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and non-peptidic inhibitor properties etc. [18]. The predominant
route adopted in this process involves an ortho-quinone methide
[o-QM] intermediate and its interaction with carbon nucleophiles
[19]. It has been reported that these intermediates mainly interact
with various dienophiles via [4 + 2] cycloaddition reactions [20].
Xanthenes and benzoxanthenes are usually formed by an in situ
trapping of the o-QM with carbon nucleophiles like phenols and
napththols.
[24] and inorganic salts such as InCl3 [25], Zr(HSO4)4 [26], Sr(OTf)2
[27], Caro’s acid-silica [28], tungstophosphoric acid [29], CAN [30]
and Ce (IV) salts [31]. Some of the organic acids like PTSA [32], TBAF
[33], sulfamic acid [34], cyanuric acid [35] and N,Nꢀ-dibromo-N,Nꢀ-
1,2-ethanediyl-bis(p-toluenesulfonamide) [36] have also been
reported. Most of such solid acid catalyzed reactions often involve
elongated reaction time, drastic reaction conditions and formation
of non-desirous side products. The intrinsic reaction conditions
preferably utilize strong acidic media, high reaction tempera-
tures and stoichiometric ratios resulting in tedious work-up and
The inherent properties of mesoporous materials such as, their
tuneable pore size and acid-base sites have been utilized for
widespread applications in catalysis [37]. MCM-41 [38] and SBA-15
[39] have demonstrated exceptional properties by virtue of their
hexagonal pore array arrangement and narrow pore size distri-
bution [40]. It was observed from the literature that the Ce(IV)
derivatives have shown better catalytic activity among other cata-
lysts such as FeCl3, SnCl4, ZnCl2 and AlCl3 are normally employed
as single-electron oxidants [30]. The commercial availability, ease
of handling and inexpensive ceric ammonium nitrate (CAN) in
carbon carbon and carbon heteroatom bond forming reactions
has recently attracted much attention [41]. The catalyst, Ce-MCM-
41, has received considerable attention due to its low toxicity,
cost effectiveness, air and water compatibility, ease of handling,
good reactivity, experimental simplicity and remarkable ability to
suppress side reactions in acid sensitive substrates [42]. Recently,
we have explored the activity of Ce-MCM-41 catalyst towards the
regioselective dual C H bond activation under heterogeneous con-
ditions [42b]. In furtherance of our studies, we report the use
of cerium containing MCM-41 as a heterogeneous and reusable
catalyst for the multicomponent one pot synthesis of benzoxan-
thenones.
spectra were acquired on a Q STAR XL Hybrid LC/MS/MS system,
Applied Biosystems, USA. FT-IR data were acquired on a Thermo
Nicolet Nexus 670 FT-IR spectrometer with DTGS KBr detector. XPS
spectra were recorded on a Kratos AXIS 165 with a dual anode
apparatus using the Mg K␣ anode. X-ray powder diffraction data
was collected on a Siemens/D-5000 diffractometer using Cu K␣
radiation. Pore size distribution measurements were performed
on Auto sorb-1 instrument (Quanta chrome, USA) using by nitro-
gen physisorption. The particle size and external morphology of
the samples were observed on a Philips TECNAI F12 FEI transmis-
Hitachi SEM S-520, EDX-Oxford Link ISIS-300 instrument. Diffuse
reflectance UV/vis spectra for samples as KBr pellets were recorded
on a GBC Cintra 10e UV–vis spectrometer in the range 200–800 nm
with a scan speed 400 nm/min.
2.2. Synthesis of catalyst (Ce-MCM-41)
The pure MCM-41 was prepared by direct hydrothermal method
described in the previously reported procedures [43] and was
activated by heating in oven prior to use. The cerium-loaded meso-
porous material MCM-41, designated by Ce-MCM-41, was prepared
by a wet impregnation method using aqueous solution of ceric
ammonium nitrate [44]. To 10 mL of an aqueous solution of ceric
ammonium nitrate (0.12 g, 0.02 M), the MCM-41 (calcined) support
(0.21 g) was added and stirred vigorously for 24 h at room tem-
perature. The pale yellow solid, obtained after evaporation of the
solvent, was dried overnight at 100 ◦C and then again calcined at
500 ◦C in air for 5 h to obtain Ce-MCM-41 with (0.247 g) 75% yield.
The EDX analysis result showed the presence of cerium, oxygen and
silicon at 14.97 wt%, 55.18 wt% and 29.85 wt%, respectively, which
indicates the formation of ceric ion functionalized MCM-41. All the
characterization studies were carried out with 15 wt% Ce-MCM-41.
2.3. General procedure for the synthesis of benzoxanthenone
In a 25 mL round-bottom flask, aldehyde (1 mmol), naphthol
(1 mmol), 1,3 diketone (1 mmol) and Ce-MCM-41 (1.7 mol %) were
taken. The reaction mixture was stirred at 80 ◦C under solvent-free
conditions for 30–50 min. The reaction was monitored by TLC and
on completion of the reaction, the reaction mixture was cooled
to room temperature and ethyl acetate was added to dissolve all
organic components and filtered to remove catalyst. The filtrate
was concentrated and purified by silica gel column chromatogra-
phy.
3. Results and discussion
In the present work, we demonstrate Ce-MCM-41 as an inexpen-
sive, highly efficient, heterogeneous and reusable solid acid catalyst
for the preparation of aryl benzoxanthenone derivatives under
mild reaction conditions. In recent years, solvent-free reactions
have been explored extensively [45]. This convention offers sev-
work-up, recovery, reusability of catalyst and tolerance towards a
uation of our work on the applications of heterogeneous catalysts
for organic reactions [42b], we intend to utilize Ce-MCM-41 as a
promising candidate to perform the condensation of naphthols,
aldehydes with 1,3-dicarbonyls (Scheme 1).
ticomponent reaction is attributed to the inherent high surface area
of the mesoporous catalyst that offers enough space for organic
substrates to interact with active acidic sites present on the solid
surface inside the ordered mesopore [37]. In addition, compared
with pure MCM-41, a decrease in BET surface area, pore volume and
2. Experimental
2.1. Instrumentation
The thin layer chromatography (TLC) was performed on Merck
silica gel 60 F254 plates using ethyl acetate and hexane as eluting
agents. Thin layer chromatography plates were visualized by expo-
sure to UV-light/iodine and/or by immersion in an acidic staining
solution of phosphomolybdic acid followed by heating on a hot
plate. Purification of products was carried out by column chro-
matography using silica gel and a mixture of ethyl acetate and
hexane as eluting agent. All the products were characterized by
mass, 1H and 13C NMR spectroscopy. The NMR spectra of samples
were acquired on a Varian Unity Inova 500 MHz, Inova 400 MHz
and Bruker Avance 300 MHz spectrometer using TMS as an inter-
nal standard in CDCl3 and DMSO. Mass spectra were acquired on a
Thermo LCQ fleet ion trap mass spectrometer. High resolution mass