LETTER
263
Highly Efficient Transdithioacetalization of Acetals Catalyzed by Silica
Chloride
Habib Firouzabadi,*a Nasser Iranpoor,*a Babak Karimi,b Hassan Hazarkhania
a Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran.
b Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P. O. Box 45195-159, Gava Zang, Zanjan, Iran
Fax +98(071)20027; E-mail: firouzabadi@chem.susc.ac.ir
Received 25 October 1999
The present method is applicable to both open-chain ace-
tals (dimethyl and diethyl acetals) and cyclic acetals (1,3-
dioxanes and 1,3-dioxolanes). As shown in the Table,
open-chain acetals derived from aromatic and aliphatic
carbonyl compounds on treatment with silica chloride
(0.2g, 0.4 mmol) and thiols (2.2mmol) or dithiols
(1.1mmol) were all cleanly and rapidly thioacetalized giv-
ing the corresponding S,S-acetal in good to excellent
yields (Table, entries 1-6). Both 1,3-dioxanes and 1,3-di-
oxalanes on treatment with thiols and the silica chloride
were also furnished the corresponding dithioacetals in
good to excellent yields (Table, entries 7-13). By this
method sterically hindered acetals such as 1,2-diphenyl-
1,3-dioxalane were also successfully converted to 2,2-
diphenyl-1,3-dithiane in excellent yield (Table, entry 13).
Our studies show that this type of silica chloride is much
more reactive8 than that reported by Hojo et al5g, in which
a lesser number of silanol groups were replaced by chlo-
rine.9 We have used TMSCl as a catalyst for this purpose.
Our observation was that when electron withdrawing
groups are attached to the aromatic rings the rate of the re-
actions were much slower than that observed with the sil-
ica chloride. Another problem with using TMSCl is its
low boiling point which puts some restriction on its use at
high temperatures. Chemoselective catalytic activity of
the catalyst is demonstrated by transthioacetalization of
acetals in the presence of ketones. e.g. acetophenone di-
ethylacetal (1) is preferentially protected in the presence
of acetophenone (2) in a quantitative yield (Scheme 2).
Similarly, 2-methyl-2-(2-phenylethyl)-1,3-dioxane (3) in
the presence of benzylacetone (4) is converted to the cor-
responding 1,3-dithianes almost exclusively (Scheme 2).
Abstract: A modified procedure for the preparation of a silica chlo-
ride with higher chloride capacity than that reported is described.
Moreover, this silica chloride was found to be an effective catalyst
for chemoselective and highly efficient transdithioacetalization of
different classes of acetals.
Key words: silica chloride, thioacetals, protection, transdithioace-
talization, acetals
Protection of carbonyl compounds as dithioacetals or ace-
tals is an indispensable part of the synthesis of many poly-
functional molecules.1 Moreover, dithioacetals often
serve as precursors of acyl anion equivalent displaying a
reactivity umpolung,2 and masked methylene functions.3
Several types of methods that are reported for the prepa-
ration of dithioacetals include the use of some silicon re-
agents,1 protic acids,1 Lewis acids,1,4 solid inorganic acids
and supported reagents.5 Transdithioacetalization of ace-
tals has been used as an alternative method for the prepa-
ration of dithioacetals.6 Due to many useful properties of
solid reagents such as ease of handling, low costs, and
ease of work up of the reaction products, the use of hetero-
geneous reagents for functional group transformations is
of value to synthetic organic chemists.7 Nevertheless, a
literature survey shows that less attention has been paid to
the use of heterogeneous reagents in transdithioacetaliza-
tion reactions. In this communication we first introduce a
modified procedure for the preparation of a high surface
and efficient silica chloride by refluxing silica gel (for
plate chromatography) with thionyl chloride (SOCl2) for
48 h.8 In the subsequent studies we have found that this
chloride can act as a highly efficient and reactive hetero-
geneous catalyst for transdithioacetalization of different
types of acetals (Scheme1).
In summary, high rates and yields of the reactions, facile
work up, low cost, easy availability of the catalyst and
high chemoselectivity of the reaction are the strong prac-
tical points of the presented method. Further investigation
for the use of this type of silica chloride, as a reagent or
catalyst, is underway in our laboratory.
Preparation of Silica Chloride; To an oven-dried
(120°C,vacuum) silica gel (10g) in a round bottomed flask
(250mL) equipped with a condenser and a drying tube,
was added thionyl chloride (40mL) and refluxed for 48h.
The unreacted thionyl chloride was distilled off. The re-
sulting white-grayish powder was flame-dried and stored
in a tightly capped bottle. This silica chloride can be used
for months without loosing its activity.
Scheme 1
Synlett 2000, No. 2, 263–265 ISSN 0936-5214 © Thieme Stuttgart · New York