Thiamine hydrochloride (VB1)-catalyzed one-pot synthesis of (E)-N-benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3-amine derivatives

Article abstract of DOI:10.1016/j.tet.2013.02.022

An environmentally benign and efficient method has been developed for the synthesis of (E)-N-benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3- amine derivatives via a four-component reaction of 2-aminobenzimidazole, two molecules of aromatic aldehyde, and trimethylsilanecarbonitrile (TMSCN) in the presence of a catalytic amount of thiamine hydrochloride (VB₁) in EtOH at reflux temperature. The advantages of this method are the use of an inexpensive and readily available catalyst, high yields, simple workup, and easy purification.

Full text of DOI:10.1016/j.tet.2013.02.022

Tetrahedron 69 (2013) 2954e2960
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thiamine hydrochloride (VB1)-catalyzed one-pot synthesis of (E)-N-
benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3-amine
derivatives
*
*
Fanglei Chen, Min Lei , Lihong Hu
Shanghai Research Center for Modernization of Traditional Chinese Medicine, Shanghai Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An environmentally benign and efficient method has been developed for the synthesis of (E)-N-benzy-
lidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3-amine derivatives via a four-component reaction
of 2-aminobenzimidazole, two molecules of aromatic aldehyde, and trimethylsilanecarbonitrile (TMSCN)
in the presence of a catalytic amount of thiamine hydrochloride (VB1) in EtOH at reflux temperature. The
advantages of this method are the use of an inexpensive and readily available catalyst, high yields, simple
workup, and easy purification.
Received 26 October 2012
Received in revised form 24 January 2013
Accepted 4 February 2013
Available online 13 February 2013
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Multi-component reaction
Thiamine hydrochloride (VB1)
Green chemistry
2-Aminobenzimidazole
TMSCN
1. Introduction
diazabicyclo[5.4.0]undec-7-ene (DBU),14 oxalic acid dihydrate,15
and thiamine hydrochloride (VB1).16 Our group has been dedi-
Multi-component reactions (MCRs)1 have been existed about
160 years since the first MCR was reported in 1850.2 During this
period, a large number of MCRs have been reported, such as Bigi-
nelli reaction,3 Mannich reaction,4 Passerini reaction,5 and Ugi re-
action.6 These MCRs have been widely used in modern organic
chemistry due to their common advantages, such as convergence,
operational simplicity, generally good yield of products, and
structural complexity and diversity of products. Because of these
valued features, the development of new MCRs has been received
much attention in the field of organic synthesis.7
2-Aminobenzimidazole is one of important intermediates in
organic synthesis. It has been used for the synthesis of a wide di-
versity of benzimidazole derivatives of pharmacological interest.8
In addition, it has also been used for the synthesis of organo-
catalysts,9 antibiofilm agents,10 and chemosensors.11
cated to explore new MCRs catalyzed by VB1.17e20 It is well known
that VB1 is an inexpensive and eco-friendly reagent. The structure
of VB1 contains a pyrimidine ring and a thiazole ring linked by
a methylene bridge (Fig.1). In the field of organic synthesis, VB1 and
its analogs have been taken as effective catalysts to improve re-
actions.21 In continuation of our endeavor to find VB1 catalyzed
new MCRs, we wish to report here a VB1-catalyzed four-component
reaction of 2-aminobenzimidazole, two molecules of aromatic al-
dehyde, and trimethylsilanecarbonitrile (TMSCN) for the synthesis
of (E)-N-benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imida-
zol-3-amine derivatives (Scheme 1).
In recent years, organocatalysts have been widely used in or-
ganic synthesis, such as L
-proline,12 ammonium acetate,13 1,8-
*Corresponding authors. Tel.: þ86 021 2023 1000; fax: þ86 021 2023 1965 (M.L.);
tel./fax: þ86 021 2023 1965 (L.H.); e-mail addresses: mlei@simm.ac.cn (M. Lei),
lhhu@simm.ac.cn (L. Hu).
Fig. 1. The structure of thiamine hydrochloride (VB1).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.02.022

F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
2955
Table 1
Optimization of condensation of 2-aminobenzimidazole 1a, two molecules of
benzaldehyde 2a, and TMSCN 3a
Entry
Cat. (mol %)
Temp (ꢀC)
Time (h)
Yield of 4a (%)b
Scheme 1. Synthesis of (E)-N-benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imi-
dazol-3-amine derivatives 4.
1
2
3
4
5
6
7
8
VB1 (0)
VB1 (1)
VB1 (3)
VB1 (5)
VB1 (10)
VB1 (3)
VB1 (3)
VB1 (3)
Reflux
Reflux
Reflux
Reflux
Reflux
rt
10
10
6
6
6
6
6
6
55
70
90, 89, 87c
2. Results and discussion
88
90
5
10
60
Initially, we explored the multi-component condensation re-
action of 2-aminobenzimidazole 1a (3 mmol) with benzaldehyde 2a
(3 mmol), and TMSCN 3 (3.3 mmol) in 10 mL EtOH at reflux tem-
perature in the presence of VB1. Curiously, the product (E)-N-ben-
zylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3-amine
4a, shown in Scheme 2 and confirmed by NMR and LC-MS, was
obtained, while in contrast, the 2-phenyl-1H-benzo[d]imidazo
[1,2-a]imidazol-3-amine 5a was not obtained although only
1 equiv of benzaldehyde 2a was added (Scheme 2). The prob-
able reason for this phenomenon was that 5a was easy to react
with aldehyde to form 4a under this reaction conditions. En-
couraged by this result, we further optimized the reaction
conditions on the model reaction of 2-aminobenzimidazole 1a,
two molecules of benzaldehyde 2a, and TMSCN 3 (Table 1).
40
60
a
Conditions: 2-aminobenzimidazole 1a (3 mmol), benzaldehyde 2a (6.6 mmol),
and TMSCN 3 (3.3 mmol), EtOH (10 mL).
b
Isolated yields.
VB1 was reused for three times.
c
was obtained when 1 mol % of VB1 was added to the reaction
system (Table 1, entry 2). Further investigation revealed that the
yields of 4a were improved as the amount of VB1 increased from
1 mol % to 3 mol %. However, an increase in the quantity of VB1 form
3 mol % to 10 mol % did not improve the yields of 4a (Table 1, entries
3e5). Hence, 3 mol % of VB1 was considered to be the most suitable.
To study the relation between the yields of the model reaction
and temperature, we performed the reaction at temperature
ranging from room to reflux temperature in the presence of 3 mol %
VB1, founding that the yields of product were improved as the
temperature was increased (Table 1, entries 3, 6e8). Therefore, the
best reaction conditions were obtained by using 3 mol % VB1 as the
catalyst in EtOH at reflux temperature.
Furthermore, the reusability of VB1 was also examined with the
optimized experiment conditions of the model reaction. After
completion of the reaction (TLC), the desired product 4a was iso-
lated by simple filtration. Then the filtrate containing VB1 was
further used to react with the reactants, affording the product 4a in
90, 89, 87% yield after 1e3 runs, respectively (Table 1, entry 3). This
study demonstrated that VB1 could be effectively used as a re-
cyclable catalyst for this four-component reaction.
To further explore the scope and limitation of this MCR, a variety
of aromatic aldehydes were employed as substrates in this reaction
of 2-aminobenzimidazole 1, aromatic aldehyde 2, and TMSCN 3 in
the presence of 3 mol % VB1 as the catalyst in EtOH at reflux tem-
perature (Table 2).
As shown in Table 2, the steric hindrance effect from the aro-
matic aldehydes had no significant impact on the overall yields of
the products 4. For example, aromatic aldehydes carrying sub-
stituents (eMe, eCl, eOMe) at the ortho, meta, or para position
could react efficiently to give the corresponding products without
significant difference (Table 2, entries 2e4, 5e7, 9e11, 12e13).
Furthermore, nicotinaldehyde (Table 2, entry 18) was also selected
for this condensation, and 80% yield of the desired product 4r was
obtained under this conditions.
However, the electronic effect from the aromatic aldehydes had
significance impact on the yields of the products 4. As shown in
Table 2, the aromatic aldehydes with either electron-donating
(eMe, eOMe, eOH) or weak electron-withdrawing groups (eCl,
Scheme 2. Condensation of 2-aminobenzimidazole 1a, benzaldehyde 2a, and TMSCN 3.
As shown in Table 1, only 55% yield of the desired product 4a
was obtained under catalyst-free conditions when the condensa-
tion of 2-aminobenzimidazole 1 (3 mmol) with benzaldehyde 2a
(6.6 mmol), and TMSCN 3 (3.3 mmol) in 10 mL EtOH at reflux
temperature for 10 h (Table 1, entry 1). In contrast, 70% yield of 4a

2956
F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
Table 2
Synthesis of (E)-N-benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imidazol-3-
a
amine derivatives catalyzed by VB1
Scheme 3. Condensation of 1-methyl-2-aminobenzimidazole 1d, benzaldehyde 2a,
and TMSCN 3.
which is captured by VB1 to generate intermediate 6. Intermediate 6
reacts with the corresponding aromatic aldehyde 2 to give in-
termediate 7, then 7 reacts with another molecule of 3 to generate
intermediate 8 and NCꢁ, and NCꢁ participates next reaction cycle.
Next, intermediate 8 reacts with 2-aminobenzimidazole 1 to yield
intermediate 9, which is cyclized to form 10. Next, intermediate 10 is
isomerized to 11, which reacts with another molecule of aromatic
aldehyde 2 to form product 4.
Entry
1
Ar
Product 4
Time (h)
Yield of
4 (%)b
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
C6H5
4a
4b
4c
4d
4e
4f
4g
4h
4i
6
6
6
6
8
8
8
8
6
6
6
6
6
12
12
6
6
6
90
86
89
90
83
87
85
80
90
91
92
87
88
0
2-MeC6H4
3-MeC6H4
4-MeC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
3. Conclusion
In conclusion, we have developed a novel and convenient
method for the synthesis of (E)-N-benzylidene-2-phenyl-1H-benzo
[d]imidazo[1,2-a]imidazol-3-amine derivatives via the four-
component reaction of 2-aminobenzimidazole 1, two molecules
of aromatic aldehyde 2, and TMSCN 3 catalyzed by VB1 in EtOH. The
use of inexpensive and eco-friendly catalyst VB1, mild reaction
condition, high yields, and easy workup procedure, make this
method attractive to synthesize a variety of these derivatives.
4-FC6H4
9
2-MeOC6H4
3-MeOC6H4
4-MeOC6H4
3-HOC6H4
4-HOC6H4
4-NCC6H4
4- O2NC6H4
3,4-(MeO)2C6H3
3,4,5-(MeO)3C6H2
3-Pyridyl
C6H5
10
11
12
13
14
15
16
17
18
19
20
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
0
92
93
80
85
91
4. Experimental section
4.1. General
8
6
C6H5
a
Conditions: 2-aminobenzimidazole
1 (3 mmol), aromatic aldehyde 2
Melting points were measured by a WRS-1B micromelting point
apparatus and are uncorrected. NMR spectra were recorded on
a Bruker AMX 400 and Bruker Avance III/500 instruments using
solvent peaks as DMSO-d6 solutions. HRESIMS were determined on
a Micromass Q-Tof Global mass spectrometer and ESIMS were run
on a Bruker Esquire 3000 Plus Spectrometer. TLC was performed on
GF254 silica gel plates (Yantai Huiyou Inc., China). The chemicals
used in this work were obtained from commercial channels and
were used without purification.
(6.6 mmol), TMSCN 3 (3.3 mmol), VB1 (0.09 mmol, 3 mol %), EtOH (10 mL), reflux.
b
Isolated yields.
eF) afforded the corresponding products 4 in 80e93% yields (Table
2, entries 1e12, 16e17). Whereas, no desired products were ob-
served when using aromatic aldehydes with strong electron-
withdrawing groups (eNO2, eCN) as the substrates under the
same reaction conditions for 12 h (Table 2, entries 14 and 15), and
only some byproducts were detected by LC-MS (See in the
Supplementary data).
Moreover, substituted 2-aminobenzimidazole, such as 5,6-
dichloro-1H-benzo[d]imidazol-2-amine 1b and 5H-[1,3]dioxolo
[40,50:4,5]benzo[1,2-d]imidazol-6-amine 1c were used as the sub-
strates, finding the reaction can proceed smoothly to obtain the
corresponding products in good yields (Table 2, entries 19, 20). In
addition, we also used 1-methyl-2-aminobenzimidazole 1d as the
substrate to replace 2-aminobenzimidazole 1a in EtOH as solvent
under reflux temperature for 8 h, and the product (E)-N-benzyli-
dene-9-methyl-2-phenyl-9H-benzo[d]imidazo[1,2-a]imidazol-3-
amine 4u was obtained in 88% yield (Scheme 3).
4.2. Experimental procedure for the condensation of 2-
aminobenzimidazole 1, aromatic aldehyde 2, and TMSCN 3
A mixture of 2-aminobenzimidazole 1 (3 mmol), aromatic al-
dehyde 2 (6.6 mmol), TMSCN 3 (3.3 mmol) and VB1 (0.09 mmol,
3 mol %) in 10 mL EtOH was heated to reflux for 6e12 h (Table 2).
After completion of the reaction (TLC), the solid was filtered off and
washed with ethanol to yield the pure products 4.
4.2.1. (E)-N-Benzylidene-2-phenyl-1H-benzo[d]imidazo[1,2-a]imida-
zol-3-amine (4a). Isolated as a yellow powder; mp 266.4e270.0 ꢀC;
1H NMR (400 MHz, DMSO-d6): 12.11 (br s, 1H, NH), 8.92 (s, 1H, CH),
8.20 (d, J¼7.6 Hz, 2H, ArH), 8.02 (d, J¼7.6 Hz, 2H, ArH), 7.75 (d,
Although the mechanism of this reaction hasnot been established
experimentally, the proposed mechanism for the formation of 4 is
illustrated (Scheme 4). TMSCN 3 reacts with EtOH to release the NCꢁ,

F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
2957
Scheme 4. A plausible mechanistic pathway to explain the formation of compounds 4.
J¼8.0 Hz, 1H, ArH), 7.61e7.52 (m, 3H, ArH), 7.49e7.43 (m, 3H, ArH),
7.34 (t, J¼7.6 Hz, 1H, ArH), 7.29 (t, J¼7.2 Hz, 1H, ArH), 7.22 (t,
J¼7.6 Hz, 1H, ArH); 13C NMR (100 MHz, DMSO-d6): 152.03, 148.19,
136.64, 134.67, 130.60, 130.60, 128.99, 128.99, 128.24,128.24, 127.88,
127.88, 127.76, 127.34, 127.34, 126.76, 125.09, 123.63, 123.63, 119.86,
112.32, 112.22. ESI-MS m/z 337 [MþH]þ; HRMS (ESI) calcd for
C22H16N4 [MþH]þ 337.1448, found 337.1440.
13C NMR (100 MHz, DMSO-d6): 150.15,146.88,137.38,136.31, 134.17,
130.94, 130.94, 130.30, 130.23, 130.23, 128.37, 128.11, 126.40, 126.40,
126.12, 125.48, 124.79, 123.39, 123.39, 120.17, 112.33, 111.76, 19.53,
18.14; ESI-MS m/z 365 [MþH]þ; HRMS (ESI) calcd for C24H20N4
[MþH]þ 365.1761, found 365.1768.
4.2.3. (E)-N-(3-Methylbenzylidene)-2-(m-tolyl)-1H-benzo[d]imidazo
[1,2-a]imidazol-3-amine (4c). Isolated as a yellow powder; mp
237.0e237.4 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.10 (br s, 1H, NH),
8.84 (s, 1H, CH), 8.06 (s, 1H, ArH), 7.97 (d, J¼8.0 Hz, 1H, ArH), 7.81 (s,
1H, ArH), 7.79 (d, J¼8.0 Hz, 1H, ArH), 7.73 (d, J¼8.1 Hz, 1H, ArH),
7.47e7.41 (m, 2H, ArH), 7.35e7.30 (m, 3H, ArH), 7.21 (t, J¼7.8 Hz,1H,
ArH), 7.10 (d, J¼7.4 Hz, 1H, ArH), 2.42 (s, 3H, CH3), 2.38 (s, 3H, CH3);
13C NMR (100 MHz, DMSO-d6): 153.48, 150.03, 140.06, 138.91,
4.2.2. (E)-N-(2-Methylbenzylidene)-2-(o-tolyl)-1H-benzo[d]imidazo
[1,2-a]imidazol-3-amine (4b). Isolated as a yellow powder; mp
266.0e267.0 ꢀC; 1H NMR (400 MHz, DMSO-d6): 11.94 (br s, 1H, NH),
8.46 (s, 1H, CH), 8.10e8.03 (m, 1H, ArH), 7.96 (d, J¼7.8 Hz, 1H, ArH),
7.47 (d, J¼8.1 Hz, 2H, ArH), 7.39e7.28 (m, 6H, ArH), 7.25 (t, J¼7.6 Hz,
1H, ArH), 7.21e7.15 (m, 1H, ArH), 2.28 (s, 3H, CH3), 2.07 (s, 3H, CH3);

2958
F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
138.62, 136.41, 133.14, 130.77, 130.77, 130.18, 130.01, 130.01, 129.47,
129.29, 126.97, 126.97, 126.37, 125.47, 125.47, 121.72, 114.21, 114.17,
23.18, 22.88; ESI-MS m/z 365 [MþH]þ; HRMS (ESI) calcd for
C24H20N4 [MþH]þ 365.1761, found 365.1752.
115.08 (2JCF¼21.2 Hz), 112.49, 112.10; ESI-MS m/z 373 [MþH]þ;
HRMS (ESI) calcd for C22H14F2N4 [MþH]þ 373.1259, found 373.1256.
4.2.9. (E)-N-(2-Methoxybenzylidene)-2-(2-methoxyphenyl)-1H-
benzo[d]imidazo[1,2-a]imidazol-3-amine (4i). Isolated as a yellow
powder; mp 251.0e251.1 ꢀC; 1H NMR (400 MHz, DMSO-d6): 11.82
(br s, 1H, NH), 8.50 (s, 1H, CH), 8.15 (dd, J¼7.6, 1.7 Hz, 1H, ArH), 8.02
(d, J¼7.2 Hz,1H, ArH), 7.60 (dd, J¼7.6,1.7 Hz,1H, ArH), 7.47e7.40 (m,
3H, ArH), 7.31 (td, J¼7.7, 1.2 Hz, 1H, ArH), 7.24 (td, J¼7.7, 1.2 Hz, 1H,
ArH), 7.16e7.06 (m, 4H, ArH), 3.74 (s, 3H, OCH3), 3.71 (s, 3H, OCH3);
13C NMR (100 MHz, DMSO-d6): 158.49, 156.09, 148.42, 146.60,
131.85, 131.57, 129.24, 129.20, 125.42, 125.04, 124.94, 123.96, 123.20,
120.88, 120.88, 120.39, 120.39, 120.02, 112.33, 111.83, 111.81, 110.98,
55.56, 55.00; ESI-MS m/z 397 [MþH]þ; HRMS (ESI) calcd for
C24H20N4O2 [MþH]þ 397.1659, found 397.1650.
4.2.4. (E)-N-(4-Methylbenzylidene)-2-(p-tolyl)-1H-benzo[d]imidazo
[1,2-a]imidazol-3-amine (4d). Isolated as a yellow powder; mp
279.9e281.1 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.05 (br s, 1H, NH),
8.86 (s, 1H, CH), 8.06 (d, J¼8.0 Hz, 2H, ArH), 7.89 (d, J¼8.0 Hz, 2H,
ArH), 7.71 (d, J¼7.9 Hz, 1H, ArH), 7.45 (d, J¼7.9 Hz, 1H, ArH), 7.38 (d,
J¼8.1 Hz, 2H, ArH), 7.32 (t, J¼7.8 Hz, 1H, ArH), 7.26 (d, J¼8.1 Hz, 2H,
ArH), 7.20 (t, J¼7.8 Hz, 1H, ArH), 2.41 (s, 3H, CH3), 2.35 (s, 3H, CH3);
13C NMR (100 MHz, DMSO-d6): 151.83, 148.09, 140.56, 135.94,
134.12, 131.82, 129.60, 129.60, 129.28, 129.06, 128.85, 128.85, 127.84,
127.84, 127.44, 127.23, 127.23, 125.19, 123.46, 119.72, 112.30, 112.18,
21.16, 20.83; ESI-MS m/z 365 [MþH]þ; HRMS (ESI) calcd for
C24H20N4 [MþH]þ 365.1761, found 365.1770.
4.2.10. (E)-N-(3-Methoxybenzylidene)-2-(3-methoxyphenyl)-1H-
benzo[d]imidazo[1,2-a]imidazol-3-amine (4j). Isolated as a yellow
powder; mp 220.8e221.0 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.13
(br s,1H, NH), 8.86 (s,1H, CH), 7.85 (s, 1H, ArH), 7.80 (d, J¼7.9 Hz,1H,
ArH), 7.73 (d, J¼7.9 Hz, 1H, ArH), 7.61e7.56 (m, 2H, ArH), 7.50e7.44
(m, 2H, ArH), 7.37e7.32 (m, 2H, ArH), 7.22 (t, J¼7.7 Hz,1H, ArH), 7.09
(dd, J¼8.2, 2.2 Hz, 1H, ArH), 6.86 (dd, J¼8.2, 2.2 Hz, 1H, ArH), 3.86 (s,
3H, OCH3), 3.82 (s, 3H, OCH3); 13C NMR (100 MHz, DMSO-d6):
159.72, 159.14, 151.57, 148.21, 138.19, 135.97, 130.08, 129.24, 127.71,
125.07, 123.70, 120.86, 120.86, 119.92, 119.79, 116.75, 116.75, 112.69,
112.54, 112.44, 112.24, 111.87, 55.16, 54.92; ESI-MS m/z 397 [MþH]þ;
HRMS (ESI) calcd for C24H20N4O2 [MþH]þ 397.1659, found
397.1662.
4.2.5. (E)-N-(2-Chlorobenzylidene)-2-(2-chlorophenyl)-1H-benzo[d]
imidazo[1,2-a]imidazol-3-amine (4e). Isolated as a yellow powder;
mp 280.5e282.3 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.09 (br s, 1H,
NH), 8.47 (s, 1H, CH), 8.28 (d, J¼7.4 Hz, 1H, ArH), 8.05 (d, J¼7.4 Hz,
1H, ArH), 7.70e7.65 (m, 1H, ArH), 7.64e7.58 (m, 1H, ArH), 7.52e7.43
(m, 6H, ArH), 7.36 (t, J¼7.8 Hz, 1H, ArH), 7.29 (t, J¼7.8 Hz, 1H, ArH);
13C NMR (100 MHz, DMSO-d6): 147.06, 146.83, 135.11, 134.26,
133.91, 133.32, 132.44, 132.36, 131.76, 130.32, 129.91, 129.91, 129.70,
128.46, 127.71, 127.53, 126.89, 124.45, 123.81, 120.61, 112.12, 112.12;
ESI-MS m/z 405 [MþH]þ; HRMS (ESI) calcd for C22H14Cl2N4 [MþH]þ
405.0668, found 405.0664.
4.2.6. (E)-N-(3-Chlorobenzylidene)-2-(3-chlorophenyl)-1H-benzo[d]
imidazo[1,2-a]imidazol-3-amine (4f). Isolated as a yellow powder;
mp 292.8e293.1 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.27 (br s, 1H,
NH), 8.88 (s,1H, CH), 8.31 (t, J¼1.8 Hz,1H, ArH), 8.17 (d, J¼7.9 Hz,1H,
ArH), 8.07 (s, 1H, ArH), 8.00 (d, J¼7.8 Hz, 1H, ArH), 7.82 (d, J¼7.8 Hz,
1H, ArH), 7.63e7.57 (m, 2H, ArH), 7.52e7.45 (m, 2H, ArH), 7.40e7.34
(m, 2H, ArH), 7.25 (t, J¼7.5 Hz, 1H, ArH); 13C NMR (100 MHz, DMSO-
d6): 149.62, 148.49, 138.92, 133.89, 133.89, 133.02, 130.87, 130.87,
130.19, 130.14, 127.85, 127.38, 127.07, 126.51, 126.34, 126.34, 125.74,
124.84, 124.06, 120.22, 113.01, 112.07; ESI-MS m/z 405 [MþH]þ;
HRMS (ESI) calcd for C22H14Cl2N4 [MþH]þ 405.0668, found
405.0663.
4.2.11. (E)-N-(4-Methoxybenzylidene)-2-(4-methoxyphenyl)-1H-
benzo[d]imidazo[1,2-a]imidazol-3-amine (4k). Isolated as a yellow
powder; mp 285.9e286.7 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.01
(br s, 1H, NH), 8.82 (s, 1H, CH), 8.12 (d, J¼8.8 Hz, 2H, ArH), 7.95 (d,
J¼8.8 Hz, 2H, ArH), 7.68 (d, J¼8.0 Hz, 1H, ArH), 7.44 (d, J¼8.0 Hz, 1H,
ArH), 7.30 (t, J¼7.8 Hz, 1H, ArH), 7.19 (t, J¼7.8 Hz, 1H, ArH), 7.12 (d,
J¼8.7 Hz, 2H, ArH), 7.02 (d, J¼8.7 Hz, 2H, ArH), 3.86 (s, 3H, OCH3),
3.80 (s, 3H, OCH3); 13C NMR (100 MHz, DMSO-d6): 161.36, 158.19,
158.19, 151.49, 148.01, 129.55, 129.55, 128.48, 128.48, 127.20, 127.06,
125.31, 123.32, 123.32, 119.61, 119.61, 114.55, 114.55, 113.77, 113.77,
112.39, 112.10, 55.42, 55.07; ESI-MS m/z 397 [MþH]þ; HRMS (ESI)
calcd for C24H20N4O2 [MþH]þ 397.1659, found 397.1655.
4.2.7. (E)-N-(4-Chlorobenzylidene)-2-(4-chlorophenyl)-1H-benzo[d]
imidazo[1,2-a]imidazol-3-amine (4g). Isolated as a yellow powder;
mp >300 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.16 (br s, 1H, NH),
8.88 (s, 1H, CH), 8.21 (d, J¼8.4 Hz, 2H, ArH), 8.03 (d, J¼8.4 Hz, 2H,
ArH), 7.76 (d, J¼8.0 Hz, 1H, ArH), 7.62 (d, J¼8.2 Hz, 2H, ArH), 7.50
(d, J¼8.2 Hz, 2H, ArH), 7.45 (d, J¼8.0 Hz, 1H, ArH), 7.35 (t, J¼7.8 Hz,
1H, ArH), 7.22 (t, J¼7.8 Hz, 1H, ArH); 13C NMR (125 MHz, DMSO-d6)
dC: 150.88, 148.83, 136.05, 135.54, 131.82, 130.07, 130.07, 129.60,
129.60, 129.47, 129.47, 128.83, 128.83, 128.34, 128.24, 125.42,
125.39, 124.41, 124.36, 120.58, 113.19, 113.19; ESI-MS m/z 405
[MþH]þ; HRMS (ESI) calcd for C22H14Cl2N4 [MþH]þ 405.0668,
found 405.0677.
4.2.12. (E)-3-(3-((3-Hydroxybenzylidene)amino)-1H-benzo[d]imi-
dazo[1,2-a]imidazol-2-yl)phenol (4l). Isolated as a yellow powder;
mp 247.1e247.5 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.06 (br s, 1H,
NH), 9.76 (br s, 1H, OH), 9.43 (br s, 1H, OH), 8.82 (s, 1H, CH), 7.68 (d,
J¼8.0 Hz, 1H, ArH), 7.63 (d, J¼8.0 Hz, 1H, ArH), 7.61e7.57 (m, 1H,
ArH), 7.47e7.40 (m, 3H, ArH), 7.39e7.30 (m, 2H, ArH), 7.26e7.16 (m,
2H, ArH), 6.96e6.91 (m, 1H, ArH), 6.72e6.67 (m, 1H, ArH); 13C NMR
(100 MHz, DMSO-d6): 157.84, 157.21, 152.89, 147.98, 137.95, 135.89,
130.03, 130.03, 129.16, 127.81, 125.15, 123.60, 119.79, 119.60, 118.39,
118.07, 114.23, 113.93, 113.70, 113.70, 112.30, 112.11; ESI-MS m/z 369
[MþH]þ; HRMS (ESI) calcd for C22H16N4O2 [MþH]þ 369.1346, found
369.1355.
4.2.8. (E)-N-(4-Fluorobenzylidene)-2-(4-fluorophenyl)-1H-benzo[d]
imidazo[1,2-a]imidazol-3-amine (4h). Isolated as a yellow powder;
mp >300 ꢀC; 1H NMR (400 MHz, DMSO-d6): 12.14 (br s, 1H, NH),
8.87 (s, 1H, CH), 8.21 (dd, J¼8.6, 5.8 Hz, 2H, ArH), 8.06 (dd, J¼8.6,
5.8 Hz, 2H, ArH), 7.72 (d, J¼8.1 Hz, 1H, ArH), 7.46e7.18 (m, 7H, ArH);
13C NMR (100 MHz, DMSO-d6): 164.50 and 162.52 (1JCF¼247.4 Hz),
162.15 and 160.21 (1JCF¼243.0 Hz), 150.57, 148.15, 135.77 and 135.75
(4JCF¼2.8 Hz), 135.33, 133.35 and 133.32 (4JCF¼2.6 Hz), 131.27, 130.16
and 130.09 (3JCF¼8.6 Hz), 129.26 and 129.20 (3JCF¼7.8 Hz), 127.45,
125.01, 123.72, 119.94, 116.18 and 116.01 (2JCF¼21.9 Hz), 115.24 and
4.2.13. (E)-4-(3-((4-Hydroxybenzylidene)amino)-1H-benzo[d]imi-
dazo[1,2-a]imidazol-2-yl)phenol (4m). Isolated as a yellow powder;
mp >300 ꢀC; 1H NMR (400 MHz, DMSO-d6): 11.90 (br s, 1H, NH),
10.12 (br s, 1H, OH), 9.54 (br s, 1H, OH), 8.77 (s, 1H, CH), 7.97 (d,
J¼8.6 Hz, 2H, ArH), 7.84 (d, J¼8.6 Hz, 2H, ArH), 7.66 (d, J¼8.0 Hz, 1H,
ArH), 7.43 (d, J¼8.0 Hz, 1H, ArH), 7.28 (t, J¼7.6 Hz, 1H, ArH), 7.16 (t,
J¼7.6 Hz, 1H, ArH), 6.94 (d, J¼8.6 Hz, 2H, ArH), 6.84 (d, J¼8.6 Hz, 2H,
ArH); 13C NMR (100 MHz, DMSO-d6): 160.08, 156.42, 152.06, 147.91,
129.77, 129.77, 128.56, 128.56, 128.07, 126.77, 125.45, 125.45, 123.14,
119.44, 115.96, 115.96, 115.84, 115.18, 115.18, 112.60, 111.92, 111.92;

F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
2959
ESI-MS m/z 369 [MþH]þ; HRMS (ESI) calcd for C22H16N4O2 [MþH]þ
101.66,101.66, 94.96; ESI-MS m/z 381 [MþH]þ; HRMS (ESI) calcd for
C23H16N4O2 [MþH]þ 381.1346, found 381.1334.
369.1346, found 369.1343.
4.2.14. (E)-N-(3,4-Dimethoxybenzylidene)-2-(3,4-dimethoxyphenyl)-
1H-benzo[d]imidazo[1,2-a]imidazol-3-amine (4p). Isolated as a yel-
low powder; mp 271.1e273.4 ꢀC; 1H NMR (400 MHz, DMSO-d6):
11.99 (br s, 1H, NH), 8.82 (s, 1H, CH), 7.87 (d, J¼1.9 Hz, 1H, ArH), 7.73
(dd, J¼8.4, 1.9 Hz, 1H, ArH), 7.68e7.64 (m, 2H, ArH), 7.53 (dd, J¼8.4,
1.9 Hz, 1H, ArH), 7.45 (d, J¼7.6 Hz, 1H, ArH), 7.31 (t, J¼7.4 Hz, 1H,
ArH), 7.19 (t, J¼7.4 Hz, 1H, ArH), 7.13 (d, J¼8.4 Hz, 1H, ArH), 7.04 (d,
J¼8.4 Hz,1H, ArH), 3.88 (s, 3H, OCH3), 3.86 (s, 3H, OCH3), 3.83 (s, 3H,
OCH3), 3.80 (s, 3H, OCH3); 13C NMR (100 MHz, DMSO-d6): 152.07,
151.36, 149.26, 148.37, 147.98, 147.86, 129.64, 129.64, 127.35, 127.12,
125.31, 123.34, 122.86, 119.75, 119.62, 112.45, 112.05, 111.66, 111.66,
110.74, 108.89, 108.89, 55.69, 55.48, 55.46, 55.27; ESI-MS m/z 457
[MþH]þ; HRMS (ESI) calcd for C26H24N4O4 [MþH]þ 457.1870, found
457.1864.
4.2.19. (E)-N-Benzylidene-9-methyl-2-phenyl-9H-benzo[d]imidazo
[1,2-a]imidazol-3-amine (4u). Isolated as a yellow powder; mp
130.0e131.2 ꢀC; 1H NMR (400 MHz, DMSO-d6) dH: 8.85 (s, 1H, CH),
8.22 (dd, J¼8.0, 1.2 Hz, 2H, ArH), 7.99 (dd, J¼8.0, 1.2 Hz, 2H, ArH),
7.73 (d, J¼8.1 Hz, 1H, ArH), 7.60e7.51 (m, 4H, ArH), 7.44 (t, J¼7.8 Hz,
2H, ArH), 7.39 (t, J¼7.8 Hz, 1H, ArH), 7.29 (d, J¼7.4 Hz, 1H, ArH), 7.25
(d, J¼7.4 Hz, 1H, ArH), 3.76 (s, 3H, CH3); 13C NMR (125 MHz, DMSO-
d6) dC: 152.14, 148.73, 137.18, 137.13, 136.57, 135.39, 131.09, 129.48,
129.48, 128.94, 128.72, 128.72, 128.40, 128.40, 127.92, 127.92, 127.27,
124.86, 124.14, 120.67, 112.90, 110.74, 29.16; ESI-MS m/z 351
[MþH]þ; HRMS (ESI) calcd for C23H18N4 [MþH]þ 351.1604, found
351.1615.
Acknowledgements
4.2.15. (E)-N-(3,4,5-Trimethoxybenzylidene)-2-(3,4,5-
trimethoxyphenyl)-1H-benzo[d]imidazo[1,2-a]imidazol-3-amine
(4q). Isolated as a yellow powder; mp 252.1e252.7 ꢀC; 1H NMR
(400 MHz, DMSO-d6): 12.04 (br s, 1H, NH), 8.85 (s, 1H, CH), 7.65 (d,
J¼8.0 Hz, 1H, ArH), 7.56 (s, 2H, ArH), 7.46 (d, J¼8.0 Hz, 1H, ArH), 7.37
(s, 2H, ArH), 7.33 (t, J¼7.8 Hz, 1H, ArH), 7.21 (t, J¼7.8 Hz, 1H, ArH),
3.88 (s, 6H, OCH3), 3.85 (s, 6H, OCH3), 3.76 (s, 3H, OCH3), 3.71 (s, 3H,
OCH3); 13C NMR (100 MHz, DMSO-d6): 153.41, 153.41, 152.86,
152.71, 152.71, 147.98, 140.06, 136.64, 132.08, 132.08, 127.64, 125.15,
123.60, 123.60, 119.86, 112.32, 112.19, 112.19, 105.16, 105.16, 104.52,
104.52, 60.23, 60.13, 56.00, 56.00, 55.71, 55.71; ESI-MS m/z 517
[MþH]þ; HRMS (ESI) calcd for C28H28N4O6 [MþH]þ 517.2082, found
517.2087.
This work was supported by the National Natural Science
Foundation of China (grants 30925040, 81102329, 81273397), the
Chinese National Science & Technology Major Project ‘Key New Drug
Creation and Manufacturing Program’ (grants 2013ZX09508104,
2012ZX09301001-001, 2011ZX09307-002-03), and the Science
Foundation of Shanghai (grant 12XD1405700).
Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.02.022.
References and notes
4.2.16. (E)-2-(Pyridin-3-yl)-N-(pyridin-3-ylmethylene)-1H-benzo[d]
imidazo[1,2-a]imidazol-3-amine (4r). Isolated as a yellow powder;
mp 257.2e258.1 ꢀC; 1H NMR (400 MHz, DMSO-d6) dH: 12.21 (br s,
1H, NH), 9.36 (s, 1H, ArH), 9.15 (s, 1H, ArH), 8.92 (s, 1H, CH), 8.67 (d,
J¼4.5 Hz, 1H, ArH), 8.51 (d, J¼8.0 Hz, 1H, ArH), 8.48 (d, J¼4.5 Hz, 1H,
ArH), 8.37 (d, J¼8.0 Hz, 1H, ArH), 7.85 (d, J¼8.3 Hz, 1H, ArH),
7.62e7.55 (m,1H, ArH), 7.52e7.42 (m, 2H, ArH), 7.36 (t, J¼7.7 Hz,1H,
}
ꢀ
1. (a) Domling, A.; Ugi, I. Angew. Chem., Int. Ed. 2000, 39, 3168; (b) Bienayme, H.;
Hulme, C.; Oddon, G.; Schmitt, P. Chem.dEur. J. 2000, 6, 3321.
2. Strecker, A. Liebigs Ann. Chem. 1850, 75, 27.
3. (a) Bose, A. K.; Pednekar, S.; Ganguly, S. N.; Chakraborty, G.; Manhas, M. S.
Tetrahedron Lett. 2004, 45, 8351; (b) Rao, G. B. D.; Acharya, B. N.; Verma, S. K.;
Kaushik, M. P. Tetrahedron Lett. 2011, 52, 809; (c) Narahari, S. R.; Reguri, B. R.;
Gudaparthi, O.; Mukkanti, K. Tetrahedron Lett. 2012, 53, 1543.
4. (a) Liu, X. L.; Zhang, X. M.; Yuan, W. C. Tetrahedron Lett. 2011, 52, 903; (b) Kavala, V.;
Lin, C.; Kuo, C. W.; Fang, H.; Yao, C. F. Tetrahedron 2012, 68, 1321; (c) Landelle, G.;
Claraz, A.; Oudeyer, S.; Levacher, V. Tetrahedron Lett. 2012, 53, 2414.
5. (a) Brioche, J.; Masson, G.; Zhu, J. Org. Lett. 2010, 12, 1432; (b) Bousquet, T.; Jida,
M.; Soueidan, M.; Deprez-Poulain, R.; Agbossou-Niedercorn, F.; Pelinski, L.
Tetrahedron Lett. 2012, 53, 306; (c) Beasley, B. O.; Clarkson, G. J.; Shipman, M.
Tetrahedron Lett. 2012, 53, 2951.
ArH), 7.24 (t, J¼7.7 Hz, 1H, ArH); 13C NMR (100 MHz, DMSO-d6) dC
:
151.34, 150.24, 149.17, 149.10, 148.84, 148.00, 135.86, 135.82, 134.75,
134.56,132.78,130.98,128.69, 125.25, 124.57, 124.48, 123.91,120.65,
113.45, 112.48; ESI-MS m/z 339 [MþH]þ; HRMS (ESI) calcd for
C20H14N6 [MþH]þ 339.1353, found 369.1340.
6. (a) Campian, E.; Lou, B.; Saneii, H. Tetrahedron Lett. 2002, 43, 8467; (b) Kalinski, C.;
Umkehrer, M.; Gonnard, S.; Jager, N.; Rossa, G.; Hillerb, W. Tetrahedron Lett. 2006,
€
4.2.17. (E)-N-Benzylidene-6,7-dichloro-2-phenyl-1H-benzo[d]imi-
dazo[1,2-a]imidazol-3-amine (4s). Isolated as a yellow powder; mp
>300 ꢀC; 1H NMR (500 MHz, DMSO-d6) dH: 12.50 (br s, 1H, NH),
8.95 (s, 1H, CH), 8.09 (d, J¼7.6 Hz, 2H, ArH), 8.01 (d, J¼7.6 Hz, 2H,
ArH), 7.83 (s,1H, ArH), 7.74 (s,1H, ArH), 7.62e7.55 (m, 3H, ArH), 7.49
(t, J¼7.5 Hz, 2H, ArH), 7.35 (t, J¼7.5 Hz,1H, ArH); 13C NMR (125 MHz,
DMSO-d6) dC: 155.45, 149.95, 136.72, 136.72, 131.72, 131.72, 129.63,
129.63, 129.03, 129.03, 128.65, 128.65, 127.97, 127.82, 127.82, 127.73,
125.94, 125.70, 121.13, 121.06, 113.97, 113.97; ESI-MS m/z 405
[MþH]þ; HRMS (ESI) calcd for C22H14Cl2N4 [MþH]þ 405.0668,
found 405.0679.
47, 2041; (c) Mroczkiewicz, M.; Ostaszewski, R. Tetrahedron 2009, 65, 4025; (d)
Lakontseva, E.; Krasavin, M. Tetrahedron Lett. 2010, 51, 4095; (e) Evans, C. G.;
Smith, M. C.; Carolan, J. P.; Gestwicki, J. E. Bioorg. Med. Chem. Lett. 2011, 21,
2587; (f) Ge, J.; Cheng, X.; Tan, L. P.; Yao, S. Q. Chem. Commun. 2012, 48, 4453.
7. (a) Ramon, D. J.; Yus, M. Angew. Chem., Int. Ed. 2005, 44, 1602; (b) Domling,
A. Chem. Rev. 2006, 106, 17; (c) Dondoni, A.; Massi, A.; Sabbatini, S.; Berto-
lasi, V. J. Org. Chem. 2002, 67, 6979; (d) Nilsson, B. L.; Overman, L. E. J. Org.
Chem. 2006, 71, 7706; (e) Galliford, C. V.; Scheidt, K. A. J. Org. Chem. 2007,
72, 1811.
ꢀ
€
8. (a) Bharti, N.; Shailendra; Garza, M. T. G.; Cruz-Vega, D. E.; Castro-Garza, J.;
Saleem, K.; Naqvi, F.; Maurya, M. R.; Azam, A. Bioorg. Med. Chem. Lett. 2002, 12,
869; (b) Joo, T. Y.; Singh, N.; Lee, G. W.; Jang, D. O. Tetrahedron Lett. 2007, 48,
8846; (c) Tu, S.; Shao, Q.; Zhou, D.; Cao, L.; Shi, F.; Li, C. J. Heterocycl. Chem. 2007,
44, 1401; (d) Liu, G.; Shao, Q.; Tu, S.; Cao, L.; Li, C.; Zhou, D.; Han, B. J. Heterocycl.
Chem. 2008, 45, 1127; (e) Sørensen, U. S.; Strøbæk, D.; Christophersen, P.;
Hougaard, C.; Jensen, M. L.; Nielsen, E. é.; Peters, D.; Teuber, L. J. Med. Chem.
2008, 51, 7625.
4.2.18. (E)-N-Benzylidene-7-phenyl-6H-[1,3]dioxolo[40,50:4,5]benzo
[1,2-d]imidazo[1,2-a]imidazol-8-amine (4t). Isolated as a yellow
ꢀ
ꢀ
9. (a) Almas¸ i, D.; Alonso, D. A.; Gomez-Bengoa, E.; Najera, C. J. Org. Chem. 2009, 74,
powder; mp 287.1e287.3 ꢀC; 1H NMR (400 MHz, DMSO-d6) dH
:
6163; (b) Zhang, L.; Lee, M. M.; Lee, S. M.; Lee, J.; Cheng, M.; Jeong, B. S.; Park, H.;
ꢀ
Jewb, S. Adv. Synth. Catal. 2009, 351, 3063; (c) Gomez-Torres, E.; Alonso, D. A.;
11.96 (br s, 1H, NH), 8.79 (s, 1H, CH), 8.12 (d, J¼7.2 Hz, 2H, ArH), 7.98
(d, J¼7.2 Hz, 2H, ArH), 7.58e7.50 (m, 3H, ArH), 7.44 (t, J¼7.6 Hz, 2H,
ArH), 7.28 (t, J¼7.6 Hz, 1H, ArH), 7.26 (s, 1H, ArH), 7.08 (s, 1H, ArH),
6.07 (s, 2H, CH2). 13C NMR (125 MHz, DMSO-d6) dC: 152.38, 148.79,
144.64, 144.64, 141.64, 137.12, 137.12, 131.03, 129.46, 129.46, 128.76,
128.76, 128.32, 128.32, 127.74, 127.74, 127.48, 127.24, 127.24, 119.08,
ꢀ
ꢀ
Gomez-Bengoa, E.; Najera, C. Org. Lett. 2011, 13, 6106.
10. Rogers, S. A.; Huigens, R. W., III; Melander, C. J. Am. Chem. Soc. 2009, 131, 9868.
11. (a) Kang, J.; Kim, H. S.; Jang, D. O. Tetrahedron Lett. 2005, 46, 6079; (b) Moon, K. S.;
Singh, N.; Lee, G. W.; Jang, D. O. Tetrahedron 2007, 63, 9106.
12. (a) Feng, L.-C.; Sun, Y.-W.; Tang, W.-J.; Xu, L.-J.; Lam, K.-L.; Zhou, Z.; Chan, A. S. C.
Green Chem. 2010, 12, 949; (b) Mecadon, H.; Rohman, M. R.; Kharbangar, I.;
Laloo, B. M.; Kharkongor, I.; Rajbangshi, M.; Myrboh, B. Tetrahedron Lett. 2011,

2960
F. Chen et al. / Tetrahedron 69 (2013) 2954e2960
52, 3228; (c) Karimi, A. R.; Momeni, H. R.; Pashazadeh, R. Tetrahedron Lett. 2012,
18. Lei, M.; Ma, L.; Hu, L. Synth. Commun. 2011, 41, 1969.
53, 3440.
19. Lei, M.; Ma, L.; Hu, L. Synth. Commun. 2011, 41, 3424.
13. Lei, M.; Ma, L.; Hu, L. Tetrahedron Lett. 2011, 52, 2597.
14. Khurana, J. M.; Nand, B.; Saluja, P. Tetrahedron 2010, 66, 5637.
15. Sajadikhah, S. S.; Maghsoodlou, M. T.; Hazeri, N.; Habibi-Khorassani, S. M.;
Willis, A. C. Chin. Chem. Lett. 2012, 23, 569.
20. Liu, J. H.; Lei, M.; Hu, L. Green Chem. 2012, 14, 840.
21. (a) Noonan, C.; Baragwanath, L.; Connon, S. J. Tetrahedron Lett. 2008, 49,
€
4003; (b) Dunkelmann, P.; Jung, D. K.; Nitsche, A.; Demir, A. S.; Siegert, P.;
€
Lingen, B.; Baumann, M.; Pohl, M.; Muller, M. J. Am. Chem. Soc. 2002, 124,
16. Mandhane, P. G.; Joshi, R. S.; Nagargoje, D. R.; Gill, C. H. Chin. Chem. Lett. 2011,
22, 563.
17. Lei, M.; Ma, L.; Hu, L. Monatsh. Chem. 2010, 141, 1005.
12084; (c) Orlandi, S.; Caporale, M.; Benaglia, M.; Annunziata, R. Tetrahedron:
Asymmetry 2003, 14, 3827; (d) Sheenan, J. C.; Hunneman, D. H. J. Am. Chem.
Soc. 1966, 88, 3666; (e) Sheehan, J. C.; Hara, T. J. Org. Chem. 1974, 39, 1196.