N-benzyl-DABCO-tribromide as an efficient and mild reagent for deprotection of dithioacetals

Article abstract of DOI:10.1080/00397910500498952

N-Benzyl-DABCO-ammonium tribromide was found to be an efficient and recyclable reagent for the deprotection of dithioacetals in dichloromethane/ methanol at room temperature. The reaction can be performed cleanly, in short time, and in high yield. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500498952

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N‐Benzyl‐DABCO‐tribromide as an efficient
and mild reagent for deprotection of
dithioacetals
Firouz Matloubi Moghaddam a , Hassan Zali Boeini a , Dordaneh Zargarani a
Ghasem Rezanejad Bardajee a
&
a Sharif University of Technology, Department of Chemistry , Tehran, Iran
Published online: 16 Aug 2006.
To cite this article: Firouz Matloubi Moghaddam , Hassan Zali Boeini , Dordaneh Zargarani & Ghasem
Rezanejad Bardajee (2006) N‐Benzyl‐DABCO‐tribromide as an efficient and mild reagent for deprotection of
dithioacetals, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 36:8, 1093-1096, DOI: 10.1080/00397910500498952
To link to this article: http://dx.doi.org/10.1080/00397910500498952
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Synthetic Communicationsw, 36: 1093–1096, 2005
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500498952
N-Benzyl-DABCO-tribromide as an efficient
and mild reagent for deprotection of
dithioacetals
Firouz Matloubi Moghaddam, Hassan Zali Boeini,
Dordaneh Zargarani, and Ghasem Rezanejad Bardajee
Sharif University of Technology, Department of Chemistry, Tehran, Iran
Abstract: N-Benzyl-DABCO-ammonium tribromide was found to be an efficient and
recyclable reagent for the deprotection of dithioacetals in dichloromethane/methanol
at room temperature. The reaction can be performed cleanly, in short time, and in
high yield.
Keywords: Deprotection, dithioacetal, N-benzyl-DABCO-tribromide
INTRODUCTION
Protecting groups are useful tools in organic synthesis, and among them
dithioacetals are very important because of their stability under basic and
acidic conditions and also their use as acyl carbanion equivalents in organic
synthesis.[1] However, the main problem of thioacetals is the safe deprotection
and generation of the corresponding carbonyl compounds. Therefore, several
methods for the deprotection of dithioacetal have been reported in the litera-
ture.[2] The heavy metal salts such as mercury(II) chloride,[3] mercury[II]
oxide, boron trifluoride-diethyl ether,[4] cerium(IV) ammonium nitrate,[5]
selenium dioxide,[6] silver nitrate–iodine,[7] and bismuth nitrate[8] have
been used for the deprotection of dithioacetals. These reagents are very
toxic, and they are not ecofriendly. Recently benzyl triphenylphosphonium
Received in the U.K. June 14, 2005
Address correspondence to Firouz Matloubi Moghaddam, Sharif University of
Technology, Department of Chemistry, P.O. Box 11365-9516, Tehran, Iran. E-mail:
matloubi@sharif.edu
1093

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F. M. Moghaddam et al.
tribromide[9] and 1-benzyl-4-aza-1-azoniabicyclo [2.2.2] octane periodate[10]
have also been used for the deprotection of dithioacetals to the corresponding
carbonyl compounds. Herein we report a mild and efficient method for the
deprotection of dithioacetals using N-benzyl-DABCO-tribromide.
RESULTS AND DISCUSSION
Organic ammonium tribromides (OATB) have been extensively used in
organic synthesis as a mild brominating agent[11] and in the oxidative
cyclization of thioamides to benzothiazoles.[12] We have found that
N-benzyl-DABCO-tribromide is an effective reagent for the deprotection of
dithioacetals to the corresponding carbonyl compounds (Scheme 1). Interest-
ingly, this reagent selectively cleaves the S, S-acetals, and the O, O-acetals, are
not affected by this reagent.
In a typical procedure dithioacetal (1 mmol) is dissolved in a mixture of
CH2Cl2/MeOH (5 ml, 50:50) and treated with N-benzyl-DABCO-tribromide
(1 mmol) at room temperature. After conventional aqueous workup the corre-
sponding carbonyl compounds were obtained in high yields (85–98%) and in
short reaction times (3–10 min). The generality of the method was confirmed
by the successful conversion of fourteen quite different substrates to
the carbonyl compounds, and the results are summarized in Table 1. The
reaction proceeds easily and cleanly, and no overoxidation to the correspond-
ing carboxylic acid is observed. Another advantage of this methodology is
the regeneration of the reagent. After the workup, the aqueous layer of the
reaction mixture was washed with chloroform and treated with bromine.
Then the precipitated N-benzyl-DABCO-tribromide was filtered, washed
twice with water (10 ml), and dried under vacuum. It is also notable that no
bromination takes places either at the activated aromatic ring (Table 1,
entries 3, 6) or on the a-carbon in ketones (Table 1, entries 6, 14).
In conclusion a versatile reagent has been developed for the rapid, safe,
and selective deprotection of dithioacetals to the corresponding carbonyl
compounds under mild reaction conditions. This reagent is introduced by
our group for the first time, and its preparation is rather simple.[12b] Also,
the reagent can be regenerated, and the methodology has the advantages of
Scheme 1.

N-Benzyl-DABCO-Tribromide
1095
Table 1. Deprotection of dithioacetals to the corresponding carbonyl compounds
using N-benzyl-DABCO-tribromide
Time
(min)
Yield
(%)a
Substrate
R1
R2
R3
1
2
2-NO2C6H4
4-NO2C6H4
4-MeOC6H4
Ph
H
22CH2CH2CH222
22CH2CH2CH222
22CH2CH2CH222
22CH2CH222
22CH2CH222
22CH2CH222
22CH2CH222
22CH2CH222
22CH2CH222
PhCH222
3
3
98
98
90
95
90
89
85
91
97
95
87
93
92
86
H
3
H
7
4
H
5
5
4-ClC6H4
4-MeOC6H4
4-BrC6H4
H
CH3
5
6
8
7
H
8
8
PhCH55CH22
CH3(CH2)522
(CH 3)2CHCH222
CH3CH222
Ph
H
5
9
H
6
10
11
12
13
14
H
CH3CH222
8
PhCH2-
PhCH222
PhCH222
PhCH222
10
8
H
H
4-NO2C6H4
4-NO2C6H4
6
10
CH3
aAll yields refer to pure isolated products.
high yield, short reaction time, inexpensive reagent, and nontoxicity compared
to the reported methods.
EXPERIMENTAL
The compounds were characterized by comparison of their spectroscopic data
1
with authentic samples (IR, H NMR spectrum, and TLC analysis). FT IR
spectra were recorded as KBr pellets on a Nicolet spectrometer (Magna
1
550). A Bruker (DRX-500 Avance) NMR was used to record the H NMR
spectra. All 1H NMR spectra were determined in CDCl3-d6 at ambient temp-
erature. N-benzyl-DABCO-tribromide was prepared as reported in Ref. 12b.
General Procedure for the Deprotection of Dithioacetals
N-benzyl-DABCO-tribromide and appropriate dithioacetal (1 mmol) were
dissolved in a mixture of CH2Cl2/MeOH (5 ml, 50:50). The reaction

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F. M. Moghaddam et al.
mixture was stirred at room temperature for the time noted in Table 1 and
monitored by TLC (silica gel; ethyl acetate–hexane, 1:5). Then the reaction
mixture was poured in water (50 ml) and extracted with CH2Cl2 (20 ml).
The extract was washed with water (2 Â 20 ml) and dried over magnesium
sulfate, and the solvent was removed under reduced pressure to obtain pure
product.
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