A facile and practical one-pot synthesis of multisubstituted 2-aminothiophenes via imidazole-catalyzed Gewald reaction

Article abstract of DOI:10.1016/j.tet.2011.06.061

A simple and efficient procedure was developed for the synthesis of multisubstituted 2-aminothiophene derivatives. In the presence of catalytic amount of imidazole, ketones or aldehydes, dicyanomethane and elemental sulfur were converted into the corresponding 2-aminothiophene derivatives in moderate to high yields in DMF at 60 °C.

Full text of DOI:10.1016/j.tet.2011.06.061

Tetrahedron 67 (2011) 6202e6205
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A facile and practical one-pot synthesis of multisubstituted 2-aminothiophenes
via imidazole-catalyzed Gewald reaction
*
Xian-Gui Huang, Jia Liu, Jiangmeng Ren, Tao Wang, Weidong Chen, Bu-Bing Zeng
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 April 2011
Received in revised form 9 June 2011
Accepted 21 June 2011
Available online 25 June 2011
A simple and efficient procedure was developed for the synthesis of multisubstituted 2-aminothiophene
derivatives. In the presence of catalytic amount of imidazole, ketones or aldehydes, dicyanomethane and
elemental sulfur were converted into the corresponding 2-aminothiophene derivatives in moderate to
high yields in DMF at 60 ꢀC.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
2-Aminothiophenes
Imidazole
Gewald reaction
One-pot
1. Introduction
Multisubstituted 2-aminothiophenes are privileged structures,
which attracted considerable attention in the designing of bi-
ologically active molecules.1 Moreover, they were found to have
various biological applications, such as 1,1i a potent apoptosis in-
ducer; 2,1j a potential anti-inflammatory and anti-osteoporosis
agent; 3,1k an agonist of allosteric enhancers (AE) at the adeno-
sine A1 receptor (A1AR) (Fig. 1). The prevalence of 2-
aminothiophenes substituted compounds has resulted in a contin-
uous demand for the development of general and flexible synthetic
methods for this structural moiety.
Fig. 1. Biologically important multisubstituted 2-aminothiophenes derivatives.
2. Results and discussion
The most popular approach to multisubstituted 2-
aminothiophenes was Gewald reaction, which involves the multi-
components condensation of ketones or aldehydes, cyanoacetate,
and elemental sulfur. Usually, the Gewald reaction was mediated by
excess amount of base, such as morpholine, diethylamine, trie-
thylamine, KFealumina, etc.2 Many modifications of this reaction
have been developed recently, including using solid support,3 mi-
crowave irradiation combined with insoluble polymer support4 or
On the basis of our previous results,7 an effective catalytic base
needs to be identified first. Therefore, the model reactions were
conducted using butyraldehyde (4), dicyanomethane, and ele-
mental sulfur in DMF under nitrogen atmosphere at 60 ꢀC in the
presence of various catalytic amount of bases. The screening results
of the reaction were summarized in Table 1. Classical Gewald re-
action bases, such as morpholine and diethylamine2 were used to
mediate the reaction, however, giving low yields (entries 1 and 2).
The reaction also gave trace amount of desired product in the
presence of triethyl phosphate8 (entry 4). Several additional bases
containing nitrogen atoms, such as isoquoline,8 N-methyl-
imidazole,8 were tested to catalyze the multi-component conden-
sation. The results showed that the yields were not significantly
improved even prolonged the reaction time and increased the
soluble polymer support,5 Lewis acid as catalyst,1a ionic liquid,6
L-
proline,7 etc. In the course of our current research, we developed
a facile and practical method to conduct one-pot Gewald reaction
using catalytic amount of imidazole, which could give the desired
products in moderate to high yield.
amount of catalyst (entries 3 and 5e9). By using
a prolonged reaction time up to 20 h, it could have a higher yield of
L-/D
-proline7 and
* Corresponding author. Tel./fax: þ86 21 64253689; e-mail address: zengbb@
ecust.edu.cn (B.-B. Zeng).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.06.061

X.-G. Huang et al. / Tetrahedron 67 (2011) 6202e6205
6203
Table 2
Synthesis of multisubstituted 2-aminothiophenes in the presence of imidazole via
Table 1
Gewald reactiona
Optimization of reaction conditions in the presence of catalytic amount of base
R2
R1
C
N
CN
Catalyst
CN
O
Imidazole
N
C
R1
+
S8
S8
+
+
O
+
CN
R2
NH2
NH2
DMF
S
5a
N
C
DMF
S
5
60 oC
60 oC
4a
4
Entry
Catalyst
mol %
Time (h)
Yielda (%)
Entry
4
R1
Et
Me
C6H5
R2
5
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Morpholine
Diethylamine
10
10
10
10
10
10
20
20
20
10
10
10
10
5
11
10
12
12
11
13
18
12
22
10
20
10
10
11
11
49
43
48
Trace
57
49
35
59
52
67
74
64
75
52
51
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
H
H
H
H
H
H
5a
5b
5c
5d
5e
5f
10
12
18
12
10
10
15
11
12
75
37c
38
55
66
88
41
56
56d
N-Methylimidazole
Triethyl phosphite
N,N-Dimethylglycine
Isoquinoline
N,N-Dimethylglycine
N-Methylimidazole
Isoquinoline
C6H5CH2
i-Pr
t-Bu
n-C7H15
n-Bu
Et
H
Me
H
H
Me
H
H
H
5g
5h
5i
9
Me
Pr
Me
Et
Me
Et
C6H5
4-MeC6H4
4-BrC6H4
2-FC6H4
10
11
12
13
14
15
D-Proline
5i0
5j
L
-Proline
-Proline
10
4j
12
45d
L
5j0
5k
5l
Imidazole
Imidazole
Imidazole
11
12
13
14
15
16
17
18
4k
4l
12
11
18
15
14
13
17
17
39
57
20
4m
4n
4o
4p
4q
4r
5m
5n
5o
5p
5q
5r
80e
42e
77e
27e
37e
46e
a
Isolated yield.
H
H
H
4-Cl-3-FC6H3
3,4-Cl2C6H3
74% and 67%, respectively. However, if the reaction time was re-
duced to 10 h using -proline, we only gained 64% yield (entries
a
L
Carbonyl compounds 4aer (3.0 mmol), dicyanomethane (3.3 mmol), sulfur
(4.5 mmol), and imidazole (0.3 mmol) in 3 mL DMF at 60 ꢀC under nitrogen.
10e12). When this reaction was conducted with 10 mol % imidaz-
ole, the yield could raise up to 75% in a shorter time (entry 13). We
also found that the reaction gave lower yields when 5 mol % and
20 mol % of imidazole were used (entries 14 and 15). Therefore, we
chose the optimized reaction condition of 10 mol % imidazole
(entry 13) in the next step to investigate the scopes and the limi-
tations of the imidazole-catalyzed Gewald reaction.
As shown in Table 2, the imidazole-catalyzed Gewald reactions
proceeded smoothly with a wide range of ketones and aldehydes.
Acetone 4k known as its low boiling point together with other two
unstable aldehydes, 4b and 4c, were challenged using our opti-
mized reaction condition. These three compounds could give their
corresponding products with moderate yield own to their in-
stability (entries 2, 3, and 11). It can be seen that if the aldehyde
b
Isolated yield unless indicated otherwise.
c
Actual reaction temperature at 50 ꢀC.
d
Ratio (5i/5i0¼3.4:1) and (5j/5j0¼14:1) determined by 1H NMR.
e
% conversion by SiO2 gel.
good yields through the reaction of aldehydes, or ketones with
dicyanomethane and elemental sulfur in DMF in the presence of
catalytic amount of imidazole.
4. Experimental
4.1. General information
bearing electron donating group at the b-position, the yield could
Solvents and reagents were obtained from commercial sources
be significantly increased as high as 88% (entries 5 and 6). We also
found that along with the shorter carbon chain of aldehyde, our
one-pot Gewald reaction could give a higher yield in a shorter re-
action time (entries 1, 7, and 8). Next, some chain aliphatic ketones,
which were difficult to produce 2-aminothiophenes in normal one-
pot Gewald reaction, were also proceeded well under the optimized
condition (entries 9e12). It was worth to notice that the asym-
metric ketones of one-pot Gewald reaction showed some regiose-
lectivity with the ratio of 5j/5j0 up to 14:1 (entry 10). Furthermore,
some challenging aromatic ketones were also investigated (entries
13e18) especially with electron withdraw groups appearing on the
benzene ring. Although these ketones could also proceed under the
imidazole-catalyzed reaction condition to give the corresponding
2-aminothiophenes, the results showed that these electron with-
draw groups had side-effects on the yield. In the cases of 5p and 5q,
the yields were significantly lower than those of 5o and 5r. This
significant difference could be caused by the strong electronega-
tivity of fluorine atom.
and without purification. Column chromatography was carried out
on silica gel (300e400 mm). Melting points were determined using
a digital melting-point apparatus WRS-1B and are uncorrected. 1H
NMR and 13C NMR spectra were recorded on a Bruker Avance-
400 MHz spectrometer (400 MHz and 100 MHz, respectively) using
TMS as internal standard. The 19F NMR was measured with external
CF3CO2H as the standard. Chemical shifts of protons were reported
in parts per million (ppm) downfield from tetramethylsilane.
Coupling constants are reported in Hertz (Hz). Infrared spectra
were recorded on a Thermo FT-IR 200 Spectrometer. HRMS (ESI)
data were determined using a mircoOTOF-Q II HPLC/MS instrument
(Water). Elemental analyses were determined using a elementar
vario El Ⅲ Elemental Analyser.
4.2. Typical procedure for the synthesis of multisubstituted
2-aminothiophenes via imidazole-catalyzed Gewald reaction
A mixture of carbonyl compounds 4aer (3.0 mmol), dicyano-
methane (3.3 mmol), sulfur (4.5 mmol), and imidazole (0.3 mmol)
in DMF (3.0 mL) was stirred at 60 ꢀC under nitrogen atmosphere for
10e18 h. The crude materials were directly purified through flash
chromatography to give the desired products 5aer.
3. Conclusions
In summary, we described a facile and practical method for the
synthesis of multisubstituted 2-aminothiophenes in moderate to

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X.-G. Huang et al. / Tetrahedron 67 (2011) 6202e6205
4.2.1. 2-Amino-5-ethylthiophene-3-carbonitrile (5a)9. Yellow solid;
IR (KBr) nmax 3420, 3229, 2973, 2202, 1626, 1522, 1388, 821,
(s, 3H); 13C NMR (100 MHz, CDCl3)
12.7, 12.3.
d 159.1, 129.6, 117.2, 115.9, 90.6,
512 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d 6.34 (s, 1H), 4.73 (br s, 2H),
2.60 (q, J¼7.2 Hz, 2H), 1.20 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz,
4.2.11. 2-Amino-4-methylthiophene-2-carbonitrile (5k)17. Yellow
solid; IR (KBr) nmax 3418, 3310, 3207, 2213, 1633, 1514, 1508, 1443,
CDCl3)
d 161.0, 132.0, 120.2, 115.9, 86.7, 23.0, 15.2.
1195, 826 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d
5.97 (s, 1H), 4.67 (br s,
126.3, 135.7, 115.4,
4.2.2. 2-Amino-5-methylthiophene-3-carbonitrile (5b)10. Yellow solid;
IR (KBr) nmax 3420, 3332, 3227, 2916, 2199, 1626, 1519, 1385, 1276,
2H), 2.20 (s, 3H); 13C NMR (100 MHz, CDCl3)
105.1, 90.6, 15.3.
d
1106, 897, 812, 505 cmꢁ1
;
1H NMR (400 MHz, CDCl3)
d
6.34 (s, 1H),
161.4, 124.4,
4.72 (br s, 2H), 2.27 (s, 3H); 13C NMR (100 MHz, CDCl3)
122.0, 115.9, 86.8, 14.8.
d
4.2.12. 2-Amino-4-ethyl-5-methylthiophene-3-carbonitrile
(5l)18. Yellow solid; IR (KBr) nmax 3431, 3322, 3214, 2974, 2936,
2874, 2199, 1632, 1524, 1388, 1326, 1297, 498 cmꢁ1 1H NMR
;
4.2.3. 2-Amino-5-phenylthiophene-3-carbonitile (5c)11. Brown solid;
IR (KBr) nmax 3418, 3310, 3207, 2213, 1633, 1514, 1443, 1196, 826, 747,
(400 MHz, CDCl3)
d
4.0 (br s, 2H), 2.49 (q, J¼7.6 Hz, 2H), 2.18 (s, 3H),
1.16 (t, J¼7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 160.0, 136.1,
685 cmꢁ1
;
1H NMR (400 MHz, CDCl3)
d
7.42e7.33 (m, 5H), 6.93 (s,
116.6, 116.0, 89.1, 20.9, 14.3, 12.1.
1H), 4.76 (br s, 2H); 13C NMR (100 MHz, DMSO-d6)
d 164.9, 133.6,
129.4, 127.0, 124.5, 122.1, 116.7, 84.3.
4.2.13. 2-Amino-4-phenylthiophene-3-carbonitrile (5m)1d. Yellow
solid; IR (KBr) nmax 3428, 3307, 3209, 3104, 2208, 1629, 1509, 1400,
4.2.4. 2-Amino-5-benzythiophene-3-carbonitrile (5d)12. Pale brown
solid; IR (KBr) nmax 3418, 3410, 3207, 2213, 1633, 1514, 1508, 1443,
1193, 943, 774, 720, 661, 696 cmꢁ1 1H NMR (400 MHz, CDCl3)
;
d
7.60e7.58 (m, 2H), 7.44e7.40 (m, 2H), 7.38e7.34 (m, 1H), 6.32 (s,
1196, 826, 747, 685 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d
7.36e7.21 (m,
1H), 5.02 (br s, 2H); 13C NMR (100 MHz, CDCl3)
134.2, 128.8, 128.2, 127.2, 116.2, 105.9, 88.0.
d 164.1, 139.9,
5H), 6.40 (s, 1H), 4.63 (br s, 2H), 3.93 (s, 2H); 13C NMR (100 MHz,
CDCl3)
35.9.
d 161.9, 139.1, 129.0, 128.7, 128.5, 126.9, 122.3, 115.7, 87.0,
4.2.14. 2-Amino-4-p-totylthiophene-3-carbonitrile (5n)19. Yellow
solid; IR (KBr) nmax 3440, 3334, 3213, 3104, 2206, 1623, 1512, 1414,
4.2.5. 2-Amino-5-isopropylthiophene-3-carbonitrile
yellow solid; IR (KBr) nmax 3425, 3331, 3221, 2972, 2956, 2195, 1619,
1518,1387,1272, 813 cmꢁ1; 1H NMR (400 MHz, CDCl3)
6.35 (s, 1H),
4.66 (br s, 2H), 2.92e2.89 (m, 1H), 1.23 (s, 3H), 1.21 (s, 3H); 13C NMR
(100 MHz, CDCl3) 160.7, 137.9, 118.9, 115.9, 86.8, 29.6, 24.0.
(5e)13. Light
1187, 820, 738, 497 cmꢁ1 1H NMR (400 MHz, CDCl3)
; d 7.48 (d,
J¼8.0 Hz, 2H), 7.23 (d, J¼8.4 Hz, 2H), 6.31 (s, 1H), 4.85 (br s, 2H),
d
2.38 (s, 3H); 13C NMR (100 MHz, CDCl3)
129.4, 127.0, 116.0, 105.3, 88.4, 21.2.
d 163.6, 140.0, 138.1, 131.4,
d
4.2.15. 2-Amino-4-(4-bromophenyl)thiophene-3-carbonitrile
(5o)1d. Yellow solid; IR (KBr) nmax 3329, 3206, 2219, 1648, 1538,
1512,1406,1383,1070,1010, 817, 732, 691 cmꢁ1; 1H NMR (400 MHz,
4.2.6. 2-Amino-5-(tert-butyl)thiophene-3-carbonitrile (5f)14. White
solid; IR (KBr) nmax 3416, 3332, 3229, 2961, 2201, 1626, 1524, 1450,
1248, 822 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d
6.36 (s, 1H), 4.61 (br s,
160.5, 142.2, 118.4,
DMSO-d6)
d
7.61 (d, J¼8.0 Hz, 2H), 7.48 (d, J¼8.4 Hz, 2H), 7.29 (s,
2H), 1.29 (s, 9H); 13C NMR (100 MHz, CDCl3)
115.9, 34.2, 31.8.
d
2H), 6.56 (s, 1H), 4.87 (s, 2H); 13C NMR (100 MHz, DMSO-d6)
d 167.0,
137.5, 134.0, 132.0, 129.3, 121.5, 116.9, 106.1, 83.2.
4.2.7. 2-Amino-5-heptylthiophene-3-carbonitrile (5g). Yellow solid,
mp 119 ꢀC; IR (KBr) nmax 3422, 3331, 3225, 2956, 2927, 2853, 2200,
1621, 1520, 1465, 1392, 1261, 807 cmꢁ1; 1H NMR (400 MHz, CDCl3)
4.2.16. 2-Amino-4-(3-fluorophenyl)thiophene-3-carbonitrile
(5p). Pale white solid, mp 108e110 ꢀC; IR (KBr) nmax 3418, 3310,
3207, 2215, 1634, 1544, 1508, 1443, 1196, 826, 747, 685 cmꢁ1 1H
;
d
6.39 (s, 1H), 4.51 (br s, 2H), 2.58 (t, J¼8.0 Hz, 2H), 1.58e1.55 (m,
NMR (400 MHz, CDCl3)
7.08e7.04 (m, 1H), 6.39 (s, 1H), 5.02 (br s, 2H); 13C NMR (100 MHz,
DMSO-d6) 167.0, 163.8, 161.4, 137.3, 137.1, 137.0, 131.1, 131.0, 123.4,
d 7.40e7.39 (m, 2H), 7.29e7.28 (m, 1H),
2H), 1.30e1.27 (m, 8H), 0.88 (t, J¼6.8 Hz, 3H); 13C NMR (100 MHz,
CDCl3)
d
160.9, 130.6, 120.9, 115.8, 87.0, 31.7, 30.9, 29.6, 28.9, 28.9,
d
22.5, 14.0; HRMS (ESI): m/z [MþH]þ calcd for C12H19N2S: 223.1269;
found: 223.1270. Anal. Calcd for C12H18N2S: C, 64.82; H, 8.16; N,
12.60. Found: C, 64.97; H, 8.09; N, 12.40.
116.9, 115.1, 114.9, 113.9, 106.6, 83.2; 19F NMR (376 MHz, DMSO-d6)
d
ꢁ112.7 to ꢁ112.6 (m); HRMS (ESI): m/z [MꢁH]ꢁ calcd for
C11H6FN2S: 217.0236; found: 217.0239. Anal. Calcd for C11H7FN2S: C,
60.54; H, 3.23; N, 12.84. Found: C, 60.79; H, 3.18; N, 12.67.
4.2.8. 2-Amino-5-butylthiophene-3-carbonitrile (5h). Yellow solid,
mp 91e93 ꢀC; IR (KBr) nmax 3419, 3333, 3225, 2964, 2928, 1410,
4.2.17. 2-Amino-4-(3-chloro-4-fluorophenyl)thiophene-3-
carbonitrile (5q). Light yellow solid, mp 149 ꢀC; IR (KBr) nmax 3418,
2201, 1625, 1518, 1463, 1390, 813, 641 cmꢁ1 1H NMR (400 MHz,
;
CDCl3)
d
6.37 (s, 1H), 4.20 (br s, 2H), 2.59 (t, J¼7.6 Hz, 2H), 1.60e1.52
3310, 3207, 2213, 1633, 1544, 1508, 1443, 1196, 826, 747, 685 cmꢁ1
;
(m, 2H), 1.41e1.32 (m, 2H), 0.93 (t, J¼7.2 Hz, 3H); 13C NMR
1H NMR (400 MHz, DMSO-d6)
d 7.71e7.70 (m, 1H), 7.55e7.53 (t,
(100 MHz, CDCl3)
d
160.6, 130.8, 121.0, 115.7, 87.3, 33.0, 29.3, 21.9,
J¼8.8 Hz, 1H), 7.33 (s, 2H), 6.62 (s, 1H); 13C NMR (100 MHz, DMSO-
13.6; HRMS (ESI): m/z [MþH]þ calcd for C9H13N2S: 181.0799; found:
181.0796. Anal. Calcd for C9H12N2S: C, 59.96; H, 6.71; N, 15.54.
Found: C, 60.24; H, 6.23; N, 15.39.
d6) d 167.0, 166.9, 158.4, 156.0, 136.1, 132.6, 129.2, 128.0, 127.9, 120.3,
120.1, 117.7, 117.4, 116.8, 106.6, 83.2; 19F NMR (376 MHz, DMSO-d6)
d
ꢁ117.3 (s); HRMS (ESI): m/z [MꢁH]ꢁ calcd for C11H5ClFN2S:
250.9846; found: 250.9850. Anal. Calcd for C11H6ClFN2S: C, 52.28;
H, 2.39; N, 11.09. Found: C, 52.56; H, 2.66; N, 10.79.
4.2.9. 2-Amino-5-ethyl-methylthiophene-3-carbonitrile (5i) and 2-
amino-4-propylthiophene-3-carbonitrile (5i0), (5i/5i0¼3.4:1)15
.
Compound (5i) 1H NMR (400 MHz, CDCl3)
d
4.81 (br s, 2H), 2.54 (q,
4.2.18. 2-Amino-4-(3,4-dichlorophenyl)thiophene-3-carbonitrile
J¼6.4 Hz, 2H), 2.0 (s, 3H), 1.14 (t, J¼6.8 Hz, 3H); compound (5i0) 1H
(5r). Pale white solid, mp 190 ꢀC; IR (KBr) nmax 3418, 3310, 3207,
NMR (400 MHz, CDCl3)
d
5.93 (s, 1H), 4.80 (br s, 2H), 2.50e2.46 (m,
2213, 1633, 1546, 1508, 1445, 1195, 826, 747, 685 cmꢁ1 1H NMR
;
2H), 1.64e1.61 (m, 2H), 0.94 (t, J¼7.2 Hz, 3H).
(400 MHz, DMSO-d6)
d
7.77 (d, J¼2.0 Hz, 1H), 7.68 (d, J¼8.4 Hz, 1H),
7.53 (dd, J1¼1.6 Hz, J2¼8.4 Hz, 1H), 7.35 (s, 2H), 6.70 (s, 1H); 13C NMR
4.2.10. 2-Amino-4,5-dimethylthiophene-3-carbonitrile (5j)16. Brown
(100 MHz, DMSO-d6) d 167.1, 135.9, 135.3, 131.8, 131.2, 130.8, 128.9,
solid; 1H NMR (400 MHz, CDCl3)
d
4.60 (br s, 2H), 2.16 (s, 3H), 2.07
127.4, 116.8, 107.2, 82.9; HRMS (ESI): m/z [MꢁH]ꢁ calcd for

X.-G. Huang et al. / Tetrahedron 67 (2011) 6202e6205
6205
€
2. (a) Gewald, K.; Schinke, E.; Bottcher, H. Chem. Ber. 1966, 99, 94e100; (b) Sabnis,
R. W.; Rangnekar, D. W.; Sonawane, N. D. J. Heterocycl. Chem. 1999, 36,
333e345; (c) Treu, M.; Karner, T.; Kousek, R.; Berger, H.; Mayer, M.; McConnell,
D. B.; Stadler, A. J. Comb. Chem. 2008, 10, 863e868; (d) Mckibben, B. P.; Cart-
wright, C. H.; Castllhano, A. L. Tetrahedron Lett. 1999, 40, 5471e5474; (e)
C11H5Cl2FN2S: 266.9551; found: 266.9555. Anal. Calcd for
C11H6Cl2N2S: C, 49.09; H, 2.25; N, 10.41. Found: C, 49.56; H, 2.45; N,
10.26.
ꢀ
ꢀ
ꢀ
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Acknowledgements
€
Financial support of this work from Shanghai Foundation of
Science and Technology (09JC1404200) and the national ‘863’
Project of China (2007AA02Z301) is gratefully acknowledged.
€
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.06.061.
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