Selective and efficient oxidative deprotection of trimethylsilyl and tetra hydropyranyl ethers, ethylene acetals and ketals with n-butyltriphenylphosphonium peroxodisulfate (BunPPh3)2S2O8

Article abstract of DOI:10.1081/SCC-120003626

n-Butyltriphenylphosphonium peroxodisulfate in refluxing acetonitrile transforms trimethylsilyl (TMS) and tetrahydropyranyl ethers, ethylene acetals and ketals to their corresponding carbonyl compounds in excellent yields. Selective oxidative deprotection of TMS and THP ethers in the presence of ethylene acetals (ketals) is of additional importance in this method.

Full text of DOI:10.1081/SCC-120003626

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SELECTIVE AND EFFICIENT OXIDATIVE DEPROTECTION
OF TRIMETHYLSILYL AND TETRAHYDROPYRANYL
ETHERS, ETHYLENE ACETALS AND KETALS WITH n-
BUTYLTRIPHENYLPHOSPHONIUM PEROXODISULFATE (Bu
n
PPh ) S O
3
2 2 8
a
b
b
I. Mohammadpoor-Baltork , A. R. Hajipour & M. Aghajari
a
Department of Chemistry , Isfahan University , Isfahan, 81744, Iran
b
College of Chemistry , Isfahan University of Technology , Isfahan, 84156, Iran
Published online: 17 Aug 2006.
To cite this article: I. Mohammadpoor-Baltork , A. R. Hajipour & M. Aghajari (2002) SELECTIVE AND EFFICIENT OXIDATIVE
DEPROTECTION OF TRIMETHYLSILYL AND TETRAHYDROPYRANYL ETHERS, ETHYLENE ACETALS AND KETALS WITH n-
n
BUTYLTRIPHENYLPHOSPHONIUM PEROXODISULFATE (Bu PPh ) S O , Synthetic Communications: An International Journal for
3
2 2 8
Rapid Communication of Synthetic Organic Chemistry, 32:9, 1311-1317, DOI: 10.1081/SCC-120003626
To link to this article: http://dx.doi.org/10.1081/SCC-120003626
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SYNTHETIC COMMUNICATIONS, 32(9), 1311–1317 (2002)
SELECTIVE AND EFFICIENT OXIDATIVE
DEPROTECTION OF TRIMETHYLSILYL
AND TETRAHYDROPYRANYL ETHERS,
ETHYLENE ACETALS AND KETALS WITH
n-BUTYLTRIPHENYLPHOSPHONIUM
n
PEROXODISULFATE (Bu PPh ) S O
3
2 2
8
1
,
2
I.Mohammadpoor-Baltork, * A.R.Hajipour,
2
and M.Aghajari
1
Department of Chemistry, Isfahan University,
Isfahan 81744, Iran
College of Chemistry, Isfahan University of
Technology, Isfahan 84156, Iran
2
ABSTRACT
n-Butyltriphenylphosphonium peroxodisulfate in refluxing
acetonitrile transforms trimethylsilyl (TMS) and tetrahydro-
pyranyl (THP) ethers, ethylene acetals and ketals to their
corresponding carbonyl compounds in excellent yields. Selec-
tive oxidative deprotection of TMS and THP ethers in the
presence of ethylene acetals (ketals) is of additional impor-
tance in this method.
*
Corresponding author.
1
311
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

1
312
MOHAMMADPOOR-BALTORK, HAJIPOUR, AND AGHAJARI
The protection of hydroxyl groups as their trimethylsilyl and tetra-
1
–7
hydropyranyl ethers is an important reaction in organic chemistry.
Oxidative deprotection of the above mentioned ethers to their correspond-
ing carbonyl compounds under mild and aprotic conditions is also of
8
–11
synthetic value.
The protection of aldehydes and ketones by the formation of ethylene
acetals and ketals is important in the preparation of a variety of multi-
1
2–14
functional organic molecules.
Transformation of ethylene acetals and
ketals to their parent carbonyl compounds under neutral, aprotic and non-
aqueous conditions is of interest to synthetic organic chemists and several
1
5–17
methods have been reported for this purpose.
However, some of the
reported methods for the oxidative deprotection of trimethylsilyl and tetra-
hydropyranyl ethers, ethylene acetals and ketals show limitations such as
strong protic and aqueous conditions, long reaction times, low yields of the
products, tedious work-up and expensive reagents. Therefore, introduction
of a milder, more selective and inexpensive reagent for the conversion of the
above mentioned derivatives to their corresponding carbonyl compounds is
desirable.
Recently, we have reported new methods for the oxidative deprotec-
tion of trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and
1
8–20
ketals.
In continuation of our research in this area, we were interested
to find a more efficient, more selective and inexpensive method for the
oxidative deprotection of these functional groups. In this respect, we
wish to report that n-butyltriphenylphosphonium peroxodisulfate is able
to convert TMS and THP ethers, ethylene acetals and ketals to their
corresponding carbonyl compounds efficiently in refluxing acetonitrile.
This reagent is an inexpensive and stable oxidizing agent and has been
used for the cleavage of carbon–nitrogen double bonds under non-aqueous
2
1
conditions.
The treatment of a variety of trimethylsilyl and tetrahydropyranyl
ethers with n-butyltriphenylphosphonium peroxodisulfate gave the corre-
sponding carbonyl compounds in excellent yields (Table 1). Deprotection
of ethylene acetals and ketals was also investigated with this reagent and the
corresponding aldehydes and ketones were obtained in excellent yields
(
Table 2). Interestingly, overoxidation of the products was not observed
in this method. It is noteworthy that the reaction medium was almost
neutral, so that some of the sensitive functionalities such as the carbon–
carbon double bond remained intact (Table 1, Entries 7, 22 and Table 2,
Entry 6).
In order to show the better chemoselectivity of the described
method, we have also performed several competitive oxidative deprotec-
tion reactions. The experimental results show that trimethylsilyl and

OXIDATIVE DEPROTECTION OF TMS AND THP
Table 1. Oxidative Deprotection of TMS and THP Ethers with (Bu PPh ) S O
8
1313
n
3
2
2
a
Run
Substrate
OTMS
Product
PhCHO
Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
PhCH
2
10
30
10
10
30
10
10
20
20
25
15
15
30
30
30
30
15
30
15
15
15
20
30
20
20
30
30
98
96
96
98
97
97
98
96
98
98
98
99
97
93
95
96
95
96
97
99
92
96
98
97
98
94
96
2-MeOC H CH OTMS
2-MeOC H CHO
6
6
6
4
4
4
2
2
2
6
6
6
4
4
4
3-MeOC
4-MeOC
H
H
CH
CH
OTMS
OTMS
3-MeOC
4-MeOC
H
H
CHO
CHO
4- N CO H CH OTMS
6
4-O NC H CHO
4
2
4
2
2
6
Ph(CH
2
)
3
OTMS
OTMS
PhCH(Me)OTMS
Ph CHOTMS
PhCH(OTMS)CH
4-ClC H CH(Me)OTMS
Ph(CH
2
)
2
CHO
PhCH¼CHCH
2
PhCH¼CHCHO
PhCOMe
2
Ph
PhCOCH
4-ClC H COMe
2
CO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2
Ph
2
Ph
6
4
6
4
4-PhC H CH(Me)OTMS
6
4-PhC H COMe
6 4
4
a-TetralolTMS
CH (CH OTMS
CH (CH ) OTMS
a-Tetralone
CH (CH CHO
CH (CH ) CHO
3
2
)
6
3
2 5
)
3
2 7
3
2 6
CH
PhCH
3
(CH
2
)
5
CH(Me)OTMS CH (CH
PhCHO
3
2
)
5
COMe
2
OTHP
2-MeOC H CH OTHP
2-MeOC H CHO
6
6
6
4
4
4
2
2
2
6
6
6
4
4
4
3-MeOC
4-MeOC
H
H
CH
CH
OTHP
OTHP
3-MeOC
4-MeOC
H
H
CHO
CHO
4-O NC H CH OTHP
2
4-O NC H CHO
2 6 4
2
6
4
PhCH¼CHCH
2
OTHP
PhCH(Me)OTHP
4-ClC H CH(Me)OTHP
PhCH¼CHCHO
PhCOMe
4-ClC H COMe
6
4
6
4
a-TetralolTHP
a-Tetralone
CH
CH
3
(CH
(CH
2
)
6
OTHP
OTHP
CH
CH
3
(CH
2
)
5
CHO
3
2
)
7
3
(CH
2
)
6
CHO
a
Isolated yield.
tetrahydropyranyl ethers are oxidized selectively in the presence of ethylene
acetals and ketals (Table 3). These selectivities are useful achievements in
organic synthesis.
We have also shown the general applicability and efficiency of
the presented method in comparison with some of those reported with
1
-carboxypyridinium chlorochromate (CPCC) and silver bromate/alumi-
8
3
nium chloride
1
0,20
(Table 4).
Even though the reaction mechanism is interesting, at the present time
it is obscure to us, and we have not been able to assign any reasonable
mechanism for them in our studies.

1
314
MOHAMMADPOOR-BALTORK, HAJIPOUR, AND AGHAJARI
n
Table 2. Oxidative Deprotection of Ethylene Acetals and Ketals with (Bu PPh
3
)
2
S
O
2 8
Time Yield
a
Run
Substrate
Product
Benzaldehyde
2-Methoxybenzaldehyde
(h)
(%)
1
2
Benzaldehydeethylene acetal
2-Methoxybenzaldehydeethylene
acetal
1.5
2
97
95
3
4-Methoxybenzaldehydeethylene
acetal
4-Methoxybenzaldehyde
1.5
98
4
5
6
7
8
2-Nitrobenzaldehydeethylene acetal 2-Nitrobenzaldehyde
4-Nitrobenzaldehydeethylene acetal 4-Nitrobenzaldehyde
2.5
2
96
98
97
97
98
Cinnamaldehydeethylene acetal
Acetophenoneethylene ketal
4-Chloroacetophenoneethylene
ketal
Cinnamaldehyde
Acetophenone
4-Chloroacetophenone
1
1.5
1.5
9
4-Phenylacetophenoneethylene
ketal
Cyclohexanoneethylene ketal
4-Phenylacetophenone
Cyclohexanone
2.5
2.5
99
94
1
0
a
Isolated yield.
Table 3. Competitive Oxidative Deprotection of TMS (THP) Ethers and Ethylene
n
Acetals (Ketals) with (Bu PPh ) S O
3 2 2 8
Time
(min)
Yield
b
(%)
a
Run
1
Substrate
OTMS
Product
PhCH
4
2
PhCHO
4-Chloroacetophenone
98
0
-Chloroacetophenoneethylene
ketal
10
10
2
Ph(CH
-Methoxybenzaldehydeethylene
acetal
2
)
3
OTMS
Ph(CH
4-Methoxybenzaldehyde
2
)
2
CHO
96
2
4
3
4
PhCH(Me)OTMS
Benzaldehydeethylene acetal
PhCOMe
Benzaldehyde
95
3
2
0
4-
2
NC
6
HO
4
CH OTHP
2
4-O
4-Methoxybenzaldehyde
2 6
NC H
4
CHO
92
3
4
-Methoxybenzaldehydeethylene
acetal
PhCH(Me)OTHP
15
5
6
PhCOMe
Benzaldehyde
97
2
3
0
Benzaldehydeethylene acetal
4-MeOC
-Chloroacetophenoneethylene
ketal
6
H
4 2
CH OTHP
4-MeOC
4-Chloroacetophenone
6
H
4
CHO
98
0
4
15
a
Substrates/Oxidant (1 : 1 : 1).
GLC yield.
b

OXIDATIVE DEPROTECTION OF TMS AND THP
1315
Table 4. Comparison of Oxidative Deprotection of Some TMS and THP
n
Ethers with (Bu PPh
18
(I), CPCC (II) and Silver Bromate/Aluminium
3
)
2
2
S O
8
1
0,20
Chloride (III)
Yield % (Time, min)
Run
Substrate
Product
I
II
III
1
2
3
4
5
6
7
8
PhCH OTMS
PhCHO
OTMS 2-MeOC
OTMS 4-O NC
PhCH(Me)OTMS PhCOMe
Ph CO
4-O NC
98 (10) 97 (45)
CHO 96 (30) 93 (45)
CHO 97 (30)
96 (20) 93 (70)
–
–
2
2-MeOC
6
H
4
CH
CH
2
6 4
H
4- NCO
2
6
H
4
2
2
6
H
4
–
83 (30)
92 (20)
Ph
4-
2
CHOTMS
NCO CH
2
98 (20) 80 (120) 98 (25)
2
6
H
4
2
OTHP
2
6
H
4
CHO 92 (15)
–
78 (150)
90 (150)
–
2-MeOC H CH OTHP 2-MeOC H CHO 96 (30) 96 (6)
4
6
4
2
6
PhCH¼CHCH
2
OTHP
PhCH¼CHCHO
96 (20) 63 (10)
In conclusion, in this study we have introduced n-butyltriphenyl-
phosphonium peroxodisulfate as a stable, inexpensive and efficient reagent
for the selective oxidative deprotection of TMS and THP ethers, ethylene
acetals and ketals.
EXPERIMENTAL
All of the products were characterized by comparison of their physical
and spectral data with those of authentic samples. TMS and THP ethers,
ethylene acetals and ketals were prepared according to described proce-
3
,4,12
dures.
peroxodisulfate was prepared as described previously.
Yields refer to isolated products. n-Butyltriphenylphosphonium
2
1
Oxidative Deprotection of TMS and THP Ethers,
Ethylene Acetals, and Ketals: General Procedure
To a solution of substrate (1 mmol) in CH CN (15 ml), n-butyl-
3
triphenylphosphonium peroxodisulfate (0.826 g, 1 mmol) was added and
stirred magnetically under reflux condition for the appropriate time accord-
ing to Tables 1 and 2. The progress of the reaction was monitored by TLC.
The reaction mixture was cooled to room temperature and filtered. The solid
material was washed with CH CN (20 ml). The filtrates were combined and
3
evaporated. The resulting crude material was purified on a silica gel plate

1
316
MOHAMMADPOOR-BALTORK, HAJIPOUR, AND AGHAJARI
with appropriate eluent. Pure carbonyl compounds were obtained in
2–99% yields (Tables 1 and 2).
9
ACKNOWLEDGMENT
We are thankful to the Isfahan University Research Council for
partial support of this work.
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Received in Japan December 27, 2000