Polyvinylpolypyrrolidone supported triflic acid (PVPP.OTf); An efficient and recyclable heterogeneous catalyst for one-pot condensation of β-naphthol, aldehydes, and cyclic 1,3-dicarbonyl compounds

Article abstract of DOI:10.1007/s11164-013-1521-5

Polyvinylpolypyrrolidoniume triflate (PVPP.OTf) is found to be a recyclable heterogeneous catalyst for the one-pot condensation of β-naphthol, aldehydes, and cyclic 1,3-dicarbonyl compounds in good to excellent yield under reflux conditions. These catalyt

Full text of DOI:10.1007/s11164-013-1521-5

Res Chem Intermed
DOI 10.1007/s11164-013-1521-5
Polyvinylpolypyrrolidone supported triflic acid
(PVPP.OTf); an efficient and recyclable heterogeneous
catalyst for one-pot condensation of b-naphthol,
aldehydes, and cyclic 1,3-dicarbonyl compounds
•
Satra Vahdat Samad Khaksar
Received: 19 August 2013 / Accepted: 18 December 2013
Ó Springer Science+Business Media Dordrecht 2014
Abstract Polyvinylpolypyrrolidoniume triflate (PVPP.OTf) is found to be a
recyclable heterogeneous catalyst for the one-pot condensation of b-naphthol,
aldehydes, and cyclic 1,3-dicarbonyl compounds in good to excellent yield under
reflux conditions. These catalytic condensations have advantages from the view-
point of green chemistry. PVPP.OTf catalyst is air-stable, cost-effective, easy to
handle, easily separated from reaction products, and recovered in excellent purity
for direct reuse.
Keywords Polyvinylpolypyrrolidoniume triflate Á Heterogeneous Á
Benzoxanthene Á b-Naphthol
Introduction
Xanthene and their structural analogues are significant for their pharmacological
activities. They show a wide spectrum of biological activities such as anti-
inflammatory [1], antiviral [2], and antibacterial [3], and can be utilized as
antagonists for drug-resistant leukemia lines [4]. In addition, they have also been
used as dyes [5, 6] and pH-sensitive fluorescent materials [7, 8], as well as in laser
technologies [9, 10]. Moreover, several polycyclic compounds containing the
xanthene skeleton have been isolated from natural sources [11]. Consequently, wide
demands of diverse xanthene derivatives in various fields have promoted the
development of practical and diversified synthetic methods. Recently, several
elegant multicomponent strategies for the synthesis of tetrahydrobenzo[a]xanthen-
11-one derivatives by the cyclocondensation of naphthols, aldehydes, and 1,3-
diketones utilizing different types of catalysts have been reported [12–17]. These
S. Vahdat Á S. Khaksar (&)
Chemistry Department, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran
e-mail: S.khaksar@iauamol.ac.ir
123

S. Vahdat, S. Khaksar
methods show varying degrees of successes as well as limitations, such as harsh
reaction conditions, expensive and detrimental metal reagents, tedious work-up, low
product yields, long reaction times, and co-occurrence of several side products.
Considering the above reports, the development of novel methods for the synthesis
of xanthenes is of great importance because of their potential biological and
pharmaceutical activities.
In recent years, polymer-supported reagents are gaining considerable attention
because polymeric reagents facilitate work-up and product purification, reduce
environmental damage, and have easy separation, which make them promising for
both academic and industrial applications [18]. Triflic acid (TfOH) is widely used as
a homogeneous acid catalyst due to its very strong Brønsted acidity (pKa = -13.6),
and its structure and properties [19–23]. However, TfOH is highly corrosive and is a
fuming liquid, and so there are difficulties involved in storage, transportation,
handling and waste disposal, thus severely restricting its application in industry.
Despite its widespread use in acid-catalyzed reactions, these drawbacks, together
with product isolation, catalyst recovery, and toxicological concerns, still exist. The
immobilization of TfOH onto solid supports affords solid acids which can be easily
handled, as they are invariably of low toxicity, and are non-corrosive, free-flowing
powders with superior thermal and mechanical stability under catalytic conditions.
Thus, some immobilization processes for TfOH on solid supports have been
designed [24–27]. In continuing our studies on the application of new reagents or
systems for organic functional group transformations [28–30], we describe the
preparation of polyvinylpolypyrrolidone-supported TfOH (PVPP.OTf) and its
application as a catalyst for the synthesis of xanthene derivatives. In this light,
we have supported TfOH on polyvinylpolypyrrolidone to prepare an efficient, mild
catalyst for the heterogeneous preparation of tetrahydrobenzo[a]xanthen-11-one
derivatives (Scheme 1).
Experimental
Apparatus and analysis
NMR spectra were determined on an FT-NMR Bruker AV-400 spectrometer in
CDCl3 or DMSO-d6 and are expressed in d values relative to tetramethylsilane;
coupling constants (J) are measured in Hz. Melting points were determined on an
Electrothermal 9100 apparatus. Infrared spectra were recorded on a Rayleigh WQF-
510 Fourier transform instrument. Commercially available reagents were used
throughout without further purification.
Preparation of the Polyvinylpolypyrrolidoniume triflate (PVPP.OTf)
To a suspension of polyvinylpolypyrrolidone (3.0 g) in toluene (35 mL), TfOH
(2.0 g, 13 mmol) was added. The mixture was stirred magnetically for 60 min at r.t.
The toluene was removed under reduced pressure and the residue was dried at
123

Polyvinylpolypyrrolidone supported triflic acid (PVPP.OTf)
R
O
RCHO
O
O
PVPP.OTf (30 mg)
1
+
+
toluene, 110 C, 5-6 h
OH
O
3
4
*
2
n
*
-
N+
PVPP.OTf =
CF3SO3
O
H
Scheme 1 Reaction of b-naphthol, benzaldehyde, and 5,5-dimethylcyclohexane1,3-dione
110 °C for 2 h to afford PVPP.OTf as a white powder. The number of H? sites
PVPP.OTf determined by acid–base titration was 10 mequiv./g.
General procedure for the preparation of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]xanthen-11-one derivative
A toluene solution (3 mL) of aldehyde (1 mmol), b-naphthol (1 mmol) and 5,5-
dimethylcyclohexane-1,3-dione (1 mmol) was mixed with PVPP.OTf (30 mg), and
the mixture was stirred at 110 °C for an appropriate time. The reaction was
monitored by TLC. After completion of the reaction, the mixture was washed with
chloroform and filtered to recover the catalyst. The filtrate was evaporated and
dried. Products were characterized by comparison of their physical and spectral data
with those of authentic samples [22]. Spectroscopic data for selected examples are
shown below. Spectroscopic data for selected examples as follows:
12-(4-Chlorophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(4a)
White solid; mp 180–182 °C; IR (KBr, cm-1): 3,070, 2,935, 1,640, 1,368. 1H NMR
(400 MHz, CDCl3): d = 7.89 (d, J = 8.2 Hz, 1H), 7.75–7.80 (m, 2H), 7.10–7.42
(m, 7H), 5.65 (s, 1H), 2.56 (s, 2H), 2.33 (d, J = 16.1 Hz, 1H), 2.25 (d, J = 16.1 Hz,
1H), 1.12 (s, 3H), 0.95 (s, 3H); 13C NMR (100 MHz, CDCl3): d = 196.7, 164.1,
152.1, 147.2, 146.3, 132.1, 130.3, 129.2, 129.2, 128.3, 127.1, 125.1, 123.3, 123.0,
116.4, 115.3, 113.1, 50.3, 41.5, 34.7, 31.9, 29.3, 27.2.
12-(2-Chlorophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(4b)
White solid; mp 178–180 °C; IR(KBr, cm-1): 3,070, 2,929, 1,645, 1,369, 1,230,
1
1,180. H NMR (400 MHz, CDCl3): d = 8.22 (d, J = 8.3 Hz, 1H), 7.71–7.75 (m,
2H), 7.27–7.49 (m, 5H), 6.95–7.07 (m, 2H), 5.88 (s, 1H), 2.60 (s, 2H), 2.35 (d,
J = 16.0 Hz, 1H), 2.24 (d, J = 16.0 Hz, 1H), 1.12 (s, 3H), 0.98 (s, 3H); 13C NMR
(100 MHz, CDCl3): d = 196.5, 164.2, 148.1, 142.5, 133.0, 131.3,
123

S. Vahdat, S. Khaksar
131.5,131.2,129.5,129.0,128.1,127.2,127.0,126.5,124.9, 122.9,117.2,117.0, 114.1,
50.7, 41.4, 32.8, 32.1, 29.4, 27.1.
12-(3-Chlorophenyl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one
(4c): entry 6
White solid; mp 175–177 °C; IR(KBr, cm-1): 3,050, 2,927, 1,647, 1,365. 1H NMR
(400 MHz, CDCl3): d = 8.03 (d, J = 8.2 Hz, 1H), 7.88–8.01 (m, 2H), 7.40–7.50
(m, 3H), 7.36 (s, 1H), 7.19 (d, J = 4.4 Hz, 2H), 7.11 (s, 1H), 5.60 (s, 1H), 2.60 (s,
2H), 2.30 (d, J = 16.2 Hz, 1H), 2.13 (d, J = 16.2 Hz, 1H), 1.11 (s, 3H), 0.98 (s,
3H); 13C NMR (100 MHz, CDCl3): d = 195.7, 164.0, 148.3, 148.0, 134.5, 132.1,
131.3, 130.8, 130.3, 129.2, 128.8, 128.1, 127.5, 127.1, 125.9, 124.1, 117.9, 117.3,
113.4, 51.9, 42.0, 35.7, 33.7, 29.8, 26.9.
12-(4-Nitrophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo-[a]xanthen-11-one (4d)
White solid; mp 177–180 °C; IR(KBr, cm-1): 3,078, 2,933, 2,911, 1,645, 1,595,
1
1,510, 1,375, 1,347. H NMR (400 MHz, CDCl3): d = 8.03 (d, J = 8.4 Hz, 2H),
7.78–7.80 (m, 3H), 7.32–7.51 (m, 5H), 5.80 (s, 1H), 2.60 (s, 2H), 2.35 (d,
J = 16.3 Hz, 1H), 2.24 (d, J = 16.3 Hz, 1H), 1.11 (s, 3H), 0.93 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 196.2, 165.1, 51.6, 147.5, 145.9, 131.4, 130.9, 129.6,
129.3, 128.4, 127.1, 125.2, 123.2, 122.9, 117.3, 115.9, 113.0, 50.7, 41.2, 34.6, 32.1,
29.3, 27.1.
12-(3-Nitrophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo-[a]xanthen-11-one (4e)
White solid; mp 168–170 °C; IR (KBr, cm-1): 3,073, 2,950, 1,648, 1,527, 1,370,
1,352. 1H NMR (400 MHz, CDCl3): d = 8.09 (s, 1H), 7.79–7.94 (m, 5H),
7.35–7.46 (m, 4H), 5.80 (s, 1H), 2.60 (s, 2H), 2.36 (d, J = 16.2 Hz, 1H), 2.27 (d,
J = 16.2 Hz, 1H), 1.14 (s, 3H), 0.97 (s, 3H). 13C NMR (100 MHz, CDCl3):
d = 196.4, 164.2, 148.5, 147.7, 146.5, 134.5, 132.1, 130.6, 129.6, 129.1, 128.4,
127.2, 125.0, 123.2, 123.0, 121.2, 117.2, 115.6, 113.2, 51.2, 41.4, 34.5, 32.1, 29.1,
27.3.
12-(4-Fluorophenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(4f)
White solid, m.p. 184–185 °C; IR (KBr, cm-1): 3,034, 2,952, 2,880, 1,654, 1,620,
1,508. 1H NMR (400 MHz, CDCl3): d = 7.98 (d, J = 8.4 Hz, 1H), 7.76 (t,
J = 8.2 Hz, 2H), 7.30–7.45 (m, 5H), 6.86 (t, J = 8.6 Hz, 2H), 5.72 (s, 1H), 2.55 (s,
2H), 2.24 (d, J = 16.4 Hz, 1H), 2.31 (d, J = 16.4 Hz, 1H), 1.13 (s, 3H), 0.97 (s,
3H). 13C NMR (100 MHz, CDCl3): d = 196.8, 163.6, 148.1, 141.6, 132.5, 131.7,
129.9, 129.2, 128.5, 127.2, 125.1, 123.5, 117.3, 117.2, 115.3, 114.9, 114.2, 51.2,
41.4, 34.3, 32.2, 29.2, 27.2.
123

Polyvinylpolypyrrolidone supported triflic acid (PVPP.OTf)
12-(4-Bromophenyl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one
(4g)
White solid; mp 186–187 °C; IR (KBr, cm-1): 3,072, 2,936, 1,643, 1,371.1H NMR
(400 MHz, CDCl3): d = 7.88 (d, J = 8.3 Hz, 1H), 7.76–7.80 (m, 2H), 7.38–7.45 (m,
2H), 7.26–7.33 (m, 3H), 7.20–7.23 (m, 2H), 5.66 (s, 1H), 2.56 (s, 2H), 2.31 (d,
J = 16.3 Hz, 1H), 2.24 (d, J = 16.3 Hz, 1H), 1.13 (s, 3H), 0.96 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 196.8, 164.0, 147.7, 143.8, 131.5, 131.2, 131.2, 130.1, 129.2,
128.5, 127.1, 125.0, 123.4, 120.1, 117.1, 116.9, 113.7, 50.8, 41.4, 34.2, 32.3, 29.4, 27.1.
12-(2-Bromorophenyl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11
(12H)-one (4h)
White solid; mp 168–170 °C; IR (KBr, cm-1): 3,069, 2,933, 1,646, 1,377.1H NMR
(400 MHz, CDCl3): d = 8.12 (d, J = 16.1 Hz, 1H), 7.88–7.91(m, 2H), 7.46–7.52
(m, 2H), 7.40–7.42(m, 2H), 7.18–7.21 (m, 2H), 6.97–7.01 (m, 1H), 5.75 (s, 1H),
2.71 (s, 2H), 2.32 (d, J = 16.1 Hz, 3H), 2.08 (d, J = 16.1 Hz, 1H), 1.06 (s, 3H),
0.90 (s, 3H). 13C NMR (100 MHz, CDCl3): d = 196.1, 163.3, 146.6, 142.1, 132.2,
131.2, 131.5, 131.2, 129.5, 129.0, 128.3, 127.5, 127.1, 126.6, 124.1, 123.9,116.3,
117.1, 113.2, 50.5, 41.4, 33.1, 32.1, 29.1, 27.1.
12-Phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]-xanthen-11-one (4i)
White solid; mp 150–152 °C; IR (KBr, cm-1): 3,055, 2,950, 2,879, 1,650, 1,376. 1H
NMR (400 MHz, CDCl3): d = 8.01 (d, J = 8.1 Hz, 1H), 7.73–7.78 (m,
2H),7.05–7.40 (m, 8H), 5.73 (s, 1H), 2.61 (s, 2H), 2.33 (d, J = 16.1 Hz, 1H),
2.25 (d, J = 16.1 Hz, 1H), 1.12 (s, 3H), 0.93 (s, 3H). 13C NMR (100 MHz, CDCl3):
d = 196.6, 164.1, 147.5, 144.3, 132.1, 131.3, 128.8, 128.4, 128.2, 128.0, 126.6,
126.1, 124.8, 123.5, 117.3, 117.1, 114.1, 50.5, 41.5, 34.7, 32.3, 29.6, 27.2.
12-(4-Methoxyphenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(4j)
White solid; mp 205–206 °C; IR (KBr, cm-1): 3,056, 2,948, 1,642, 1,224, 1,170. 1H
NMR (400 MHz, CDCl3): d = 8.01 (d, J = 8.1 Hz, 1H), 7.71–7.76 (m, 2H),
7.21–7.42 (m, 5H), 6.71 (d, J = 8.2 Hz, 2H), 5.64 (s, 1H), 3.68 (s, 3H), 2.56 (s, 2H),
2.35 (d, J = 16.2 Hz, 1H), 2.27 (d, J = 16.2 Hz, 1H), 1.13 (s, 3H), 0.98 (s,3H). 13
C
NMR (100 MHz, CDCl3): d = 196.5, 164.1, 155.7, 146.3, 136.1,133.6, 130.6,
130.2, 128.5, 127.2, 126.5, 125.1, 123.6, 122.4, 117.1, 116.8, 115.4, 113.3, 112.7,
53.4, 49.7, 40.2, 32.6, 31.1, 28.4, 26.1.
12-(4-Methylphenyl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one
(4k)
1H NMR (400 MHz, CDCl3): d = 8.0 (d, J = 8.4 Hz, 1H), 7.33–7.77 (m, 2H),
7.40–7.44 (m, 1H), 7.32–7.37 (m, 2H), 7.20–7.30 (m, 2H), 6.96 (d, J = 8.0 Hz,
123

S. Vahdat, S. Khaksar
2H), 5.66 (s, 1H), 2.54 (s, 2H), 2.30 (d, J = 16.2 Hz, 1H), 2.22 (d, J = 16.2 Hz,
1H), 2.19 (s, 3H), 1.11 (s, 3H), 0.98 (s, 3H). 13C NMR (100 MHz, CDCl3):
d = 196.8, 163.7, 147.6, 141.8, 135.6, 131.5, 131.4, 128.9, 128.6, 128.3, 128.3,
126.9, 124.8, 123.6, 117.9, 117.1,114.4, 50.9, 41.5, 34.2, 32.5, 29.3, 27.2, 20.9.
12-(3,4-dimethylphenyl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11
(12H)-one (4l)
1H NMR (400 MHz, CDCl3): d = 8.02 (d, J = 8.5 Hz, 1H), 7.77–7.72 (m, 2H),
7.42–7.25 (m, 3H), 7.11–7.04 (m, 2H), 6.91 (d, J = 8.0 Hz, 1 H), 5.63 (s, 1H), 2.56
(s, 2H), 2.26 (dd, J1 = 4.0 Hz, J2 = 16.0 Hz, 2H), 2.13 (s, 3H), 2.09 (s, 3H), 1.11
(s, 3H), 0.99 (s, 3H). 13C NMR (100 MHz, CDCl3): 195.2, 164.1, 147.6, 142.1,
135.2, 134.4, 132.5, 129.7, 129.3, 128.7, 128.4, 127.1, 125.6, 124.9, 123.5, 117.8,
117.1, 114.5, 52.1, 42.4, 34.3, 32.6, 29.2, 26.4, 21.1, 19.2.
12-(3-Hydroxyphenyl)-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
(4m)
White solid; mp 239–240 °C; IR (KBr, cm-1): 3,410, 3,050, 2,960, 1,642, 1,590,
1
1,370, 1,225, 1,70. H NMR (400 MHz, CDCl3): d = 8.01 (d, J = 8.1 Hz, 1H),
7.73–7.79 (m, 2H),7.30–7.44 (m, 3H), 7.03 (t, J = 7.7 Hz, 1H), 6.95 (s, 1H), 6.81
(d, J = 7.5 Hz, 1H), 6.58 (d, J = 6.1 Hz, 1H), 5.67 (s, 1H), 5.45 (s, 1H), 2.55 (s,
2H), 2.36 (d, J = 16.1 Hz, 1H), 2.31 (d, J = 16.1 Hz, 1H), 1.12 (s, 3H), 0.98(s,
3H). 13C NMR (100 MHz, CDCl3): d = 198.2, 164.6, 148.2, 147.1, 146.3, 135.15,
131.4, 130.6, 129.5, 129.2,128.1,127.1, 126.1,123.2, 123.0, 120.7,117.3, 115.5,
113.2, 50.6, 42.0, 34.7, 32.1, 29.1, 27.1.
9,9-Dimethyl-12-styryl-8,9,10,12-tetrahydrobenzo[a]-xanthen-11-one (4n)
White solid; mp 149–150 °C; IR (KBr, cm-1): 3,078, 2,930, 1,646, 1,465, 1,382. 1H
NMR (400 MHz, CDCl3): d = 8.04 (d, J = 8.3 Hz, 1H), 7.74–7.83 (m, 2H),
7.18–7.50 (m, 8H), 6.43–6.46 (m, 1H), 6.18 (d, J = 15.8 Hz, 1H), 5.35 (d,
J = 6.3 Hz, 1H), 2.55 (s, 2H), 2.37 (s, 2H), 1.06 (s, 3H), 0.89 (s, 3H). 13C NMR
(100 MHz, CDCl3): d = 197.2, 165.1, 147.7, 137.1, 131.4, 130.8, 130.3, 128.7,
128.3, 128.0, 127.4, 127.0, 126.1, 125.1, 123.6, 117.1, 113.1, 51.1, 41.4, 32.4, 31.3,
29.1, 27.4.
Results and discussion
In order to optimize the reaction conditions, we chose the condensation of the
reaction of b-naphthol, 4-chlorobenzaldehyde, and dimedone catalyzed by
PVPP.OTf under different conditions both in the absence and the presence of
PVPP.OTf, and results are given in Table 1. Then, the effect of temperature, the
amount of catalyst, and the reaction time on the yield of the product were examined.
It was found that this condensation reaction was affected by various solvents
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Polyvinylpolypyrrolidone supported triflic acid (PVPP.OTf)
(Table 1, entries 2–11). Among them, toluene provided the highest yield at refluxed
temperature after 5 h (Table 1, entry 5). In addition, no conversion to the product
was obtained in the absence of the catalyst even after 10 h (Table 1, entry 1). Next,
we examined the optimal amount of catalyst using the same model reaction. We
observed that 30 mg PVPP.OTf was sufficient to catalyze the reaction smoothly.
Increasing either the amount of the catalyst and/or prolonging the reaction time did
not improve the yield (Table 1, entry 11), while reducing these factors led to a
reduction in product yield (Table 1, entries 3, 4). Surprisingly, when the reaction
was carried out in toluene medium, solid product was separated at the end of the
reaction.
Using these optimized reaction conditions, the scope and efficiency of this
approach was explored for the synthesis of a wide variety of substituted
benzo[a]xanthenes and results are summarized in Table 2. A wide range of
structurally varied aldehyde reacted smoothly and quickly to give the corresponding
benzo[a]xanthenes in high yield and purity as listed in the table 2. In all cases,
aromatic aldehydes substituted with either electron-donating or electron-withdraw-
ing groups underwent the reaction smoothly and gave the products in good yields.
It could also be concluded that the aldehydes bearing electron-withdrawing
groups required shorter times and gave higher yields (Table 2, entries 1–8). In
addition, a,b-unsaturated aldehyde also afforded good yields (Table 2, entry 14). In
most cases, no undesired side products such as aryl-14H-dibenzo[a,j]xanthene were
obtained under these conditions [15]. However, the reaction failed with aliphatic
aldehydes such as propionaldehyde and cyclohexanecarbaldehyde (Scheme 2). In
these cases, alkyl-14H-dibenzo[a,j]xanthene, 5a, b, was the exclusive product.
Another important feature of this procedure is the survival of a variety of functional
groups such as ethers, alkyls, nitros, and halides under the present reaction
conditions.
Table 1 Effect of different PVPP.OTf and solvent on formation of 4
Entry
PVPP.OTf amount (mg)
Condition/solvent
Time (h)/yield
1
0
r.t./toluene
24/0
2
20
10
20
30
30
30
30
30
30
50
r.t./toluene
10/60
10/60
8/90
3
Reflux/toluene
Reflux/toluene
Reflux/toluene
r.t./CH2Cl2
r.t./THF
4
5
5/95
6
12/30
12/20
10/60
24/10
24/10
5/95
7
8
r.t./ethanol
9
r.t./H2O
10
11
r.t./diethyl ether
Reflux/toluene
123

S. Vahdat, S. Khaksar
Table 2 Synthesis of benzo[a]xanthenes in the presence of PVPP.OTf
Entry
1
Aldehyde
Product
Time (h)
5
Yield
95
%
4a
CHO
Cl
2
4b
6
90
CHO
Cl
CHO
3
4
4c
5
5
90
90
Cl
4d
CHO
CHO
O2N
O2N
5
6
4e
4f
5
5
85
92
CHO
F
7
8
4g
4h
5
5
90
85
CHO
Br
CHO
Br
9
4i
4j
5
6
85
85
CHO
10
CHO
MeO
11
12
4k
4l
6
6
85
80
CHO
CHO
Me
Me
Me
HO
13
14
4m
4n
6
5
85
85
CHO
CHO
123

Polyvinylpolypyrrolidone supported triflic acid (PVPP.OTf)
Characterizing the Brønsted acid sites presented on the polymer was performed
by recording the FT-IR spectrum of PVPP.OTf, which shows a strong broad
absorption at 3,432 cm-1 for the O–H bonds and a moderate absorption at
1,648 cm-1 corresponding to the imine group on the backbone (Fig. 1). Respec-
tively [31], the bands at 1,225 and 1,174 cm-1 were assigned to the S=O
-
asymmetric and symmetric stretching vibrations of the –SO3 group. The loading
capacity of the reagent was determined by titration and found to be 10 mmol/g,
whereas its silica-supported analogue has a loading capacity of less than 1 mmol/g.
As PVPP.OTf is not soluble in toluene, no PVPP.OTf leaching as well as no
contribution of homogeneous catalysis in the course of reaction were expected. To
prove this, after 3 h, the catalyst was removed from the toluene by filtration and the
supernatant was tested for activity. No activity was observed, indicating that there
was no contribution of homogeneous catalysis in this reaction. After the reaction,
the catalyst can be easily separated (by filtration) and reused after washing with
dichloromethane with gradual decrease in its activity. For example, the reaction of
b-naphthol, 4-chlorobenzaldehyde, and dimedone afforded the corresponding
xanthene derivative in 95, 95, and 90 % isolated yield over three cycles.
R
RCHO
+
O
O
PVPP.OTf (30 mg)
toluene, 110 C, 6 h
+
OH
O
R= Propyl, t-Butyl
Scheme 2 Reaction of b-naphthol, aliphatic aldehyde, and 5,5-dimethylcyclohexane1,3-dione
Fig. 1 The FT-IR spectrum of the polyvinylpolypyrrolidone (PVPP) and (PVPP.OTf) catalyst
123

S. Vahdat, S. Khaksar
Conclusion
In conclusion, we have developed an efficient synthesis of tetrahydrobenzo[a]xan-
then-11-one derivatives via one-pot condensation of b-naphthol, aldehydes, and
cyclic 1,3-dicarbonyl compounds in the present of polyvinylpolypyrrolidoniume
triflate. In contrast to the existing methods using potentially hazardous catalysts/
additives, the present method offers the following competitive advantages: (1)
PVPP.OTf is easy to prepare from commercially available polyvinylpolypyrroli-
done and TfOH, (2) short reaction time, (3) ease of product isolation/purification by
non-aqueous work-up, (4) no side reaction, (5) low costs and simplicity in process
and handling, and (6) tetrahydrobenzo[a]xanthen-11-one is produced by an
environmentally benign process.
Acknowledgment This research is supported by the Islamic Azad University, Ayatollah Amoli Branch.
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