An organocatalytic domino thia-michael/aldol condensation reaction: Highly enantioselective synthesis of functionalized dihydrothiophenes

Article abstract of DOI:10.1002/adsc.201000245

An organocatalytic domino thia-Michael/ aldol condensation reaction of a, β-unsaturated aldehydes with 1, 4-dithiane-2,5-diol catalyzed by chiral diphenylprolinol TMS ether has been developed, which provides a new practical and direct route to chiral dihydrothiophenes with high yields (up to 90%) and excellent enantioselectivities (up to>99% ee). The catalytic mechanism of the domino reaction was further confirmed through the APCI-MS detection of proposed reaction intermediates.

Full text of DOI:10.1002/adsc.201000245

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DOI: 10.1002/adsc.201000245
An Organocatalytic Domino Thia-Michael/Aldol Condensation
Reaction: Highly Enantioselective Synthesis of Functionalized
Dihydrothiophenes
Jie Tang,a Dan Qian Xu,a,* Ai Bao Xia,a Yi Feng Wang,a Jun Rong Jiang,a
Shu Ping Luo,a and Zhen Yuan Xua
a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology,
Hangzhou 310014, Peopleꢀs Republic of China
Fax : (+86)-0571-8832-0609; e-mail: chrc@zjut.edu.cn
Received: March 29, 2010; Revised: July 27, 2010; Published online: September 7, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000245.
Abstract: An organocatalytic domino thia-Michael/
aldol condensation reaction of a, b-unsaturated al-
dehydes with 1, 4-dithiane-2,5-diol catalyzed by
chiral diphenylprolinol TMS ether has been devel-
oped, which provides a new practical and direct
route to chiral dihydrothiophenes with high yields
(up to 90%) and excellent enantioselectivities (up
to>99% ee). The catalytic mechanism of the
domino reaction was further confirmed through the
APCI-MS detection of proposed reaction inter-
mediates.
Keywords: aldol reaction; asymmetric catalysis; di-
hydrothiophenes; domino reactions; Michael reac-
Figure 1. Biologically active thiophene compounds.
tion; organocatalysis
co-workers developed a stereoselective cycloisomeri-
zation reaction of a-thioallenes to chiral 2,5-dihydro-
thiophenes using gold as the catalyst.[11] Herein, we
Thiophene compounds have attracted particular at- disclose an asymmetric domino reaction between 1,4-
tention due to their special place as building blocks in dithiane-2,5-diol and a,b-unsaturated aldehydes using
natural products, pharmaceutical agents and materi- organocatalysts, which leads for the first time to the
als. These valuable S-heterocycles include, for in- highly enantioselective formation of 2,5-dihydrothio-
stance, antihypertensive compounds,[1] mimics of peni- phenes in a single reaction pot.
cillins,[2] an essential coenzyme (biotin),[3] potential in-
Organocatalysis has been developed as a powerful
hibitors of HIV,[4] glucosidase inhibitors[5] and dihy- tool for the preparation of enantiomerically pure
droorotate dehydrogenase (DHODH) inhibitors[6] compounds in the past decade.[12] As a fast-growing
(Figure 1). Moreover, S-heterocycles can also be uti- subfield, the synthetic potential of organocatalyst-
lized as photochromic compounds.[7] Therefore much mediated asymmetric domino reactions has been in-
effort has been devoted to the development of effi- tensively researched.[13] Some research works on
cient synthetic approaches to thiophenes,[8] dihydro- sulfa-Michael-initiated domino processes under orga-
thiophenes[9] and tetrahydrothiophenes,[10] Neverthe- nocatalysis have also been reported.[14] Herein, we en-
less, the enantioselective synthesis of dihydrothio- visaged that a,b-unsaturated aldehydes could be pro-
phenes, which are quite amenable to further function- moted by chiral amines to participate, as shown in
alization in natural products and pharmaceuticals, are Scheme 1, in the asymmetric thia-Michael/aldol con-
still demanding research areas. Recently, Krause and densation reaction with 1,4-dithiane-2,5-diol to pro-
Adv. Synth. Catal. 2010, 352, 2121 – 2126
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Scheme 1. Synthesis of chiral dihydrothiophenes by organocatalyst promoted asymmetric domino process.
vide chiral 2,5-dihydrothiophenes, which is interest- withdrawing substituents at the aromatic ring or the
ingly different from the transformation reported by substitution pattern (entries 1–13). The reaction was
De Risi and co-workers.[14e,15] Therefore, the investiga- also applicable to an oxa-heterocyclic enal, producing
tion was started with the transformation of 1,4-di- the desired product with 90% yield and 90% ee
thiane-2,5-diol with cinnamaldehyde employing the (entry 14). In addition, while the straight chain ali-
most efficient chiral secondary amines 1[15] at room phatic a,b-unsaturated aldehyde gave a less efficient
temperature in toluene, and the results are shown in result, the domino reaction of 1,4-dithiane-2,5-diol
Table 1. It was found that proline 1a and imidazolidi- with a branched aliphatic a,b-unsaturated aldehyde
none 1b catalyzed the reaction smoothly to afford the also exhibited promising results, leading to 86% yield
dihydrothiophene 5a in high conversions but with and 94% ee (entries 15 and 16).
only low to moderate enantioselectivities (entries 1
The absolute configuration of the domino reaction
and 2). To our delight, diphenylprolinol silyl ether 1c products was determined by the single-crystal analysis
led to a significant improvement, providing 97% con- of 5c, the stereochemistry of the newly created stereo-
version and 95% ee value after 12 h (entry 3). The di- center was revealed to possess the R-configuration as
arylprolinol TMS ether 1d could also catalyze the re- shown in Figure 2.[16]
action with 90% ee, but the reaction proceeded much
more slowly compared to the reaction using the cata- of the product, the possible mechanistic proposal for
lyst 1c (entry 4). this domino process is given in Scheme 2. The enal
Therefore, based on the observed stereochemistry
Further investigation showed that the domino reac- was activated by the catalyst 1c and formed an imini-
tion could be carried out in various polar and apolar um ion 6, while the 2-mercaptoacetaldehyde, generat-
solvents to achieve 81–99% conversions and 80–95% ed from 1,4-dithiane-2,5-diol under equilibrium condi-
ee after 12 h. However, toluene might be the best suit- tions, approached from he Re face to the b position of
able solvent in terms of both reactivity and enantiose- the iminium ion for carbon-sulfur bond formation.
lectivity (entry 3 vs. entries 5–16). It also revealed that The resulting enamine 7 then underwent intramolecu-
a Brønsted acid co-catalyst was required for the reac- lar aldol reaction to form the intermediate 8. Dehy-
tion to proceed smoothly and, 10 mol% or more of an dration (8 to 9) and followed by imine hydrolysis
acid additive would be appropriate, which provided yielded the corresponding final product 5 and regen-
96–97% conversions and 90–95% ee (entry 3 and en- erated the catalyst. The beneficial role of the acid ad-
tries 17, 18). In addition, of the acid additives investi- ditive might lie throughout the domino process, in-
gated, 4-nitrobenzoic acid proved to be the most ef- cluding assisting the formation of the iminium ion 6
fective for the asymmetric reaction with > 99% ee and activating successively the aldehyde and hydroxy
enantioselectivity (entry 3 and entries 19–23). Conse- moieties in the intramolecular aldol addition and de-
quently, the conditions employed in entry 19 should hydration steps through protonation, and accelerating
be the most suitable for the domino process.
imine (9) hydrolysis as well. As shown in Figure 3, the
Having established the best protocol for the reac- APCI-MS (atmospheric pressure chemical ionization
tion, we decided to explore the scope of this domino mass spectrometry) showed the signals for the key in-
thia-Michael/aldol condensation reaction. Thus, 1,4- termediates 6–9 involved in this domino process.[17]
dithiane-2,5-diol 3 was treated with a variety of a,b- These results were further confirmed by accurate
unsaturated aldehydes 4 bearing different substituents mass data using Fourier-transform ion cyclotron reso-
in the presence of catalyst 1c and additive 4-nitroben- nance mass spectrometry as given in Table 3.[16]
zoic acid 2b at room temperature in toluene. As
In summary, we have developed a powerful enan-
shown in Table 2, the addition of 1,4-dithiane-2,5-diol tioselective domino thia-Michael/aldol condensation
to aromatic a,b-unsaturated aldehydes afforded the reaction to synthesize functionalized chiral dihydro-
domino reaction products in good to high yields (70– thiophenes from readily available starting materials of
90%) and excellent enantioselectivities (89–>99% a,b-unsaturated aldehydes and 1,4-dithiane-2,5-diol.
ee), regardless of the electron-donating or electron- The reaction was efficiently catalyzed by a commer-
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Adv. Synth. Catal. 2010, 352, 2121 – 2126

An Organocatalytic Domino Thia-Michael/Aldol Condensation Reaction
Table 1. Catalyst screening and reaction optimization.[a]
Table 2. The domino reaction of 1,4-dithiane-2,5-diol with
a,b-unsaturated aldehydes.[a]
Entry
R
Products t [h] C[b] Y[c] ee[d]
1
2
3
4
5
6
7
8
Ph
4-MeC6H4
2-MeOC6H4 5c
3-MeOC6H4 5d
4-MeOC6H4 5e
2-BrC6H4
4-BrC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-FC6H4
4-CF3C6H4
4-NO2C6H4
2-furanyl
propyl
5a
5b
12
12
18
12
12
24
12
12
12
12
12
12
12
11
10
24
93
94
89
98
98
95
96
90
97
99
98
98
98
99
99
97
78
78
74
79
84
70
87
80
86
88
88
90
86
90
83
86
>99
94
>99
94
95
93
95
98
92
92
94
94
89
90
72
94
Entry Cat Additive
Solvent
t
C[b] ee[c]
[h]
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
1
2
3
4
5
6
7
8
1a PhCO2H
1b PhCO2H
1c PhCO2H
1d PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
(2a) PhMe
(2a) PhMe
(2a) PhMe
(2a) PhMe
(2a) DMF
(2a) DMSO 12 81 94
(2a) EtOH
(2a) iPrOH
(2a) EtOAc 12 90 94
(2a) Et2O
(2a) THF
(2a) 1,4-diox- 12 93 80
ane
24 98
5
24 95 À68
12 97 95
168 65 90
12 85 90
9
10
11
12
13
14
15
16
12 95 94
12 84 92
9
i-propyl
10
11
12
12 87 93
12 88 94
[a]
All reactions were conducted in toluene (1 mL) using
1,4-dithiane-2,5-diol 3 (0.230 mmol) and a,b-unsaturated
aldehydes 4 (0.384 mmol) in the presence of 20 mol%
catalyst 1c and 10 mol% additive 2b (amount used based
on 4) at room temperature with vigorous stirring.
Conversion to the product, as determined by GC-MS.
Isolated yield.
13
14
15
16
17
18
19
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c PhCO2H
1c[d] PhCO2H
(2a) CH2Cl2 12 97 94
(2a) CHCl3
(2a) PhCl
(2a) xylene
(2a) PhMe
PhMe
12 95 93
12 98 86
12 99 92
12 96 90
12 42 65
12 93 >99
[b]
[c]
[d]
Determined by HPLC analysis on OD-H.
1c
1c 4-
NO2C6H4CO2H
1c 4-
CF3C6H4CO2H
1c 4-FC6H4CO2H (2d) PhMe
–
(2b) PhMe
20
(2c) PhMe
12 97 96
21
22
12 95 97
12 88 97
1c 2-
HOC6H4CO2H
1c 4-TsOH
(2e) PhMe
23
(2f) PhMe
12 40 87
[a]
Unless otherwise specified, all reactions were conducted
in solvent (1 mL) using 1, 4-dithiane-2, 5-diol
3
(0.230 mmol) and cinnamaldehyde 4a (0.384 mmol) in
the presence of 20 mol% catalysts 1 and 10 mol% addi-
tive 2 (amount used based on 4a) at room temperature
with vigorous stirring.
Conversion to the product, as determined by GC-MS.
Determined by HPLC analysis on OD-H.
[b]
[c]
[d]
Figure 2. X-ray crystal structure of the product 5c.
In the presence of 20% mol% additive 2a.
cially available chiral diphenylprolinol TMS ether to the in situ generated 2-mercaptoacetaldehyde from
give the products with good to high yields (70–90%) 1,4-dithiane-2,5-diol is employed to foster the domino
and excellent enantioselectivities (up to >99% ee). reaction, which differs from those using aromatic
The notable features of the process include (i) to the thio-aldehydes as both the Michael donors and the
best of our knowledge, this is the first example of aldol acceptors.[14b–d] We hope that the strategy ap-
highly enantioselective synthesis of functionalized di- plied in this study may give some hints in the devel-
hydrothiophenes catalyzed by a secondary amine; (ii) opment of important S-heterocyclic compounds.
Adv. Synth. Catal. 2010, 352, 2121 – 2126
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Jie Tang et al.
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Scheme 2. A possible reaction pathway for the domino process.
Figure 3. APCI-mass spectrum (positive mode) of the domino process, 3 h after the start of the reaction.
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An Organocatalytic Domino Thia-Michael/Aldol Condensation Reaction
Table 3. High-resolution mass data of detected intermediates
6–9.
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Species Formula
Mass
Mass (cal- Error
(measured) culated)
(ppm)
6
A
C29H34NOSi
440.2420
440.2410
516.2393
À2.35
[7+H]+ C31H38NO2SSi 516.2354
7.46
8
9
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C31H36NOSSi 498.2298
498.2287
À2.23
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Typical Procedure for the Organocatalytic
Asymmetric thia-Michael/Aldol Condensation
Reaction
Toluene (1 mL) was added to a mixture of 1,4-dithiane-2,5-
diol 3 (0.230 mmol) and various a,b-unsaturated aldehydes 4
(0.384 mmol) in the presence of 0.0768 mmol secondary
amine catalyst 1c and 0.0384 mmol acid additive 2b at room
temperature with vigorous stirring. The reaction conversion
was monitored by GC-MS. After completion, the reaction
mixture was extracted with EtOAc (3ꢂ10 mL), washed with
water, dried and concentrated. The residue was purified by
flash chromatography on silica gel using ether/petroleum
ether as the eluent to give the slightly yellow solid the thia-
Michael/aldol condensation products 5a–5n and the slightly
yellow liquid products 5o–5p. The enantiomeric ratio was
determined by HPLC analysis on a chiral column.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 20772110).
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