Xanthan sulfuric acid: An efficient and recyclable solid acid catalyst for Pechmann condensation

Article abstract of DOI:10.1080/00397911.2010.543964

In this report, substituted coumarins are formed via Pechmann condensation using various substituted phenols and ethyl acetoacetates in the presence of xanthan sulfuric acid as a solid acid catalyst under solvent-free conditions. This method is very simple, costeffective, and has shorter reaction times, and the catalyst could be reused. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.543964

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Xanthan Sulfuric Acid: An Efficient
and Recyclable Solid Acid Catalyst for
Pechmann Condensation
B. Suresh Kuarm a , J. Venu Madhav a & B. Rajitha a
a Department of Chemistry, National Institute of Technology,
Warangal, India
Available online: 17 Nov 2011
To cite this article: B. Suresh Kuarm, J. Venu Madhav & B. Rajitha (2012): Xanthan Sulfuric Acid: An
Efficient and Recyclable Solid Acid Catalyst for Pechmann Condensation, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:12, 1770-1777
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Synthetic Communications1, 42: 1770–1777, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.543964
XANTHAN SULFURIC ACID: AN EFFICIENT AND
RECYCLABLE SOLID ACID CATALYST FOR
PECHMANN CONDENSATION
B. Suresh Kuarm, J. Venu Madhav, and B. Rajitha
Department of Chemistry, National Institute of Technology, Warangal, India
GRAPHICAL ABSTRACT
Abstract In this report, substituted coumarins are formed via Pechmann condensation using
various substituted phenols and ethyl acetoacetates in the presence of xanthan sulfuric acid
as a solid acid catalyst under solvent-free conditions. This method is very simple, cost-
effective, and has shorter reaction times, and the catalyst could be reused.
Keywords Biodegradable xanthan sulfuric acid (XSA); coumarins; Pechmann conden-
sation; solvent-free conditions
INTRODUCTION
The synthesis of coumarins and their derivatives has received much attention
from organic and medicinal chemists because of their wide range of biological and
pharmaceutical properties. This heterocyclic system has been employed in the prep-
aration of important drugs required for treatment of platelet aggregation,[1]
bacteria,[2] and cancer.[3] Some of the coumarin analogs are used to inhibit steroid
5a-reductase[4] and HIV-1 protease.[5] Moreover, these derivatives are used as anti-
coagulants,[6] additives in food and cosmetics,[7] and in the preparation of insecti-
cides, optical brighteners,[8] and dispersed fluorescent and laser dyes.[9] To
synthesize coumarins and their derivatives, several methods are reported, including
Pechmann,[10a] Perkin,[10b] Knoevenagel,[10c] Reformatsky,[10e] and Wittig[11] reac-
tions. Among them, the Pechmann reaction is the most widely applied method for
synthesis of coumarins as it involves the condensation of phenols with b-ketoesters
in the presence of a variety of acidic condensing agents to afford good yields of cou-
marins.[12,13] To prepare this important moiety, various condensing agents are
Received August 30, 2010.
Address correspondence to B. Rajitha, Department of Chemistry, National Institute of
Technology, Warangal, AP, India. E-mail: rajithabhargavi@ymail.com
1770

XANTHAN SULFURIC ACID
1771
reported in literature such as polyphosphoric acid (PPA),[14] InCl3,[15] ZrCl4,[16]
Yb(OTf)3,[17] p-TsOH,[18] BiCl3,[19] I2 or AgOTf,[20] and Sm(NO3)3,[21] as well as
chloroaluminate ionic liquids.[22,23] The main disadvantages of these methods are
longer reaction times, loading of large amount of the catalyst, and tedious workup
procedures. Some of the catalysts are highly expensive and after work up cannot
be recovered or reused. These shortcomings surely show the need for a safe, eco-
friendly, and efficient method to synthesize these important compounds.
Recently, the direction of science and technology has been shifting more toward
ecofriendly, natural product resources and reusable catalysts. Thus, natural biopoly-
mers are attractive candidates in the search for such solid support catalysts.[24,25]
Among different biopolymers, xanthan is one of the most common biopolymers
and has some unique properties that make it an attractive alternative to conventional
organic or inorganic supports for catalytic applications.[26] Recently, sulfonated
xanthan has been utilized as a biopolymeric solid support acid catalyst for the syn-
thesis of a-amino nitriles.[27] This polymer has unlimited availability as a renewable
agro-resource and is biodegradable.
RESULTS AND DISCUSSION
For the aforementioned reasons and in light of our general interest in synthesiz-
ing coumarins,[28] in this article, we report a simple procedure for the synthesis of
coumarins by the condensation of phenols with ethylacetoacetates under solvent-free
conditions using xanthan sulfuric acid (XSA) (Scheme 1). The reactions were carried
out at room temperature for 20–30 min using phenol 1 (1 mmol) and b-ketoester 2
(1 mmol) and in the presence of XSA (0.08 g). The results are summarized in Table 1.
We then investigated the efficiency of the XSA compared to various sulfur
analog acidic catalysts. The results are summarized in Table 2. XSA was found to
be the most effective catalyst based on the product yield. The reaction did not
proceed in the absence of catalyst (yield less than 10%).
We examined the amount of catalyst in this reaction. The best results were
obtained using 0.08 g of catalyst (96%). Using lower amounts of catalyst resulted
in lower yields, and in the absence of catalyst the product yield was found to be very
poor (Table 3).
EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected. The
reactions were monitored by thin-layer chromatography (TLC) and visualized with
Scheme 1. Synthesis of coumarin by xanthan sulfuric acid as a solid acid catalyst.

1772
B. S. KUARM, J. V. MADHAV, AND B. RAJITHA
Table 1. Xanthan sulfuric acid–catalyzed sunthesis of coumarins
Entry
Phenol
Ester
Coumarin
Time (min) Yielda (%)
Mp (ꢀC)
81[28b]
1
2
3
4
5
6
30
25
20
30
25
30
88
90
96
91
92
92
164–165[29]
257[30]
185[31]
280[31]
187[32]
7
20
94
242[31]
8
9
25
20
20
95
91
95
174[33]
153[30]
137[28b]
10
11
12
25
30
91
88
115–116[29]
260–262
aYields refer to pure products and all products were characterized by comparison of their physical data,
1H NMR 13C NMR, IR, and mass.
ultraviolet light. Infrared (IR) spectra (KBr) were recorded on a Shimadzu FTIR
1
model 8010 spectrometer, and the H NMR spectra were measured on a Varian
Gemini 200-MHz spectrometer using tetramethylsilane (TMS) as an internal stan-
dard. Mass spectra were recorded on a Jeol JMS D-300 spectrometer. CHN analysis
was done by Carlo Erba EA 1108 automatic elemental analyzer. All solvents and
reagents were purchased from Aldrich and Fluka.

XANTHAN SULFURIC ACID
1773
Table 2. Effect of catalysts on yielda
Entry
Catalyst
Quantity
Yieldb (%)
1
2
3
4
5
Xanthan sulfuric acid
Silica sulfuric acid
Methane sulfonic acid
Sulfuric acid in acetic acid
No catalyst
0.08 g
0.08 g
96
92
86
56
10
0.1 mmol
0.1 mmol
None
aMixture of phenol 1 (1 mmol) and b-ketoester 2 (1 mmol)
was stirred at room temperature.
bIsolated yield.
Catalyst Preparation
Chlorosulfonic acid (1.00 g) was added dropwise to a magnetically stirred
mixture of xanthan (5.00 g) in CHCl3 (15 mL) at 0 ꢀC during 2 h. After completion
of the addition, the mixture was stirred for 3 h. Then, the mixture was filtered,
washed with methanol (25 mL), and dried at room temperature to obtain XSA as
white powder (5.30 g). Sulfur content of the samples by conventional elemental
analysis was 0.62 mmol=g for XSA. The number of Hþ site of xanthan–SO3H deter-
mined by acid–base titration was 0.6 meq=g. This value corresponds to about 92% of
the sulfur content, indicating that most of the sulfur species on the sample are in the
form of the sulfonic acid groups.
FT-IR Spectrum of Xanthan Sulfuric Acid
The FT-IR spectrum of the catalyst showed a strong absorption band at around
1200 cmꢁ1 that was attributable to stretching vibrations of the SO2 group. Two strong
absorption bands at around 1400 and 1200 cmꢁ1 were observed and attributed to
asymmetric and symmetric stretching vibration of SO2 group, respectively. For
S–O functional group, the stretching absorption as at around 600–700 cmꢁ1. The
spectrum also showed a strong broad band for OH stretching absorption at around
3400–3700 cmꢁ1. This result indicates that reaction of xanthan with chlorosulfonic
acid succeeded in incorporating sulfated groups in XSA.
Table 3. Influence of the catalytic amounts of xanthan sulfuric acida
Entry
Catalyst (g)
Time (min)
Yieldb (%)
1
2
3
4
5
6
None
0.01
0.03
0.05
0.08
0.08
60
20
20
20
40
20
Trace
28
51
79
96
96
aMixture of phenol 1 (1 mmol), b-ketoester 2 (1 mmol), and xanthan
sulfuric acid was stirred at room temperature.
bIsolated yield.

1774
B. S. KUARM, J. V. MADHAV, AND B. RAJITHA
General Procedure for Synthesis of Coumarins
A mixture of the phenol 1 (1 mmol), b-ketoester 2 (1 mmol), and XSA (0.08 g)
was stirred at room temperature for the appropriate time according to Table 1. Com-
pletion of the reaction was confirmed by thin-layer chromatography (TLC). After
completion of the reaction, CHCl3 (10 ml) was added to the reaction mixture. The
reaction mixture was filtered and washed with CHCl3 (5 ml), combined organic layers
were concentrated in vacuum to afford the crude compound, and it was recrystallized
from ethanol to afford the pure product. The reusability of the catalyst was checked
by separating the XSA from the reaction mixture by simple filtration and drying it in
an oven (50 mm Hg pressure) at 60 ꢀC for 3 h prior to use in the other reaction.
The recovered catalyst can be reused at least three additional times in subsequent
reactions without significant decrease in product yield (Table 4).
Spectral Data
Table 1, Entry 2. IR (KBr, cmꢁ1): 2935, 1683, 1626, 1485; 1H NMR (CDCl3):
d 2.41 (3H, s), 3.89 (3H, s), 6.36 (1H, s), 7.15 (1H, d), 7.26 (1H, s), 7.35 (1H, d). 13
C
NMR (CDCl3): d 160.1, 155.4, 153.2, 146.9, 120.2, 119.7, 117.4, 115.3, 108.3, 56.1,
18.4; EIMS (m=z) 190 (Mþ). Anal. calcd. for C11H10O3: C, 69.43; H, 5.30. Found:
C, 69.45; H 5.27.
Table 1, Entry 4. IR (KBr, cmꢁ1): 3400, 1725, 1530; 1H NMR (CDCl3) d 2.39
(3H, s), 3.28 (br s, 1H), 6.06 (s, 1H), 6.8 (s, 1H), 6.82 (d, 1H), 7.44 (d, 1H). 13C NMR
(CDCl3): d ¼ 161.7, 159.6, 152.6, 151.80, 128.9, 113.9, 112.5, 111.5, 109.7, 22.0;
EIMS (m=z) 176 (Mþ). Anal. calcd. for C10H8O3: C, 68.10; H, 4.71. Found: C,
68.18; H, 4.55.
Table 1, Entry 7. IR (KBr, cmꢁ1): 3417, 1676, 1620, 1585; 1H NMR (CDCl3):
d 2.37 (3H, s), 6.12 (1H, s), 6.86 (1H, d), 7.11 (1H, d); 13C NMR (CDCl3): d 161.6,
155.5, 153.3, 149.6, 145.21, 122.3, 114.2, 112.5, 110.3, 22.5; EIMS (m=z) 192 (Mþ).
Anal. calcd. for C10H8O4: C, 62.50; H, 4.20. Found: C, 62.54; H, 4.15.
1
Table 1, Entry 12. IR (KBr, cmꢁ1): 1745, 1648, 1619; H NMR (CDCl3): d
7.32 (d, 1H), 7.24 (s, 1H), 6.91 (d, 1H), 6.14 (s, 1H), 2.95 (s, 3H), 2.41 (s, 3H); 13C
NMR (CDCl3): 189.9, 161.0, 158.4, 151.3, 149.9, 127.2, 115.7, 113.2, 110.8, 108.5,
Table 4. Effect of reusability of catalyst on yield
Run
Cycle
Yielda (%)
1
2
3
4
0
1
2
3
96
95
91
86
Note. Reaction conditions: phenol 1 (1 mmol),
b-ketoester 2 (1 mmol), and xanthan sulfuric acid
(0.08 g) were stirred at room temperature.
aYields refer to the pure isolated recovered
catalyst.

XANTHAN SULFURIC ACID
1775
29.4, 22.1. EIMS (m=z) 202 (Mþ). Anal. calcd. for C12H10O3: C, 71.28; H, 4.98.
Found: C, 71.11; H, 4.86.
CONCLUSION
In conclusion, we have developed an efficient synthesis of substituted coumar-
ins via Pechmann condensations using XSA catalyst under solvent-free conditions.
Moreover, inexpensive catalyst, solvent-free condition, low toxicity of the catalyst,
fast reaction times, simple experimental procedure, recyclablity of the catalyst, and
good yields of the products are the advantages. We believe this procedure will find
important applications in the synthesis of coumarins.
ACKNOWLEDGMENT
One of the authors, B. S. K., is grateful to the Council of Scientific and Indus-
trial Research (CSIR), New Delhi, Republic of India, for providing financial support
in the form of a senior research fellowship.
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