Microwave-induced protection and deprotection of 4-oxo-(4H)-1-benzopyran-3-carbaldehydes catalysed by Envirocat EPZ10R

Full text of DOI:10.1016/j.mencom.2007.01.017

Mendeleev
Communications
Mendeleev Commun., 2007, 17, 43–44
Microwave-induced protection and deprotection of 4-oxo-(4H)-
1
-benzopyran-3-carbaldehydes catalysed by Envirocat EPZ10R
Santosh S. Shindalkar, Balaji R. Madje and Murlidhar S. Shingare*
Department of Chemistry, Dr. B. A. Marathwada University, Aurangabad (M.S.) 431 004, India.
Fax: +91 240 240 0491; e-mail: profmsshingare@yahoo.co.in
DOI: 10.1016/j.mencom.2007.01.017
An efficient, solvent-free and chemoselective procedure for the synthesis of acylals and their deprotection to 4-oxo-(4H)-
-benzopyran-3-carbaldehydes catalysed by reusable Envirocat EPZ10R under microwave irradiation has been carried out in
1
good yield.
Acylals are useful protecting groups for aldehydes as they are
stable in neutral and basic media. Acylals are important
building blocks for the synthesis of dienes in Diels–Alder
Table 1 Preparation of acylals catalysed by Envirocat EPZ10R.
1,2
Conventional heating
Microwave irradiation
Compound
mp/°C
3
4
5
reactions, homoallylic acetates and nitriles. In addition, they
t/h
Yield (%)
t/min
Yield (%)
6
are useful as cross-linking reagents for cellulose in cotton and
serve as activators. Acylals are synthesised using protic acids
such as H SO or Lewis acids such as PCl and Sc(OTf) .
2
a
2
2
1
6
1
2
67
70
65
60
80
65
1
2
1
5
1
2
84
80
75
75
90
80
150
170
147
144
189
162
7
2b
8
9
10
2
4
3
3
2c
2d
2e
Montmorillonite K-10 clay under microwave irradiation has been
1
1
12
reported for the synthesis of acylals. Recently, Zn(BF4)2 and
2f
InCl313 have also been reported for this conversion.
R
20
EPZ10 catalyst as a protecting reagent, we have developed
R1
R1
†
a microwave-induced procedure for the protection and depro-
R2
R3
O
O
EPZ10, Ac2O
MW, 300 W
R2
O
‡
tection of 4-oxo-(4H)-1-benzopyran-3-carbaldehydes catalysed
R
by Envirocat EPZ10 . 4-Oxo-(4H)-1-benzopyran-3-carbaldehydes
O
OAc
OAc
EPZ10
MW, 800 W R3
†
O
Starting 4-oxo-(4H)-1-benzopyran-3-carbaldehydes were prepared by
the Vilsmeir–Haack reaction of 2-hydroxyacetophenones. Acetic anhydride
was double distilled before using. The uncorrected melting points of all
compounds were taken in an open capillary in a paraffin bath. The
1
a–f
2a–f
1
2
3
1
2
3
a R = R = H, R = Me
d R = R = R = H
1
2
3
1
3
2
R
b R = R = H, R = Cl
e R = R = Cl, R = H
progress of the reaction was monitored by TLC. Envirocat EPZ10 from
1
3
2
1
2
3
c R = R = H, R = Me
f R = Me, R = H, R = Cl
Contract Chemicals, Merseyside, England, was used. All experiments
under microwave irradiation were carried out in a Model 800T domestic
microwave oven (BPL, Appliances and Utilities Ltd., India) having a
maximum power output of 800 W and a 2450 MHz frequency. IR spectra
Microwave-assisted organic reactions have attracted atten-
tion of chemists due to high reaction rates with the formation
of cleaner products and the operational simplicity.
catalysis of organic reactions by inorganic reagents supported on
high-surface-area inorganic materials was described. Envirocat ,
a new family of supported reagents, is a breakthrough in envi-
ronmentally friendly chemistry. These supported reagents are
non-toxic powders. Envirocat EPZ10 (clayzic) is such a sup-
ported catalyst that exhibits both Brönsted and Lewis acid
characteristics.20 The catalytic activity of EPZ10 was reported
for various reactions such as selective transdithioacetalization
1
5–18
1
13
The
were recorded in KBr discs on a FTIR instrument. H and C NMR
spectra were recorded on 300 MHz and 75 MHz spectrometers, respec-
tively, using CDCl3 as a solvent and TMS as an internal standard.
General procedure for the preparation of acylals.
1
9
R
2
0
A. Conventional heating. In a round-bottom flask, a mixture of 4-oxo-
(4H)-1-benzopyran-3-carbaldehyde (10 mmol), acetic anhydride (20 mmol)
R
R
and Envirocat EPZ10 (50 mg) was refluxed for a prescribed time (Table 1).
After completion of reaction monitored by TLC, 10 ml of ethyl acetate
was added to the reaction mixture. The catalyst was filtered off and
washed with ethyl acetate (2×15 ml) for further uses. The filtrate was
neutralised by a 2 M sodium bicarbonate (2×15 ml) solution. The organic
R
21
of acetals, ketals, oxathioacetals and oxathioketals, Biginelli
reaction22 and Friedel–Crafts acylation. Other applications
23
layer was separated, dried over anhydrous MgSO and concentrated on a
4
R
24
of EPZ10 include the olefination of benzaldehyde and the
rotary evaporator to dryness. The residue was recrystallised in ethanol to
afford pure acylals.
2
5
formation of β-iodo ethers and iodohydrin. Here, we report a
microwave-induced, efficient and chemoselective procedure for
the protection and deprotection of 4-oxo-(4H)-1-benzopyran-
-carbaldehydes catalysed by reusable Envirocat EPZ10 . The
reactions are completed in a very short time as compared to
conventional methods (Table 1). In each conversion, the catalyst
was successfully recovered by filtration and recycled without
significant loss in conversion.
In continuation of our work on chemoselective protection
and deprotection of aldehydes and ketones, study of 4-oxo-
B. Microwave irradiation. A mixture of 4-oxo-(4H)-1-benzopyran-3-
carbaldehyde (10 mmol) and acetic anhydride (20 mmol) was placed in
R
R
a 25 ml conical flask containing Envirocat EPZ10 (50 mg) and exposed
3
to microwave irradiation at an output of 350 W for 45 s intervals to avoid
quantitative loss of acetic anhydride (Table 1). After completion of
reaction monitored by TLC, 10 ml of ethyl acetate was added to the
reaction mixture. The catalyst was filtered off and washed with ethyl
acetate (2×15 ml). The filtrate was neutralised by a 2 M sodium bi-
carbonate (2×15 ml) solution. The organic layer was separated, dried over
2
6
anhydrous MgSO and concentrated on a rotary evaporator. The residue
4
2
7–29
(
4H)-1-benzopyran-3-carbaldehydes
and utility of Envirocat
was recrystallised from ethanol to afford pure acylals.
–
43 –

Mendeleev Commun., 2007, 17, 43–44
Table 2 Deprotection of acylals catalysed by Envirocat EPZ10R.
We are grateful to Dr. Babasaheb Ambedkar, Head of the
Department of Chemistry, Marathwada University, Aurangabad,
for providing laboratory facilities. We also thank Contract
Chemicals, England, for the generous gift of Envirocat EPZ10 .
Without acidic Al O
2 3
2
With acidic Al O3
Compound
R
t/min
Yield (%)
t/min
Yield (%)
2
2
2
2
2
2
a
b
c
d
e
f
4
6
6
8
3
5
90
80
75
85
90
90
7
10
9
10
6
85
75
70
80
85
85
References
1
T. W. Greene and P. G. M. Wuts, Protective Groups in Organic
rd
Synthesis, 3 edn., Wiley, New York, 1999.
2
3
4
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have three reactive centres: a carbon–carbon double bond, an
α,β-unsaturated carbonyl group (pyrone ring) and a formyl
group. The reaction chemoselectively occurs at the formyl group
5
6
7
M. Sandberg and L. K. Sydnes, Tetrahedron Lett., 1998, 39, 6361.
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(
Table 1). The acylals were isolated by quenching in water and
1
02, 64010K).
neutralization with sodium bicarbonate, while solvent extrac-
tion is sufficient for deprotection.
In view of green chemistry, the efficient recovery and reuse
of the catalyst is highly preferable. In our process, Envirocat
EPZ10 was recovered conveniently from the reaction mixture
§
8
9
M. Tomita, T. Kikuchi, K. Bessho, T. Hori and Y. Inubushi, Chem.
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R
11 D. Karmakar, D. Prajapati and J. S. Sandhu, J. Chem. Res. (S), 1998,
through filtration and subsequent washing with ethyl acetate.
In the synthesis of acylals, the catalytic activity of recovered
Envirocat EPZ10 was slightly lower than that of the fresh
catalyst, and it remained unchanged in two further attempts
without a loss of yield.
382.
1
2 B. C. Ranu, J. Dutta and A. Das, Chem. Lett., 2003, 32, 366.
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R
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1
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1
1
5 L. Bai, J.-X. Wang and Y. Zhang, Green Chem., 2003, 5, 615.
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‡
General procedure for the deprotection of acylals to 4-oxo-(4H)-
17 A. Dandia, M. Sati, K. Arya, P. Sarawgi and A. Loupy, Arkivoc, 2005,
1-benzopyran-3-carbaldehyde.
i, 105.
A. With acidic alumina. In a 25 ml conical flask, 10 mmol of acylals
18 L. D. S. Yadav, S. Singh and A. Singh, Tetrahedron Lett., 2002, 43,
R
and 1 g of acidic alumina were added to 50 mg of Envirocat EPZ10 and
mixed thoroughly. The reaction mixture was exposed to microwave
irradiation (800 W) for a prescribed time (Table 2). After completion
of reaction monitored by TLC, 10 ml of ethyl acetate was added to the
reaction mixture. The catalyst and acidic alumina were filtered off and
washed with ethyl acetate (2×15 ml) for further uses. The organic layer
8551.
19 J. H. Clark, A. P. Kybett and D. J. Macquarrie, Supported Reagents,
VCH, New York, 1992.
20 J. H. Clark and D. J. Macquarrie, Chem. Soc. Rev., 1996, 303; Product
information of Contract Chemicals, England, 1994.
21 A. S. Gajare, M. S. Shingare and B. P. Bandgar, J. Chem. Res. (S),
1
998, 452.
was dried over anhydrous MgSO and concentrated on a rotary evaporator.
4
22 K. Y. Lee and K. Y. Ko, Bull. Korean Chem. Soc., 2004, 25, 1929.
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Engl., 1994, 33, 1611.
The residue was recrystallised from a proper solvent to afford the pure
product.
B. Without acidic alumina. In a 25 ml conical flask, 10 mmol of acylals
was added to 50 mg of Envirocat EPZ10R and mixed thoroughly. The
reaction mixture was exposed to microwave irradiation (800 W) for a
prescribed time (Table 2). After completion of reaction monitored by TLC,
2
2
2
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Mendeleev Commun., 2002, 209.
1
0 ml of ethyl acetate was added to the reaction mixture. The catalyst
was filtered off and washed with ethyl acetate (2×15 ml). The organic
layer was dried over anhydrous MgSO and concentrated on a rotary
4
evaporator. The residue was recrystallised from a proper solvent to afford
the pure product.
28 S. S. Shindalkar, B. R. Madje and M. S. Shingare, Indian J. Chem.,
2005, 44B, 1519.
§
–
Acetoxy(6-methyl-4-oxo-4H-chromen-3-yl)methyl acetate 2a. IR (n/cm
29 S. S. Shindalkar, B. R. Madje and M. S. Shingare, J. Korean Chem.
Soc., 2005, 49, 377.
1)
: 3050 (C–H ), 1656 (C=OChromone), 1755 (OCOMe). H NMR
1
Ar
(
CDCl ) d: 2.14 (s, 6H), 2.45 (s, 3H), 7.35–7.38 (d, 1H, J 8 Hz), 7.48–
3
7
.49 (d, 1H, J 8 Hz), 7.80 (s, 1H), 7.51–7.52 (s, 1H), 8.08 (s, 1H).
1
3
C NMR (CDCl ) d: 175.46, 168.76, 155.14, 154.88, 136.21, 135.78,
3
1
25.67, 124.27, 119.79, 118.31, 85.63, 21.34, 21.17. Calc. for C H O
1
5
14
6
(
%): C, 62.07; H, 4.86. Found (%): C, 62.11; H, 4.82.
Acetoxy(6-chloro-4-oxo-4H-chromen-3-yl)methyl acetate 2b. IR (n/cm ):
–1
1
3
050 (C–H ), 1643 (C=O
), 1750 (OCOMe), 1013 (C–Cl). H NMR
Ar
Chromone
(
CDCl ) d: 2.13 (s, 6H), 7.42–7.45 (d, 1H, J 8.7 Hz), 7.61–7.63 (dd, 1H,
3
J 2.2 and 8.7 Hz), 7.77 (s, 1H), 8.10 (s, 1H), 8.15 (d, 1H, J 2.2 Hz).
1
3
C NMR (CDCl ) d: 174.23, 168.68, 155.43, 154.90, 134.81, 132.16,
3
1
25.73, 125.49, 120.37, 120.17, 85.23, 20.09. Calc. for C H ClO (%):
1
4
11
6
C, 54.12; H, 3.57. Found (%): C, 54.10; H, 3.55.
Acetoxy(7-methyl-4-oxo-4H-chromen-3-yl)methyl acetate 2c. IR (n/cm ):
–1
1
3
050 (C–H ), 1656 (C=O
), 1758 (OCOMe). H NMR (CDCl )
Ar
Chromone 3
d: 2.10 (s, 6H), 2.40 (s, 3H), 7.35 (s, 1H), 7.45 (d, 1H, J 8.7 Hz), 7.78 (d,
13
1
H, J 8.7 Hz), 7.94 (s, 1H), 8.06 (s, 1H). C NMR (CDCl ) d: 175.40,
3
168.69, 155.12, 154.83, 136.15, 135.75, 125.55, 124.17, 119.77, 118.68,
8
5.49, 21.31, 21.11. Calc. for C H O (%): C, 62.07; H, 4.86. Found (%):
15
14 6
C, 62.08; H, 4.84.
Received: 9th October 2006; Com. 06/2790
–
44 –