A mild procedure for rapid and selective deprotection of aryl acetates using natural kaolinitic clay as a reusable catalyst

Article abstract of DOI:10.1016/S0040-4039(00)02198-5

A variety of aryl acetates are selectively cleaved to the corresponding phenols using natural kaolinitic clay in methanol under mild conditions (25°C).

Full text of DOI:10.1016/S0040-4039(00)02198-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1163–1165
A mild procedure for rapid and selective deprotection of aryl
acetates using natural kaolinitic clay as a reusable catalyst
B. P. Bandgar,* L. S. Uppalla, A. D. Sagar and V. S. Sadavarte
Organic Chemistry Research Laboratory, School of Chemical Sciences, Swami Ramanand Teerth Marathwada University,
Nanded 431 606, India
Received 25 August 2000; revised 15 November 2000; accepted 29 November 2000
Abstract—A variety of aryl acetates are selectively cleaved to the corresponding phenols using natural kaolinitic clay in methanol
under mild conditions (25°C). © 2001 Published by Elsevier Science Ltd.
Phenolic hydroxyl groups occur widely in plant and
animal life, both land-based and aquatic, as demon-
strated by the vast number of natural products that
contain this group.1 In developing a synthesis of any
phenol-containing product, protection is often manda-
tory to prevent reaction with oxidizing agents and
electrophiles or reaction of the nucleophilic phenoxide
ion with even mild alkylating and acylating agents. The
protection of phenols as O-acyl derivatives has long
played a key role in organic synthesis, because of the
ease with which they are formed and cleaved. In spite
of several efforts, the methods available for selective
removal of aryl acetates in the presence of other sensi-
tive functional groups are very few,2 not general3 and
most involve homogenous conditions. The recently
reported ytterbium triflate,4 unlike hafnium triflate,5
cleaves aryl acetates without Fries migration. However,
it requires long reaction times, drastic conditions,
aqueous work-up and does not discriminate between
aryl and alkyl acetates. Recently, there has been an
upsurge in the application of clay as a catalyst for
organic reactions.6 In continuation with our ongoing
research in this area,6c,d we wish to report in this
communication a preliminary account of use of natural
kaolinitic clay (N.C.) as a catalyst for the selective
removal of aryl acetates in the presence of other sensi-
tive functional groups under mild conditions (Scheme
1).
Trivandrum, and it was used as obtained without any
pretreatment.
A series of aryl acetates were subjected to the deprotec-
tion conditions with natural kaolinitic clay catalyst in
methanol at 25°C to obtain the corresponding phenols.
The results are presented in Table 1. It is important to
note that this environmentally benign and manipula-
tively simple protocol allows chemoselective deprotec-
tion of aryl acetates in the presence of several other
protecting groups, such as oximes (entry 3), THP ethers
(entry 8), thioacetals (entry 9), NH-acetyl (entries 10
and 14), and t-butoxycarbonyl groups6a (entry 15).
Functional groups, such as aldehydes (entries 2 and 6),
imines (entry 4), and benzoates (entry 17), remain unal-
tered under these reaction conditions. Furthermore, it is
notable that aryl acetate groups are removed selectively
in the presence of benzyl acetates (entries 7, 12, and 13).
No deacylation was observed for alkyl acetates (entries
14, 15, and 18) and phenyl thioacetate (entry 16). An
important feature of this method is that the optical
purities of the products (entries 13–15) remain unal-
tered (based on optical rotation). The present reaction
conditions allow exclusive aryl deacetylation with no
trace of Fries migration products. The tolerance of
different protecting groups to these reaction conditions
illustrates the flexibility and generality of the protocol.
In conclusion, the present results demonstrate the nov-
elty of natural kaolinitic clay that shows unique selec-
Natural kaolinitic clay was obtained from the Padap-
pakara mine of Quilon District, Kerala, India and it
was purified and supplied by Dr. Lalithambika, RRL,
OCH3
N.C. / Methanol
25 °C
ArO
ArOH +
O
O
Keywords: catalyst; aryl acetates; deprotection; phenols.
* Corresponding author. Fax: 0091-2462-29245; e-mail: upekam@
hotmail.com
Scheme 1.
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(00)02198-5

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B. P. Bandgar et al. / Tetrahedron Letters 42 (2001) 1163–1165
Table 1. Deprotection of aryl acetates using natural kaolinitic clay under mild conditions
Reaction
time (min)
Yield %a,b
Entry
Substrate
Product
Phenyl acetate
1
2
Phenol
20
30
96
90
4-Acetoxy-3-methoxy benzaldehyde
Vaniline
N OH
N OH
30
91
3
AcO
HO
MeO
MeO
N
OMe
N
OMe
AcO
MeO
HO
MeO
4
30
89
β-Naphthyl acetate
β-Naphthol
5
6
20
30
95
90
4-Acetoxy benzaldehyde
4-Hydroxy benzaldehyde
OAc
OAc
30
20
20
94
92
90
7
8
AcO
HO
OTHP
AcO
OTHP
HO
S
S
AcO
HO
9
S
S
O
O
20
20
89
89
10
11
AcO
N
HO
N
Hydroquinone
O
1,4-Phenylenediacetate
O
OAc
OH
20
92
12
13
OAc
OAc
NHMe
OAc
NHMe
AcO
AcO
HO
HO
30
30
30
88
(99% ee)
OAc
MeO2C
MeO2C
14
15
93
(99% ee)
NHAc
NHAc
OH
OH
OAc
OAc
MeO2C
MeO2C
93
(98% ee)
BocHN
BocHN
Phenyl thioacetate
--
--
--
16
17
18
No reaction
No reaction
No reaction
12 h
12 h
12 h
4-Benzoyloxy benzaldehyde
CH3(CH2)4COOCH3
a
Yield of pure isolated products. b Products characterized by spectral analysis and physical properties.
tivity and constitutes a useful alternative to commonly
accepted deacylation procedures. Moreover, the superi-
ority and flexibility of the protocol lies in its ease of
operation and simplicity in work-up, which involves
mere filtration of the catalyst that can be reused at least
three times without appreciable loss of activity.
.

B. P. Bandgar et al. / Tetrahedron Letters 42 (2001) 1163–1165
1165
General procedure:
References
The aryl acetate (5 mmol) and natural clay (100 mg)
were stirred in methanol (10 ml) at 25°C. After comple-
tion of the reaction (TLC), the catalyst was filtered off
and washed with ethyl acetate (2×5 ml). The solvent
was removed under vacuum to furnish the phenol in
almost pure form, which was further purified by
column chromatography on SiO2 (hexane:ethyl ace-
tate=9:1) where necessary.
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Chem. 1999, 64, 8027 and references cited therein.
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therein.
4. Sharma, G. V. M.; Ilangovan, A. Synlett 1999, 12, 1963.
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Lett. 1996, 37, 2053.
Acknowledgements
6. (a) Shaikh, N. S.; Gajre, A. S.; Deshpande, V. H.;
Bedekar, A. V. Tetrahedron Lett. 2000, 41, 385; (b) Cor-
nelis, A.; Laszlo, P. Synlett 1994, 155; (c) Bandgar, B.
P.; Uppalla, L. S.; Kurule, D. S. Green Chem. 1999, 243;
(d) Bandgar, B. P.; Kasture, S. P. Green Chem. 2000, 2,
154.
V.S.S. thanks CSIR, New Delhi for the Junior
Research Fellowship and we thank Dr. Lalithambika,
RRL, Trivandrum for the supply of natural clay.
.
.