4
770
N. Iranpoor et al. / Tetrahedron Letters 44 (2003) 4769–4773
1
13
Table 1. Conversion of 2-phenyl-1,3-dithiane (1.0 mmol)
(m 147), and H and C NMR spectral data with
to benzaldehyde with various catalysts (NBS, NCS,
23–25
those reported in the literature.
spectral data and GC retention time were also com-
pared with that of sample obtained from
intramolecular coupling of 1,3-propandithiol. In the
work-up, due to the polymerization of this com-
pound, it can be easily removed from the reaction
mixture by chromatography.
In addition, its
a
TABCO, TCCA, Br and I ) in the presence of DMSO in
2
2
chloroform at room temperature
a
2
5
Yieldb (%)
Entry
Catalyst (molar
equiv.)
Time (min)
1
2
3
4
5
6
7
8
9
NBS (0.05)
NBS (0.2)
TABCO (0.05)
TABCO (0.2)
NCS (0.05)
NCS (0.2)
TCCA (0.1)
TCCA (0.2)
Br2 (0.1)
45
5
60
7
75
10
60
30
10
5
94
98
93
95
93
97
95
95
98
95
35
We also studied selective dethioacetalization in the
presence of different protected carbonyl groups using
TABCO, NBS or Br2 as catalysts. All the catalysts
showed similar results and high to excellent selectivity
was observed. The results of selective deprotection of
dithioacetals and 1,3-oxathiolanes in the presence of
acyclic acetals, cyclic acetals and diacetates (acylals)
in 1:1 mixtures using NBS as a representative catalyst
are shown in Table 3.
1
1
0
1
Br2 (0.2)
I2 (0.2)
24 h
a
The molar ratio of DMSO/2-phenyl-1,3-dithiane is 5/1.
Isolated yield.
b
Using this method, 2-phenyl-1,3-dithiane was depro-
tected in the presence of benzaldehyde dimethyl acetal
with a ratio of 100/5 (Table 3, entry 1). When 2-
phenyl-1,3-dioxane was used which is more stable
than benzaldehyde dimethyl acetal, again excellent
chemoselectivity was observed with a ratio of 100/2
and TCCA (Table 1, entries 4–8), the yields are more
or less the same. We also examined the possibility of
using molecular bromine and iodine as catalyst for
this reaction (Table 1, entries 9–11). Our findings
showed that the conversion of 2-phenyl-1,3-dithiane
to benzaldehyde with catalytic amounts of bromine
occurs in a similar manner to those reactions using
NBS and TABCO as catalyst (Table 1, entries 9, 10).
However, the reaction with iodine is very slow and
was not completed (Table 1, entry 11).
(
Table 3, entry 2). 2-Phenyl-1,3-oxathiolane was
deprotected in the presence of 2-(4-nitrophenyl)-1,3-
dioxane with high chemoselectivity of 100/17 (Table
3
, entry 3). 2-Phenyl 1,3-dithiolane was also depro-
tected in the presence of 4-methyl-benzyl diacetate
with a ratio of 100/23 (Table 3, entry 4). The same
reaction in the presence of 4-nitro benzyldiacetate
occurred with a high selectivity ratio of 100/5 (Table
3
, entry 5).
We therefore chose TABCO, NBS, and Br and con-
2
tinued our study on the deprotection of different
dithioacetals and also 1,3-oxathioacetals derived from
carbonyl compounds carrying no enolizable hydrogen.
The results obtained are presented in Table 2.
Thioacetals and 1,3-oxathioacetals derived from eno-
lizable carbonyl compounds under similar reaction
conditions were converted to their ring-expanded-
brominated products and are currently under further
investigation.
In conclusion, in this method, the use of electrophilic
halogens provides an efficient, novel and mild proce-
dure for the deprotection reactions of cyclic and
acyclic dithioacetals and ketals and also 1,3-
oxathioacetals. In addition to the selectivity of the
method for the deprotection of S,S- and S,O-acetals
in the presence of O,O-analogs, the low cost and
availability of the reagents, simplicity of the method,
short reaction times, and excellent yields can also be
considered as strong points for this method.
In Scheme 2, the suggested mechanism for dethioac-
etalization using TABCO as catalyst is shown. Bromi-
nation of I occurs to produce the intermediate II,
which then reacts with DMSO to give the sulfenyl
bromide intermediate III. This is in equilibrium with
General procedure for the deprotection of dithioacetals
and 1,3-oxathioacetals to their parent carbonyl
compounds:
23
sulfonium and bromide ions in polar solvents and
produces the corresponding carbonyl compound and
Dithioacetal or 1,3-oxathioacetal (1.0 mmol) was
added to a stirred solution of 0.05–0.3 mmol of cata-
1
,2-dithiacyclopentane IV together with the formation
lyst (NBS, TABCO, or Br ) in 3 ml of chloroform
2
of intermediate V. In this reaction, the formation of
(Table 2). Dimethyl sulfoxide (5 mmol) was then
added to this mixture. The reaction was monitored by
thin layer chromatography (silica-gel/n-hexane) and
GC analysis. Upon completion of the reaction, 25 ml
of chloroform was added to the reaction mixture. The
solution was washed with cold 5% NaOH (20 ml)
followed with brine solution (10 ml) and water (2×10
ml). The organic phase was separated and dried with
Br possibly through the reaction of intermediate III
2
and V was also observed and which could also be
incorporated in the continuation of the catalytic cycle.
The easily polymerizable 1,2-dithiacyclopentane IV,
which must be handled only in dilute solution, was
isolated from the reaction mixture and identified by
the comparison of its UV absorption band at 330 mm