2,6-Dicarboxypyridinium chlorochromate. An efficient and selective reagent for the mild deprotection of acetals, thioacetals, and 1,1-diacetates to carbonyl compounds

Article abstract of DOI:10.1007/s00706-004-0193-7

2,6-Dicarboxypyridinium chlorochromate (2,6-DCPCC) was found to be an efficient reagent for the conversion of acetals, thioacetals, and 1,1-diacetates to their corresponding carbonyl compounds under neutral and anhydrous conditions in good to excellent yields. Selective deprotection of acetals or 1,1-diacetates in the presence of thioacetals at room temperature is also observed with this reagent. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-004-0193-7

Monatshefte f €u r Chemie 135, 1243–1249 (2004)
DOI 10.1007/s00706-004-0193-7
2
,6-Dicarboxypyridinium Chlorochromate.
An Efficient and Selective Reagent
for the Mild Deprotection of Acetals,
Thioacetals, and 1,1-Diacetates
to Carbonyl Compounds
Ã
and Mohammad Yazdani Niaki
Ã
Rahman Hosseinzadeh , Mahmood Tajbakhsh , Alireza Shakoori,
Department of Chemistry, Mazandaran University, Babolsar, Iran
Received January 20, 2004; accepted January 21, 2004
Published online June 30, 2004 # Springer-Verlag 2004
Summary. 2,6-Dicarboxypyridinium chlorochromate (2,6-DCPCC) was found to be an efficient re-
agent for the conversion of acetals, thioacetals, and 1,1-diacetates to their corresponding carbonyl com-
pounds under neutral and anhydrous conditions in good to excellent yields. Selective deprotection of
acetals or 1,1-diacetates in the presence of thioacetals at room temperature is also observed with this
reagent.
Keywords. 2,6-DCPCC; Deprotection; Acetals; Thioacetals; 1,1-Diacetates.
Introduction
The protection of carbonyl compounds as their acetals and ketals has found wide
application in multistep syntheses [1]. In recent years, several reagents have been
used for the conversion of acetals and ketals into carbonyl compounds such
as phosphorous triiodide [2a], titanium(IV) chloride [2b], bismuth nitrate [2c],
cerium(III) chloride [2d], oxone [2e], and decaborane [2f]. However, some of
the reported methods suffer from one or more disadvantages, such as long reaction
times, low yields, and tedious work-ups.
Being stable under both acidic and basic conditions, the dithioacetal group
is suitable as a protecting group of carbonyl compounds [1a, 1b]. In addition to
carbonyl protection, they behave as masked acyl anions [3] in carbon–carbon bond
forming reactions. A number of methods for dethioacetalization, such as clay sup-
ported ammonium nitrate (clayan) [4a], zirconium sulfonyl phosphonate [4b],
à Corresponding authors. E-mail: r.hosseinzadeh@umz.ac.ir

1
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R. Hosseinzadeh et al.
hv=pyrylium=O2 [4c], isoamyl nitrite [4d], diacetoxyiodobenzene [4e],
CF CO ) IPh [4f], tetrabutyl ammonium tribromide [4g], and FeCl [4h] have
(
3
2 2
3
been described. However, some major disadvantages are low chemoselectivity,
stoichiometric use of expensive reagents, the presence of a strong Lewis acid, and
necessity for an aqueous solvent.
The formation of diacetyl acetals constitutes one of the simplest methods to
derivatize the aldehyde moiety [1a]. These compounds are stable to oxidants [5],
easily prepared [6], and they are starting materials for Diels-Alder reactions [7].
A number of methods have been documented for the conversion of 1,1-diacetates
to corresponding aldehydes. The following methods have been used: alcoholic
sulfuric [8a] or hydrochloric acid [8b], sodium hydroxide or potassium carbonate
in aqueous THF [5], boron triiodide N,N-diethylaniline complex [8c], ceric am-
monium nitrate coated on silicagel in dichloromethane [8d], neutral alumina under
microwave irradiation [8e], potassium phenoxides [8f], montmorillonite K10 [8g]
and KSF [8h] under microwave irradiation, montmorillonite K10 and KSF in
refluxing dichloromethane [8i], expensive graphite [8j], scandium triflate [8k],
bismuth(III) chloride [8l], envirocoat EPZG under microwave irradiation [8m],
CBr in refluxing acetonitrile [8n], and zirconium sulfophenyl phosphonate [8o].
4
Recently, we have introduced 2,6-dicarboxypyridinium chlorochromate (2,6-
DCPCC) as an efficient reagent for oxidation of alcohols, trimethylsilyl ethers,
THP ethers, and oximes to the corresponding carbonyl compounds under non-
aqueous conditions [9]. We now report a new and convenient method for the
deprotection of acetals, thioacetals, and 1,1-diacetates to carbonyl compounds in
high yields.
Results and Discussion
A series of acetals and ketals dissolved in acetonitrile were deprotected with 2,6-
DCPCC (Scheme 1) [10]. The results shown in Table 1 indicate that the reaction
was successful for a variety of acetals of aromatic aldehydes (entries 1a–5a) and
ketals of aromatic (entries 6a–9a) and aliphatic ketones (entries 10a–11a). It is
noteworthy that, unlike other methods, the major drawback of over-oxidation of the
aldehydes to their carboxylic acids was not observed.
Some representative thio derivatives of aldehydes and ketones were readily
deprotected into their parent carbonyl compounds in good yields in acetonitrile
(
Scheme 2) [11]. Although acetals underwent deprotection at room temperature in
less than 30 min, the deprotection of thioacetals was relatively slow and proceeded
in refluxing acetonitrile only. The reaction is general for propanediyl S,S-acetals of
aliphatic, araliphatic, and aromatic carbonyl compounds (Table 2).
Scheme 1

2
,6-Dicarboxypyridinium Chlorochromate
Table 1. Deprotection of acetals with 2,6-DCPCC
Entry Substrate
1245
a
b
Time=min
Yield=%
1a
2a
3a
4a
5a
6a
12
93
15
16
15
16
15
93
94
94
95
95
7
8
9
a
a
a
14
25
94
92
16
15
90
89
1
0a
1a
1
28
88
a
b
Reactions are performed at room temperature using 1:1 molar ratio of substrates to reagent; yields
refer to isolated products
Scheme 2
1,1-Diacetates dissolved in acetonitrile were deprotected using 2,6-DCPCC
to give the corresponding aldehydes at room temperature (Scheme 3). The re-
sults shown in Table 3 indicate that the reaction is successful for a variety of

1
246
R. Hosseinzadeh et al.
a
Table 2. Deprotection of thioacetals with 2,6-DCPCC
b
Entry
Substrate
Time=min
Yield=%
1b
2b
3b
4b
5b
6b
30
95
95
93
90
88
87
45
88
115
128
120
7
8
b
b
120
100
60
70
a
Reactions are performed at reflux temperature using 1:2.5 molar ratio of substrates to reagent;
yields refer to isolated products
b
Scheme 3
1,1-diacetates of aromatic (entries 1c–15c) and aliphatic aldehydes (entries 16c–18c).
It should be noted that the phenolic acetate function (entry 15c) remains unaffected
under these reaction conditions.
It is interesting to mention that 2,6-DCPCC can selectively cleave acetals or
1,1-diacetates in the presence of thioacetals at room temperature. When mixtures
of equimolar amounts of acetals 1 or 5 in the presence of thioacetals 2 or 6 were
treated with 2,6-DCPCC, only the acetals were deprotected and the thioacetals
remained unchanged (Scheme 4). Similarly when a mixture of equimolar amounts
of 1,1-diacetate 9 in the presence of thioacetal 6 was treated with 2,6-DCPCC,
only the 1,1-diacetate was deprotected and thioacetal remained unchanged
(
Scheme 5).
In conclusion, we have shown that 2,6-DCPCC is an efficient, rapid, mild,
and inexpensive reagent for conversion of aliphatic and aromatic acetals, ketals,
and 1,1-diacetates to their corresponding carbonyl compounds. Deprotection of

2
,6-Dicarboxypyridinium Chlorochromate
1247
a
Table 3. Deprotection of 1,1-diacetates with 2,6-DCPCC in MeCN
b
Entry
Substrate
Time=min
Yield=%
1c
2c
3c
4c
5c
6c
7c
8c
Ph–CH(OAc)2
12
8
96
96
93
96
96
93
90
90
4-MePh–CH(OAc)2
2-OMePh–CH(OAc)2
2,5-di-OMePh–CH(OAc)2
3,4-di-OMePh–CH(OAc)2
4-OBzPh–CH(OAc)2
1-Naphthyl–CH(OAc)2
7
5
5
8
14
11
9
c
4-ClPh–CH(OAc)2
2-ClPh–CH(OAc)2
4-BrPh–CH(OAc)2
4-PhPh–CH(OAc)2
23
30
22
30
10
90
88
88
70
84
1
1
1
1
0c
1c
2c
3c
1
1
1
1
1
4c
5c
6c
7c
8c
4-(AcO) CH–Ph–CH(OAc)
2
20
15
15
15
15
94
95
96
96
95
2
4-AcOPh–CH(OAc)2
n-Hexyl–CH(OAc)2
Et HC–CH(OAc)
2
2
n-Pr–CH(OAc)2
a
Reactions are performed at room temperature using 1:0.5 molar ratio of substrates to reagent;
yields refer to isolated products
b
Scheme 4
thioacetals with 2,6-DCPCC requires a higher molar ratio of the oxidant, much
longer reaction times, and higher temperature. This reagent deprotects acetals and
1,1-diacetates quantitatively irrespective of the presence of thioacetals.

1
248
R. Hosseinzadeh et al.
Scheme 5
Experimental
1
Infrared spectra were recorded on Pye-unicam SP 1100 spectrometer. H NMR spectra were obtained
at 250 MHz using a Bruker spectrometer in CDCl as the solvent and tetramethyl silane (TMS) as
3
internal reference. Melting points were measured with Electro thermal 9100. Purification was achieved
by distillation or silica gel column chromatography (mesh 27–30). Acetals [10], thioacetals [11], and
1
,1-diacetates [5] were prepared according to literature.
General Procedure for the Deprotection of Acetals with 2,6-DCPCC
3
In a round-bottomed flask, a solution of 1 mmol of acetal in 10cm of MeCN was treated with 1 mmol
of 2,6-DCPCC and the mixture was stirred at room temperature. The progress of the reaction was
monitored by GC or TLC. The reaction mixture was filtered and the solid material was washed with
3
3
2
0cm of n-hexane. The filtrate was washed with 3Â10cm of water. Purification of the crude product
on a silica gel plate or silica gel column (eluent: n-hexane=EtOAc) afforded the pure carbonyl
1
compound. All products (Table 1) were characterized by their melting points, IR, H NMR spectro-
scopic data, and also by comparison with authentic samples.
General Procedure for the Deprotection of Thioacetals with 2,6-DCPCC
In a round-bottomed flask, a solution of 1 mmol of thioacetal was treated with 2.5 mmol of 2,6-DCPCC
and the mixture was stirred under reflux in acetonitrile. The progress of the reaction was monitored by
3
GC or TLC. The reaction mixture was filtered and the solid material was washed with 20cm of n-
3
hexane. The filtrate was washed with 3Â10cm of water. Purification of the crude product on a silica
gel plate or silica gel column (eluent: n-hexane=CH Cl ) afforded the pure carbonyl compound. All
2
2
1
products (Table 2) were characterized by their melting points, IR, H NMR spectroscopic data, and
also by comparison with authentic samples.
General Procedure for Deprotection of 1,1-Diacetates with 2,6-DCPCC
3
In a round-bottomed flask, a solution of 1 mmol of 1,1-diacetate [5] in 10cm of MeCN was treated with
0
.5 mmol of 2,6-DCPCC and the mixture was stirred at room temperature. The progress of the reaction
was monitored by GC or TLC. The reaction mixture was filtered and the solid material was washed with
3
0cm of n-hexane. Purification of the crude product on a silica gel column (eluent: n-hexane=ether
3=1)) afforded the pure carbonyl compound. All products (Table 3) were characterized by their melting
2
(
1
points, IR, H NMR spectroscopic data, and also by comparison with authentic samples.
Acknowledgement
Financial support of this work from the Research Council of Mazandaran University is gratefully
acknowledged.

2
,6-Dicarboxypyridinium Chlorochromate
1249
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