Vanadium(III) chloride as an effective catalyst for the Pechmann reaction

Article abstract of DOI:10.1007/s10593-006-0066-6

Good yields of substituted coumarins were obtained by a synthetic method involving the Pechmann reaction using vanadium(III)chloride (VCl₃) reagent to effect this condensation under solvent-free conditions. 2006 Springer Science+Business Media, Inc.

Full text of DOI:10.1007/s10593-006-0066-6

Chemistry of Heterocyclic Compounds, Vol. 42, No. 2, 2006
VANADIUM(III) CHLORIDE AS AN EFFECTIVE
CATALYST FOR THE PECHMANN REACTION
B. Sunil Kumar1, P. S. Kumar2 N. Srinivasulu1 B. Rajitha2, Y. Thirupathi Reddy2,
,
,
P. Narsimha Reddy2, and R. H. Udupi1
Good yields of substituted coumarins were obtained by a synthetic method involving the Pechmann
reaction using vanadium(III)chloride (VCl3) reagent to effect this condensation under solvent-free
conditions.
Keywords: coumarins, vanadium(III) chloride, Pechmann reaction.
Coumarins and chromones are naturally occurring important plant constituents and occupy a unique
place in natural and synthetic organic chemistry. Plants belonging to the natural orders of orchidaceae,
leguminoceae, rutaceae, umberlliferae, and labiatae are rich sources of naturally occurring coumarins. Members
of this group have a broad range of applications as fragrances, pharmaceuticals, and additives to food and
cosmetics [1, 2]. Methyl coumarins occur in the heartwood of various plants and some of them possess antibiotic
[3] and antiviral [4] properties, while others are used as fluorescent brighteners, photographic sensitizers, and
dyes [5]. Coumarin derivatives have also found applications as CNS depressants [6], antitumour agents [7], HIV
inhibitors [8], and as intermediates for the synthesis of the cardiac drug carbochromene [9]. The potent
antibiotics chartrecisin [10] and coumarmycine [11] are also coumarin derivatives. Coumarins act as
intermediates for the synthesis of polysubstituted heterocycles, which are most interesting compounds as
potential biodegradable agrochemicals.
There are some well-known synthetic methods used for the synthesis of coumarins. All these methods
center around the possibility of building up the pyrone ring on suitable benzene derivative. However, some of
the classified methods, which have attained significant importance for the synthesis of coumarin derivatives, are
Perkin [12], Pechmann [13], Pechmann–Duisburg, Knoevenagel [l4], Reformatsky [15], and Wittig reactions
[16, 17]. The rapid development in the chemistry of coumarins is mainly due to the synthetic method universally
known as the Pechmann reaction, which consists in the reaction of phenol with β-keto ester in the presence of
sulfuric acid as a cyclizing agent. Since the Pechmann reaction proceeds from very simple starting materials and
gives good yields of coumarin derivatives, it is the most widely used method for the preparation of various
substituted coumarins. Various cyclizing reagents like H2SO4, P2O5, FeCl3, ZnCl2, POCl3, AlCl3, HCl,
phosphoric acid, and trifluoroacetic acid [18-22] are known to affect the Pechmann condensation. A mixture of
phenol, β-keto ester, and the acidic catalyst was allowed to stand for long time or was heated above 150°C in all
the methods. Recently an attempt was made to use a solid acid catalyst for this reaction [23]. Application of ionic
liquids [24] and sulfamic acid was also reported [25]. However, all these reactions require high
__________________________________________________________________________________________
1
Department of Chemistry, N. E. T. Pharmacy College, Raichur (Karnataka), India; e-mail:
2
penthala_nr@yahoo.co.in. Department of Chemistry, National Institute of Technology, Warangal (AP), India.
Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 197-200, February, 2006. Original article
submitted August 07, 2005.
172
0009-3122/06/4202-0172©2006 Springer Science+Business Media, Inc.

TABLE 1. Synthesis of Substituted Coumarins from Phenols and β-Keto
Esters* Catalyzed by VCl3
Entry
1
Phenol 1
Product 3
mp, °C*2
4
Yield, %
5
2
3
O
O
HO
HO
O
O
HO
HO
OH
OH
1
2
181-183
283-284
92
90
OH
OH
OH
OH
O
HO
O
HO
OH
3
4
5
254-255
154-155
248-250
90
86
91
O
O
OH
O
O
HO
O
HO
OH
HO
HO
O
O
HO
HO
OH
OH
6
7
241-242
243-244
92
84
Ph
O
OH
OH Ph
O
HO
O
HO
OH
8
244-246
88
Ph
O
OH
O
9
240-241
245-246
87
90
Ph
O
O
HO
HO
OH
10
173

TABLE 1 (continued)
1
2
3
4
5
O
OH
O
11
217-218
88
O
O
HO
HO
O
O
HO
HO
OH
12
245-246
251-253
94
92
OH
13
OH
OH
_______
* β-Keto ester 2: ethyl acetoacetate (1-5), ethyl benzoylacetate (6-9),
2-сarbethoxycyclopentanone (10, 11), and 2-carbethoxycyclohexanone (12,
13).
*2 mp according to the data of [25].
temperatures (150°C or above) or long reaction times; an excess amount of the catalyst is also a requirement for
such reactions, and sometimes the yields are low. Several reports are also available on the use of microwaves for
the accelerated synthesis of different coumarins [26-29].
We report in this communication a facile one-step synthesis of coumarin derivatives by the Pechmann
reaction using vanadium(III) chloride as a new condensing agent. Simple workup, less reaction time, and
solvent-free conditions are the advantages of using vanadium(III) chloride as a catalyst in the synthesis of
coumarin derivatives by the application of the Pechmann reaction. This method is very effective and superior to
earlier reported methods as it affords good yields without causing any side reaction.
R1
OH
O
O
O
O
VCl3
R
R2
R
+
R2
1
R1
EtO
2
3
The reaction was studied by using different concentrations of reactants and catalyst at different
temperatures under neat conditions. The best results were achieved when the reactions were carried out at 50-
55°C for 2 h using 10% of the catalyst with respect to the quantity of reactants in the absence of the solvent. The
generality and scope of the reaction are summarized in Table 1.
174

EXPERIMENTAL
General Procedure for the Preparation of Coumarin Derivatives Using VCl3. A mixture of the
appropriate phenol (1 mmol) and β-keto ester (1 mmol) was stirred at 50-55°C for 2 h with VCl3 (0.1 mmol).
The progress of the reaction was monitored by TLC. After completion of the reaction the reaction mass was
cooled to room temperature, water (25 ml) was added, and the whole stirred for 30 min. The solid obtained was
filtered off, washed with water, and recrystallized from ethanol.
The authors are thankful to UGC, New Delhi, for financial assistance and to the Director, IICT,
Hyderabad for 1H NMR and mass spectral analysis.
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