Tungsten Hexachloride (WCl6) as a Mild and Efficient Reagent for Deprotection of Acetals and Ketals

Article abstract of DOI:10.1039/a803977a

A variety of acetals and ketals are efficiently and rapidly converted to the corresponding carbonyl compounds by using WCl₆ in dichloromethane or acetonitrile at room temperature.

Full text of DOI:10.1039/a803977a

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6
64 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
Tungsten Hexachloride (WCl ) as a Mild and
6
1998, 664±665$
Efficient Reagent for Deprotection of Acetals and
Ketals$
Habib Firouzabadi,* Nasser Iranpoor* and Babak Karimi
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
A variety of acetals and ketals are efficiently and rapidly converted to the corresponding carbonyl compounds by using
WCl in dichloromethane or acetonitrile at room temperature.
6
Acetals are one of the most useful and versatile protecting
groups in organic syntheses. They ®nd widespread appli-
cation for example in the protection of carbonyl, hydroxy,
1
and diol functions. Therefore, regeneration of the parent
carbonyl group from their masked form seems to be a
useful synthetic process. There are several methods for
the deprotection of acetals and ketals such as aqueous
Scheme 1
1
acid hydrolysis, and K-10 montmorillonite in aqueous
2
methanol, and non-aqueous methods including those based
the corresponding carbonyl compounds under mild reaction
conditions (Scheme 1).
3
on TeCl4, [Ru(CH CN) (triphos)](OTf ) , silica-supported
4
3
3
2
5
6
guanidinium chloride±acetyl chloride, SiCl ±NaI, K-10
4
As shown in Table 1 a variety of dimethyl acetals (entry
1) and diethyl acetals (entries 2±6), of structurally di€erent
carbonyl compounds as well as 1,3-dioxolanes of aldehydes
(entries 7±10) can be cleanly deprotected at room tempera-
ture using 0.15±0.2 equivalent of WCl6 in dry CH Cl .
On the other hand, cleavage of 1,3-dioxolanes derived from
aromatic and aliphatic ketones was achieved in dry CH CN
in the presence of 0.5±0.8 equivalent of WCl6 (entries
11±16). Nitro and methoxy groups are conserved in this
method (entries 5, 8). In conclusion, mild reaction con-
ditions, easy work-up, and excellent yields of the desired
products are worthy of mention as advantages of the
present method.
7
8
9
10
montmorillonite,
1
DDQ,
CeCl
Ph
12
3
P±CBr
4
,
SnCl
O, oxidation methods etc.
But the synthetic application of this transformation is so
important that the introduction of improved methods
continues to attract attention. New applications of tungsten
2
Á 2H
2
O,
1
13
1
CuSO
4
±SiO
2
,
3
Á 7H
2
2
2
hexachloride (WCl
6
) have been of interest to us in recent
3
years. Halo-dehydroxylation, dihalo-de-bisubstitution reac-
tions and chemoselective dithioacetalization of carbonyl
compounds and transthioacetalization of acetals have been
1
4
reported. In continuation of our studies we have found
that tungsten hexachloride eciently converts various types
of acetals and ketals (dimethyl, diethyl and cyclic acetals) to
Table 1 Deprotection of acetals and ketals with WCl
6
1
2
R
a
Yield (%)
Entry
R
XX
Subst.:WCl
6
ratio
Solvent
Time/min
1
2
3
4
5
Ph
Ph
p-MeC H
6 4
p-ClC
p-NO
H
H
H
H
H
(OMe)
2
1:0.15
1:0.15
1:0.15
1:0.15
1:0.2
CH
CH
CH
CH
CH
2
2
2
2
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
5
5
3
5
7
92
93
94
91
98
(OEt)
(OEt)
(OEt)
(OEt)
2
2
2
2
6 4
H
C H
2 6 4
6
(OEt)
2
1:0.2
CH
2
Cl
2
5
98
7
8
9
0
1
2
3
4
p-MeC
p-MeOC
PhCH.CH
n-C
Ph
p-ClC
p-PhC
PhCH
6
H
4
H
H
H
H
Me
Me
Me
Me
-OCH
-OCH
-OCH
-OCH
-OCH
-OCH
-OCH
-OCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
2
2
2
2
2
2
2
O-
O-
O-
O-
O-
O-
O-
O-
1:0.2
1:0.2
1:0.2
1:0.25
1:0.7
1:0.8
1:0.8
1:0.6
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
3
3
3
3
Cl
Cl
Cl
Cl
CN
CN
CN
CN
2
2
2
2
5
5
5
15
10
10
10
10
90
89
86
81
94
89
82
91
6
H
4
1
1
1
1
1
6
H
13
6
H
4
H
4
CH
2
6
2
1
5
-OCH
2
CH
2
O-
1:0.6
CH
3
CN
20
75
1
6
-OCH
2
CH
2
O-
1:0.8
CH
3
CN
15
90
a
The yields refer to isolated pure products.
Experimental
General Procedure for Deprotonation of Acetals with WCl
solution of acetal 1 (2 mmol) in dry CH Cl or CH CN (10 ml),
WCl (0.3±1.6 mmol) was added. The solution was stirred at room
*
To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
6
.ÐTo a
2
2
3
6

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J. CHEM. RESEARCH (S), 1998 665
temperature, and the progress of the reaction was monitored by
TLC. On completion (3±20 min), the reaction was quenched with
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NaOH aqueous solution (10%; 15 ml), and extracted with CH
3Â 30 ml). The organic layer was washed successively with
saturated NaCl solution (2Â 15 ml), and water (15 ml) and dried
over anhydrous Na SO . Evaporation of the solvent under reduced
2 2
Cl
(
2
4
pressure gave almost pure product. Further puri®cation was
achieved by column chromatography on silica gel or recrystalliza-
tion from the appropriate solvent to give the desired product in
good to excellent yields (Table 1).
Financial support by the Shiraz University Research
Council is gratefully acknowledged.
9 C. Johnstone, W. J. Kerr and J. S. Scott, Chem. Commun., 1996,
341.
1
1
1
1
0 K. L. Ford and E. J. Roskamp, J. Org. Chem., 1993, 58, 4142
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1 G. M. Caballero and E. G. Gros, Synth. Commun., 1995, 25,
Received, 27th May 1998; Accepted, 23rd June 1998
Paper E/8/03977A
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