New applications of solid silica chloride (SiO2-Cl) in organic synthesis. Efficient preparation of diacetals of 2,2-bis(hydroxymethyl)-1,3-propanediol from different substrates and their transthioacetalization reactions. Efficient regeneration of carbonyl compounds from acetals and acylals

Article abstract of DOI:10.1080/10426500214879

A new application of solid silica chloride, an easily available and efficient catalyst for the preparation of diacetal of 2,2-bis-(hydroxymethyl)-1,3-propanediol from aldehydes, acetals, acylals, and oximes, is described. Transthioacetalization of diacetals of 2,2-bis-(hydroxymethyl)-1,3-propanediol into their corresponding 1,3-dithianes and 1,3-dithiolanes in the presence of silica chloride is presented. Efficient regeneration of carbonyl compounds from their corresponding acetals, ketals, diacetals, and acylals in the presence of this catalyst also is described.

Full text of DOI:10.1080/10426500214879

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Phosphorus, Sulfur, and Silicon and the Related
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New Applications of Solid Silica Chloride (SiO 2 -Cl) in
Organic Synthesis. Efficient Preparation of Diacetals of
2,2-Bis(Hydroxymethyl)-1,3-propanediol from Different
Substrates and Their Transthioacetalization Reactions.
Efficient Regeneration of Carbonyl Compounds from
Acetals and Acylals
Habib Firouzabadi a , Nasser Iranpoor a & Hassan Hazarkhani a
a Shiraz University , Shiraz, Iran
Published online: 27 Oct 2010.
To cite this article: Habib Firouzabadi , Nasser Iranpoor & Hassan Hazarkhani (2002) New Applications of Solid Silica Chloride
(SiO 2 -Cl) in Organic Synthesis. Efficient Preparation of Diacetals of 2,2-Bis(Hydroxymethyl)-1,3-propanediol from Different
Substrates and Their Transthioacetalization Reactions. Efficient Regeneration of Carbonyl Compounds from Acetals and
Acylals, Phosphorus, Sulfur, and Silicon and the Related Elements, 177:12, 2847-2858, DOI: 10.1080/10426500214879
To link to this article: http://dx.doi.org/10.1080/10426500214879
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:2847–2858
C
Copyright 2002 Taylor & Francis
1042-6507/02 $12.00 + .00
DOI: 10.1080/10426500290146893
NEW APPLICATIONS OF SOLID SILICA CHLORIDE
(SiO2-Cl) IN ORGANIC SYNTHESIS. EFFICIENT
PREPARATION OF DIACETALS OF
2,2-Bis(HYDROXYMETHYL)-1,3-PROPANEDIOL
FROM DIFFERENT SUBSTRATES AND THEIR
TRANSTHIOACETALIZATION REACTIONS.
EFFICIENT REGENERATION OF CARBONYL
COMPOUNDS FROM ACETALS AND ACYLALS
Habib Firouzabadi, Nasser Iranpoor, and Hassan Hazarkhani
Shiraz University, Shiraz, Iran
(Received May 10, 2002)
A new application of solid silica chloride, an easily available and effi-
cient catalyst for the preparation of diacetal of 2,2-bis-(hydroxymethyl)-
1,3-propanediol from aldehydes, acetals, acylals, and oximes, is
described. Transthioacetalization of diacetals of 2,2-bis-(hydroxy-
methyl)-1,3-propanediol into their corresponding 1,3-dithianes and
1,3-dithiolanes in the presence of silica chloride is presented. Efficient
regeneration of carbonyl compounds from their corresponding acetals,
ketals, diacetals, and acylals in the presence of this catalyst also is
described.
Keywords: Acetals; acylals; carbonyl compounds; oximes; silica
chloride; transthioacetalization
Solid supports have found wide applications in organic reactions from
different views.1 They mostly facilitate the work-up of the reaction
mixtures and usually high selectivity accompanied with high yields
of the products is observed. Silica gel is one of the extensively used
support for different purposes in organic chemistry.2 Modified silica
supports for functional group transformation also is of interest. Silica
chloride has been reported to be an efficient reagent for the selec-
tive thioacetalization of carbonyl compounds and transformation of
sulfoxides to thioethers.3 Literature survey shows that less attention
The authors are thankful to the Shiraz University Research Council for the partial
support of this work.
Address correspondence to Habib Firouzabadi or Nasser Iranpoor, Department
of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran. E-mail:
firouzabadi@chem.susc.ac.ir or iranpoor@chem.susc.ac.ir
2847

2848
H. Firouzabadi et al.
has been paid to silica chloride as a potential heterogeneous inorganic
polymeric bed in organic reactions. Recently we paid attention to sil-
ica chloride in functional group transformation reactions and reported
a new procedure for the preparation of high capacity SiO2-Cl.4a We
have applied this heterogeneous catalyst for efficient transdithioacetal-
ization of acetals, transformation of acylals into thioacetals, dehydra-
tion of tert-benzylic alcohols to their olefins, and dethioacetalization of
thioacetals into their carbonyl compounds and transformation of silyl
and THPethers to their iodides, ring-expansion annelation and ring-
f
expansion-chlorination of cyclic thioacetals.4a Solid SiO2-Cl recently
has been used as an efficient catalyst for tetrahydropyranylation of
hydroxy functional groups.4e In this article we report further applica-
tions of this high capacity solid SiO2-Cl in some important functional
group transformations.
A) PREPARATION OF DIACETALS OF
2,2-Bis-(HYDROXYMETHYL)-1,3-PROPANEDIOL
FROM DIFFERENT SUBSTRATES AND THEIR
TRANSTHIOACETALIZATION REACTIONS
Carbonyl functional groups often play important roles in the synthesis
of complex organic molecules, and a great deal of synthetic work has
been done on the protection and masking of carbonyl compounds.5 Ac-
etals are one of the most useful and versatile protecting groups for car-
bonyl functions and the most popular ones are 1,3-dioxolanes and 1,3-
dioxanes because of their relative stability.6 They usually are liquid and
their purification and handling is difficult. In recent years, attention has
been paid to the synthesis of diacetals of 2,2-bis (hydroxymethyl)-1,3-
propanediol, which are crystalline substances and have sharp melting
points.7 We now report on the efficient and high yielding preparation
of diacetals of 2,2-bis-(hydroxymethyl)-1,3-propanediol from aldehydes
and their cyclic and open-chain O,O-acetals, oxathioacetals, acylals,
and oximes catalyzed by SiO2-Cl at room temperature in CH3CN
(Scheme 1, Table I). However, ketones do not react under similar
SCHEME 1

New Applications of Solid Silica Chloride (SiO -Cl)
2849
2
TABLE I Synthesis of Diacetals of 2,2-Bis-(hydroxymethyl)-1,3-propanediol
from Different Substrates Using SiO2-Cl as a Catalyst in CH3CN at Room
Temperature
Yieldb
%
1H-NMR (CDCl3, 250 MHz, ) and some
of the 13C-NMR (CDCl3, 62.90 MHz,
Substratea
)
(1a) 95 3.75 (2H, d), 3.85 (4H, m), 4.88 (2H, d), 5.44 (2H, s),
7.52 (10H, m)
(1b) 96 2.33 (6H, s), 3.67 (2H, d), 3.80 (4H, m), 4.81 (2H, d),
5.42 (2H, s), 7.18 (4H, d), 7.38 (4H, d)
(1c) 95 3.69 (2H, d), 3.81 (6H, s), 3.86 (4H, m), 4.86 (2H, d),
5.40 (2H, s), 6.97 (4H, d), 7.42 (4H, d)
(1d) 96 3.60 (2H, d), 3.82 (6H, s), 3.84 (4H, m), 4.85 (2H, d),
5.39 (2H, s), 6.90 (2H, d), 7.07 (4H, d),
7.24 (2H, t). 13C-NMR (CDCl3, 50 MHz)
= 32.91, 56.11, 70.92, 71.42, 102.43, 111.74,
115.53, 118.88, 130.48, 140.08, 160.06.
(1e) 89 2.21 (6H, s), 2.45 (12H, s), 3.52 (2H, d), 3.74 (4H, m),
4.92 (2H, d), 5.71 (2H, s), 6.79 (4H, s).
13C-NMR (CDCI3, 50 MHz) = 20.98, 21.38,
32.91, 71.14, 73.09, 102.59, 130.41, 130.92
137.12, 138.86.
(1f) 94 3.39 (2H, d), 3.59 (4H, m), 4.38 (2H, d), 5.14 (2H, s),
7.08 (4H, d), 7.22(4H, d).
(1g) 95 3.53 (2H, d), 3.79 (4H, m), 4.79 (2H, d), 5.41 (2H, s),
7.35 (4H, d), 7.41(4H, d).
(1h) 95 3.70 (2H, d), 3.84 (4H, m), 4.80 (2H, d), 5.50 (2H, s),
7.49 (2H, d), 7.77 (2H, d), 8.13 (2H, t),
8.29 (2H, d).
(1i)
96 3.49 (2H, d), 3.70 (4H, m), 4.68 (2H, d), 5.05 (2H, d),
6.20 (2H, d of d), 6.75 (2H, d), 7.32 (10H, m).
13C-NMR (CDCl3, 50 MHz) = 32.85, 70.62,
71.13, 101.87, 125.49, 127.30, 128.73,
129.02, 134.16, 136.33
(1j)
85 1.58 (6H, d), 3.10 (2H, m), 3.33 (2H, d), 3.58 (4H, m),
4.63 (4H, d), 7.39 (10H, m). 13C-NMR
(CDCl3, 50 MHz) = 16.07, 16.34, 64.29,
65.70, 105.45, 126.99, 128.70, 143.06.
(1k) 96 3.75 (2H, d), 3.85 (4H, m), 4.88 (2H, d), 5.41 (2H, s),
7.52 (10H, m)
(Continued on next page)

2850
H. Firouzabadi et al.
TABLE I Synthesis of Diacetals of 2,2-Bis-(hydroxymethyl)-1,3-propanediol
from Different Substrates Using SiO2-Cl as a Catalyst in CH3CN at Room
Temperature (Continued)
Yieldb
%
1H-NMR (CDCI3, 250 MHz, ) and some
of the 13C-NMR (CDCl3, 62.90 MHz,
Substratea
)
(1l) 95 3.75 (2H, d), 3.85 (4H, m), 4.88 (2H, d),
5.45 (2H, s), 7.52 (10H, m)
(1m) 94 2.36 (6H, s), 3.71 (2H, d), 3.88 (4H, m),
4.88 (2H, d), 5.45 (2H, s), 7.20 (4H, d),
7.38 (4H, d)
(1n) 92 2.33 (6H, s), 3.67 (2H, d), 3.80 (4H, m),
4.81 (2H, d), 7.18 (4H, d), 7.38 (4H, d)
(1o) 90 3.53 (2H, d), 3.79 (4H, m), 4.79 (2H, d),
5.41 (2H, s), 7.35 (4H, d), 7.41 (4H, d)
(1p) 79 3.58 (2H, d), 3.78 (4H, m), 4.82 (2H, d),
5.38 (2H, s), 7.42 (10H, m)
(1q) 77 3.60 (2H, d), 3.82 (6H, s), 3.84 (4H, m),
4.85 (2H, d), 5.39 (2H, s), 6.90 (2H, d),
7.07 (4H, d), 7.24 (2H, t)
aThe ratios of SiO2-Cl: subs.: diol were 0.3 g: 1 mmol: 0.55–0.6 mmol.
bIsolated yields.
reaction conditions and remained almost intact in the reaction mix-
tures. Therefore, this method easily can be used for the selective pro-
tection of aldehydes in the presence of ketones. Ketals in the presence
of this catalyst under similar reaction conditions were deprotected eas-
ily to their corresponding ketones rather than undergo transacetaliza-
tion reactions with 2,2-bis-(hydroxymethyl)-1,3-propanediol.
Transthioacetalization of diacetals of 2,2-bis-(hydroxymethyl)-1,3-
propanediol with 1,3-propanedithiol and ethanedithiol also was
achieved very easily using this catalyst at room temperature in CHCl3
SCHEME 2

New Applications of Solid Silica Chloride (SiO -Cl)
2
2851
TABLE II Transthioacetalization of Acetals of
2,2-Bis(hydroxymethyl)-1,3-propanediol in the Presence of
SiO2-Cl with 1,3-Propane and 1,3-Ethanedithiol in CHCl3
at Room Temperature
Entry
R1
n
SiO2-Cl (g)
Time (min) Yield %a
4a
4b
4c
4d
4e
4f
4g
4h
4i
Ph
Ph
0
1
0
1
1
1
1
0
1
1
1
1
1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.3
0.3
0.2
0.2
15
20
15
15
10
10
20
25
25
30
50
15
25
95
94
95
96
94
95
90
92
93
94
92
95
92
4-CH3-C6H4
4-CH3-C6H4
4-CH3O-C6H4
3-CH3O-C6H4
2,4,6,-(CH3)3-C6H2
4-Br-C6H4
4-Br-C6H4
4-Cl-C6H4
3-NO2-C6H4
Cinnamyl
Ph(CH3)CH
4j
4k
4l
4m
aIsolated yields.
(Scheme 2, Table II). All reactions cleanly proceeded under mild re-
action conditions to give the corresponding 1,3-dithianes and 1,3-
dithiolanes in excellent yields.
B) DEPROTECTION OF ACETALS
Acetals are one of the most useful and versatile protecting groups
in organic syntheses. They found widespread applications in the pro-
tection of carbonyl, hydroxyl, and diol functions.6 In this study, we
paid attention to high yielding regeneration of carbonyl groups from
their acetals under mild reaction conditions. A plethora of meth-
ods are reported for this purpose such as aqueous acid hydrolysis,5
K-10 in a aqueous methanol,8 and nonaqueous methods including
TeCl4,9 [Ru(CH3CN)3(triphos)](OTf )2,10 silica-supported guanidinium
chloride/acetyl chloride,11 SiCl4/NaI,12 K-10 montmorileonite,13 DDQ,14
Ph3P/CCl4,15 SnCl2 2H2O,16 CuSO4-SiO2,17 WC16,18 trimethylsilyl
bis(fluorosulfuryl)imide,19 and CAN.20 Synthetic importance of this
transformation is the attention to and introduction of new methods
that show selectivity; being mild and using simple and easily available
catalysts. In this study we report that silica chloride is able to convert
various types of acetals and ketals (dimethyl, diethyl, cyclic acetals, and
diacetals of pentaerythritol) to the corresponding carbonyl compounds
under mild reaction conditions (Scheme 3, Table III).

2852
H. Firouzabadi et al.
SCHEME 3
As shown in Table III, a variety of acyclic acetals (entries 1–6) of
structurally different carbonyl compounds as well as cyclic dioxolanes
of aldehydes (entries 7–10) can be cleanly deprotected at room tem-
perature using 0.5–0.6 g of silica chloride in dry CH2Cl2. On the other
hand, cleavage of cyclic dioxolanes derived from aromatic and aliphatic
ketones was also achieved in dry CH2Cl2 in the presence of 0.5–0.6 g of
silica chloride (entries 11–18). Diacetal of 2,2-bis-(hydroxymethyl)-1,3-
propanediol derived from aldehydes were successfully deprotected into
their corresponding aldehydes in excellent yields (entries 19–23). Diac-
etals of 2,2-bis-(hydroxymethyl)-1,3-propanediol derived from ketones
are unstable compounds and have not been reported yet; therefore, we
could not study their behavior in the presence of silica chloride.
C) REGENERATION OF ALDEHYDES FROM ACYLALS
UNDER MILD REACTION CONDITIONS
Regeneration of aldehydes from their diacetates (acylals) under mild
conditions, with high yields and selectivity, is a useful chemical trans-
formation. A literature survey reveals that a variety of methods are
available for this purpose.21 Some of the reported procedures have its
own merit and also its own drawbacks. Some require the use of strong
acids which other functional groups in the molecule may not tolerate,
some need long reaction times and provide low yields. In some other
reported methods, the use of microwave oven is obligatory that may
not be available in any laboratories. Very recently, AlCl3 in refluxing
CH3CN has been used for the efficient regeneration of aldehydes from
acylals.22 AlCl3 suffers from being a strong Lewis acid and may affect
sensitive functional groups in refluxing CH3CN. We recently reported
that silica chloride promotes highly efficient transformation of acylals
into 1,3-dithiolanes, 1,3-dithianes and 1,3-oxathiolanes under mild re-
action conditions.4b Now we report efficient deprotection of aryl and

New Applications of Solid Silica Chloride (SiO -Cl)
2
2853
TABLE III Deprotection of Acetals and Ketals with SiO2-Cl
Entry
R1
R2
X, XX
SiO2-Cl (g) Time (min) Yield (%)a,b,c
1
2
3
4
5
6
Ph
Ph
p-MeC6H4
p-ClC6H4
H
H
H
H
H
OMe
OEt
OEt
OEt
OEt
OEt
0.5
0.5
0.5
0.6
0.6
0.5
15
10
10
20
40
15
95
93
94
91
88
98
p-NO2C6H4
7
8
9
10
11
12
13
14
15
p-MeC6H4
p-MeOC6H4
PhCH CH
n-C6H13
H
H
H
H
OCH2CH2O
OCH2CH2O
OCH2CH2O
OCH2CH2O
0.5
0.5
0.5
0.6
0.5
0.5
0.5
0.6
0.6
20
25
15
30
10
30
10
35
30
90
94
96
81
94
89
88
91
85
Ph
Me OCH2CH2O
Me OCH2CH2O
Me OCH2CH2O
Me OCH2CH2O
OCH2CH2O
p-ClC6H4
p-PhC6H4
PhCH2CH2
16
17
OCH2CH2O
0.5
1.1
15
30
90
85
18
19
20
21
22
23
Ph
Ph
Et
H
H
H
H
H
OCH2CH2O
0.5
1
1
1
1
20
20
15
15
10
40
92
90
92
94
95
93
C(CH2O
C(CH2O
C(CH2O
C(CH2O
C(CH2O
)
4
)
4
)
4
)
4
)
4
p-MeC6H4
p-MeOC6H4
PhCH CH
p-ClC6H4
1.1
aThe yields refer to isolated pure products.
bBenzaldehyde was isolated as its 2,4-nitrophenylhydrazine derivative.
cIsolated yield.
allyl acylals in CH2Cl2 at room temperature in the presence of this
catalyst (Scheme 4, Table IV).
4-Nitrobenzylidene diacetate, which usually resists deprotection re-
actions under nonsolvolytic conditions, was deprotected in refluxing
CH3CN after 90 min in a good yield in the presence of silica chloride
(entry 8, Table IV). Phenolic acetates also were deprotected easily in
the presence of this catalyst and this is demonstrated by the reaction
of acetyl salicylaldehyde diacetate (entry 9, Table IV). Phenolic ethers
survived under similar reaction conditions (entries 3–5, Table IV).

2854
H. Firouzabadi et al.
SCHEME 4
A furanoid acylal (entry 10, Table IV) also was converted to its
corresponding aldehyde without polymerization in the presence of
silica chloride. Aliphatic acylals remain unreacted even in refluxing
CH3CN.
The selectivity of the method is demonstrated by the following re-
actions. 4-Methylbenzylidene diacetate was converted to its carbonyl
compound almost exclusively in the presence of 0.5 g of silica chloride
in dry CH2Cl2 at room temperature whereas, phenylethylidene diac-
etate remained almost intact. 4-Methylbenzylidene diacetate preferen-
tially was converted to the corresponding aldehyde in the presence of
4-methylbenzaldehyde 1,3-dithiane under similar reaction conditions
(Scheme 5).
In summary, in this study, we have presented some new applications
of silica chloride as an efficient catalyst or reagent, which work
under nonhydrolytic conditions for the deprotection of benzylic and
TABLE IV Deprotection of Acylals by SiO2-Cl in CH2Cl2 at
Room Temperature
Entry
Substrate
Time (min) SiO2-Cl (g) Yield %a
1
2
3
4
5
6
7
8
9
PhCH(OAc)2
30
15
20
20
15
40
60
90
70
30
30
30
0.5
0.5
0.5
0.5
0.5
0.6
0.7
0.7c
0.6
0.5
0.5
0.5
91b
92
94
95
95
90
90
80
87
91
92
95
4-H3C-C6H4-CH(OAc)2
4-H3CO-C6H4-CH(OAc)2
2-CH3O-C6H4-CH(OAc)2
2,5-di-CH3O-C6H4-CH(OAc)2
4-Br-C6H4-CH(OAc)2
4-Cl-C6H4-CH(OAc)2
4-NO2-C6H4-CH(OAc)2
2-OAc-C6H4-CH(OAc)2
5-CH3-Furyl-CH(OAc)2
1-Naphthyl-CH(OAc)2
PhCH CHCH(OAc)2
10
11
12
aIsolated yield.
bBenzaldehyde was isolated as its 2,4-dinitrophenylhydrazine derivative.
cReaction was performed in refluxing CH3CN in the presence of 0.7 g of the
catalyst.

New Applications of Solid Silica Chloride (SiO -Cl)
2
2855
SCHEME 5
allylic acylals and also a wide variety of acetals and ketals. We also
have introduced facile and high yielding preparation of diacetals of
2,2-bis-(hydroxymethyl)-1,3-propanediol from aldehydes, acetals,
oxathioacetals, acylals, and oximes in the presence of this heteroge-
neous catalyst in excellent yields at room temperature. Transthioac-
etalization of the diacetals successfully was achieved in excellent
yields by this method. The yields of the reactions were excellent and
work-up of the reaction mixtures was easy and not time-consuming.
High chemoselectivity, facile work-up, easily available and cheap silica
chloride, high rates, and yields of the reactions are the strong practical
points of the presented method.
EXPERIMENTAL
General
Silica chloride was prepared in a quantitative yield according to our
previously reported procedure4 by the reaction of thionyl chloride and
silica gel. Silica chloride is a grayish and stable powder that should
be stored in the absence of moisture. IR spectra were recorded on a
1
Perkin-Elmer 781 spectrophotometer. H-NMR and 13C-NMR spectra
were run on a Bruker Avance DPX 250 MHz instrument. Mass spectra
were run on a Shimadzu GC MS-QP 1000 EX. Products are known and

2856
H. Firouzabadi et al.
they were identified by the comparison of their spectral and physical
data with those reported for authentic samples.
General Procedure for Preparation of Diacetal
of 2,2-Bis-(hydroxymethyl)-1,3-propanediol
To a solution of an aldehyde (5 mmol) in CH3CN (25 ml), 2,2-bis-
(hydroxymethyl)-1,3-propanediol (3 mmol) and silica chloride (1.5 g)
were added. The resulting mixture was stirred at room temperature.
After completion of the reaction (12–14 h), the solvent was evaporated
in vacuo. Then CH2Cl2 (75 ml) was added to the mixture and washed
with an aqueous solution of NaOH (10%, 25 ml) and H2O (3 25 ml).
The organic layer was separated and dried over anhydrous MgSO4 and
filtered. Evaporation of the solvent in vacuo gave the desired product
in good to excellent yields. Further purification was achieved by recrys-
tallization in an appropriate solvent (Table I).
Transthioacetalization of Diacetal of 2,2-Bis-
(hydroxymethyl)-1,3-propanediol; A Typical Procedure
To a solution of 4-bromobenzaldehyde (0.37 g, 2 mmol) in CHCl3 (25 ml),
1,3-propanedithiol (2.2 mmol) and silica chloride (0.6 g) were added.
The resulting mixture was stirred at room temperature for 25 min.
After completion of the reaction (monitored by TLC) the reaction was
quenched with an aqueous solution of NaOH (10%, 25 ml). Then CHCI3
(25 ml) was added into the mixture and the organic layer was separated
and washed with H2O (2 25 ml). The organic layer was dried over
anhydrous MgSO4 and filtered. Evaporation of the solvent in vacuo gave
the pure 4-bromo-1,3-dithiane (0.53 g, 92%) as white needle crystals
from petroleum ether; m.p 92–94 C (uncorrected); 1H-NMR (CDCl3,
250 MHz) = 1.89–2.12 (m, 2H), 2.82–3.03 (t, 4H), 5.04 (s, 1H), 7.33
(d, 2H), 7.46 (d, 2H) ppm; 13C-NMR (CDCl3, 63 MHz) = 26.80, 32.30,
51.07, 122.73, 129.95, 132.22, 138.51 ppm; MS (20 eV) m/z (relative
intensity) 276 (M+ + 2, 37.3), 274 (M+ 35.4), 201 (M+-SC2H4, 32.7), 130
(14.6), 105 (40.2), 74 (45.8), 45 (100), CH analysis: %C (calculated =
43.64, found = 43.60), %H (calculated = 4.03, found = 4.10) (Table II,
entry 4i).
General Procedure for Deprotection of Acetals
with SiO2-Cl
To a solution of acetal (2 mmol) in dry CH2Cl2 (10 ml) SiO2-Cl (0.5–1.1 g)
was added. The resulting mixture was stirred at room temperature and

New Applications of Solid Silica Chloride (SiO -Cl)
2857
2
the progress of the reaction was monitored by TLC. After completion of
the reaction (10–40 min), the reaction was quenched with 10% NaOH
aqueous solution (15 ml) and extracted with CH2Cl2 (3 30 ml). The
organic layer was washed with saturated NaCl solution (2 15 ml) and
water (15 ml) and dried over anhydrous Na2SO4. Evaporation of the
solvent under reduced pressure gave almost pure product. Further pu-
rification was achieved by column chromatography on silica gel or re-
crystallization from an appropriate solvent to give the desired product
in good to excellent yields (Table III).
General Procedure for Deprotection of Acylals
in the Presence of Silica Chloride
To a solution of acylal (1 mmol) in dry CH2Cl2 (25 ml), silica chloride
(0.5–0.7 g) was added and the resulting mixture was stirred at room
temperature for the appropriate reaction time. After completion of the
reaction (TLC), silica gel (1 g) was added to the reaction mixture and
the solvent was evaporated under reduced pressure. To the resulting
powder an extra amount of silica gel (2 g) was added and the solid
mixture was washed with petroleum ether (b.p. 60–80 C)/EtOAc (5:1)
(75 ml) and filtered. The solvent was evaporated on a rotary evapo-
rator to afford the desired aldehyde in excellent yields. The partially
volatile aldehydes were isolated as their 2,4-dinitrophenyl hydrazine
derivatives (Table IV).
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