Facile synthesis of substituted dihydro-1,4-dithiins and -1,4-dithiepins from α-oxo ketene cyclic dithioacetals

Article abstract of DOI:10.1016/j.tetlet.2005.08.128

A novel and facile synthesis of substituted 2,3-dihydro-1,4-dithiins and 6,7-dihydro-5H-1,4-dithiepins based on the reactions of α-bromo/hydroxy ketones with α-oxo ketene cyclic dithioacetals has been developed. A general mechanism for the reactions is pr

Full text of DOI:10.1016/j.tetlet.2005.08.128

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7331–7335
Facile synthesis of substituted dihydro-1,4-dithiins
and -1,4-dithiepins from a-oxo ketene cyclic dithioacetals
Dewen Dong,* Ran Sun, Haifeng Yu, Yan Ouyang, Qian Zhang and Qun Liu*
Department of Chemistry, Northeast Normal University, Changchun 130024, PR China
Received 1 July 2005; revised 25 August 2005; accepted 25 August 2005
Available online 12 September 2005
Abstract—A novel and facile synthesis of substituted 2,3-dihydro-1,4-dithiins and 6,7-dihydro-5H-1,4-dithiepins based on the reac-
tions of a-bromo/hydroxy ketones with a-oxo ketene cyclic dithioacetals has been developed. A general mechanism for the reactions
is proposed.
Ó 2005 Published by Elsevier Ltd.
Thioacetals have been extensively investigated as car-
bonyl protection groups and versatile intermediates in
the synthesis of multi-functional complex molecules
and natural products.1,2 To date, a variety of methods
are available for the preparation of thioacetals from car-
bonyl compounds or O,O-acetals with thiols employing
various acidic catalysts.3,4 However, the high stability
of thioacetals makes their deprotection requiring drastic
conditions or toxic reagents such as mercury salts,
heavy metals, ceric ammonium nitrate (CAN), SeO2, or
Pb(OAc)4.5–7 Recently, a number of Lewis acids such
as Bi(NO3)3, clay–Fe(NO3)3, and some nonmetallic
reagents including the oxide of nitrogen, triethyloxonium
tetrafluoroborate, and methyl fluorosulfonate have also
been applied for the selective deprotection of dithioace-
tals.8–10 During the course of these studies, a range of
ring-expansion reactions of 1,3-dithiolanes and 1,3-dithi-
anes to produce substituted dihydro-1,4-dithiins and
-1,4-dithiepins had been achieved in the presence of
TeCl4, WCl6, MoCl5, SiO2–Cl, o-iodoxybenzoic acid,
m-chloroperbenzoic acid, 2,4,6-trichloro-1,3,5-triazine,
halogens (Br2 and Cl2), or N-halosuccinimides (NBS,
NCS, and NIS) (Scheme 1).11–13 Indeed, 2,3-dihydro-
1,4-dithiins were reported to undergo oxidation easily,
affording dienophiles for the convenient use in Diels–
Alder reactions.14 Also, 2,3-dihydro-1,4-dithiins have
proven to be useful precursors to mimic cis-configurated
double bonds, in the preparation of simple alkenes and
other unsaturated compounds as well.15 Additionally,
some derivatives of the dihydro-1,4-dithiins and -1,4-di-
thiepins show activities as nonpeptide antagonists of
human Galanin Hgal-1 receptor.16
Recently, we developed a novel thioacetalization reaction
using nonthiolic odorless cyclic ketene dithioacetals, for
example,
2-(2-chloro-1-(1-chloroethenyl)-2-propenyl-
idene)-1,3-dithiane and 3-(1,3-dithian-2-ylidene)pentane-
2,4-dione, as 1,3-propanedithiol equivalents.17 During
the course of our studies on the thioacetalization of
a-bromo/hydroxy carbonyl compounds using 2-(1,3-
dithiolan/dithian-2-ylidene)-3-oxobutanoic acids 1a/1b
as dithiol equivalents, surprisingly, we found that the
n
n
n
O
S
S
N-halosuccinimide
HS
SH
S
S
R1
CH3
Catalyst
R1
R1
CH3
X
X = H, Cl, Br, I
n = 1, 2
Scheme 1.
Keywords: Acetyl chloride; 2,3-Dihydro-1,4-dithiins; 6,7-Dihydro-5H-1,4-dithiepines; a-Oxo ketene cyclic dithioacetals.
*
Corresponding authors. Tel.: +86 431 5099759; fax: +86 431 5098966 (D.D.); e-mail addresses: dongdw663@nenu.edu.cn; liuqun@nenu.edu.cn
0040-4039/$ - see front matter Ó 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.08.128

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D. Dong et al. / Tetrahedron Letters 46 (2005) 7331–7335
main products were substituted dihydro-1,4-dithiins and
-1,4-dithiepins. We herein wish to report the new findings
on these investigations.
the cleavage of 1a and the subsequent reaction with 2a
in methanol (entries 1–4). The reaction rate is signifi-
cantly speeded up when increasing the feed ratio of ace-
tyl chloride/1a. It appears that there might be a critical
ratio for the reaction, in other words, the reaction rate
changes very slightly beyond the ratio. The high reaction
rate is attained when the reaction proceeds with a 1:5
molar ratio of 1a/acetyl chloride (entry 3). An appropri-
ately excess of 1a to 2a can result in slightly higher yield
(entries 3 and 5). The reaction can proceed in ethanol to
afford 3a (entry 7), but fails in benzene, THF, or CH2Cl2
(entries 8–10). For the comparison with acetyl chloride,
other acidic reagents such as HCl(aq) and H3PO4 were
selected and employed in the novel reactions (entries
11 and 12). The results reveal that acetyl chloride is
the most effective reagent among those examined. It
should be mentioned that only very faint odor of dithiol
can be perceived during both reaction and work-up
process.
There are many reports including some review articles
available regarding the synthesis and application of a-
oxo ketene dithioacetals.18 According to the procedure
described in the literature,19 ethyl 2-(1,3-dithiolan/
dithian-2-ylidene)-3-oxobutanoate were prepared from
ethyl 3-oxobutanoate, carbon disulfide, 1,2-dibromo-
ethane/1,3-dibromopropane in the presence of K2CO3
in nearly quantitative yields (99%). They were then con-
verted into 1a and 1b via a hydrolysis process, respec-
tively.18 It is worth noting that compounds 1a and 1b
are odorless solids and stable under ambient atmo-
sphere. Also they are associated with some synthetic
advantages including simple procedure, mild conditions,
high yields and commercial starting materials without
foul smell.
The initial study was performed on the reaction between
1a and 2-bromo-1-phenylethanone 2a (Table 1, entry 1)
via a very simple procedure described as following: 1a
(1.0 mmol), 2a (1.0 mmol), methanol (10 mL), and ace-
tyl chloride (1.5 mmol) were added into a flask equipped
with a condenser. The mixture was heated to reflux and
stirred for about 10 h when 2a was consumed as indi-
cated by TLC. The reaction mixture was cooled to ambi-
ent temperature and neutralized with 10% aq NaHCO3.
After extractive work-up, chromatography over silica
gel (eluent: petroleum ether) afforded a pure product
in 56.5% yield, which was identified as 5-phenyl-2,3-
dihydro-1,4-dithiin 3a (mp: 59–61 °C).
Under the conditions described in Table 1 (entry 5), a
range of reactions were performed on a variety of a-bro-
mo ketones 2 with compounds 1a and 1b.20 All reactions
proceed smoothly under the essentially mild acidic con-
ditions to afford the corresponding substituted dihydro-
1,4-dithiins and -1,4-dithiepins 3b–k in good yields, and
some results are listed in Table 2. The results exhibit the
scope and generality of the reaction with respect to dif-
ferent a-bromo ketones 2. To extend the scope of this
novel protocol, the reactions of 1 with other carbonyl
compounds were examined. To our delight, when 2-hy-
droxy-1,2-diphenylethanone was subjected to identical
conditions, the substituted dihydro-1,4-dithiins and
-1,4-dithiepins 3l and 3m were obtained in 72.5% and
70.1% yields, respectively (Table 2, entries 12 and 13).
Therefore, we present here a convenient and facile pro-
tocol for the synthesis of dihydro-1,4-dithiins and -1,4-
dithiepins. To the best of our knowledge, this method
To optimize the yield of 3a, a range of reactions between
1a and 2a were carried out under various conditions,
and some results are summarized in Table 1. Appar-
ently, the amount of acetyl chloride affects the rate of
Table 1. Reactions between 2-(1,3-dithiolan-2-ylidene)-3-oxobutanoic acid 1a and 2-bromo-1-phenylethanone 2a
O
O
O
Acidic reagent
MeOH
S
Br
OH
+
S
S
Reflux
S
1a
2a
3a
Entry
Acidic reagent
1a/2a/Acidic reagenta
Solvent (h)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
HCl(aq)
2:2:3
1:1:3
1:1:5
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
Benzene
THF
CH2Cl2
MeOH
MeOH
10.0
8.0
5.0
4.5
5.0
5.0
8.0
5.0
5.0
5.0
5.0
5.0
56.5
58.1
58.2
59.4
60.1
61.3
47.7
0
0
0
26.8
0
1:1:10
1.2:1:5
2:1:5
1.2:1:5
1.2:1:5
1.2:1:5
1.2:1:5
1.2:1:5
1.2:1:5
9
10
11
12
H3PO4
a Molar ratio.
b Isolated yields after silica gel chromatography.

D. Dong et al. / Tetrahedron Letters 46 (2005) 7331–7335
7333
Table 2. The reactions of a-bromo/hydroxy ketones 2 with 2-(1,3-dithiolan/dithian-2-ylidene)-3-oxobutanoic acids 1
O
O
S
n
X
MeCOCl
MeOH
O
S
S
OH
n
+
R2
S
R1
Reflux
R1
R2
1
2
3
Entry
Substrates 1, 2
Time (h)
Product 3
Mp (°C)
Yielda (%)
n
X
R1
R2
1
2
3
4
5
6
7
8
1
2
1
2
1
1
2
1
2
1
2
1
2
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
OH
OH
Ph
Ph
4-ClPh
4-ClPh
4-CH3Ph
4-CH3OPh
4-CH3OPh
4-Biphenyl
4-Biphenyl
Naphenyl
Naphenyl
Ph
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
5.0
5.5
6.0
5.0
6.5
6.0
5.0
4.5
3.5
4.5
3.5
6.5
7.5
3a
3b
3c
3d
3e
3f
59–61
Oil
60.1
63.7
60.9
54.8
61.5
64.5
65.6
63.9
64.8
80.3
79.8
72.5
70.1
71–73
69–71
57–59
76–78
56–58
103–105
106–108
81–83
66–68
96–98
134–136
3g
3h
3i
9
10
11
12
13
3j
3k
3l
Ph
3m
a Isolated yields over silica gel chromatography.
O
O
S
O
S
O
S
MeOH
H3O+
OH
O
SH
S
S
Decarboxylation
( ) n
( ) n
( ) n
4
5
1
( ) n
S
( ) n
X
O
S
S
5/H+
5/H+
S
R1
Br
R2 (X=Br)
HO
R1
(X=OH)
R1
R2
O
H
R2
2
9
6
HO
-HBr
-H2O
( ) n
S
( ) n
S
( ) n
S
R1
S
S
S
R2
R1
R2
O
R2
R1
OH
7
10
3
-H2O
-H2O
( ) n
S
( ) n
( ) n
H+
S
S
S
S
S
O
HO
R1
OH
R2
R2
R1
R2
R1
8
11
Scheme 2. A mechanism proposed for the reaction of 1 with 2.
12
is the first example of a one-step synthesis of 2,3-
dihydro-1,4-dithiins and 6,7-dihydro-5H-1,4-dithie-
pins directly from carbonyl compounds without using
dithiols.
On the basis of the results together with our previous
finding,17 a mechanism was proposed as depicted in
Scheme 2. The reaction starts from the generation of
HCl based on the esterification of acetyl chloride with

7334
D. Dong et al. / Tetrahedron Letters 46 (2005) 7331–7335
9. (a) Tanemura, K.; Dohya, H.; Imamura, M.; Suzuki, T.;
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MeOH. Prompted by HCl generated, compound 1
undergoes decarboxylation to give a ketene dithioacetal
4. With the attacks by methanol and H2O, 4 is trans-
formed into a thiol-bearing intermediate 5, which reacts
with a-bromo/hydroxy ketones 2. Most likely the mech-
anism for a-bromo ketones is different from that for a-
hydroxy ketones although they are finally converted into
the same compounds of type 3.
In summary, a facile one-step synthesis of substituted
dihydro-1,4-dithiins and -1,4-dithiepins 3 based on the
reactions of a-bromo/hydroxy ketones 2 with a-oxo
ketene cyclic dithioacetals 1a and 1b has been developed.
This novel protocol is associated with simple procedure,
mild conditions, and good yields, especially in relation
to recent environmental concerns. Further investiga-
tions of the scope of the reaction and application are
in progress.
Acknowledgments
14. Nakayama, J.; Nakamura, Y.; Hoshino, M. Heterocycles
1985, 23, 1119–1122.
Financial supports of this research by NNSFC
(20272008) and the Key Project of the Ministry of Edu-
cation of China (105061) are greatly acknowledged.
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methanol (10 mL), and acetyl chloride (0.36 mL, 5 mmol)
were added into a flask equipped with a condenser. The
mixture was heated to reflux and stirred for about 5 h
when 2a was consumed as indicated by TLC. The reaction
mixture was cooled down to ambient temperature and
neutralized with 10% aq NaHCO3. After extractive
workup, separation was carried out over silica gel chro-
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5-Phenyl-2,3-dihydro-1,4-dithiine 3a, white solid, mp:
1
59–61 °C; H NMR (400 MHz, CDCl3): d: 3.22–3.25 (m,
2H), 3.29–3.33 (m, 2H), 6.39 (s, 1H), 7.30 (m, 3H), 7.40 (m,
2H); 13C NMR (125 MHz, CDCl3): 26.6, 27.6, 113.1, 126.7,
127.1, 128.5, 133.4, 138.7; IR (KBr, neat): 3070, 2972, 1687,
1643, 1597, 1493, 1241, 1142, 762 cmÀ1; Anal. Calcd for
C10H10S2: C, 61.81; H, 5.19. Found: C, 61.71; H, 5.22.

D. Dong et al. / Tetrahedron Letters 46 (2005) 7331–7335
7335
2-Phenyl-6,7-dihydro-5H-1,4-dithiepine 3b, colorless oil;
1H NMR (400 MHz, CDCl3): d: 2.20–2.25 (m, 2H), 3.56–
3.64 (m, 4H), 6.11 (s, 1H), 7.25–7.30 (m, 3H), 7.46–7.49 (m,
2H); 13C NMR (125 MHz, CDCl3): 30.4, 31.0, 32.4, 118.0,
127.3, 127.7, 127.9, 128.1, 129.2, 135.2, 141.2; IR (KBr,
neat): 3027, 2955, 1601, 1494, 1453, 1236, 766 cmÀ1; Anal.
Calcd for C12H14OS2: C, 60.46; H, 5.92. Found: C, 60.57;
H, 5.85.
5-(4-Diphenyl)-2,3-dihydro-1,4-dithiine 3h, white solid,
mp: 103–105 °C; 1H NMR (400 MHz, CDCl3): d: 3.25–
3.27 (t, J = 4.0 Hz, 2H), 3.31–3.34 (t, J = 4.0 Hz, 2H), 6.46
(s, 1H), 7.34–7.61 (m, 9H); IR (KBr, neat): 3070, 2971,
1687, 1596, 1493, 1449, 1241, 761 cmÀ1; Anal. Calcd for
C16H14S2: C, 71.07; H, 5.22. Found: C, 71.16; H, 5.18.
2-(4-Diphenyl)-6,7-dihydro-5H-1,4-dithiepine 3i, white
solid, mp: 106–108 °C; 1H NMR (300 MHz, CDCl3): d:
2.22–2.25 (m, 2H), 3.58–3.66 (m, 4H), 6.18 (s, 1H), 7.33–
7.59 (m, 9H); IR (KBr, neat): 3029, 2918, 1675, 1531, 1481,
1299, 762 cmÀ1; Anal. Calcd for C17H16S2: C, 71.78; H,
5.67. Found: C, 71.66; H, 5.73.
neat): 3063, 2922, 1676, 1578, 1487, 1276, 1156, 749 cmÀ1
;
Anal. Calcd for C11H12S2: C, 63.41; H, 5.81. Found: C,
63.52; H, 5.77.
5-(4-Chlorophenyl)-2,3-dihydro-1,4-dithiine 3c, white
solid, mp: 71–73 °C; 1H NMR (400 MHz, CDCl3): d:
3.41–3.46 (m, 4H), 6.38 (s, 1H), 7.30 (d, J = 8.0 Hz, 2H),
7.36 (d, J = 8.0 Hz, 2H); IR (KBr, neat): 3010, 2923, 1669,
1567, 1484, 1286, 1089, 787 cmÀ1; Anal. Calcd for
C10H9ClS2: C, 52.50; H, 3.97. Found: C, 52.58; H, 3.92.
2-(4-Chlorophenyl)-6,7-dihydro-5H-1,4-dithiepine
3d,
1
white solid, mp: 69–71 °C; H NMR (400 MHz, CDCl3):
d: 2.20–2.24 (m, 2H), 3.57–3.64 (m, 4H), 6.08 (s, 1H), 7.24
(d, J = 8.0 Hz, 2H), 7.41 (d, J = 8.0 Hz, 2H); 13C NMR
(125 MHz, CDCl3): 30.2, 30.8, 32.5, 118.6, 128.2, 128.5,
133.6, 133.8, 139.6; IR (KBr, neat): 2920, 1614, 1536, 1482,
1301, 790 cmÀ1; Anal. Calcd for C11H11ClS2: C, 54.42; H,
4.57. Found: C, 54.51; H, 4.53.
5-(Naphthalen-2-yl)-2,3-dihydro-1,4-dithiine 3j, white
solid, mp: 81–83 °C; 1H NMR (400 MHz, CDCl3): d:
3.28–3.30 (t, J = 4.0 Hz, 2H), 3.35–3.37 (t, J = 4.0 Hz,
2H), 6.55 (s, 1H), 7.41–7.46 (m, 2H), 7.56 ( d, J = 8.0 Hz,
1H), 7.78–7.83 (m, 3H), 7.91 (s, 1H); IR (KBr, neat): 3052,
2922, 1627, 1559, 1460, 1284, 7.53 cmÀ1; Anal. Calcd for
C14H12S2: C, 68.81; H, 4.95. Found: C, 68.69; H, 5.01.
5-p-Tolyl-2,3-dihydro-1,4-dithiine 3e, white solid, mp: 57–
59 °C; 1H NMR (400 MHz, CDCl3): d: 2.34 (s, 3H), 3.22–
3.24 (t, J = 4.0 Hz, 2H), 3.30–3.31 (t, J = 4.0 Hz, 2H), 6.34
(s, 1H), 7.12 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.0 Hz, 2H);
IR (KBr, neat): 3021, 2918, 1643, 1552, 1451, 1384,
781 cmÀ1; Anal. Calcd for C11H12S2: C, 63.41; H, 5.81.
Found: C, 63.51; H, 5.76.
5-(4-Methoxyphenyl)-2,3-dihydro-1,4-dithiine 3f, white
solid, mp: 76–78 °C; 1H NMR (300 MHz, CDCl3): d:
3.20–3.22 (t, J = 4.0 Hz, 2H), 3.28–3.32 (t, J = 4.0 Hz,
2H), 3.81 (s, 3H), 6.27 (s, 1H), 6.84–6.87 (q, J = 8.0 Hz,
2H), 7.08–7.45 (q, J = 8.0 Hz, 2H); IR (KBr, neat): 2959,
1605, 1505, 1251, 789; Anal. Calcd for C11H12OS2: C,
58.89; H, 5.39. Found: C, 58.76; H, 5.45.
2-(Naphthalen-2-yl)-6,7-dihydro-5H-1,4-dithiepine
3k,
1
white solid, mp: 66–68 °C; H NMR (300 MHz, CDCl3):
d: 2.24–2.28 (m, 2H), 3.62–3.65 (m, 2H), 3.67–3.69 (m, 2H),
6.25 (s, 1H), 7.43–7.49 (m, 2H), 7.62 (d, 1H), 7.79–7.83 (m,
3H), 7.96 (s, 1H); 13C NMR (125 MHz, CDCl3): 30.4, 31.0,
32.5, 118.7, 125.4, 126.0, 126.1, 126.2, 127.4, 127.6, 128.1,
132.9, 133.1, 135.2, 138.5; IR (KBr, neat): 3051, 2907, 1626,
1533, 1450, 1299, 742 cmÀ1; Anal. Calcd for C15H14S2: C,
69.72; H, 5.46. Found: C, 69.63; H, 5.50.
5,6-Diphenyl-2,3-dihydro-1,4-dithiine 3l, white solid, mp:
1
96–98 °C; H NMR (400 MHz, CDCl3): d: 3.41–3.47 (m,
4H), 7.12–7.22 (m, 10H); IR (KBr, neat): 3055, 2920, 1647,
1572, 1443, 698 cmÀ1; Anal. Calcd for C16H14S2: C, 71.07;
H, 5.22. Found: C, 71.18; H, 5.16.
2-(4-Methoxyphenyl)-6,7-dihydro-5H-1,4-dithiepine 3g,
white solid, mp: 56–58 °C; H NMR (500 MHz, CDCl3):
d: 2.18–2.22 (m, 2H), 3.53–3.57 (m, 2H), 3.58–3.65 (m, 2H),
3.80 (s, 3H), 6.01 (s, 1H), 6.82 (d, J = 8.0 Hz, 2H), 7.42 (d,
J = 8.0 Hz, 2H); 13C NMR (125 MHz, CDCl3): 30.7, 31.3,
32.6, 55.6, 113.7, 116.5, 128.9, 134.0, 135.6, 159.8; IR (KBr,
2,3-Diphenyl-6,7-dihydro-5H-1,4-dithiepine 3m, white
solid, mp: 134–136 °C; 1H NMR (300 MHz, CDCl3): d:
2.17–2.21 (m, 2H), 3.74–3.79 (m, 4H), 7.13–7.24 (m, 10H);
IR (KBr, neat): 2929, 1646, 1563, 1442, 695 cmÀ1; Anal.
Calcd for C17H16S2: C, 71.78; H, 5.67. Found: C, 71.66; H,
5.72.
1