H4SiW12O40 Catalyzed one-pot synthesis of 12-Aryl-8,9,10,12-tetrahydrobenzo[a] xanthen-11-ones under solvent-free conditions

Article abstract of DOI:10.1155/2012/930251

An efficient and environmentally benign protocol for the one-pot, three-component synthesis of 12-aryl-8,9,10,12-tetrahydro-benzo[a]xanthen- 11-one derivatives by condensation of aryl aldehydes, 2-naphthol and dimedone using H₄SiW₁₂O

Full text of DOI:10.1155/2012/930251

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
http://www.ejchem.net
2012, 9(2), 786-790
H4SiW12O40 Catalyzed One-Pot Synthesis of
12-Aryl-8,9,10,12-tetrahydrobenzo[a]
Xanthen-11-ones Under Solvent-Free Conditions
A. HASSANKHANIa* E. MOSADDEGH a and S. Y. EBRAHIMIPOURb
aDepartment of Materials Science, International Center for Science,
High Technology & Environmental Sciences, P O Box 76315-117, Kerman, Iran
bDepartment of Chemistry, Payame Noor University (PNU), Mashhad, Iran
hassankhani_a@yahoo.com
Received 21 September 2011; Accepted 13 November 2011
Abstract: An efficient and environmentally benign protocol for the one-pot,
three-component synthesis of 12-aryl-8,9,10,12-tetrahydro-benzo[a]xanthen-
11-one derivatives by condensation of aryl aldehydes, 2-naphthol and
dimedone using H4SiW12O40 (SiWA) as a reusable catalyst with high catalytic
o
activity was reported. The reaction was carried out at 100 C under solvent-
free conditions within 15-40 min in 82-91% yield.
Keywords: 12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones, Dimedone, 2-Naphthol, Aldehyde,
Tungstosilicic acid, Multicomponent reaction.
Introduction
Organic reactions under green media have attracted much interest from chemists,
particularly from the viewpoints of green chemistry. Green chemistry approaches are
significant due to the reduction in byproducts and waste chemicals and lowering of energy
costs. The possibility of performing multi-component reactions (MCRs) under green media
could enhance their ability from the economical aspect as well as ecological point of view1.
Xanthenes and benzoxanthenes are important biologically active heterocyclic
compounds, which possess antiviral2, anti-inflammatory3 and antibacterial4 activities. These
are being utilized as antagonists for paralyzing action of zoxazolamine5 and in
photodynamic therapy6. Furthermore, these compounds can be used as dyes7 in laser
technologies8 and as pH sensitive fluorescent materials for visualization of biomolecules9.
Tetrahydroxanthenone derivatives are generally synthesized by the three-component
condensation of 2-naphthol, aryl aldehydes and cyclic 1,3-dicarbonyl compounds under
different conditions. There are several reports for the synthesis of 12-aryl-8,9,10,12-Tetrahy-
drobenzo[a]xanthen-11-ones using catalysts such as NaHSO4.SiO210, Strontium triflate11,
pTSA12, Indium(III) chloride or P2O513, Dodecatungstophosphoric acid14, Iodine15,
21
HBF4/SiO216, HClO4-SiO217, Sulfamic acid18, Cyanuric chloride19, Zr(HSO4)420, Cu/SiO2
,

H4SiW12O40 Catalyzed One-Pot Synthesis of Conditions 787
Ruthenium chloride22, Caro's acid–silicagel23, Surfactant24, Scandium triflate/MW25 and
Camphor sulphonic acid26. However, in spite of their potential utility, some of these methods
suffer drawbacks such as the use of toxic and hazardous solvents, unsatisfactory product
yields, expensive catalyst, requirement of huge amounts of catalyst, purification of the
products by column chromatography and prolonged reaction times. So the development of a
clean, high yielding and ecofriendly approach is still desirable. In this regard, a simple and
efficient method for the one-pot synthesis of 12-aryl-8,9,10,12-tetrahydro-benzo[a]xanthen-
11-one derivatives using H4SiW12O40 (SiWA) is reported herein.
In recent years, solid acids have found increased application in organic synthesis, as
they may be easily recovered and recycled. Heteropolyacids are strong solid acids, harmless
to the environment, and highly stable toward humidity, with flexibility in modifying acid
strength27. Moreover, solvent-free reactions often provide clean, efficient and high-yielding
organic processes in heterocyclic synthesis28. In continuation of our efforts to develop new,
green chemistry methods29-31, we describe herein a clean and convenient synthesis of
12-aryl-8,9,10,12-tetrahydrobenzo[a] xanthen-11-ones by three-component condensation
reaction of 2-naphthol, aromatic aldehydes, and dimedone using catalytic amount of
tungstosilicic acid as a recyclable catalyst under solvent-free conventional heating
conditions (Scheme 1) 15-40 min.
X
O
CHO
OH
+
Tungstosilicic acid
Solvent-free, 100 oC
15-40 min
+
O
O
X
O
2
3
1
4
Scheme 1
Experimental
Melting points recorded on an open capillary and IR spectra were measured on a Shimadzu
1
IR-470 spectrophotometer. The H NMR spectra were recorded on a Bruker-400 MHz. All
of the products are known and were characterized by their spectral and physical data. The
monitoring of the progress of all reactions was carried out by TLC. TLC was runned using
TLC aluminum sheets silica gel 60 F254 (Merck).
General procedure for the preparation of 12-aryl-8,9,10,12-tetrahydrobenzo[a]
xanthen-11-one 4a
A mixture of 4-chlorobenzaldehyde (2 mmol), dimedone (2 mmol), 2-naphthol (2 mmol)
o
and H4SiW12O40 (3.5 mol%, ca. 0.2 g) was heated with stirring at 100 C for an appropriate
time (TLC). After completion of the reaction, the mixture was cooled to room temperature
and washed with water. The solid product was purified by recrystallization from EtOH to
afford the pure product. All compounds are known and their physical and spectroscopic data
were in good agreement with those of authentic samples10-26
.

788 A. HASSANKHANI et al.
Results and Discussion
Aromatic aldehydes 1a-l, 2-naphthol 2 and dimedone 3 in the presence of a catalytic amount
o
of SiWA undergo a fast 1:1:1 addition reaction at 100 C under solvent-free conditions for
several minutes to produce 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones 4a-l
(Table 1). The results were excellent in terms of yields and product purity in the presence of
SiWA, while without it for long period of time (1-2 h), the yields of products were trace.
Next, the optimum amount of SiWA was evaluated. The highest yield was obtained with
3.5 mol% of the catalyst. A further increase in the amount of SiWA did not have any
significant effect on the product yield. The generality of this reaction was examined using
different aldehydes having electron-donating as well as electron-withdrawing groups. In all
cases, the reactions gave the corresponding products in good to excellent yields (Table 1)
and in short reaction times. Substituent on the aromatic ring had no obvious effect on yield
or reaction time under the above optimal conditions. This method offers significant
improvements with regard to the scope of the transformation, simplicity and green aspects
by avoiding toxic solvents.
Table 1. H4SiW12O40 catalyzed synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a] xanthen-
11-ones.
Product
4a
Ar
4-Cl-C6H4
Time (min)
Yield, %a
15
20
20
15
30
15
15
15
15
15
30
40
89, 87, 86, 83b
4-Br-C6H4
2,4-Cl-C6H3
C6H5
91
90
89
85
83
87
82
87
85
88
91
4b
4c
4d
4e
4-H3C-C6H4
4-CH3O-C6H4
3-CH3O-C6H4
4-O2N-C6H4
3-O2N-C6H4
2-O2N-C6H4
2-Cl-C6H4
4f
4g
4h
4i
4j
4k
4l
2-CH3O-C6H4
aYield refers to isolated pure product. bCatalyst was reused three times after drying.
The recovery and reusability of the catalyst was examined in the synthesis of 12-aryl-
8,9,10,12-tetrahydrobenzo[a] xanthen-11-ones. When the reaction was completed, water
was added and the product was filtered. The aqueous solution containing soluble catalyst
was evaporated under reduced pressure, and the obtained powder was washed with diethyl
ether, dried and reused for the same reaction again. It was found that the catalyst could be
reused three times with slight decreasing in catalytic activity, 87, 86 and 83% (Table 1, entry
1).
A possible mechanism for the formation of the products is shown in Scheme 2. The
reaction occurs via initial formation of the ortho-quinone methides intermediate, which was
formed by the nucleophilic addition of 2-naphthol to aldehyde catalyzed by SiWA.
Subsequent Michael addition of the o-QM with dimedone and followed by addition of the

H4SiW12O40 Catalyzed One-Pot Synthesis of Conditions 789
phenolic hydroxyl moiety to the carbonyl of ketone provides cyclic hemiketal which on
dehydration afforded 4.
OH
2
OH
Ar
O
H
H
H
+
+
O
5
H
R
H
O
O
O
O
+
O
Ar
H
H
HO
+
3
H
1
R
SiWA
Ar
O
OH
OH
O
Ar
O
4
Scheme 2
Conclusion
We have developed a simple, efficient, one-pot and green protocol for the synthesis of
12-aryl-8,9,10,12-tetrahydro-benzo[a] xanthen-11-ones using SiWA as a reusable heterogeneous
catalyst under solvent-free conditions. The present approach offers several advantages such
as high yields and purity, shorter reaction times and low cost.
Acknowledgment
We thank the International Center for Science, High Technology & Environmental Sciences,
for financial support of this investigation.
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