Synthesis and cytotoxic activity of novel hexahydropyrrolo[2,3-b]indole imidazolium salts

Article abstract of DOI:10.1016/j.bmcl.2015.11.092

A series of novel hexahydropyrrolo[2,3-b]indole-1H-imidazolium salts were synthesized and evaluated in vitro against a panel of human tumor cell lines. The results suggest that the 5,6-dimethyl-benzimidazole ring, and substitution of the imidazolyl-3-posi

Full text of DOI:10.1016/j.bmcl.2015.11.092

Bioorganic & Medicinal Chemistry Letters 26 (2016) 460–465
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and cytotoxic activity of novel hexahydropyrrolo[2,3-b]
indole imidazolium salts
Yunjing Zhou a, Kunyun Duan a, Liang Zhu a, Zhengfeng Liu a, Chaobo Zhang a, Lijuan Yang b, Minyan Li a,
Hongbin Zhang a, , Xiaodong Yang a,
⇑
⇑
a Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of Education, School of Chemical Science and Technology, Yunnan University,
Kunming 650091, PR China
b School of Chemistry & Environment, Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University, Kunming 650500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel hexahydropyrrolo[2,3-b]indole–1H-imidazolium salts were synthesized and evaluated
in vitro against a panel of human tumor cell lines. The results suggest that the 5,6-dimethyl-benzimida-
zole ring, and substitution of the imidazolyl-3-position with a 2-bromobenzyl or 2-naphthylmethyl
group, were important for the cytotoxic activity. Notably, Compound 43, bearing a 2-bromobenzyl sub-
stituent at position-3 of 5,6-dimethyl-benzimidazole, was found to possess the most potent derivative
Received 27 August 2015
Revised 21 November 2015
Accepted 25 November 2015
Available online 25 November 2015
against five human tumor cell lines with IC50 values below 2.68
SMMC-7721, A549 and SW480 cell lines. Compounds 25 and 39 were more selective to HL-60 and
MCF-7 cell lines with IC50 values of 0.47 and 1.46 M.
lM and more selective towards
Keywords:
Hexahydropyrrolo[2,3-b]indole
Imidazolium salts
l
Ó 2015 Published by Elsevier Ltd.
Structure–activity relationships
Hexahydropyrrolo[2,3-b]indoles and indolines represent the
important classes of biologically active nitrogen-containing hete-
rocycles. Natural compounds and biologically active agents pos-
sessing the indoline and hexahydropyrrolo[2,3-b]indole moieties
exhibit a wide range of remarkable biological activities, especially
antitumor activity.1 As exemplified in Figure 1, ICNP with indoline
moiety showed strong cytotoxic activities against NCI-H23 and
PC-3 cell lines,2 while Luteoalbusin A with hexahydropyrrolo[2,3-
b]indole moiety exhibited potent cytotoxic activity against
SF-268, MCF-7, NCI-H460 and HepG-2 cell lines.3 On the other
hand, imidazole and their derivatives have gained considerable
interests due to their significant biological activities,4 especially
antitumor activity.5 For instance, natural imidazolium chlorides
Lepidiline A and B (Fig. 1) showed potent cytotoxic activity against
four human cancer cell lines.6 In this respect, we have previously
reported the synthesis of a series of novel imidazolium salts, such
as NMIB (Fig. 1), and their potential antitumor activity.7 Mecha-
nism study demonstrated that the imidazolium salts can induce
the cell cycle arrest and apoptosis in tumor cells.7
the preparation of the hybridizing compounds of hexahydropy-
rrolo[2,3-b]indoles with imidazole moieties. To the best of our
knowledge, no reports concerning antitumor activity of hexahy-
dropyrrolo[2,3-b]indoles–imidazole hybrids have been found in
the literature.
In this Letter, a series of novel hexahydropyrrolo[2,3-b]indole
imidazolium salts were synthesized to investigate the cytotoxic
activity of hexahydropyrrolo[2,3-b]indoles–imidazole hybrids with
the ultimate aim of developing potent antitumor agents.
To synthesize the hexahydropyrrolo[2,3-b]indole–1H-imida-
zolium salts, 2-iodobenzenamine 1 was chosen as the starting
material for the preparation of a series of hybrid derivatives
(21–78, Scheme 1). Acylation of 2-iodobenzenamine (1) with
methacryloyl chloride (2) gave anilide 3, which was N-benzylated
or N-methylated to afford 4 or 5 in 85% or 90% yield. Treatment
of 4 or 5 under palladium-catalysis conditions provided oxindole
6 or 7 in 50% or 52% yield. Reductive cyclization of oxindole 6 or
7 using LiAlH4 provided key hexahydropyrrolo[2,3-b]indole 8 or
9.9 N-acylation of 8 or 9 with 2-chloroacetyl chloride in the pres-
ence of pyridine gave the corresponding amide 10 or 11 in 90%
or 87% yield. Subsequently, chloride 10 or 11 was transformed to
the respective nine hexahydropyrrolo[2,3-b]indole–imidazole
hybrids 12–20 with various substituted imidazole or benzimida-
zole (imidazole, 2-methyl-imidazole, benzimidazole, 2-methyl-
benzimidazole or 5,6-dimethyl-benzimidazole) by heating under
DMF with 70–88% yields.10 Finally, sixty hexahydropyrrolo[2,3-b]
indole–1H-imidazolium salt derivatives (21–80) were synthesized
Molecular hybridization is a useful tool in new drug design and
development during the past two decades.8 Considering the anti-
cancer activities of hexahydropyrrolo[2,3-b]indoles, as well as the
potent cytotoxic activities of imidazole derivatives, we focus on
⇑
Corresponding authors. Tel.: +86 871 65031119; fax: +86 871 65035538.
E-mail addresses: zhanghbyd@gmail.com (H. Zhang), xdyang@ynu.edu.cn
(X. Yang).
http://dx.doi.org/10.1016/j.bmcl.2015.11.092
0960-894X/Ó 2015 Published by Elsevier Ltd.

Y. Zhou et al. / Bioorg. Med. Chem. Lett. 26 (2016) 460–465
461
Indoline moiety
As shown in Table 2, the structures of hexahydropyrrolo[2,3-b]
indole–imidazole derivatives have an obvious influence on the
inhibitory activities. Hexahydropyrrolo[2,3-b]indole (8 and 9), imi-
dazole, hexahydropyrrolo[2,3-b]indoles 12–20 lacked activities
against all tumor cell lines investigated at the concentration of
Imidazolium salts
Cl
HN
N
N
Cl
N
O
Cl
R
20 lM. However, their imidazolium salts 21–80 exhibited some
O
NHBoc
ICNP
Lepidiline A R = H
Lepidiline B R = Me
degree or high cytotoxicity. This difference in inhibitory activities
between neutral compounds and imidazolium salts may be due
to the changes of molecular structure, charge distribution and
water solubility.14 Hexahydropyrrolo[2,3-b]indole imidazolium
salts improved the water solubility and have high solubility in
DMSO and MeOH. Recent researches have reported that imida-
zolium salt hybrids can dissolve in many organic solvents and pos-
sess good water solubility.15
For the N-substituents of indole moiety, N-benzylated imida-
zolium salts 21–46 showed more active than N-methylated imida-
zolium salts 47–80. Many of N-methylated salts, such as 47–60,
lacked activities against all tumor cell lines, while most of N-ben-
zylated salts, such as 29–46, exhibited medium or high inhibitory
activities.
For the imidazole rings (imidazole, 2-methyl-imidazole, benz-
imidazole, 2-methyl-benzimidazole or 5,6-dimethyl-benzimida-
zole), imidazolium salts 47–51 with imidazole rings and 21–
25/52–57 with 2-methyl-imidazole rings lacked activities or dis-
played weak inhibitory activities. Meanwhile, compounds 26–
32/58–65 with benzimidazole rings and 33–39/66–72 with 2-
methyl-benzimidazole rings exhibited medium or high inhibitory
activities. Among them, compounds 30, 32, 36, 38, 39, 65, 69 and
72, bearing a 2-bromobenzyl, 4-methylbenzyl, 2-naphthylmethyl
or 2-naphthylacyl substituent at position-3 of the benzimidazole
or 2-methyl-benzimidazole rings, displayed similar or higher
inhibitory activity compared with DDP. However, compounds
40–46/73–80 with 5,6-dimethyl-benzimidazole rings showed
powerful inhibitory activities. Most of these kinds of derivatives
were found to be more active than DDP. Among them, compounds
Hexahydropyrrolo[2,3-b]indole moiety
H
N
O
O
N
N
S
S
N
OH
Br
N
H
H
O
NMIB
Luteoalbusin A
Figure 1. Representative structures of indolines, hexahydropyrrolo[2,3-b]indoles
and imidazolium salts.
with good yields by reaction of hexahydropyrrolo[2,3-b]indole–
imidazoles with the various alkyl bromides in refluxing toluene
(51–97%).11 The structures and yields of hexahydropyrrolo[2,3-b]
indole–1H-imidazolium salt derivatives are listed in Table 1.
To verify the structures of the hexahydropyrrolo[2,3-b]indole–
1H-imidazolium salts, imidazolium salts 70 was selected as repre-
sentative compounds and characterized by X-ray crystallography
(CCDC 1419385),
12
as shown in Figure 2.
The potential cytotoxicity of all newly synthesized hexahy-
dropyrrolo[2,3-b]indole–1H-imidazolium salts were evaluated
in vitro against a panel of human tumor cell lines, including leu-
kaemia (HL-60), myeloid liver carcinoma (SMMC-7721), lung carci-
noma (A549), breast carcinoma (MCF-7) and colon carcinoma
(SW480), on the basis of the procedures in the literature.13 DDP
(Cisplatin), as well as hexahydropyrrolo[2,3-b]indole (8 and 9)
and imidazole, were chosen as positive controls. The results are
shown in Table 2.
Method A:
BnBr, NaH, THF
I
O
I
I
Et3N, DCM
rt, 3h
+
Cl
rt, 3h
85%
Method B:
MeI, NaH, THF
rt, 3h
N
R1
NH2
N
H
3
1
2
4 R = Bn 85%
5 R = Me 90%
O
CN
Cl
Pd(OAc)2, K4[Fe(CN)6]
LiAlH4, THF
rt, 1h
Cl
NH
O
Na2CO3, DMF
120oC, 12 h
50%
N
Pyd, DCM
N
R1
R1
0oC, 30 min
8 R = Bn 55%
9 R = Me 52%
6 R = Bn 50%
7 R = Me 52%
R
R
HN
N
imidazole ring
N
N
N
Cl
imidazole
N
N
N
DMF
O
O
2-methyl-imidazole
benzimidazole
120oC, 24-36 h
70-88%
R1
R1
2-Methylbenzimidazole
5,6-Dimethylbenzimidazole
10 R = Bn 90%
11 R = Me 87%
12-20
R
R2
X
N
N
N
Br
R2
toluene, reflux
R1 = Bn, Me
R2 = phenacyl, benzyl
N
24-48 h
O
R1
51-97%
21-80
Scheme 1. Synthesis of hexahydropyrrolo[2,3-b]indole imidazolium salts 21–80.

462
Y. Zhou et al. / Bioorg. Med. Chem. Lett. 26 (2016) 460–465
Table 1
Structures and yields of compounds 12–80
Entry
Compound
R1
Imidazole ring
R2
Yields (%)
1
2
3
4
5
6
7
8
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
Bn
Bn
Bn
Bn
Me
Me
Me
Me
Me
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
2-Methyl-imidazole
Benzimidazole
2-Methyl-benzimidazole
5,6-Dimethyl-benzimidazole
Imidazole
—
—
—
—
—
—
—
—
70
80
72
74
73
76
75
88
84
51
60
65
81
60
67
65
66
60
55
51
67
87
90
90
93
97
91
92
67
62
61
56
80
80
85
82
88
83
81
88
92
94
95
96
85
88
74
63
71
71
73
73
71
80
73
65
77
67
73
73
84
80
77
78
72
63
65
77
80
2-Methyl-imidazole
Benzimidazole
2-Methyl-benzimidazole
5,6-Dimethyl-benzimidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
9
—
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
4-Methylbenzyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacy
2-Bromobenzyl
4-Bromobenzyl
4-Methylbenzyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Bromobenzyl
4-Bromobenzyl
4-Methylbenzyl
2-Naphthylmethyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Bromobenzyl
4-Bromobenzyl
4-Methylbenzyl
2-Naphthylmethyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
4-Bromobenzyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
4-Bromobenzyl
4-Methylbenzyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
2-Bromobenzyl
4-Bromobenzyl
4-Methylbenzyl
2-Naphthylmethyl
Phenacyl
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
2-Bromobenzyl
4-Methylbenzyl
2-Naphthylmethyl
Phenacyl
Benzimidazole
Benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
2-Methyl-imidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
Benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
2-Methyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
5,6-Dimethyl-benzimidazole
Bn
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
4-Bromophenacyl
4-Methoxyphenacyl
2-Naphthylacyl
2-Bromobenzyl
4-Bromobenzyl
4-Methylbenzyl
2-Naphthylmethyl
43, 45 and 46, with a 2-bromobenzyl, 4-methylbenzyl or 2-naph-
thylmethyl, displayed potent higher inhibitory activity in vitro
For the substituents of imidazolium salts, a phenacyl sub-
stituent at position-3 of imidazole ring, such as 21, 26, 33, 47, 52,
58 and 66, decreased the inhibitory activities against five tumor
than DDP with IC50 values below 4.84
lines.
lM against five tumor cell
cell lines, while
a 4-bromophenacyl or 4-methoxyphenacyl

Y. Zhou et al. / Bioorg. Med. Chem. Lett. 26 (2016) 460–465
463
substituent, such as 27, 28, 34, 35, 41, 42, 74 and 75, could slightly
improve the inhibitory activities. Contrastingly, a 2-naphthylacyl
substituent in 24, 29, 69 and 76, 4-bromobenzyl substituent in
31, 44 and 78 or a 4-methylbenzyl substituent in 25, 32, 38, 45
and 79 have positive effects on the inhibitory activities against
tumor cell lines. Especially, compound 25 was more selective to
HL-60 cell lines with IC50 values of 0.47 lM. Interestingly, com-
pared with above substituents, a 2-bromobenzyl substituent at
position-3 of imidazole ring, such as 30, 36, 43 and 77, or a 2-naph-
thylmethyl substituent in 39, 46, 72 and 80 could cause significant
improvement of the antitumor activity. Most of these kinds of slats
exhibited strong cytotoxic activities and were more active than
DDP. Notably, compound 43, with a 2-bromobenzyl substituent
at position-3 of 5,6-dimethyl-benzimidazole, was found to be the
Figure 2. X-ray crystal structures of compound 70.
Table 2
Cytotoxic activities of compounds 12–80 in vitrob (IC50
a
, lM)
Entry
Compound
HL-60
SMMC-7721
A-549
MCF-7
SW480
1
2
3
4
5
6
7
8
8
9
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
7.75
1.30
2.20
0.47
5.72
2.93
5.94
1.64
1.21
1.42
1.20
4.64
4.25
4.18
1.33
1.35
1.25
1.38
1.47
1.55
1.64
1.27
1.29
1.23
1.21
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
5.98
5.07
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
4.79
19.49
14.46
8.43
10.03
7.62
4.69
7.53
4.98
9.28
5.99
8.83
2.27
5.10
2.97
1.80
5.78
6.22
5.07
1.72
3.59
1.74
2.27
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
11.23
>20
>20
8.07
>20
7.40
6.76
7.59
6.23
14.66
9.87
12.54
7.34
7.91
6.63
6.26
6.54
8.01
7.40
2.68
6.37
4.84
4.80
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
10.55
5.67
7.26
8.16
8.15
7.83
5.13
2.23
3.87
2.72
6.35
7.37
6.99
2.50
2.40
1.98
1.46
3.51
2.78
2.35
1.78
1.72
1.86
1.68
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
11.02
14.03
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
7.78
9.71
8.43
9.18
8.35
6.88
6.35
7.43
6.57
8.31
8.86
8.38
4.62
6.11
1.62
1.64
7.09
1.68
1.50
1.44
1.41
1.77
1.76
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
8.56
>20
Imidazole
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
(continued on next page)

464
Y. Zhou et al. / Bioorg. Med. Chem. Lett. 26 (2016) 460–465
Table 2 (continued)
Entry
Compound
HL-60
SMMC-7721
A-549
MCF-7
SW480
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
DDP
6.96
5.13
2.80
>20
>20
>20
5.95
3.72
2.25
1.49
8.03
7.31
6.23
1.60
1.67
1.73
1.52
1.35
1.16
>20
>20
7.45
>20
>20
>20
>20
14.06
>20
>20
>20
7.64
>20
>20
16.69
>20
8.93
9.47
6.86
>20
>20
>20
9.45
8.95
10.83
5.09
13.44
8.28
8.15
4.88
6.28
3.08
5.79
1.84
10.45
>20
>20
7.95
>20
>20
>20
10.11
>20
>20
7.36
>20
8.52
11.12
5.01
7.24
6.90
7.57
4.54
8.88
>20
7.09
10.76
8.03
4.34
>20
8.72
7.49
2.85
5.61
5.69
4.52
4.03
8.08
>20
>20
6.47
12.66
17.36
10.48
6.18
7.10
a
Cytotoxicity as IC50 for each cell line, is the concentration of compound which reduced by 50% the optical density of treated cells with respect to untreated cells using the
MTT assay.
In conclusion, a number of novel hexahydropyrrolo[2,3-b]in-
dole–1H-imidazolium salts were synthesized and proved to be
potent cytotoxic activity. Compounds 30, 36, 39, 43, 46 and 80,
bearing a substituted benzimidazole ring, and 2-bromobenzyl or
2-naphthylmethyl substituent at position-3 of the benzimidazole
ring, were found to be the most potent activity. Notably,
Compound 43 was found to possess the most potent derivative
against five human tumor cell lines with IC50 values below
2.68 lM and more selective towards SMMC-7721, A549 and
SW480 cell lines, with IC50 values 4.7-fold, 2.6-fold and 6.2-fold
lower than DDP. Compounds 25 and 39 were more selective to
HL-60 and MCF-7 cell lines with IC50 values of 0.47 and 1.46 lM.
R
R
N
N
R2
N
N
N
N
Br
N
N
O
O
R1
R1
(A)
(B)
2-substituted indoline derivatives:
imidazolium salts (B) > imidazole hybrids (A)
Better
R1
=
Bn > Me
Better
imidazole ring:
benzimidazole
The hexahydropyrrolo[2,3-b]indole–1H-imidazolium salts 30, 36,
39, 43, 46 and 80 are promising leads for further structural
modifications, guided by the valuable information obtained from
our SARs.
imidazole
5,6-dimethyl-benzimidazole >
>
2-methyl-benzimidazole 2-methyl-imidazole
Best
Best
R' = 2-bromobenzyl ≈ 2-naphthylmethyl > 2-naphthylacyl ≈ 4-bromobenzyl
≈ 4-methylbenzyl > 4-bromophenacyl ≈ 4-methoxyphenacyl > phenacyl
Acknowledgments
Best Potent Compounds
This work was supported by grants from the Program for
Changjiang Scholars and Innovative Research Team in University
(IRT13095), Natural Science Foundation of China (21462049,
21332007 and U1402227), Yunnan Province (2013FA028 and
2012FB113), Education Department of Yunnan Province
(ZD2014010), and Program for China Scholarship Council
(201408535034) and excellent young talents of Yunnan University.
Authors thank Prof. Yan Li in Kunming Institute of Botany for
biological evaluation.
N
N
N
Br
Br
N
O
Bn
43
Scheme 2. Structure analysis of hexahydropyrrolo[2,3-b]indole–1H-imidazolium
salts.
Supplementary data
most potent derivative against five human tumor cell lines investi-
gated with IC50 values below 2.68 lM, and exhibited inhibitory
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.11.
092.
activity selectively against SMMC-7721, A549 and SW480 cell
lines, with IC50 values 4.7-fold, 2.6-fold and 6.2-fold lower than
DDP. Additionally, compound 39 with a 2-naphthylmethyl sub-
stituent was more selective to MCF-7 cell lines with IC50 value
7.2-fold lower than DDP.
References and notes
This finding suggests that the existence of 5,6-dimethyl-benz-
imidazole ring, and substitution of the imidazolyl-3-position with
a 2-bromobenzyl or 2-naphthylmethyl group, were important for
the antitumor activity. Moreover, an N-benzyl group at indole moi-
ety also led to enhance of the inhibitory activity. In general, the
structure–activity relationship (SAR) results are summarized in
Scheme 2.
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J = 17.4 Hz), 5.05 (1H, d, J = 17.4 Hz), 4.68 (1H, d, J = 16.3 Hz), 4.52 (1H, d,
J = 16.3 Hz), 3.97–3.92 (1H, m), 3.40–3.31 (1H, m), 2.28 (3H, s), 2.26 (3H, s),
2.28–2.25 (1H, m), 2.16–2.09 (1H, m), 1.34 (3H, s). 13C NMR (75 MHz, DMSO) d:
166.51, 149.06, 143.89, 141.54, 139.17, 133.98, 133.10, 130.75, 129.61, 128.28,
127.99, 126.83, 126.60, 122.29, 119.21, 117.41, 110.46, 106.06, 85.83, 50.85,
49.80, 46.08, 45.19, 39.39, 25.04, 19.97, 19.79. HRMS (ESI-TOF) m/z calcd for
C
29H31N4O [M+1]+ 451.2498. Found: 451.2493.
11. General procedure for the preparation hexahydropyrroloindole–1H-imidazolium
salts 21–80: A mixture of compound 12–20 (1 mmol) and alkyl bromides
(1.2 mmol) was stirred in toluene (10 mL) at reflux for 24–48 h. An insoluble
substance was formed. After completion of the reaction, as indicated by TLC,
the precipitate was filtered through a small pad of Celite, and washed with
toluene (3 Â 10 mL), then dried to afford imidazolium salts 21–80 in 51–97%
yields. Compound 43: white powder, yield 56%, mp 251–253 °C. IR m
max (cmÀ1):
3013, 1668, 1548, 1417, 1336, 1190, 1082, 1017, 901, 747. 1H NMR (300 MHz,
DMSO) d: 9.50 (1H, s), 7.78–7.76 (3H, m), 7.44–7.36 (2H, m), 7.32–7.13 (7H, m),
6.97 (1H, t, J = 7.5 Hz), 6.64 (1H, t, J = 7.5 Hz), 6.30 (1H, d, J = 7.8 Hz), 5.84 (2H,
s), 5.74 (1H, d, J = 17.1 Hz), 5.56 (1H, d, J = 17.1 Hz), 5.46 (1H, s), 4.66 (1H, d,
J = 16.2 Hz), 4.48 (1H, d, J = 16.2 Hz), 3.99–3.94 (1H, m), 2.36 (6H, s), 2.35–2.29
(1H, m), 2.19–2.13 (1H, m), 1.33 (3H, s); 13C NMR (75 MHz, DMSO) d 164.48,
148.98, 142.55, 139.01, 136.63, 133.89, 133.33, 132.63, 131.04, 130.40, 129.10,
128.45, 128.31, 128.05, 126.91, 126.68, 123.10, 122.36, 117.61, 113.50, 113.08,
106.29, 85.96, 51.04, 50.26, 49.86, 48.52, 45.35, 39.42, 25.04, 19.95. HRMS (ESI-
TOF) m/z calcd for C36H36BrN4O [M+1]+ 619.2067. Found: 619.2065.
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10. General procedure for the preparation of hexahydropyrroloindole–imidazoles 12–
20: A mixture of amide 10 or 11 (2 mmol) and imidazole or various substituted
benzimidazole (6 mmol) was stirred in DMF (20 mL) at 120 °C for 24–36 h
(monitored by TLC). After cooling to room temperature, the solvent was
concentrated, and the residue was diluted with EtOAc (20 mL). The organic
layer was washed with water (20 mL) and brine (20 mL), dried over anhydrous
Na2SO4 and concentrated. The residue was purified by column
chromatography (silica gel, petroleum ether 60–90 °C/EtOAc/Et3N = 1:1:0.1)
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white power, yield 74%, mp 103–104 °C. IR mmax (cmÀ1): 3309, 2956, 1755,
1659, 1462, 1327, 1191, 1010, 919, 861, 746, 666. 1H NMR (300 MHz, DMSO): d
7.88 (1H, s), 7.42 (1H, s), 7.35–7.18 (6H, m), 7.13 (1H, d, J = 6.9 Hz), 7.00–6.95
(1H, m), 6.64 (1H, t, J = 7.5 Hz), 6.28 (1H, d, J = 7.8 Hz), 5.52 (1H, s), 5.28 (1H, d,
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