Oxidative deprotection of trimethylsilyl ethers, tetrahydropyranyl ethers, and ethylene acetals with benzyltriphenylphosphonium peroxymonosulfate under microwave irradiation

Article abstract of DOI:10.1081/SCC-100103980

This paper describes the oxidation deprotection of trimethylsilyl ethers, tetrahydrophyranyl ethers and deprotection of ethylene acetals to afford carbonyl compounds using benzyltriphenylphosphonium peroxymonosulfate (1) (PhCH₂PPh₃HSO₅) in the presence of a catalytic amount of bismuth chloride under microwave irradiation. This reagent is a stable white powder, which has been prepared by the drop-wise addition of an aqueous solution of benzyltriphenyl-phosphonium chloride to an aqueous solution of oxone at room temperature.

Full text of DOI:10.1081/SCC-100103980

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OXIDATIVE DEPROTECTION
OF TRIMETHYLSILYL ETHERS,
TETRAHYDROPYRANYL
ETHERS, AND ETHYLENE
ACETALS WITH
BENZYLTRIPHENYLPHOSPHONIUM
PEROXYMONOSULFATE UNDER
MICROWAVE IRRADIATION
A. R. Hajipour a , S. E. Mallakpour a , I. M. Baltork
b & H. Adibi a
a Pharmaceutical Research Laboratory , College
of Chemistry, Isfahan University of Technology ,
Isfahan, IR, 84156, Iran
b Department of Chemistry , Isfahan Univversity ,
Isfahan, IR, 81744, Iran
Published online: 09 Nov 2006.
To cite this article: A. R. Hajipour , S. E. Mallakpour , I. M. Baltork
& H. Adibi (2001) OXIDATIVE DEPROTECTION OF TRIMETHYLSILYL
ETHERS, TETRAHYDROPYRANYL ETHERS, AND ETHYLENE ACETALS WITH
BENZYLTRIPHENYLPHOSPHONIUM PEROXYMONOSULFATE UNDER MICROWAVE
IRRADIATION, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 31:11, 1625-1631, DOI: 10.1081/
SCC-100103980

To link to this article: http://dx.doi.org/10.1081/SCC-100103980
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SYNTHETIC COMMUNICATIONS, 31(11), 1625–1631 (2001)
OXIDATIVE DEPROTECTION OF
TRIMETHYLSILYL ETHERS,
TETRAHYDROPYRANYL ETHERS, AND
ETHYLENE ACETALS WITH
BENZYLTRIPHENYLPHOSPHONIUM
PEROXYMONOSULFATE UNDER
MICROWAVE IRRADIATION
A. R. Hajipour,1, S. E. Mallakpour,1 I. M. Baltork,2
*
and H. Adibi1
1Pharmaceutical Research Laboratory,
College of Chemistry, Isfahan University of Technology,
Isfahan, 84156, IR, Iran
2Department of Chemistry, Isfahan University,
Isfahan 81744, IR, Iran
ABSTRACT
This paper describes the oxidation deprotection of
trimethylsilyl ethers, tetrahydrophyranyl ethers and depro-
tection of ethylene acetals to afford carbonyl compounds
using benzyltriphenylphosphonium peroxymonosulfate (1)
(PhCH2PPh3HSO5) in the presence of a catalytic amount of
bismuth chloride under microwave irradiation. This reagent
is a stable white powder, which has been prepared by the drop-
wise addition of an aqueous solution of benzyltriphenyl-
phosphonium chloride to an aqueous solution of oxoneÕ at
room temperature.
* Corresponding author. E-mail: haji@cc.iut.ac.ir
1625
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

1626
HAJIPOUR ET AL.
The protection and deprotection of the hydroxy functional groups can
play an essential role in synthetic strategy.1 The transformation of alcohols
to the corresponding trimethylsilyl ethers is a very common way to protect
hydroxy groups.1–3 In recent years a wide variety of methods have been
reported for oxidative cleavage of trimethylsilyl ethers to the corresponding
carbonyl compounds,4–7 but some of the mentioned methods encounter
drawbacks such as the requirement for aqueous reaction conditions, use
of expensive reagents, long reaction times, low yields of the products and
tedious work-up. Therefore, introduction of new methods and inexpensive
reagents for such functional group transformations is still in demand. The
tetrahydropyranyl (THP) group is one of the most frequently used groups to
protect alcohols and phenols.1,8 Owing to the outstanding stability of tetra-
hydropyranyl ether under a variety of reaction conditions, 3,4-dihydro-
2H-pyran is still a reagent of choice for protection of the alcohol group in
peptide, nucleotide and carbohydrate chemistry.9 Since direct transforma-
tion of tetrahydropyranyl ethers to their carbonyl compounds is rare in
the literature,5,7,10–13 introduction of new methods is desirable. Acetal
formation is the most widely used protecting method for aldehydes and
ketones,1 however, deprotection of acetals and ketals to the corresponding
carbonyl compound is a useful transformation in organic synthesis.13–18
Organic reactions that are assisted by microwave irradiation have
recently attracted attention.19 The most advantage of these methods over
conventional classical method is that they show cleaner reactions, decreased
reaction time and easier work-up. In continuation of our ongoing program
to develop environmentally benign methods using solid supports,20 we wish
to report a novel and efficient method for the oxidative deprotection of
trimethylsilyl and tetrahydropyranyl ethers to the corresponding carbonyl
compounds by using benzyltriphenylphosphonium peroxymonosulfate (1)
in the presence of a catalytic amount of bismuth chloride under microwave
irradiation. This compound can also react as an efficient reagent to trans-
form ethylene acetals to the corresponding carbonyl compounds in the pres-
ence of a catalytic amount of bismuth chloride under microwave irradiation.
Benzyltriphenylphosphonium peroxymonosulfate (1) is a mild, effi-
cient, stable and cheap reagent, which has been used for our studies under
microwave irradiation. This reagent is a white powder, which is prepared by
the dropwise addition of an aqueous solution of oxone,Õ to an aqueous
solution of benzyltriphenylphosphonium chloride at room temperature.20b
Filtration and drying of the precipitates resulted in a white powder, which
could be stored for months without losing its oxidation ability. The amounts
of HSO5À in this reagent have been determined by an iodometric titration
method.21 The measurements are consistent with almost 99% by weight of
active oxidizing agent.

BENZYLTRIPHENYLPHOSPHONIUM PEROXYMONOSULFATE
1627
Table 1. Oxidative Deprotection of Trimethylsilyl and Tetrahydropyranyl Ethers
with Reagent (1) Under Microwave Irradiationa
Time
(min)
Yield
(%)b
Entry
Substrate
PhCH2OTMS
2-MeOC6H4CH2OTMS
3-MeOC6H4CH2OTMS
3-O2NC6H4CH2OTMS
2,5-(MeO)2C6H3CH2OTMS
Product
1
2
3
4
5
PCHO
3
3
3
5
90
85
91
90
80
2-MeOC6H4CHO
3-MeOC6H4CHO
3-O2NC6H4CHO
2,5-(Me)2C6H3CHO
3.5
99
4
5
5
3
3.5
4
3.5
3.5
3
4
3.5
5
5
4
6PhCH(Me)OTMS
7
8
PhCOMe
3
4-ClC6H4CH(Me)OTMS
Ph2CHOTMS
4-PhC6H4CH(Me)OTMS
a-Tetralol TMS
PhCH2OTHP
2-MeOC6H4CH2OTHP
3-MeOC6H4CH2OTHP
4-MeOC6H4CH2OTHP
3,4-(MeO)2C6H3CH2OTHP
4-ClC6H4COMe
Ph2CO
4-PhC6H4COMe
a-Tetralone
PhCHO
2-MeOC6H4CHO
3-MeOC6H4CHO
4-MeOC6H4CHO
3,4-(MeO)2C6H3CHO
2,5-(MeO)2C6H3CHO
PhCOMe
93
94
92
95
90
88
83
96
80
85
80
90
84
88
9
10
11
12
13
14
15
162,5-(MeO)
2C6H3CH2OTHP
17
18
19
20
PhCH(Me)OTHP
4-ClC6H4CH(Me)OTHP
Ph2CHOTHP
4-ClC6H4COMe
Ph2CO
a-Tetralone
a-Tetralol THP
aSubstrate/Oxidant/BiCl3, (1:1:0.4). bYields based on the isolated products after
column chromatography.
At first, the oxidative deprotection was performed by mixing one
equivalent of the reagent (1) and bismuth chloride with benzyltrimethylsilyl
ether in a mortar and ground the mixture with a pestle until the formation of
a homogeneous mixture (1 min). Then, the reaction mixture was transferred
to an Erlenmeyer flask and irradiated in a microwave oven, until TLC
showed complete disappearance of the starting material. However, the start-
ing material remained intact during the solid phase irradiation. So, we tested
several solvents such as carbon tetrachloride, hexane, toluene, acetonitrile
and dichloromethane. Finally, dichloromethane was found to be a micro-
wave active solvent for this aim. In order to evaluate the effect of bismuth
chloride in this reaction, we tried the oxidative deprotection of the desired
silyl ether with the reagent (1) without using any catalyst. The reaction does
not complete under the reaction condition, for this aim, the catalytic effects
of several Lewis acids such as ZnCl2, FeCl3 6H2O, AlCl3 and BiCl3 were

1628
HAJIPOUR ET AL.
thoroughly investigated. Surprisingly, only BiCl3 was found to be an effec-
tive catalyst during this investigation. The optimum molar ratio of bismuth
chloride for this purpose was 0.4 molar ratio. Primary and secondary
trimethylsilyl ethers were converted to their corresponding carbonyl
compounds with benzyltriphenylphosphonium peroxymonosulfate (1) in the
presence of 0.4 molar ratio of bismuth chloride in dichloromethane under
microwave irradiation in high yields and short reaction times (Table 1 and
Scheme 1). Interestingly, overoxidation of the products was not observed by
this method. In order to evaluate the ability of this reagent, we also decided
to convert tetrahydropyranyl ethers to their carbonyl compounds with the
reagent (1). The reaction of primary and secondary tetrahydropyranyl ethers
was completed in the presence of bismuth chloride in dichloromethane
under microwave irradiation and the corresponding products were obtained
in good to excellent yields (Table 1 and Scheme 1).
Scheme 1.
Benzyltriphenylphosphonium peroxymonosulfate (1) was also used to trans-
form ethylene acetals to the corresponding carbonyl compounds in dichlor-
omethane under microwave irradiation in the presence of 0.4 molar ratio of
bismuth chloride in good to high yields (Table 2 and Scheme 1).
In conclusion, we report here the preparation of benzyltriphenyl-
phosphonium peroxymonosulfate (1) as a mild, inexpensive and stable
oxidizing agent. The oxidizing reagent is very easily prepared from
commercially available starting materials and can be stored for months
without losing its activity. This reagent is an efficient, mild and inexpensive
reagent for oxidative deprotection of trimethylsilyl and tetrahydropyranyl
ethers and deprotection of ethylene acetals to the corresponding carbonyl
compounds in high yields under microwave irradiation.
EXPERIMENTAL
Trimethylsilyl ethers, tetrahydropyranyl ethers, ethylene acetals were
prepared according to described procedures.3a,14,23,24 The reagent (1) was

BENZYLTRIPHENYLPHOSPHONIUM PEROXYMONOSULFATE
1629
Table 2. Deprotection of Ethylene Acetals with Reagent (1) Under Microwave
Irradiationa
Time Yield
(min) (%)b
Entry
Substrate
Product
1
2
Actophenoneethylene acetal
2-Methoxybenzaldehydeethylene 2-Methoxybenzaldehyde
acetal
Acetophenone
4
4
91
90
3
4
5
2-Nitrobenzaldehydeethylene
acetal
2-Chloroactophenoneethylene
acetal
2-Nitrobenzaldehyde
680
5
2-Chloroactophenone
85
a-Tetraloneethylene acetal
a-Tetralone
4-Phenylacetophenone
4
5
95
88
64-Phenylacetophenoneethylene
acetal
a Substrate/Oxidant/BiCl3 (1:1:0.4). b Yields based on the isolated products after
purification with column chromatography.
prepared according to described procedures.20b All of the yields refer to
isolated products. All of the products were characterized by comparison
1
of their spectral (IR, H-NMR and TLC) and physical data (melting and
1
boiling point) with those of authentic samples.5,13,22 All H-NMR spectra
were recorded at 90 MHz in CDCl3 and CCl4 relative to TMS (0.00 ppm).
The Research Institute of Petroleum Industry, Tehran, I.R. Iran performed
elemental analysis. The apparatus used for the oxidative deprotection was a
Samsung domestic microwave oven (2450 MHz, 900 W) without any mod-
ification, but all of the reactions were carried out in a hood with strong
ventilation.
Oxidative Deprotection of Trimethylsilyl and Tetrahydropyranyl
Ethers with Reagent (1) Under Microwave Irradiation
General Procedure
Trimethylsilyl ether or tetrahydropyranyl ether (1 mmol) and BiCl3
(0.13 g, 0.4 mmol) was added to benzyltriphenylphosphonium peroxomono-
sulfate (0.466 g, 1 mmol) and the mixture was ground with a pestle in a
mortar to form a homogeneous mixture (1 min). After transferring the
mixture to an Erlenmeyer flask and adding 3 mL CH2Cl2, it was irradiated
with microwave oven (900 W), until TLC showed complete disapprearance
of starting material (Table 1). Then CCl4 was added to the reaction mixture

1630
HAJIPOUR ET AL.
and after vigorous stirring, the mixture was filtered off. Evaporation of the
solvent gave pure carbonyl compound.
Deprotection of Ethylene Acetals with Reagent (1)
Under Microwave Irradiation
General Procedure
Ethylene acetal (1 mmol) and BiCl3 (0.13 g, 0.4 mmol) was added to
benzyltriphenylphosphonium peroxomonosulfate (0.466 g, 1 mmol) and the
mixture was ground with a pestle in a mortar to form a homogeneous
mixture. After transferring the mixture to an Erlenmeyer flask and adding
3 ml CH2Cl2, it was irradiated with microwave oven (900 W), until TLC
showed complete disappearance of starting material (Table 2). Then CCl4
(20 ml) was added to the reaction mixture and after vigorous stirring, the
mixture was filtered off. The filtrate was evaporated on a rotary evaporator
to afford pure carbonyl compound.
ACKNOWLEDGMENT
The authors are thankful of the Isfahan University of Technology
(IUT), I.R. Iran for the financial support.
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Received in Japan June 7, 2000