Phosphoric acid supported on alumina (H3PO4/Al 2O3) as an efficient and reusable catalyst for the one-pot synthesis of benzoxanthene pigments

Article abstract of DOI:10.1007/s11164-013-1047-x

Phosphoric acid supported on alumina (H₃PO₄/Al ₂O₃) is an efficient catalyst for the catalytic multi-component condensation reaction and a wide variety of syntheses of benzoxanthene pigments in good yields. The remarkable features of this new procedure are high conversions, shorter reaction times, cleaner reaction, and simple experimental and work-up procedures.

Full text of DOI:10.1007/s11164-013-1047-x

Res Chem Intermed
DOI 10.1007/s11164-013-1047-x
Phosphoric acid supported on alumina (H3PO4/Al2O3)
as an efficient and reusable catalyst for the one-pot
synthesis of benzoxanthene pigments
•
Hamid Reza Shaterian Kobra Azizi Nafiseh Fahimi
•
Received: 17 November 2012 / Accepted: 11 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Phosphoric acid supported on alumina (H3PO4/Al2O3) is an efficient
catalyst for the catalytic multi-component condensation reaction and a wide variety
of syntheses of benzoxanthene pigments in good yields. The remarkable features of
this new procedure are high conversions, shorter reaction times, cleaner reaction,
and simple experimental and work-up procedures.
Keywords Phosphoric acid Á Alumina Á Benzoxanthene Á
Heterogeneous catalyst Á Solvent-free
Introduction
Multi-component processes have recently gained considerable economic and
ecological interest, as they direct to specific destinations essential principles of
synthetic efficiency and reaction design. Multi-component reactions (MCRs) have
been proven to be a very refined and rapid way to access complex structures in a
single synthetic operation from simple building blocks and show high atom
economy and high selectivity [1].
Dyes have played an essential role in human history since primeval days. Dyes
are used in almost every commercial product such as food, clothing, pigments and
paints, etc. [2].
Derivatives of xanthene are commonly referred to collectively as xanthenes, and
among other uses are the basis of a class of dyes which includes fluorescein, eosins,
and rhodamines. Xanthene dyes tend to be fluorescent, yellow to pink to bluish red,
brilliant dyes [3].
H. R. Shaterian (&) Á K. Azizi Á N. Fahimi
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan,
PO Box 98135-674, Zahedan, Iran
e-mail: hrshaterian@chem.usb.ac.ir
123

H. R. Shaterian et al.
Xanthenes are crystalline organic compounds in which two benzene rings are
fused to a central pyran ring [4]. It is a parent chemical structure of many medicinal
elements and is a useful intermediate in organic synthesis [5]. Xanthenes nuclei are
used chiefly in dyeing textile fibers, coloring paper, producing fluorescent effects,
and as organic pigments [6].
Thus, the synthesis of a variety of xanthene heterocyclic molecules is important
for researchers. Phosphoric acid-supported alumina was prepared for the first time
by Araujoa et al. [7] by the mixing of alumina with phosphoric acid. This
heterogeneous catalyst was characterized and the catalytic evaluation of oleic acid
conversion to biofuels and biolubricants was studied. The best catalytic performance
was achieved with the highest surface area alumina impregnated with H3PO4, which
was the solid that allied high total acidity with a large quantity of mesopore surface
area. According to the 31P NMR data, two structures for the catalyst were confirmed
[8]: (1) different phosphorous and aluminum interactions in bridging structures, and
(2) linear phosphorous and aluminum bonding (Scheme 1).
In the mainstream of the current interest of research on solid heterogeneous acidic
catalysts, we wish to describe a new protocol for the rapid synthesis of benzoxanthenes
from the three-component condensation reaction under thermal solvent-free condi-
tions with a catalytic amount of H3PO4/Al2O3 (50 % w/w) as a catalyst (Scheme 2).
Experimental
Materials and methods
All reagents were purchased from Merck and Sigma–Aldrich and used without
further purification. H3PO4/Al2O3 (50 % w/w) was prepared according to the
reported procedure [7]. All yields refer to isolated products after purification. The
NMR spectra were recorded on a Bruker Avance DPX 500, 400 MHz instrument.
IR spectra were recorded on a JASCO FT-IR 460 plus spectrophotometer. Melting
points were determined in open capillaries with a BUCHI 510 melting point
apparatus. TLC was performed on silica–gel polygram SILG/UV 254 plates.
Synthesis of 3,4,6,7-tetrahydro-3,3,6,6-tetramethyl-9-aryl-2H-xanthene-
1,8(5H,9H)-dione(1a–e)
Aldehyde (1.0 mmol) and dimedone (2 mmol) were ground and 100 mg of (H3PO4/
Al2O3) (50 % w/w) was added to the reaction mixture, which was heated in an oil
bath at 120 °C for the appropriate times (1a–e) which was monitored by TLC. After
Scheme 1 Two structures for
phosphoric acid-supported
alumina
OH
P
OH
O
Al
Al
Al O P(OH)2
O
O
Structure a
Structure b
123

Phosphoric acid supported on alumina as an efficient and reusable catalyst
O
Ar
O
O
O
Ar
O
1a-e
7a-f
O
O
Ar
O
O
O
O
O
Ar
O
O
N
O
O
N
O
NMe
O
HO
NMe
O
2a-c
OH
Ar
Me
O
6a-f
O
Me
O
O
O
H
O
O
O
O
HO
OH
OH
Ar
O
Ar
O
O
O
O
Conditions:
HO
H3PO4 /Al2O3(catalyst)
Solvent-free
O
O
5a-g
120 o
C
3a-i
O
Ar
O
O
O
O
4a-g
Scheme 2 Synthesis of benzoxanthenes
satisfactory completion, the reaction mass was cooled to 25 °C, then the solid
residue was dissolved in dichloromethane. The catalyst was washed twice with
(2 9 5 ml) dichloromethane. The recovered catalyst was dried in 100 °C. The
filtrate solution was evaporated and the solid product was purified by a
recrystallization procedure in aqueous EtOH (25 %).
Synthesis of 3,4-dihydro-3,3-dimethyl-12-aryl-2H-benzo[b]xanthene-1,6,11
(12H)-trione (2a–i) and 14-aryl-14H-dibenzo[a,i]xanthene-8,13-diones(3a–i)
A stirred mixture of arylaldehydes (1 mmol), dimedone (2a–i) or b-naphthol (3a–i)
(1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol), and (H3PO4/Al2O3) (50 %
w/w, 100 mg) was reacted in an oil bath at 120 °C for the appropriate times.
Completion of the reaction was indicated by TLC, after which it was cooled to room
temperature and the crude solid product was dissolved in dichloromethane, and
filtered for separation of the catalyst. The catalyst was washed twice with
dichloromethane (2 9 5 ml), and then the recovered catalyst was dried in an oven at
100 °C for 3 h. The filtered organic solution was concentrated. The solid product
was purified by recrystallization procedure in aqueous EtOH (15 %).
123

H. R. Shaterian et al.
Select characterizations of the orange–red products are given below.
3,4-Dihydro-3,3-dimethyl-12-phenyl-2H-benzo[b]xanthene-1,6,11(12H)-trione(2a)
Orange powder; mp: 264–265 °C; 1H NMR (500 MHz, DMSO-d6): d = 0.94 (3H, s),
1.07 (3H, s), 2.15 and 2.31 (2H, s), 2.67 (2H, s), 4.88 (1H, s), 7.10–7.15 (1H, m),
7.21–7.32 (2H, m), 7.43–7.46 (2H), 7.80–7.91 (3H, m), 7.99–8.07 (1H, m). 13C NMR
(125 MHz, DMSO-d6): d = 26.9, 28.9, 32.4, 32.7, 40.1, 113.6, 124.2, 126.2, 126.5,
127.2, 128.7, 128.8, 129.0, 130.9, 131.4, 134.6, 135.0, 143.2, 149.5, 163.4, 177.5,
183.2, 196.3 ppm; IR (KBr) (tmax, cm-1): 2,926, 1,678, 1,606.
14-(3,4,5-Trimethoxylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione(3h)
Orange powder; mp: 288–290 °C; 1H NMR (500 MHz, DMSO-d6): d = 3.71 (s, 3H),
3.73 (s, 6H), 6.57 (s, 2H), 7.41–7.56 (m, 4H), 7.74–7.77 (m, 1H), 7.82–7.89 (m, 2H),
7.95 (d, J = 8.01, 1H), 8.06–8.13 (m, 2H) ppm; 13C NMR (125 MHz, DMSO-d6):
d = 178.4, 178.38, 157.16, 153.18, 147.35, 138.70, 136.82, 135.21, 131.9, 131.3,
131.12, 130.85, 129.96, 129.7, 129.45, 128.60, 127.56, 125.69, 124.59, 123.84,
116.73, 116.56, 116.34, 105.88, 77.51, 77.09, 77.66, 60.72, 56.11,35.28 ppm; IR
(KBr) (tmax, cm-1): 3,035, 1,700, 1,655, 1,636, 1,617, 1,589, 1,543, 1,507, 1,458,
1,286, 1,213, 1,126, 1,092.
14-(2,5-Dimethoxylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione(3i)
1
Orange powder; mp: 260 °C; H NMR (500 MHz, DMSO-d6): d = 3.63 (s, 3H),
3.92 (s, 3H), 6.11 (s, 1H), 6.61–6.58 (m, 1H), 6.77 (d, J = 8.91 1H), 6.84 (d,
J = 2.82, 1H), 7.37–7.50 (m, 3H), 7.55 (t, J = 7.56, 1H), 7.74–7.80 (m,3H), 8.09
(d, J = 7.59, 1H), 8.15 (d, J = 7.77, 1H), 8.26 (d, J = 8.37, 1H) ppm; 13C NMR
(125 MHz, DMSO-d6): d = 178.5, 178.18, 157.4, 153.7, 151.1, 147.0, 135.1, 133.4,
131.7, 131.9, 131.1, 130.1, 129.3, 129.1, 128.4, 127.3, 125.4, 124.5, 124.1, 117.5,
117.2, 116.8, 116.2, 113.1, 112.1, 57.0, 55.5, 30.0 ppm; IR (KBr) (tmax, cm-1):
3,034, 1,698, 1,655, 1,637, 1,590, 1,542, 1,508, 1,499, 1,458, 1,289, 1,214, 1,182,
1,155.
Synthesis of 13-aryl-5H- dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones(4a–i):
A stirred mixture of aldehyde (1 mmol), 2-hydroxy-1,4-naphthoquinone (2 mmol)
and (H3PO4/Al2O3) (50 % w/w, 100 mg) was reacted in an oil bath at 120 °C for the
appropriate times (Table 1). Completion of the reaction was indicated by TLC, after
which it was cooled to room temperature and the crude solid product was dissolved
in dichloromethane, and filtered for separation of the catalyst. The catalyst was
washed twice times with dichloromethane (2 9 5 ml), and then the recovered
catalyst was dried in an oven at 100 °C for 3 h. The filtered organic solution was
concentrated. The solid product was purified by recrystallization procedure in
aqueous EtOH (15 %).
Select characterizations of the orange–red products are given below.
123

Phosphoric acid supported on alumina as an efficient and reusable catalyst
13-Phenyl-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone(4a)
1
Red powder; mp: 305–307 °C; H NMR (400 MHz, CDCl3) d = 5.09 (1H, s),
7.16–8.08 (13H, m) ppm. IR (KBr, cm-1): 3,035, 1,660, 1,569.
13-(4-chlorophenyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone(4b)
1
Orange powder; mp: 330–332 °C. H NMR (400 MHz, CDCl3) d = 5.10 (1H, s),
7.26–8.07 (12H, m) ppm. IR (KBr, cm-1): 3,028, 1,663, 1,610.
13-(p-Tolyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone(4e)
Brick-red powder; mp: 304–307 °C; 1H NMR (400 MHz, CDCl3) d = 2.21 (3H, s),
5.09 (1H, s), 7.07–8.12 (12H, m) ppm. IR (KBr, cm-1): 3,088, 1,663, 1,608.
13-(3-Nitrophenyl)-5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraone(4g)
1
Orange powder; mp: 340–342 °C; H NMR (400 MHz, CDCl3) d = 5.47 (1H, s),
7.11–8.13 (12H, m) ppm. IR (KBr, cm-1): 3,035, 1,662, 1,605.
Synthesis of 13-aryl-indeno[1,2-b]naphtha[1,2-e]pyran-12(13H)-ones(5a–g):
A stirred mixture of b-naphthol (1 mmol), aldehyde (1 mmol), 2H-indene-1,3-dione
(1 mmol) and 100 mg of (H3PO4/Al2O3) was reacted in an oil bath at 120 °C for the
appropriate times. When the reaction was completed, it was cooled to room
temperature and the crude solid product was dissolved in dichloromethane, and
filtered for separation of the catalyst. The catalyst was washed twice times with
dichloromethane (2 9 5 ml), and then the recovered catalyst was dried in an oven at
100 °C for 3 h. The filtered organic solution was concentrated. The solid product
was purified by recrystallization procedure in aqueous EtOH (15 %).
Select characterizations of the orange–red products are given below.
13-(4-Chlorophenyl)-indeno[1,2-b]naphtho[1,2-e]pyran-12(13H)-one(5b)
Yellow solid; mp: 225–226 °C; 1H NMR (400 MHz, CDCl3) d = 7.88–7.84 (m, 2H),
7.75 (t, J = 9.2 Hz, 1H), 7.51 (d, J = 8.8 Hz, 1H), 7.45–7.25 (m, 8H), 7.18 (d,
J = 8.4 Hz, 2H), 5.63 (s, 1H) ppm. 13C NMR (100 MHz, CDCl3) d = 192.3, 167.3,
149.0, 142.1, 136.7, 132.3, 132.2, 131.9, 131.6, 130.2, 129.9, 129.5, 128.7, 128.6,
127.3, 125.4, 124.2, 121.7, 118.4, 117.7, 116.0, 110.4, 35.1 ppm.
13-(4-Methylphenyl)-indeno[1,2-b]naphtho[1,2-e]pyran-12(13H)-one(5d)
1
Yellow solid; mp: 192–193 °C; H NMR (400 MHz, CDCl3) d = 7.88–7.82 (m,
3H), 7.50 (d, J = 9.2 Hz, 1H), 7.43–7.28 (m, 6H), 7.21 (d, J = 8.0 Hz, 2H), 7.02
(d, J = 8.0 Hz, 2H), 5.61 (s, 1H), 2.24 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3)
d = 192.4, 167.2, 149.0, 140.8, 136.9, 136.1, 132.4, 132.2, 131.9, 130.0, 129.5,
123

H. R. Shaterian et al.
Table 1 Synthesis of benzoxanthene pigments using H3PO4/Al2O3 (50 % w/w) as a catalyst at 120 °C
under solvent-free conditions
Entry
Aldehyde
Time (min)
Yield (%)a
Melting points (oC)
Found
Lit [Ref]
1a
1b
1c
1d
1e
2a
2b
2c
3a
3b
3c
3d
3e
3f
Benzaldehyde
25
30
25
20
20
30
25
35
20
15
15
8
90
84
82
90
92
87
90
84
95
90
95
89
93
87
90
86
89
91
93
95
84
90
92
93
82
85
78
75
80
73
88
85
82
80
87
78
75
90
91
206–208
240–242
226–227
221–223
168–170
264–265
283
205–206 [15]
241–243 [15]
226–228 [15]
221–223 [15]
167–168 [15]
263–265 [16]
282–284 [16]
268–270 [16]
319–320 [17]
332–333 [17]
304–305 [17]
301–302 [17]
305–306 [17]
255–256 [17]
279–280 [17]
288–290 [18]
260 [18]
4-Methoxybenzaldehyde
2-Cholorobenzaldehyde
4-Nitrobenzaldehyde
3-Nitrobenzaldehyde
Benzaldehyde
4-Cholorobenzaldehyde
4-Bromobenzaldehyde
Benzaldehyde
268–270
319–320
333–335
305–306
302–303
305–307
256–258
279–281
288–290
260
4-Nitrobenzaldehyde
3-Nitrobenzaldehyde
2,4-Dicholorobenzaldehyde
4-Cholorobenzaldehyde
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
3,4,5-Trimethoxybenzaldehyde
2,5-Dimethoxybenzaldehyde
Benzaldehyde
5
18
15
25
15
10
10
10
8
3g
3h
3i
4a
4b
4c
4d
4e
4f
305–307
332–334
334–336
270–272
304–306
308–310
340–342
200–202
225–226
243–245
192–193
251
305–307 [19]
330–332 [19]
333–335 [19]
270–272 [19]
304–307 [19]
307–309 [19]
340–342 [19]
202–203 [20]
225–226 [20]
240–241 [20]
192–193 [20]
252–253 [20]
240–241 [20]
225–226 [20]
226–228 [21]
196–198 [21]
222–224 [21]
288–290 [21]
270–272 [21]
243–245 [21]
149–150 [22]
178–181 [22]
4-Cholorobenzaldehyde
4-Bromobenzaldehyde
4-Flourobenzaldehyde
4-Methylbenzaldehyde
2-Cholorobenzaldehyde
3-Nitrobenzaldehyde
Benzaldehyde
15
12
8
4g
5a
5b
5c
5d
5e
5f
40
28
45
40
45
40
35
55
50
60
50
55
55
20
25
4-Cholorobenzaldehyde
3-Nitrobenzaldehyde
4-Methylbenzaldehyde
2,4-Dicholorobenzaldehyde
2-Cholorobenzaldehyde
4-Methoxybenzaldehyde
Benzaldehyde
242–243
225–226
226–228
198
5g
6a
6b
6c
6d
6e
6f
4-Methylbenzaldehyde
2,4-Dicholorobenzaldehyde
4-Nitrobenzaldehyde
4-Cholorobenzaldehyde
4-Bromobenzaldehyde
Benzaldehyde
223–225
288–290
270–272
244–245
151–153
178–180
7a
7b
4-Nitrobenzaldehyde
123

Phosphoric acid supported on alumina as an efficient and reusable catalyst
Table 1 continued
Entry
Aldehyde
Time (min)
Yield (%)a
Melting points (oC)
Found
Lit [Ref]
7c
7d
7e
7f
a
3-Nitrobenzaldehyde
30
20
25
30
87
90
92
85
168-170
205–207
185–187
175–177
168-172 [22]
205–208 [22]
184–185 [22]
175–176 [22]
4-Methoxybenzaldehyde
4-Cholorobenzaldehyde
4-Methylbenzaldehyde
Yields refer to the pure isolated products
129.2, 128.4, 128.0, 127.1, 125.2, 124.4, 121.6, 118.2, 117.7, 116.8, 111.2, 35.3,
21.0 ppm.
13-(4-Methoxylphenyl)-indeno[1,2-b]naphtho[1,2-e]pyran-12(13H)-one(5g)
Yellow solid; mp: 225–226 °C; 1H NMR (400 MHz, CDCl3) d = 7.87–7.82 (m, 3H),
7.49 (d, J = 9.2 Hz, 1H), 7.43–7.27 (m, 6H), 7.23 (d, J = 8.8 Hz, 2H), 6.75 (d,
J = 8.8 Hz, 2H), 5.58 (s, 1H), 3.71 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3)
d = 192.5, 167.0, 158.1, 148.9, 136.9, 136.1, 132.4, 132.2, 131.9, 131.8, 130.0, 129.5,
129.1, 128.4, 127.1, 125.2, 124.4, 121.6, 118.2, 117.7, 116.8, 113.9, 111.2, 55.1,
34.8 ppm.
Synthesis of 8,12-dihydro-8,10-dimethyl-12-(aryl)-7-oxa-8,10-
diazabenzo[a]anthracene-9,11-dione(6a–f) and 9,10-dihydro-9,9-dimethyl-12-
aryl-8H-benzo[a]xanthen-11(12H)-one(7a–f)
A
stirred mixture of arylaldehydes (1 mmol), b-naphthol (1 mmol), and
1,3-dimethylbarbituric acid (6a–f) or dimedone (7a–f) (1 mmol) and H3PO4/
Al2O3 (50 % w/w, 100 mg) was reacted in an oil bath at 120 °C for the appropriate
times (Table 1). Completion of the reaction was indicated by TLC, after which it
was cooled to room temperature and the crude solid product was dissolved in
dichloromethane, and filtered for separation of the catalyst. The catalyst was washed
twice with dichloromethane (2 9 5 ml), and then the recovered catalyst was dried
in an oven at 100 °C for 3 h. The filtered organic solution was concentrated. The
solid product was purified by recrystallization procedure in aqueous EtOH (15 %).
Select characterizations of the orange–red products are given below.
8,12-Dihydro-8,10-dimethyl-12-(phenyl)-7-oxa-8,10-diazabenzo[a]anthracene-
9,11-dione(6a)
1
White solid; mp: 226–228 °C; H NMR (400 MHz, CDCl3): d = 7.96 (d, J =
7.8 Hz, 1H), 7.85 (m, 2H), 7.46–7.35 (m, 5H), 7.23–7.10 (m, 3H), 5.81 (s, 1H), 3.61
(s, 3H), 3.33 (s, 3H). 13C NMR (100 MHz, CDCl3): d = 161.9, 152.2, 150.6, 147.1,
143.8, 131.7, 130.9, 129.4, 128.5, 128.4, 128.2, 127.4, 126.7, 125.4, 123.9, 117.4,
116.2, 91.4, 35.9, 29.0, 28.1. IR (KBr, cm-1): 2,928, 1,706, 1,651, 1,485, 1,233, 1,179.
123

H. R. Shaterian et al.
8,12-Dihydro-8,10-dimethyl-12-(p-tolyl)-7-oxa-8,10-diazabenzo[a]anthracene-
9,11-dione(6b)
White solid; mp: 196–198 °C; 1H NMR (400 MHz, CDCl3): d = 7.98 (d,
J = 8.4 Hz, 1H), 7.84–7.80 (m, 2H), 7.46–7.37 (m, 3H), 7.26 (d, J = 7.5 Hz,
2H), 7.02 (d, J = 7.5 Hz, 2H), 5.78 (s, 1H), 3.60 (s, 3H), 3.32 (s, 3H), 2.21 (s, 3H).
13C NMR (100 MHz, CDCl3): d = 161.9, 152.1, 150.6, 147.1, 140.9, 136.3, 131.7,
130.9, 129.3, 129.1, 128.4, 128.0, 127.3, 125.4, 123.9, 117.5, 116.2, 91.5, 35.5,
28.9, 28.1, 20.9. IR (KBr, cm-1): 2,924, 1,700, 1,643, 1,487, 1,232, 1,179.
8,12-Dihydro-8,10-dimethyl-12-(4-nitrophenyl)-7-oxa-8,10-
diazabenzo[a]anthracene-9,11-dione(6d)
White solid; mp: 288–290 °C; 1H NMR (400 MHz, CDCl3): d = 8.07 (d, J = 8.4 Hz,
2H), 7.91–7.79 (m, 3H), 7.56–7.42 (m, 5H), 5.90 (s, 1H), 3.63 (s, 3H), 3.33 (s, 3H). 13
C
NMR (100 MHz, CDCl3): d = 161.7, 152.4, 150.7, 150.4, 147.1, 146.5, 131.7, 130.4,
130.2, 129.2, 128.7, 127.8, 125.7, 123.7, 123.3, 116.3, 115.7, 89.9, 36.0, 29.1, 28.2. IR
(KBr, cm-1): 3,070, 2,924, 2,855, 1,709, 1,667, 1,596, 1,227, 1,175.
8,12-Dihydro-8,10-dimethyl-12-(4-chlorophenyl)-7-oxa-8,10-
diazabenzo[a]anthracene-9,11-dione(6e)
White solid; mp: 270–272 °C; 1H NMR (400 MHz, CDCl3): d = 7.88–7.81
(m, 3H), 7.47–7.14 (m, 7H), 5.78 (s, 1H), 3.60 (s, 3H), 3.33 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 161.9, 152.2, 150.5, 147.1, 142.3, 132.5, 131.7, 130.7,
129.7, 129.6, 128.5, 127.6, 125.6, 123.7, 116.7, 116.3, 90.9, 35.4, 29.1, 28.2. IR
(KBr, cm-1): 2,924, 2,855, 1,708, 1,671, 1,648, 1,225, 1,174, 1,081.
9,10-Dihydro-9,9-dimethyl-12-phenyl-8H-benzo[a]xanthen-11(12H)-one(7a)
White solid; mp: 151–153 °C; 1H NMR (400 MHz, CDCl3): d = 7.99 (d, J =
8.1 Hz, 1H), 7.78–7.74 (m, 2H), 7.42–7.04 (m, 8H), 5.70 (s, 1H), 2.57 (s, 2H), 2.33 (d,
J = 16.2 Hz, 1H), 2.26 (d, J = 16.2 Hz, 1H), 1.11 (s, 3H), 0.96 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 196.8, 163.8, 147.7, 144.7, 131.4, 131.3, 128.7, 128.4, 128.3,
128.1, 126.9, 126.1, 124.8, 123.6, 117.6, 117.0, 114.2, 50.8, 41.3, 34.6, 32.2, 29.2,
27.1. IR (KBr, cm-1): 3,055, 2,954, 2,884, 1,651, 1,374, 1,229, 1,178, 1,076, 811.
9,10-Dihydro-9,9-dimethyl-12-(4-nitrophenyl)-8H-benzo[a]xanthen-11(12H)-
one(7b)
1
White solid; mp: 178–180 °C; H NMR (400 MHz, CDCl3): d = 8.04 (d, J =
8.4 Hz, 2H), 7.82–7.79 (m, 3H), 7.52–7.33 (m, 5H), 5.81 (s, 1H), 2.59 (s, 2H), 2.36
(d, J = 16.2 Hz, 1H), 2.26 (d, J = 16.2 Hz, 1H), 1.13 (s, 3H), 0.94 (s, 3H). 13C
NMR (100 MHz, CDCl3): d = 196.6, 164.5, 151.8, 147.7, 146.2, 131.5, 130.9,
129.5, 129.3, 128.6, 127.3, 125.2, 123.5, 123.0, 117.0, 115.9, 112.9, 50.7, 41.3,
123

Phosphoric acid supported on alumina as an efficient and reusable catalyst
34.8, 32.2, 29.2, 27.0. IR (KBr, cm-1): 3,077, 2,934, 1,645, 1,597, 1,516, 1,377,
1,343, 1,225, 1,177, 1,026, 827.
9,10-Dihydro-9,9-dimethyl-12-(3-nitrophenyl)-8H-benzo[a]xanthen-
11(12H)-one(7c)
1
White solid; mp: 168–170 °C; H NMR (400 MHz, CDCl3): d = 8.10 (s, 1H),
7.95–7.80 (m, 5H), 7.47–7.35 (m, 4H), 5.81 (s, 1H), 2.61 (s, 2H), 2.36 (d,
J = 16.2 Hz, 1H), 2.26 (d, J = 16.2 Hz, 1H), 1.13 (s, 3H), 0.95 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 196.7, 164.5, 148.3, 147.7, 146.7, 134.7, 131.5, 130.9,
129.6, 129.0, 128.6, 127.3, 125.1, 123.1, 123.0, 121.5, 117.1, 115.9, 113.1, 50.7,
41.3, 34.7, 32.2, 29.2, 27.1. IR (KBr, cm-1): 3,071, 2,955, 1,649, 1,528, 1,372,
1,356, 1,224, 1,172, 1,025, 809.
9,10-Dihydro-9,9-dimethyl-12-(4-methoxyphenyl)-8H-benzo[a]xanthen-
11(12H)-one(7d)
Colorless crystals; mp: 181–182 °C; 1H NMR (400 MHz, CDCl3): 1.91–2.08 (m, 2H),
2.31–2.48 (m, 2H), 2.57–2.76 (m, 2H), 3.69 (s, 3H), 5.69 (s, 1H), 6.71 (d, J = 8.0 Hz,
2H), 7.23–7.45 (m, 5H), 7.77 (t, J = 8.4 Hz, 2H), 7.96 (d, J = 8.4 Hz, 1H). IR (KBr,
cm-1): 2,945, 2,833, 1,651, 1,595, 1,460, 1,378, 1,251, 1,227, 1,188, 1,175, 1,130,
1,034, 952, 829, 748.
Results and discussions
Xanthenes and benzoxanthenes are important classes of biologically active
heterocycles [9, 10] showing fluorescence activities [11], and are used in industrial
applications and as precursors in the synthesis of heterocyclic compounds [12].
Also, these compounds have been shown to have antioxidant activity by 2,2-
diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay [13]. These compounds
are being utilized as antagonists for paralyzing action of zoxazolamine and in
photodynamic therapy. Xanthenes have also been employed as dyes and pH-
sensitive fluorescent materials for visualization of biomolecules and in laser
technologies [14]. Thus, the development of an environmentally benign method-
ology for the synthesis of benzoxanthene derivatives is in great demand. Here, we
investigated the synthesis of various benzoxanthene pigments under catalytic
amounts of H3PO4/Al2O3.
In the initial experiment, a test reaction using benzaldehyde (1 mmol), and
dimedone (2 mmol) at 120 °C under solvent-free conditions and without any
catalyst was performed in order to establish the effectiveness of the catalysts. The
reaction in the absence of catalyst did not produce the desired product. The reaction
in the presence of H3PO4/Al2O3 (50 % w/w) in different solvents did not complete
(yield % = trace-50 % range) and starting materials were observed even after
24 h. The effects of the catalyst at different reaction temperatures (60, 80, 100, and
120 °C) and different amounts of catalyst (60, 80, 100, and 120 mg) were
123

H. R. Shaterian et al.
investigated. The best result was obtained by carrying out the reaction with 100 mg
of H3PO4/Al2O3 (50 % w/w) at 120 °C under solvent-free conditions, because of the
high yields of the product and short reaction times. Using optimized reaction
conditions, the scope and efficiency of the reaction were explored for the synthesis
of benzoxanthenes (Table 1). Thus, we examined the condensation of arylaldehydes
(1 mmol) and dimedone (2 mmol) under the mentioned optimized conditions. The
reaction proceeded efficiently with various kinds of aromatic aldehydes in good to
excellent yields within short reaction times. Several structurally diverse aldehydes
(Table 1) were subjected to condensation with dimedone under the catalytic
influence of H3PO4/Al2O3 (50 % w/w) and thermal solvent-free conditions. 3,4,6,7-
tetrahydro-3,3,6,6-tetramethyl-9-aryl-2H-xanthene-1,8(5H,9H)-diones (1a–e) were
obtained in high yields.
In continuation of our ongoing research program, we found that H3PO4/Al2O3 is
useful for the synthesis of 3,4-dihydro-3,3-dimethyl-12-aryl-2H-benzo[b]xanthene-
1,6,11(12H)-triones (2a–i) by using of arylaldehydes, dimedone and 2-hydroxy-1,4-
naphthoquinone. The three component reaction of arylaldehydes, b-naphthol and
2-hydroxy-1,4-naphthoquinone cause to prepare 14-aryl-14H-dibenzo [a,i]xan-
thene-8,13-diones (3a–i). The application of this protocol was extended to a variety
of aromatic aldehydes. The reactions proceeded smoothly with different aldehydes
substituted with electron-donating/electron-withdrawing groups giving excellent
yields of the desired products. Interestingly halo-substituted aromatic aldehydes
required shorter reaction times than their electron-rich counterparts. In this
conditions, H3PO4/Al2O3 is able to efficiently catalyze the synthesis of 13-aryl-
5H-dibenzo[b,i]xanthene-5,7,12,14(13H)-tetraones (4a–g) from pseudo-three-com-
ponent reaction of arylaldehydes (1 mmol), and 2-hydroxy-1,4-naphthoquinone
(2 mmol). 13-Aryl-indeno[1,2-b]naphtha[1,2-e]pyran-12(13H)-ones (5a–g) were
prepared from the condensation of b-naphthol (1 mmol), arylaldehydes (1 mmol),
and 2H-indene-1,3-dione(1 mmol) utilizing H3PO4/Al2O3 under solvent-free con-
ditions at 120 °C.
In order to broaden the scope of this protocol, we decided to synthesis 8,12-dihydro-
8,10-dimethyl-12aryl-7-oxa-8,10-diazabenzo[a]anthracene-9,11-diones (6a–f) from the
reaction of arylaldehydes (1 mmol), b-naphthol (1 mmol), and 1,3-dimethylbarbituric
acid (1 mmol). Also, the successful condensation of aldehydes, b-naphthol and dimedone
under similar conditions produced the 9,10-dihydro-9,9-dimethyl-12-aryl-8H-benzo[a]-
xanthen-11(12H)-ones (7a–f).
Interestingly, a variety of aldehydes including ortho-, meta-, and para-substituted
aryl aldehydes participated well in this reaction and gave the corresponding
benzoxanthene derivatives in good to excellent yields without the formation of any
by-products.
We also investigated the recycling of the catalyst under solvent-free conditions
using a model reaction of benzaldehyde, b-naphthol, and 2-hydroxy-1,4-naphtho-
quinone. After completion of the reaction, the reaction mixture was cooled to room
temperature, and the crude solid product was dissolved in dichloromethane. The
mixture was filtered for separation of the catalyst. The catalyst was washed twice
(2 9 5 ml) using dichloromethane. The recovered catalyst was dried in vacuo and
123

Phosphoric acid supported on alumina as an efficient and reusable catalyst
Fig. 1 Investigation of the recycling of H3PO4/Al2O3
was used for the subsequent catalytic runs (3a). The recovered catalyst was reused
four times without any loss of its activities (Fig. 1).
Conclusion
We have developed a green and straightforward protocol for the synthesis of
benzoxanthene derivatives using H3PO4/Al2O3(50 % w/w) as catalyst under thermal
solvent-free conditions. This procedure provides several advantages such as cleaner
reactions, easier work-up, reduced reaction times, reusable catalyst, and an eco-
friendly promising strategy.
Acknowledgments We are grateful to the University of Sistan and Baluchestan Research Council for
the partial support of this research.
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