task specific ionic liquids (TSIL) are valuable tools for the
environmentally friendly preparation of structurally diverse
chemical libraries of drug-like heterocyclic compounds [7].
All chemicals were purchased from Sigma–Aldrich, Spectro- 48
chem and were used as received. F254 precoated aluminium plates 49
with silica gel 60 from Merck were used to monitor reaction 50
progress. IR (KBr) spectra were recorded on a Perkin Elmer FTIR 51
spectrophotometer, and the values are expressed as n
max cmÀ1. The 52
NMR (1H and 13C) spectra were recorded on a Jeol JNM ECX-400P at 53
400 MHz and 100 MHz, respectively. The chemical shift values are 54
*
Corresponding author.
E-mail address: jmkhurana@chemistry.du.ac.in (J.M. Khurana).
recorded on d scale and the coupling constants (J) are in Hertz. Ionic 55
http://dx.doi.org/10.1016/j.cclet.2014.05.014
1001-8417/ß 2014 Jitender M. Khurana. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: H. Singh, et al.,
A new green approach for the synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene-11-one derivatives using task specific acidic ionic liquid [NMP]H2PO4, Chin. Chem. Lett. (2014),
http://dx.doi.org/10.1016/j.cclet.2014.05.014

G Model
CCLET 2992 1–5
2
H. Singh et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Table 1
56
57
liquid [NMP]H2PO4 was prepared according to the literature
procedure [6].
Effect of solvent and temperature on the reaction of 2,7-dihydroxynaphthlaene, 4-
bromobenzaldehyde and dimedone.
No.
Solvent
Temp
Time
(min)a
Yield
(%)a
Standard
deviation (SD)c
58
59
2.1. General procedure for the synthesis of 12-aryl-8,9,10,12-
(8C)
tetrahydrobenzo[a]xanthene-11-one derivatives (1a–1y)
1
2
3
4
5
6
7
8
9
Ethanol
80
80
82
58
41
73
70
60
14
22
14
44b
67b
81.5
62b
65.5b
72b
90
1.414
2.828
2.121
1.414
3.535
4.242
1.414
2.828
0.070
Water
60
61
62
63
64
65
66
67
68
69
70
71
72
73
A mixture of 2,7-dihydroxynaphthalene/2-naphthol/2,6-dihy-
droxynaphthalene (1.0 mmol), substituted aromatic aldehydes
(1.0 mmol), dimedone/cyclohexane-1,3-dione (1.1 mmol) and task
specific ionic liquid [NMP]H2PO4 (20 mol%) was placed in a 50 mL
round-bottomed flask mounted over a magnetic stirrer. The
reaction mixture was stirred at 80 8C for the appropriate time as
mentioned in Table 3. The progress of the reaction was monitored
by TLC using ethyl acetate: petroleum ether (20:80, v/v) as the
eluent. After completion of the reaction as indicated by TLC, water
(10 mL) was added to the reaction mixture. The precipitate formed
was collected by vacuum filtration and washed with water. The
product was recrystallized from ethanol to give the pure product
(1a–1y) in high yield. The products were characterized by mp, IR,
NMR and mass spectra.
DMF
80
Methanol
80
THF
80
Acetonitrile
80
–
–
–
80
60
85
100
86.5
a
Results were averaged for two measurements to minimize random errors.
Mixture of two products.
b
c
S.D. for yield measurement.
ArH), 7.37–7.33 (m, 3H, ArH), 7.20 (d, 2H, J = 8.04 Hz, ArH), 7.13–
7.10 (m, 1H, ArH), 7.05–7.02 (m, 1H, ArH), 5.48 (s, 1H, ArCH), 2.64
and 2.53 (AB system, 2H, J = 17.45 Hz, CHaHb), 2.30 (d, 1H,
J = 16.2 Hz, CHaCO), 2.10 (d, 1H, J = 16.2 Hz, CHbCO), 1.03 (s, 3H,
116
117
118
119
120
121
122
123
124
125
74
75
76
77
78
79
80
81
82
83
84
85
86
2.1.1. 12-(4-Bromophenyl)-2-hydroxy-9,9-dimethyl-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-one (1a) [15b]
CH3), 0.85 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6):
d 195.96,
White solid; mp: 180–181 8C; 1H NMR (400 MHz, DMSO-d6):
d
164.10, 154.70, 145.01, 144.43, 132.70, 131.02, 130.66, 130.30,
127.48, 124.78, 124.51, 119.40, 119.23, 117.30, 116.73, 112.53,
110.01, 50.08, 33.78, 31.90, 28.73, 28.60, 26.50, 26.20; IR (KBr,
cmÀ1): nmax 3340, 2950, 1614, 1547, 1379, 1220; MS (ESI) [M+]
calcd. for C25H21BrO3: 448, found: 449 [M++H], 451 [M++H+2].
9.89 (s, 1H, OH), 7.77–7.74 (m, 2H, ArH), 7.40 (d, 2H, J = 6.9 Hz,
ArH), 7.19 (d, 4H, J = 8.7 Hz, ArH), 6.98 (d, 1H, J = 8.7 Hz, ArH), 5.32
(s, 1H, ArCH), 2.67 and 2.56 (AB system, 2H, J = 17.7 Hz, CHaHb),
2.34 (d, 1H, J = 16.2 Hz, CHaCO), 2.13 (d, 1H, J = 16.2 Hz, CHbCO),
1.05 (s, 3H, CH3), 0.86 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6):
d
195.92, 164.02, 156.52, 147.63, 144.00, 132.32, 131.00, 130.28,
129.01, 125.48, 119.26, 117.19, 114.74, 113.51, 112.75, 105.06,
3. Results and discussion
126
50.04, 40.34, 33.79, 31.85, 28.78, 26.21; IR (KBr, cmÀ1):
n
max 3328,
We present herein an efficient and environmentally benign
protocol for the synthesis of 12-aryl-8,9,10,12-tetrahydroben-
zo[a]xanthene-11-one derivatives by one pot condensation of 2-
naphthols, aldehydes and cyclic 1,3-dicarbonyl compounds using
task specific acidic ionic liquid [NMP]H2PO4 at 80 8C. The most
suitable reaction conditions for the proposed reaction were
achieved by investigating the model reaction of 4-bromobenzal-
dehyde (1.0 mmol), 2,7-dihydroxynaphthalene (1.0 mmol), and
dimedone (1.1 mmol) under different reaction conditions as
shown in Table 1. The best result was obtained when the reaction
was carried at 80 8C in the absence of any solvent using 20 mol% of
task specific ionic liquid NMP[H2PO4]. The reaction was complete
in 14 min and yielded 90% of 12-(4-bromophenyl)-2-hydroxy-9,9-
dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one (1a) af-
ter a simple workup (Table 1, entry 7). Reactions attempted in
solvents such as ethanol, water, DMF, methanol, THF and
acetonitrile in the presence of ionic liquid [NMP]H2PO4
(20 mol%) as the catalyst required longer reaction time and
resulted in inferior yields of the product 1a (Table 1, entries 1–6).
Reactions repeated at 60 8C required longer times while reactions
attempted at 100 8C required the same reaction times (Table 1,
entries 8 and 9). Thus, ionic liquid [NMP]H2PO4 with dual roles of
solvent and catalyst was found to be most suitable for this three-
component reaction for the synthesis of benzo[a]xanthene
derivatives.
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
2959, 1626, 1597, 1380, 1225; MS (ESI) [M+] calcd. for C25H21BrO3:
448, found: 449 [M++H], 451 [M++H+2].
87
88
89
90
91
92
93
94
95
96
97
98
2.1.2. 12-(4-Bromophenyl)-2-hydroxy-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-one (1k) [15f]
White solid; mp: 234–236 8C; 1H NMR (400 MHz, DMSO-d6):
d
9.89 (s, 1H, OH), 7.73–7.78 (m, 2H, ArH), 7.41 (d, 2H, J = 8.4 Hz,
ArH), 7.13–7.21 (m, 4H, ArH), 6.95–6.99 (m, 1H, ArH), 5.35 (s, 1H,
ArCH), 2.73–2.69 (m, 2H, CH2), 2.37–2.26 (m, 2H, CH2), 1.86–1.80
(m, 2H, CH2); 13C NMR (100 MHz, DMSO-d6):
d 196.19, 165.90,
156.49, 147.65, 144.27, 132.33, 131.06, 130.32, 130.28, 128.99,
125.44, 119.28, 117.16, 114.70, 113.96, 113.46, 105.01, 36.36,
33.70, 26.90, 19.88; IR (KBr, cmÀ1): nmax 3326, 2944, 1626, 1597,
1379, 1231, 1197; MS (ESI) [M+] calcd. for C23H17BrO3: 421, found:
422 [M++H], 424 [M++H+2].
99
2.1.3. 12-(4-Bromophenyl)-9,9-dimethyl-8,9,10,12-
100
101
102
103
104
105
106
107
108
109
110
111
tetrahydrobenzo[a]xanthen-11-one (1p) [15g]
White solid; mp: 186–187 8C; 1H NMR (400 MHz, DMSO-d6):
d
7.89 (d, 1H, J = 8.3 Hz, ArH), 7.80–7.76 (m, 2H, ArH), 7.45–7.38 (m,
2H, ArH), 7.33–7.23 (m, 3H, ArH), 7.23–7.20 (m, 2H, ArH), 5.66 (s,
1H, ArCH), 2.60 and 2.50 (AB system, 2H, J = 17.60 Hz, CHaHb), 2.24
(d, 1H, J = 16.2 Hz, CHaCO), 2.12 (d, 1H, J = 16.2 Hz, CHbCO), 1.12 (s,
3H, CH3), 0.96 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6):
d
196.75, 164.0, 147.85, 143.7, 131.45, 131.22, 130.10, 129.12,
128.56, 127.20, 125.23, 123.48, 120.55, 117.25, 116.89, 113.52,
We study the comprehensive effect of temperature and amount
of acidic ionic liquid [NMP]H2PO4 on the yield of 1a using factorial
analysis. Factorial analysis was carried out by considering two
experimental factors with three levels, i.e. temperature (with levels
60 8C, 80 8C, 100 8C) and amount of ionic liquid (with levels
10 mol%, 20 mol%, 30 mol%). The model reaction for the synthesis
of 1a was performed under different conditions as obtained by
fully designed factorial array for these factors (Table 2).
50.23, 41.23, 34.69, 32.87, 29.78, 26.98; IR (KBr, cmÀ1):
nmax 2955,
1652, 1596, 1373, 1228, 1160; MS (ESI) [M+] calcd. for C25H21BrO2:
432, found: 433 [M++H], 435 [M++H+2].
112
113
114
115
2.1.4. 12-(4-Bromophenyl)-3-hydroxy-9,9-dimethyl-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-one (1u) [15e]
White solid; mp: 208–209 8C; 1H NMR (400 MHz, DMSO-d6):
d
The factorial analysis was performed using Minitab 17 software,
and the results of factorial analysis are shown in Fig. 1. From Fig. 1
9.79 (s, 1H, OH), 7.82 (d, 1H, J = 9.5 Hz, ArH), 7.67 (d, 1H, J = 9.5 Hz,
Please cite this article in press as: H. Singh, et al.,
A
new green approach for the synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene-11-one derivatives using task specific acidic ionic liquid [NMP]H2PO4, Chin. Chem. Lett. (2014),
http://dx.doi.org/10.1016/j.cclet.2014.05.014

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H. Singh et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Table 2
Full designed factorial array for the study of temperature and amount of catalyst
over yield of 1a.
Run
Temp
Amount of
Yield (%)
(8C)
catalyst (mol%)
of 1a
1
2
3
4
5
6
7
8
9
80
100
60
20
10
10
10
30
20
30
20
30
90
79
72
77
79
85
83
86
88
80
60
60
100
100
80
Fig. 1. Results of factorial analysis for the effect temperature and amount of ionic
liquid over yield of 1a.
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
it can be concluded that the combination of two experimental
parameters, i.e., temperature of 80 8C and ionic liquid NMP[H2PO4]
(20 mol%), gave the best yield of compound 1a (90%). Thus, the
three-component reaction of 4-bromobenzaldehyde (1.0 mmol),
2,7-dihydroxynaphthalene (1.0 mmol), and dimedone (1.1 mmol)
in the presence of NMP[H2PO4] (20 mol%) at 80 8C was chosen as
the most suitable set reaction conditions for this reaction, as these
gave the product in the highest yield (90%) in the shortest reaction
time.
Subsequently, reactions of variously substituted aromatic
aldehydes with 2,7-dihydroxynaphthalene and dimedone were
carried out under these most suitable reaction conditions. The
reactions proceeded smoothly for different aromatic aldehydes to
afford the corresponding 12-aryl-2-hydroxy-8,9,10,12-tetrahydro-
benzo[a]xanthene-11-one derivatives in high yields (1b–1j,
Table 3). The protocol was further extended by exploring the
cyclocondensation of aromatic aldehydes and 2,7-dihydroxy-
naphthalene with cyclohexane-1,3-dione. The reaction of 4-
The scope of the three-component reaction was further 188
investigated with other naphthols. It was observed that 2-naphthol 189
(1.0 mmol) also underwent reactions with dimedone (1.1 mmol) 190
and substituted aromatic aldehydes (1.0 mmol) in the presence of 191
[NMP]H2PO4 (20 mol%) at 80 8C giving high yields of corresponding 192
12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one
deriva- 193
tives in good yield (1p–1t, Table 3). Similarly, reaction of 2,6- 194
dihydroxynaphthalene (1.0 mmol) with dimedone (1.1 mmol) and 195
various aromatic aldehydes (1.0 mmol) under the above optimized 196
reaction conditions proceeded smoothly to afford the correspond- 197
ing
12-aryl-3-hydroxy-8,9,10,12-tetrahydrobenzo[a]xanthene- 198
11-ones derivatives in high yields (1u–1y, Table 3). However, no 199
xanthene derivatives were obtained when the reactions were 200
attempted with sesamol, 1-naphthol, 1,5-dihydroxynaphthalene 201
and 1,6-dihydroxynaphthalene. Also all our efforts to achieve bis- 202
condensation of 2,6- and 2,7-dihydroxynaphthalenes using two 203
molar equivalents of aldehyde and active methylene compounds 204
have also been unsuccessful under these conditions. The general- 205
bromobenzaldehyde
(1.0 mmol),
2,7-dihydroxynaphthalene
(1.0 mmol), and cyclohexane-1,3-dione (1.1 mmol) in the presence
of ionic liquid [NMP]H2PO4 (20 mol%) at 80 8C yielded 88% of 12-
(4-bromophenyl)-2-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(1k) after 20 min. Other substituted benzaldehyde derivatives also
underwent successful condensation, giving excellent yields of
corresponding 12-aryl-2-hydroxy-8,9,10,12-tetrahydrobenzo[a]x-
anthene-11-one derivatives (1l–1o, Table 3).
ized reaction scheme is shown in Scheme 1.
206
The recyclability of ionic liquid [NMP]H2PO4 was also 207
investigated, as it is an important aspect of green chemistry. After 208
completion of the reaction for synthesis of 1a, the reaction mixture 209
was cooled to room temperature and water (5 mL) was added. The 210
Table 3
Synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one derivatives (1a–1u).
No.
Ar
R1
R2
R3
Product
Yield (%)
Time (min)
Mp (8C)
Lit. Mp (8C)
1
2
4-BrC6H4
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
1a
1b
1c
1d
1e
If
89
91
88
85
90
85
87
90
92
84
88
83
91
86
93
84
83
86
88
87
85
84
83
87
15
20
15
20
30
20
15
25
20
20
20
35
25
30
40
40
35
45
30
40
45
35
40
35
180 –181
184–185
182 [15b]
4-FC6H4
H
186 [15b]
3
4-NO2C6H4
4-CH3O C6H4
3,4(CH3O)2C6H3
4-CH3C6H4
3-NO2C6H4
C6H5
H
176–178
178 [15g]
4
H
202–204
205 [15b]
5
H
256 –258
173–175
258 [15g]
6
H
176 [15g]
7
H
1g
1h
1i
>300 (decom)
152–153
>300(decom) [15g]
153 [15g]
8
H
9
4-ClC6H4
H
180–182
181 [15g]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2-Naphthyl
4-BrC6H4
H
1j
231–233
233 [15f]
H
1k
1l
234–236
237 [15f]
2-Naphthyl
4-CH3C6H4
4-(CH3)2CHC6H4
C6H5
H
H
286–288 (decom)
286–288 (decom)
258–260
288 (decom) [15g]
288 (decom) [15g]
258 [15g]
H
H
1m
1n
1o
1p
1q
1r
1s
1t
H
H
H
H
202–205
203 [15b]
4-BrC6H4
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
186–187
187 [15g]
4-CH3O C6H4
4-NO2C6H4
4-CH3C6H4
4-ClC6H4
H
H
202–203
204 [15h]
H
H
178–180
180 [15h]
H
H
176–178
177 [15b]
H
H
180–182
183 [15b]
4-BrC6H4
H
OH
OH
OH
OH
1u
1v
1w
1y
208–209
208 [15e]
4-CH3OC6H4
4-ClC6H4
H
237–239
238 [15e]
H
250–251
250 [15e]
4-CH3C6H4
H
245–247
246 [15e]
Please cite this article in press as: H. Singh, et al.,
A new green approach for the synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene-11-one derivatives using task specific acidic ionic liquid [NMP]H2PO4, Chin. Chem. Lett. (2014),
http://dx.doi.org/10.1016/j.cclet.2014.05.014

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CCLET 2992 1–5
4
H. Singh et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 1. Synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one derivatives in ionic liquid [NMP]H2PO4.
Scheme 2. Plausible reaction mechanism for synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene derivatives in task specific ionic liquid [NMP]H2PO4 at 80 8C.
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
ionic liquid dissolved in water, and the solution was filtered to
isolate the product. The ionic liquid was recovered by evaporating
the water containing the ionic liquid, and the remaining viscous
liquid was washed with CH2Cl2 (5 mL) and dried under reduced
pressure. The recovered ionic liquid was tested to study its
catalytic activity in the subsequent runs. No appreciable loss in the
yield of 1a was observed after four cycles, as 1a was obtained in
89%, 88%, 85%, and 81% yield after the first, second, third and fourth
cycle, respectively. However, yield of 1a was reduced to 74% in the
fifth cycle.
The plausible reaction mechanism is given in Scheme 2. The
reaction proceeds through the in situ formation of ortho-quinone
methide intermediate by the nucleophilic addition of 2-naphthol
derivative to aldehyde in the presence of [NMP]H2PO4 which is
further attacked by cyclic 1,3-dicarbonyl compound followed by
cyclization and elimination of water to yield the 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene derivatives.
Acknowledgments
238
HS thank UGC, New Delhi, India for the grant of Junior and
Senior Research Fellowship, SK thank UGC, New Delhi, India for the
grant of Junior Research Fellowship.
239
240
241
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In conclusion, we have reported a new and improved protocol
for the eco-friendly one pot synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene-11-one derivatives by three-compo-
nent reaction of aromatic aldehydes, 2,7-dihydroxynaphthalene/2-
naphthol/2,6-dihydroxynaphthalene and dimedone/cyclohexan-
1,3-dione in task specific acidic ionic liquid [NMP]H2PO4 at 80 8C.
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Please cite this article in press as: H. Singh, et al.,
A
new green approach for the synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthene-11-one derivatives using task specific acidic ionic liquid [NMP]H2PO4, Chin. Chem. Lett. (2014),
http://dx.doi.org/10.1016/j.cclet.2014.05.014