Caro's acid supported on silica gel. Part VI. A mild reagent for regeneration of carbonyl compounds from acetals, ketals, and 1,1-diacetates

Article abstract of DOI:10.1080/10426500490494750

Efficient conversion of acetals, ketals, and diacetates to carbonyl compounds is described using Caro's acid supported on silica gel. The deacetalization reactions are carried out in acetonitrile at room temperature. Reaction of diacetates is performed in refluxing dichloromethane, and their parent carbonyl compounds are obtained in good-to-excellent yields.

Full text of DOI:10.1080/10426500490494750

This article was downloaded by: [University of Saskatchewan Library]
On: 08 October 2012, At: 20:57
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
CARO'S ACID SUPPORTED ON
SILICA GEL. PART VI. A MILD
REAGENT FOR REGENERATION
OF CARBONYL COMPOUNDS
FROM ACETALS, KETALS, AND
1
,1-DIACETATES
a
a
a
M. M. Lakouraj , M. Tajbakhsh , V. Khojasteh
&
a
M. H. Gholami
a
Department of Chemistry, Faculty of Sciences,
Mazandaran University, Babolsar, Iran
Version of record first published: 16 Aug 2010.
To cite this article: M. M. Lakouraj, M. Tajbakhsh, V. Khojasteh & M. H. Gholami
(2004): CARO'S ACID SUPPORTED ON SILICA GEL. PART VI. A MILD REAGENT FOR
REGENERATION OF CARBONYL COMPOUNDS FROM ACETALS, KETALS, AND 1,1-
DIACETATES, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:12,
2645-2651
To link to this article: http://dx.doi.org/10.1080/10426500490494750
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-
and-conditions
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,

sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden.
The publisher does not give any warranty express or implied or make any
representation that the contents will be complete or accurate or up to
date. The accuracy of any instructions, formulae, and drug doses should be
independently verified with primary sources. The publisher shall not be liable
for any loss, actions, claims, proceedings, demand, or costs or damages
whatsoever or howsoever caused arising directly or indirectly in connection
with or arising out of the use of this material.

Phosphorus, Sulfur, and Silicon, 179:2645–2651, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490494750
CARO’S ACID SUPPORTED ON SILICA GEL.
PART VI. A MILD REAGENT FOR REGENERATION
OF CARBONYL COMPOUNDS FROM ACETALS,
KETALS, AND 1,1-DIACETATES
M. M. Lakouraj, M. Tajbakhsh, V. Khojasteh,
and M. H. Gholami
Department of Chemistry, Faculty of Sciences, Mazandaran
University, Babolsar, Iran
(
Received March 16, 2004; accepted June 1, 2004)
Efficient conversion of acetals, ketals, and diacetates to carbonyl
compounds is described using Caro’s acid supported on silica gel.
The deacetalization reactions are carried out in acetonitrile at
room temperature. Reaction of diacetates is performed in refluxing
dichloromethane, and their parent carbonyl compounds are obtained
in good-to-excellent yields.
Keywords: 1,1-Diacetate; acetal and ketal; Caro’s acid; regeneration of
carbonyls
Acetals, ketals, and diacetates are frequently used to protect carbonyl
compounds in the course of total synthesis. The shielding of carbonyls
from nucleophilic attack is usual practice in multistep synthesis to ex-
1
ploit its electrophilic properties. Therefore, deprotection to their par-
ent carbonyl group is important in organic synthesis. The protection
for carbonyls is often served as cyclic and acyclic acetals, which have
proved to be more serviceable, and hence considerable efforts has been
made for their introduction and removal.2
Most of the methods used for the cleavage of these protective groups
employ aqueous or nonaqueous media acidified with mineral or or-
3
ganic acids. However, this is very often incompatible with the pres-
ence of some other protected functional groups. To overcome the prob-
lem, a number of mild and nonaqueous cleaving methods have been
4
reported. Some examples involve wet silica gel or LiBF4 in acetonitrile,
We thank the Mazandaran University Council for partial support of this work.
Address correspondence to M. M. Lakouraj, Department of Chemistry, Faculty of Sci-
ences, Mazandaran University, Babolsar 47415, Iran. E-mail: lakouraj@umz.ac.ir
2
645

2
646
M. M. Lakouraj et al.
5
phosphorous triiodide and diphosphorus tetraiodide, titanium (IV)
6
7
8
chloride, boron trifluoride-iodide ion, and cerium(III) chloride.
But many of these methods failed to deprotect the dioxalane moi-
ety, and some procedures suffer from lack of selectivity; unsatisfactory
yields; toxic, corrosive, and expensive reagents; elevated temperature
or sever reaction condition; and formation of considerable amounts of
side products. Hence, several milder methods that use neutral condi-
tions and inorganic supported reagents have also been developed for
deprotection of acetals and ketals.9
On the other hand, 1,1-diacetates have attracted considerable at-
tention during recent years because these compounds are moderately
stable, easily prepared, and can act as protecting groups for aldehy-
des. They are also important starting materials for the synthesis of
dienes for Diels-Alder cycloaddition.10 So far, various methods have
been reported for the conversion of 1,1-diacetates to the correspond-
1
1–21
ing aldehydes.
However, some of these methods have disadvan-
tages such as high acidity, long reaction time, low yield of product, and
the requirement of an additional microwave oven or inert atmosphere.
The identification of a mild reagent for regeneration of aldehyes and
ketones from acetals, ketals, and diacetates formed the basis of this
investigation.
Recently, we have used Caro’s acid supported on silica gel as an ef-
ficient oxidizing agent for some oxidative transformation in organic
2
2
chemistry. This reagent is easily prepared in situ from potassium
23
persulfate and sulfuric acid. We herein have examined Caro’s acid
supported on silica gel for deprotection of cyclic acetals, ketals, and di-
acetates. It was found that Caro’s acid/SiO2 can be used to deprotect
acetals and ketals to the corresponding carbonyl compounds in acetoni-
trile at room temperature in good-to-excellent yields (Scheme 1).
SCHEME 1
RESULTS AND DISCUSSION
The deprotection of acetals and ketals is studied in different sol-
vents such as acetonitrile, tetrahydrofuran (THF), dichloromethane,

Regeneration of Carbonyls
2647
chloroform, dioxane, hexane, and cyclohexane. Acetonitrile turned out
to be a suitable solvent for this transformation. Deprotection of ac-
etals and ketals proceed at room temperature by stirring the acetals
with equimolar quantity of the reagent in acetonitrile (Table I). Using
TABLE I Deprotection of Acetals and Ketals Using Caro’s Acid
Supported on Silica Gel
Entry
1
Substrate
Time (min)
20
Product
Yield (%)a
95b
2
3
20
20
95b
95b
4
75
80
5
6
50
60
90
84
7
8
60
45
93
78
9
25
86
1
1
0
1
40
95b
62
100
12
22
90
a
Refers to isolated yield.
b
Purity >98% (based on 1H NMR), thus it was not purified further.

2
648
M. M. Lakouraj et al.
a catalytic or less than stiochiometric amounts were also tried. Fast
completion and high yields were obtained in equimolar ratio. Sur-
prisingly, in the case of acetals overoxidation of the parent aldehy-
des is not observed. As seen in Table I, a variety of cyclic acetals and
ketals were treated with this reagent at room temperature. In this
study we have found that acetals derived from aliphatic aldehydes and
aromatic aldehydes bearing electron-withdrawing groups were depro-
tected slower than ketals, and deprotection of these compounds was not
completed under above conditions, whereas ketals completely depro-
tected to their corresponding carbonyl compounds in good-to-excellent
yields. The acid-sensitive tetrapyranyl ether group is unaffected under
these conditions.24 This method shows that Caro’s acid that is impreg-
nated onto silica gel support is efficient and has attained importance
because of its good selectivity and ease of manipulation.
Finally, deprotection of 1,1-diacetates investigated using this
reagent. As shown in Table II, different type of 1,1-diacetates, including
those bearing electron-withdrawing substituents, are converted to the
corresponding aldehydes in the presence of 0.5 molar ratio of Caro’s acid
TABLE II Deprotection of 1, 1-diacetates in the Presence of
Caro’s Acid Supported on Silica Gel
Entry
R
Time (min)
Yield (%)a,b
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C6H5
10
4
2
4
2
98
98
98
98
98
80
90
50
100
100
90
98
80
98
90
80
90
87
90
95
4-MeOC6H4
3,4-(MeO)2C6H3
3,4-(MeO)2C6H3
2-MeOC6H4
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
5-Me-2-Furyl
5-NO2-2-Furyl
C6H5CH CH
4-BrC6H4
4-ClC6H4
1-Naphtyl
CH3CH2
CH3CH2CH2
4-AcetoxyC6H4
CH3(CH2)4CH2
Ph CH2CH2
4-HOC6H4
120
170
200
2
5.5
7.5
84
45
7
300
140
7
170
120
20
1
1
1
1
1
1
1
1
1
1
2
aAll products were identified by comparison of their physical data
with those of authentic samples.
b
Isolated yields.

Regeneration of Carbonyls
2649
supported on silica gel in dichloromethane. As shown in the Table II,
caro’s acid supported on silica gel can also deprotect the aliphatic diac-
etates to corresponding aldehydes in good yields (Scheme 2).
SCHEME 2
The carbon–carbon double bound and methoxy group remained in-
tact in this method. It is important to note that aryl aldehyde diacetate
is selectively deprotected in the presence of phenolic acetate function
(
Table II, entry 17). These selectivity is of value in organic synthesis.
In conclusion, we have developed a simple and efficient method for
the selective deprotection of aryl diacetates. In addition, high yield
of the products, short reaction time, ease of workup and nontoxic-
ity of the reagent make this method a useful addition to the present
methodologies.
EXPERIMENTAL SECTION
All yields refer to the isolated products and are characterized by
1
1
H NMR, IR, and comparison with authentic samples. H NMR spec-
tra were recorded with a EM 360 A (60 MHz) spectrometer. IR spec-
tra were determined on a SP-1100, P-UV-Com instrument. Acetals and
2
5
diacetates were prepared according to the reported methods. Caro’s
acid/SiO2 was prepared as detailed below.
Preparation of Caro’s Acid
To ice cooled 98% sulfuric acid (4.7 g) is added in small portions potas-
sium persulfate (4.5 g) with stirring. Crushed ice (13 g) and water (4 g)
◦
is added to this, and the temperature is kept below 15 C. Silica gel
(
5 g, thin layer chromatography (TLC) grade, kieselgel 60G, particle
size 15 µm) is added in portions to the mixture and stirred for 4 h in an
ice water bath. The mixture is then filtered under suction and dried in
a desiccator to give a white free-flowing powder.
Deprotection of Acetals and Ketals: General Procedure
A suspension of acetals (1 mmol) and Caro’s acid/silica gel (0.6 g,
1
mmol) in CH3CN (4 ml) is prepared. The mixture is stirred

2
650
M. M. Lakouraj et al.
magnetically at room temperature for 20–100 min. After completion
of the reaction, the reaction mixture is filtered and acetonitrile is evap-
orated, and then the residue is washed with dilute NaHCO3 and ex-
tracted with dichloromethane (20 ml). Evaporation of the solvent gives
the parent carbonyl compounds in good-to-excellent yields.
Deprotection of 1,1-Diacetate: General Procedure
To a solution of 1,1-diacetates (1 mmol) in dichloromethane (5 ml) was
added Caro’s acid/silica gel (0.5 mmol), and the mixture was stirred
under reflux condition for the time indicated in the Table I. The reac-
tion was followed by TLC. On completion of the reaction, the reaction
mixture was filtered and washed with CH2Cl2. The combined filtrates
were evaporated to give the crude products. Purification of the crude
material on a silica-gel plate (eluent: n-C6H14/Et2O: 4/1) afforded the
pure aldehydes in 50–100% yields (Table II).
REFERENCES
[
[
[
1] T. W. Green and P. G. M. Wuts, Protective Groups in Organic Synthesis (John Wiley
New York, 1999), 2nd ed., pp. 297–327.
2] K. J. Eash, M. S. Pulia, L. C. Wieland, and R. S. Mohan, J. Org. Chem., 65, 8399
(
2000).
3] a) E. W. Colvin, R. A. Raphael, and J. S. Roberts, J. Chem. Soc. Chem. Commun.,
5, 858 (1971); b) G. M. Coppola, Synthesis, 12, 1021 (1984); c) R. A. Elison, E. R.
1
Lukenbach, and C.W. Chin, Tetrahedron Lett., 8, 499 (1975); d) T. Kametani, H.
Kondoh, T. Honda, H. Ishizone, Y. Suzuki, and W. Mori , Chem. Lett., 7, 901 (1989).
4] a) B. H. Lipshutz and D. F. Harvey, Synth. Commun., 12, 267 (1982); b) F. Huet,
A. Lechevealier, M. Pellet, and J. M. Conia, Synthesis, 1, 63 (1978).
[
[
[
[
[
5] J. N. Denis and A. Krief, Angew. Chem. Int. Ed., 19, 1006 (1980).
6] G. Balme and J. Gore, J. Org. Chem., 48, 3336 (1983).
7] A. K. Mandal, P. Y. Shrotri, and A. D. Ghogare, Synthesis, 2, 221 (1985).
8] E. Marcantoni, F. Nobili, G. Bartoli, M. Bosco, and L. Sambri, J. Org. Chem., 62,
4183 (1997).
[
9] a) I. E. Marko, A. Ates, A. Gautier, B. Leroy, J. M. Plancher, Y. Quesnel, and J. C.
Vanherk, Angew. Chem. Int. Ed., 38, 3207 (1999); b) C. Johnstone, W. J. Kerr, and J.
S. Scott, J. Chem. Soc. Chem. Commun., 3, 341 (1996); c) D. S. Bose, B. Jayalacshmi,
and A. V. Narsaiah, Synthesis, 1, 67 (2000); d) K. S. Kim, Y. H. Song, B. H. Lee, and
C. S. Hahn, J. Org. Chem., 51, 404 (1986); e) W. A. Szarek, A. Zamojski, K. N. Tiwari,
and E. R. Ison, Tetrahedron Lett., 27, 3827 (1986).
[
[
[
[
10] B. B. Sinder and S. G. Amin, Synth. Commun., 8, 117 (1978).
11] S. V. Lieberma and R. Connor, Org. Syn. Coll., 2, 441 (1951).
12] S. M. Tsang and E. H. Wood, J. Org. Syn., 3, 641 (1955).
13] C. Narayana, S. Padmanabhan, and G. W. Kabalka, Tetrahedron Lett., 31, 6977
(
1990).
[
14] P. Cotelle and J. P. Catteau, Tetrahedron Lett., 33, 3855 (1992).

Regeneration of Carbonyls
2651
[
[
[
[
[
[
15] R. S. Varma, A. K. Chatterhee, and M. Varma, Tetrahedon Lett., 34, 3207 (1993).
16] Y. Y. Ku, R. Patel, and D. Sawich, Tetrahedron Lett., 34, 8037 (1993).
17] T. S. Li, Z. H. Zhang, and C. G. Fu, Tetrahedron Lett., 38, 3285 (1997).
18] T. S. Jin, Z. H. Zhang, and T. S. Li, Synth. Commun., 27, 3379 (1997).
19] D. Villemin and B. Martin, J. Chem. Res. (S), 2, 146 (1994).
20] E. R. Perez, A. L. Marrero, R. Perez, and M. A. Autie, Tetrahedron Lett., 36, 1779
(
1995).
[
[
21] a) I. Mohammadpour-Baltork and H. Aliyan, J. Chem., Res (S), 4, 272 (1999); b) V.
K. Aggarwal, S. Fonquerna, and G. P. Vennall, Synlett, 8, 849 (1998) .
22] a) M. M. Lakouraj, B. Movassagh, and K. Ghodrati, Synth. Commun., 32, 847 (2002);
b) B. Movassagh, M. M. Lakouraj, and K. Ghodrati, Synth. Commun., 32, 2353
(
3
2000); c) B. Movassagh, M. M. Lakouraj, and K. Ghodrati, Synth. Commun., 29,
597 (1999).
[
[
23] K. Ghodrati, M.S. Thesis, University of Razi, Kermanshah (2000).
24] M. M. Lakouraj, M. Tajbakhsh, and V. Khojasteh, Phosphorus, Sulfur, and Silicon,
178, 1865 (2003).
[
25] F. A. J., Meskaes, Synthesis, 7, 501 (1981).