Formation of dioxolane on the surface of silica-sulphuric acid in dry media - Chemoselective protection of aryl aldehydes

Article abstract of DOI:10.1080/00397910701356421

Chemoselective protection of aryl aldehydes can be accomplished through the formation of dioxolane on the surface of silica-sulphuric acid under solvent-free conditions where aliphatic aldehydes, dialkyl ketones, aryl alkyl ketones, and diaryl ketones rem

Full text of DOI:10.1080/00397910701356421

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Formation of Dioxolane on the
Surface of Silica–Sulphuric Acid in Dry
Media—Chemoselective Protection of Aryl
Aldehydes
a
a
a
Manabendra Saha , Sanchita Roy , Subrata Kumar Chaudhuri & Sanjay
a
Bhar
a
Department of Chemistry, Organic Chemistry Section , Jadavpur
University , Kolkata, India
Published online: 22 Jun 2007.
To cite this article: Manabendra Saha , Sanchita Roy , Subrata Kumar Chaudhuri & Sanjay Bhar (2007)
Formation of Dioxolane on the Surface of Silica–Sulphuric Acid in Dry Media—Chemoselective Protection of
Aryl Aldehydes, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 37:12, 1989-1993
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Synthetic Communications , 37: 1989–1993, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701356421
Formation of Dioxolane on the Surface of
Silica–Sulphuric Acid in Dry Media—
Chemoselective Protection of Aryl
Aldehydes
Manabendra Saha, Sanchita Roy, Subrata Kumar Chaudhuri,
and Sanjay Bhar
Department of Chemistry, Organic Chemistry Section,
Jadavpur University, Kolkata, India
Abstract: Chemoselective protection of aryl aldehydes can be accomplished through
the formation of dioxolane on the surface of silica–sulphuric acid under solvent-free
conditions where aliphatic aldehydes, dialkyl ketones, aryl alkyl ketones, and diaryl
ketones remain intact.
Keywords: acetal, chemoselectivity, solvent-free reaction, supported reagent
[1]
Protection of carbonyl groups is a very important organic transformation in
a synthetic sequence for the construction of complicated molecular architec-
ture. Selective protection of one type of carbonyl functionality in the
presence of other types is often a formidable task for a synthetic chemist,
and numerous methods (especially through the formation of the corresponding
[
2]
dioxolanes ) have been reported in the literature. Many of them involve the
use of costly and toxic chemicals, harshly acidic reaction conditions (causing
various side reactions of sensitive moieties), and large amounts of organic
solvents as reaction media most of which are highly toxic. Therefore, there
is a need to develop a new methodology that would utilize relatively less
toxic and readily available acidic reagents in milder conditions as well as
Received in India October 26, 2006
Address correspondence to Sanjay Bhar, Department of Chemistry, Organic
Chemistry Section, Jadavpur University, Kolkata 700 032, India. E-mail:
sanjaybharin@yahoo.com or sssbhar@lycos.com
1
989

1
990
M. Saha et al.
accomplish good chemoselectivity in the course of carbonyl protection in eco-
[
3a]
friendly conditions. Silica–sulfuric acid
times as a mild, heterogeneous, and ecofriendly acid catalyst to accomplish
has been recognized in recent
[3a–3g]
various important organic transformations.
As a part of our continuous
[
4]
effort to develop a new methodology for important and selective organic
reactions, we report herein a mild, cost-effective, chemoselective, and eco-
friendly protocol for the protection of aryl aldehydes through the formation
of the dioxolanes on the surface of silica–sulfuric acid under solvent-free
conditions (Scheme 1). The detailed results are furnished in Table 1.
As shown in Table 1, several aryl aldehydes bearing electron-
withdrawing (entries 2 – and in Table 1) and electron-donating substituents
(
entries 6–8 in Table 1) at various locations smoothly underwent acetalization
in the presence of 1.5 mol equivalents of ethane-1,2-diol on the surface of
silica–sulfuric acid under solvent-free conditions. Interestingly, vinylogous
aryl aldehyde (entry 9 in Table 1), aliphatic aldehyde (entry 12 in Table 1),
dialkyl ketone (entry 13 in Table 1), and aryl alkyl ketones (entries 10 and
11 in Table 1) remained completely unaffected under the present reaction
condition. Diaryl ketones have also been reluctant to produce any ketal in
this condition. It is very important to note that the substrates not producing
any dioxolane in the previously mentioned protocol were recovered totally
unchanged without any side reaction. Therefore, the aforementioned
protocol can be used to protect aryl aldehydes with high chemoselectivity in
the presence of alkyl aldehydes, vinylogous aryl aldehydes, dialkyl ketones,
aryl alkyl ketones, and diaryl ketones. This protocol completely eliminates
the use of toxic and harsh reagents and organic solvents in the reaction
medium. Therefore, it can be considered a relatively greener process in com-
parison to most of the existing methods. Silica–sulphuric acid can be recycled
after product isolation with little variation of yield.
In conclusion, a novel, mild, cost-effective, and highly chemoselective
protocol has been developed in dry media on the surface of silica–sulfuric
Scheme 1.

Chemoselective Protection of Aryl Aldehydes
1991
Table 1. Selective formation of dioxolane from aryl aldehydes and 1,2-ethanediol on
the surface of silica–sulphuric acid under solvent-free conditions
Time
(min)
Yield
(%)
Entry
1
Substrate
Product
60
61
2
45
70
3
4
45
45
71
70
5
6
7
8
30
90
68
64
62
60
120
120
9
No reaction
720
—
1
1
0
1
No reaction
No reaction
720
720
—
—
1
1
2
3
No reaction
No reaction
720
720
—
—

1
992
M. Saha et al.
acid for chemoselective protection of aryl aldehydes in the presence of other
types of oxo- and formyl functionalities.
EXPERIMENTAL
General Procedure for Acetalization of Aldehydes on the Surface of
Silica–Sulfuric Acid
[
3g]
Silica–sulfuric acid (1 g) was added slowly with stirring to a mixture of the
appropriate aldehyde (2 mmol) and freshly distilled ethylene glycol (3 mmol)
at room temperature, and the entire mass was thoroughly mixed to obtain an
easy-flowing powder. After completion of reaction (monitored with thin-layer
chromatography [TLC] on silica gel) the reaction mixture was eluded with
ether or ethyl acetate (10 mL). The organic extract was washed with
saturated aqueous bicarbonate solution (2 Â 10 mL) and dried over
anhydrous sodium sulfate, and the solvent was evaporated to furnish the
product in pure form.
ACKNOWLEDGMENTS
Financial support from Council of Scientific and Industrial Research (CSIR),
New Delhi, Government of India, [Grant No. 01(1673)/00/EMR-II] is grate-
fully acknowledged. M. S. thanks CSIR, New Delhi, for a senior research fel-
lowship and S. R. thanks University Grants Commission (UGC), New Delhi,
Government of India, for a senior research fellowship. The authors express
profound thanks to N. Dutta of Indian Association for the Cultivation of
Science (IACS), Jadavpur, Kolkata, India for assistance.
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1993
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