Chemoselective deprotection of thioacetals/thioketals using HIO3 in the presence of wet SiO2 under mild solvent-free conditions

Article abstract of DOI:10.1080/104265090921146

HIO₃ in the presence of wet SiO₂ is highly efficient and a mild reagent for selective deprotection of a variety of thioacetals/thioketals to the corresponding parent carbonyl compounds at room temperature and under solvent-free conditions. Copyright Taylor & Francis Inc.

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Chemoselective Deprotection
of Thioacetals/Thioketals Using
HIO in the Presence of Wet
3
SiO Under Mild Solvent-Free
2
Conditions
a
a
Moslem M. Lakouraj , Mahmood Tajbakhsh
,
b
b
Farhad Shirini & Mohammad V. Asady Tamami
a
Department of Chemistry , University of
Mazandaran , Babolsar, Iran
b
Department of Chemistry , Guilan University ,
Rasht, Iran
Published online: 16 Aug 2006.
To cite this article: Moslem M. Lakouraj , Mahmood Tajbakhsh , Farhad Shirini &
Mohammad V. Asady Tamami (2005) Chemoselective Deprotection of Thioacetals/
Thioketals Using HIO in the Presence of Wet SiO Under Mild Solvent-Free Conditions,
3
2
Phosphorus, Sulfur, and Silicon and the Related Elements, 180:11, 2423-2429, DOI:
0.1080/104265090921146
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Phosphorus, Sulfur, and Silicon, 180:2423–2429, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090921146
Chemoselective Deprotection of Thioacetals/Thioketals
Using HIO in the Presence of Wet SiO Under Mild
3
2
Solvent-Free Conditions
Moslem M. Lakouraj
Mahmood Tajbakhsh
Department of Chemistry, University of Mazandaran, Babolsar, Iran
Farhad Shirini
Department of Chemistry, Guilan University, Rasht, Iran
Mohammad V. Asady Tamami
Department of Chemistry, University of Mazandaran, Babolsar, Iran
HIO in the presence of wet SiO is highly efficient and a mild reagent for selec-
3
2
tive deprotection of a variety of thioacetals/thioketals to the corresponding parent
carbonyl compounds at room temperature and under solvent-free conditions.
Keywords Carbonyl compounds; deprotection; HIO3/wetSiO2; solvent-free; thioacetals/
thioketals
Carbonyl compounds often are protected as thioacetals and thioke-
1
tals in organic synthesis, particularly in multistep natural product
2
synthesis, due to their stability in both acidic and basic conditions.
Several procedures are available in the literature for preparation of
3
thioacetals and thioketals, but their deprotection to the parent car-
bonyl compounds is not always an easy process. Many of the depro-
4
tection reactions require strong acidic or oxidizing conditions using
corrosive or toxic reagents.5
In addition, some chemical methods often require the use of heavy
metal reagents such as Hg2+, Ag , Tl , Cd , and Se , which are in-
+
3+
6
2+
4+
herently toxic and/or expensive to use. In view of economical, practical,
and recent environmental demands, there is a need for a simple, less
Received August 24, 2004; accepted October 26, 2004.
We are thankful to the Mazandaran University Research Council for their partial
support of this work.
Address correspondence to Moslem M. Lakouraj, University of Mazandaran, Depart-
ment of Chemistry, Faculty of Science, Babolsar 47415, Iran. E-mail: Lakouraj@umz.ac.ir
2423

2
424
M. M. Lakouraj et al.
expensive, and safer method for deprotection of thioacetals and thioke-
tals. On the other hand, recently, there has been an increasing interest
in reactions that proceed in the absence of solvents due to their reduced
7
pollution, low costs, and simplicity in process and handling. More re-
cently, we have introduced HIO3 as a mild, inexpensive, and efficient
reagent for the coupling of thiols and convenient regeneration of car-
8
bonyls from oximes and hydrazones. We, herein, explored the utility
of this reagent for selective deprotection of thioacetals and thioketals
in the presence of wet SiO2 under solvent-free condition (Scheme 1).
This reagent is readily prepared by adding HIO3 to wet SiO2, which
is stable and could be stored for months without any activity lost at
room temperature. This easily handled reagent allows the conversion
of thioacetals and thioketals to the corresponding carbonyl compounds
under mild conditions.
SCHEME 1
RESULTS AND DISCUSSION
As shown in Table I, a wide variety of thioacetals and thioketals are eas-
ily deprotected to the corresponding carbonyl compounds in high yields.
The deprotection of 2,2-diphenyl-1,3-dithiane as the model compound
was examined under solvent-free conditions at room temperature. The
optimum ratio of thioacetals and thioketals to reagent (1:1) is found to
be ideal for complete conversion of thioacetals and thioketals to aldehy-
des and ketones. By using this reagent, a variety of alkyl aryl, dialkyl,
and diaryl thioacetals and thioketals were transformed to their corre-
sponding aldehydes and ketones.
As evident from the results in Table I, most reactions proceed im-
mediately and give excellent yields of products (entries 1, 2, 4–13).
Electron-withdrawing groups such as NO2 on the aromatic ring de-
crease the reaction rate (entry 3). We have also carried out this reac-
tion in solvents such as dichloromethane, n-hexane, acetonitrile, and
diethyl ether; the results show that the reaction proceeds more slowly
and the yields are less than satisfactory even after hours.

Selective Deprotection of Thioacetals/Thioketals
2425
TABLE I Deprotection of Thioacetals/Thioketals With HIO
3 2
/Wet SiO
Under Solvent-Free Conditionsa
m.p. or b.p. m.p. or b.p.
Time byields
( C)
◦
( C)
◦
1
2
Entry
1
Substrat
Products
(sec)
(%)
Found
Reported
50
95
48–50
48.5–49
212–214
2
75
93
212–213
3
4
5
520
50
90
94
93
104.5–106
34–36.5
106
35–37
106.10
55
105–106
6
7
25
92
90
—
113–116/6
mm
130
81.5–83
81–83
8
9
60
45
92
94
196–198
196–199
228–230
227.5–230
(
Continued on next page)

2
426
M. M. Lakouraj et al.
TABLE I Deprotection of Thioacetals/Thioketals With HIO
3
/Wet
a
SiO
2
Under Solvent-Free Conditions (Continued)
m.p. or b.p. m.p. or b.p.
Time byields
( C)
◦
( C)
◦
1
2
Entry
Substrat
Products
(sec)
(%)
Found
Reported
1
0
30
90
46–47
47
1
1
47
93
247–248
248
1
1
2
3
40
25
93
94
59–61
59–60
153
153–155
aReactions are carried out at room temperature using substrate/reagent (1:1) molar
ratio.
bYields of isolated pure products.
We also have investigated the selective deprotection of thioacetals
and thioketals in the presence of sulfides, acetals, and ketals. When one
equimolar amount of a thioacetal or a thioketal is mixed with one equiv-
alent of HIO3/wet SiO2, only thioketal or thioacetal is transformed to
its carbonyl compound and the sulfide remained intact (Table II, entries
1
–4). Similarly, when 2,2-diphenyl-1,3-dithiane in the presence of ac-
etal and ketal was treated with HIO3/wet SiO2, only benzophenone was
formed and the acetal or ketal remained unchanged (Schemes 2 and 3).
SCHEME 2

Selective Deprotection of Thioacetals/Thioketals
2427
TABLE II The Competitive Reactions Between Deprotection of the
Thioacetals and Thioketals, with Oxidation of Sulfides to Sulfoxides
Using HIO
3
/Wet SiO
Under Solvent-Free Conditiona
2
Entry
1
Substrate
Products
Time (s)
Yields (%)
50
95 + 0
2
3
4
75
93 + 0
93 + 0
92 + 0
55
25
aReactions are carried out at room temperature using substrate/reagent (1:1) molar
ratio.
SCHEME 3

2
428
M. M. Lakouraj et al.
TABLE III Comparison of Our Method With Some of Those
Methods Reported in the Literature for Deprotection of
2
,2-diphenyl-1,3- dithiane(thioketal)13
Reagent/(Molar Ratio of
Yield (%)/
Entry
Substrate/Reagent)
Condition
Time (min)
1
3a
1
2
PhCH2Ph3PHSO5/BiCl3 (1:2)
Fe(NO3)3/Montmorillonite
K.10 (1:1)
IBX/ß-CD, (1:1)
TT, (1:0.5)
CH3CN, reflux
Hexane, 50 C
95/180
98/10
◦
13b
3
4
5
6
H2O, r.t.13c
DMSO, r.t.
88/240
96/150
90/45
1
3d
◦
13e
2
NaNO2,(1:1)
AcCl/H2O, 0–5 C CH2Cl
HIO /wet SiO (1:1)
Solvent-free, r.t.
95/0.83
3
2
Deprotection of acetals and ketals need longer reaction time and give a
low yield of the corresponding carbonyl compound.
In order to show the advantages of the method, we have compared
the results of deprotection of 2,2-diphenyl-1,3-dithiane(thioketal) with
some of those reported in the literature (Table III). As indicated in
Table III, our method is very simple and gives excellent yields of prod-
ucts immediately after mixing the starting materials together.
In conclusion, We have introduced a new application for HIO3 in
the presence of wet SiO2. A high yield of the products, the absence of
the toxic waste disposal, short reaction times, mild reaction conditions,
and selectivity of the method make this protocol an alternative to the
existing methods.
EXPERIMENT
Chemicals were purchased from Merck, Fluka, and Aldrich Chemical
companies. The carbonyl derivatives were prepared from the corre-
sponding carbonyl compounds and 1,3-propanedithiol according to the
9
reported procedures. Products were separated and purified by differ-
ent chromatographic techniques and were characterized by comparison
1
of their spectral (IR, H-NMR, and TLC) and physical data (m.p. and
10−12
b.p.) with those of authentic samples.
General Procedure
To a mixture of HIO3, (0.176 g. 1 mmol) and wet SiO2 [(SiO2/H2O;
0% ww), 0.08 g], was added thioacetal or thioketal (1 mmol). The
resulting mixture was mixed throughly at room temperature for the
2

Selective Deprotection of Thioacetals/Thioketals
2429
specified time (Table I). The reaction was monitored by TLC. After com-
pletion of the reaction, dichloromethane or diethyl ether (5 mL) and
sodium thiosulfate solution were added to the mixture and the resultant
mixture was filtered after 15 min. Anhydrous Na2SO4 was added to the
filtrate and filtered. Evaporation of the solvent gave the corresponding
aldehydes and ketones.
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