Unprecedented C-2 arylation of indole with diazonium salts: Syntheses of 2,3-disubstituted indoles and their antimicrobial activity

Article abstract of DOI:10.1016/j.bmcl.2011.06.081

A novel reaction of indole with aryldiazonium salts leading to the formation of 2-aryl-3-(arylazo)indoles was discovered. The products were found to possess potent anti-MRSA and anti-LLVRE activities. The SAR studies indicate that the potentially metaboli

Full text of DOI:10.1016/j.bmcl.2011.06.081

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4720–4723
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Unprecedented C-2 arylation of indole with diazonium salts: Syntheses
of 2,3-disubstituted indoles and their antimicrobial activity
Seth Daly a, Kathryn Hayden a, Indranil Malik a, Nikki Porch a, Hong Tang a, Snezna Rogelj a, Liliya V. Frolova b,
Katrina Lepthien b, Alexander Kornienko b, Igor V. Magedov b,
⇑
a Department of Biology, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
b Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel reaction of indole with aryldiazonium salts leading to the formation of 2-aryl-3-(arylazo)indoles
was discovered. The products were found to possess potent anti-MRSA and anti-LLVRE activities. The SAR
studies indicate that the potentially metabolically labile azo functionality can be replaced with ether oxy-
gen and thioether sulfur atoms without any loss of activity.
Received 2 May 2011
Revised 15 June 2011
Accepted 17 June 2011
Available online 25 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
MRSA
Low-level VRE
Indole
Arylation
Nosocomial infections caused by multi-drug resistant bacteria,
such as methicillin-resistant Staphylococcus aureus (MRSA) or van-
comycin-resistant Enterococcus faecalis (VRE) present a consider-
able challenge in the clinic.1,2 Therefore, the search for new
antibacterial agents capable of combating these microorganisms
continues unabated.3–5 Because a number of azo-containing com-
pounds have been reported to exhibit promising antimicrobial
properties,6–10 we initiated a project aimed at the synthesis of
diverse azo-containing heterocycles and their evaluation for anti-
bacterial and antifungal activities with the emphasis on hospital-
acquired infections. These efforts led to the identification of a
compound active against MRSA, whose structure was originally as-
signed as 2,3-di(p-chlorophenylazo)indole (compound A, Fig. 1).
This compound was originally synthesized by the reaction of in-
dole with 2.5 equiv of p-ClPhN2þClꢀ, which gave 87% of the C-3
monoazo product (1a in Scheme 1) and only 3% of compound A.
The subsequent optimization of this procedure resulted in the
To confirm the structure of 2a unequivocally, this compound
was synthesized by an independent route, involving the Fisher syn-
ꢀ
thesis of indole 3 and its subsequent reaction with p-ClPhN2þBF4
(Scheme 2a). The spectral data for the obtained compound were
indistinguishable with those for 2a. The reaction in Scheme 1 also
allowed for the synthesis of analogues 2b–d, while the unsymmet-
rical derivative 2e was obtained via a stepwise introduction of the
substituents (Scheme 2b).
It could be surmised that mechanistically this new process in-
volves an ipso-attack at C-3 of the indole ring11 with the forma-
tion of the geminal diazo compound B (‘path a’ in Scheme 3).
The homolytic bond breakage and elimination of nitrogen can
Cl
Cl
ꢀ
use of 4 equiv of p-ClPhN2þBF4 and afforded an improved 56%
yield of this desired product. Although the NMR spectra of com-
pound A were consistent with its originally assigned structure as
2,3-di(p-chlorophenylazo)indole, the mass spectral analysis sug-
gested that only one azo group was present. While either C-2 or
C-3 positioning of the azo functionality was theoretically possible,
the mechanistic considerations led us to propose structure 2a
(Fig. 1, Scheme 1) for this indole derivative.
N
N
N
N
N
Cl
N
Cl
N
N
H
H
original erroneous structure
correct structure 2a
Compound A
⇑
Corresponding author. Tel.: +1 575 835 6886; fax: +1 575 835 5364.
E-mail address: imagedov@nmt.edu (I.V. Magedov).
Figure 1. The originally assigned erroneous and corrected structures for compound
A.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.06.081

S. Daly et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4720–4723
4721
X
Ar
N
Ar
N
N
N
+
Ar'N2
N
Ar'
N
N
"path a"
N
N
N
H
B
−N2
N
O
O
H
Ar
N
"path b"
+
Ar'N2
+
BF4
X
N2
1a-d
N
Ar'
+
X
(4 equiv)
Ar'
N
O
N
H
AcONa,dioxane/H2O
Ar
N
C
−N2 +
+ H+
N
Ar
N
O
N
N
H
N
Ar'
X
N
O
O
H
N
Ar'
H
N
F
2a-d
2a X = Cl,
D
56%
O
O
2b X = Br, 36%
Ar
N
2c X = F,
38%
Ar
N
2d X = OMe, 43%
N
N
1,5 H-Shift
H
Scheme 1.
Ar'
Ar'
N
E
N
H
then lead to radical recombination at C-3 (C) or more sterically
accessible C-2 (D). The 1,5 hydrogen shift results in the observed
product 2. However, this mechanism is inconsistent with the for-
mation of 2e (in Scheme 2b), which proceeds without scrambling
of the halogens. Therefore, a more likely mechanism is based on
the Meerwein-type arylation shown as ‘path b’. It involves an
aromatic radical formation from the diazonium salt and addition
of this radical at C-2 position of the indole to form azo-stabilized
radical F, which results in product 2. The aryl radical formation
from the diazonium salt can be promoted by either acetate or
dioxane (shown is Scheme 3) as has been reported previously
for the metal-free Meerwein arylations.12,13 Although the classi-
cal Meerwein arylations of olefins are promoted with copper or
palladium salts,14 the use of Cu(OAc)2 or Pd(OAc)2 did not result
in higher yields in our case.
2
Scheme 3.
bility of the azo functionality under physiological conditions.
Therefore, to explore the SAR within this series of compounds we
first addressed the issue of whether this moiety is absolutely re-
quired for activity. Thus, we synthesized the ‘carba’ analogues 4
and 5, utilizing Fujiwara/Heck chemistry and varying the number
of equivalents of the olefin (Scheme 4).15 Distyrylindole 5 was then
converted to carbazole 6 upon reflux in xylenes in the presence of
Pd/C. We believe this approach provides a useful synthetic access
to the carbazole skeleton.16
We also explored a bioisosteric substitution of the azo group
with sulfur and oxygen atoms. Such compounds were prepared
using the Fisher indole synthesis starting with the requisite aryl-
thia- and aryloxa-acetophenones (Scheme 5). To our knowledge,
these syntheses constitute the first examples of the preparation
of 2-aryl-3-aryloxa- and 2-aryl-3-arylthia-indoles (7a–c and
8a–g) using the Fisher reaction.
The evaluation of compounds 2a–e for antibacterial activity
showed promising results, specifically against gram-positive
organisms (see Table 1). However, we had concerns about the sta-
O
MeSO3H
Finally, compound 9, containing the sulfone isostere of the azo
moiety, was prepared by oxidation of 7a with MCPBA (Scheme
6a),17 while compound 10, the carbono analogue, was obtained
by acylation of 3 with p-Cl-PhCOCl (Scheme 6b).18,19
+
a)
NH2
100 0C
82%
N
H
Cl
BF4
N2
Cl
The data in Table 1 indicate that all our compounds that show
antibacterial activity inhibit the growth of gram-positive microor-
ganisms only. Furthermore, there is no discrimination between the
bacterial strains on the basis of their resistance status. Thus, all
3-azo-indole derivatives synthesized (1a,b and 2a–e) are equally
potent toward the susceptible S. aureus strain (S. aureus 29213),
its resistant counterpart MRSA (S. aureus BAA-44) or low-level van-
comycin-resistant Enterococcus (LLVRE, E. faecalis 51299). Although
the activity is lost when the 3-azo functionality is replaced by ‘car-
ba’ bridges (4–6), or sulfono (9) and carbono (10) groups, even
more potent compounds result from substituting the 3-azo moiety
with the thioether sulfur (7a–c) and ether oxygen (8a–g) atoms.
For example, aryloxyindole 8b is submicromolar toward MRSA.
These results compare favorably with the clinically used antibiotics
vancomycin and penicillin G, which are less potent in this mini-
AcONa
Cl
+
2a
N
H
dioxane/H2O
74%
3
Cl
BF4
N2
Br
AcONa
N
N
b) 1a
+
dioxane/H2O
40%
Br
N
H
2e
Scheme 2.

4722
S. Daly et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4720–4723
Table 1
Antimicrobial (MICs,
l
M) and cancer cell growth inhibitory (GI50
,
l
M) activities of the synthesized indole derivativesa
Compound
Gram-(+) bacterial strains
Gram-(ꢀ) bacterial strains
Fungus
Cancer cell line
HeLa
S. aureusb BAA-44
S. aureusc 29213
E. faecalisd 51299
P. aeruginosa 27853
A. baumannii 15151
C. albicans 26555
1a
1b
2a
2b
2c
2d
2e
3
4
5
6
7a
7b
7c
8a
8b
8c
8d
8e
8f
8g
9
10
12.5
6.3
1.6
12.5
1.6
50
12.5
6.3
0.6
6.3
1.3
25
12.5
8
6.3
3.1
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
NDf
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
ND
12.5
25
23 3e
49
26
9
5
0
1
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
ND
12
23
13
1
1
0
3.1
6.3
>100
6.3
6.3
>100
>100
>100
>100
3.1
3.1
1.3
3.1
0.6
3.1
1.6
6.3
1.3
>100
25
>100
>100
1.6
1.6
1.6
1.6
1.6
>100
>100
>100
>100
3.1
3.1
6.3
12.5
12.5
12.5
6.3
25
3.1
50
>100
>100
86.3
11.9
>100
64
69
16
6
5
5
1
1
12
8
9
8
3
2
1
0
2
0
3.1
1.6
13
24
13
19
14
19
ND
ND
3
2
4
1
1
1
12.5
1.6
12.5
>100
>100
2.8
6.3
>100
>100
2.8
Vancomycin
Penicillin G
>100
23.9
ND
ND
ND
a
Bacterial suspensions were adjusted to ꢁ5 ꢂ 105 CFU/mL.20,21 The suspensions were treated with compounds, serially diluted 2-fold and were incubated according to
ATCC recommendations for 18 h. Fungal testing was accomplished according to a previously published method.22 Bacterial and fungal viability was determined using the MTT
method due to coloring of compounds.23 HeLa testing was also accomplished according to a previously published method.24
b
Methicillin-resistant S. aureus.
Susceptible S. aureus strain.
Low-level vancomycin-resistant Enterococcus.
The data are presented as GI50 SD from four experiments.
c
d
e
f
ND = not determined.
Cl
Cl
O
Br
O
XH
X
NaOH
+
Cl
EtOH/H2O
Cl
N
H
Y
Cl
Cl
Y
Pd(OAc)2/Cu(OAc)2
DMF/DMSO 9:1
4
Y = Cl, X = S. 74%
Y = Br, X = S, 76%
Y = Cl, X = O, 69%
37%
N
H
Cl
Cl
NH2
O
X
4
HN
X
HCl
5
N
4 equiv
H
Y
+
EtOH
5
Cl
6
N
H
30%
7
Z
Z
7a-c, 8a-g
Y
Cl
7a X = S, Y = Cl, Z = H,
7b X = S, Y = Br, Z = H,
7c X = S, Y = Cl, Z = 5-Cl,
8a X = O, Y = Cl, Z = H,
89%
78%
54%
49%
8b X = O, Y = Cl, Z = 5-Cl, 73%
8c X = O, Y = Cl, Z = 5-Br, 45%
Pd/C 10%
Cl
5
8d X = O, Y = Cl, Z = 5-F,
88%
xylenes, reflux
8e X = O, Y = Cl, Z = 5-OMe, 44%
8f X = O, Y = Cl, Z = 7-F, 55%
8g X = O, Y = Cl, Z = CO2Et 60%
N
H
6
88%
Scheme 5.
Scheme 4.
panel of gram-positive bacteria and show organism-dependent
growth inhibitory properties.
The synthesized indole derivatives were also tested against a
nosocomial fungus Candida albicans (Table 1). Only the 3-monoazo

S. Daly et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4720–4723
4723
Cl
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O
S
O
Cl
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compounds 1a and 1b exhibited modest levels of activity, which is
consistent with an earlier report of antifungal activities associated
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paralleled with an antiproliferative effect against cultured human
cancer cells, such as HeLa (Table 1). These findings warrant future
rodent studies, in which the toxicity of these compounds can be
properly evaluated.
In conclusion, an unprecedented reaction of indole with aryldi-
azonium salts affords 2-aryl-3-(arylazo)indoles, which display
promising anti-MRSA and anti-LLVRE activities. The successful bio-
isosteric substitution of the labile azo group with ether oxygen and
thioether sulfur atoms indicates that the azo functionality is not re-
quired for activity and, thus, the potential metabolic instability of
these indole-based antibacterial agents is not of concern. Experi-
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dioxane (2 mL) were added a solution of sodium acetate (0.004 mol) in water
(0.5 mL) and an appropriate diazonium tetrafluoroborate (0.004 mol) at room
temperature. The reaction mixture was stirred at room temperature for 6 h and
then water was added (10 mL). After the extraction with AcOEt, the organic
layer was dried (Na2SO4) and the solvent was removed under reduced
pressure. The product was purified by flash chromatography on silica gel
(hexane–ethyl acetate, 15:1). Selected characterization data (2a): 56%; 1H NMR
(CDCl3) d 8.60–8.59 (m, 1H), 7.90–7.87 (d, J = 8.52 Hz, 2H), 7.80–7.77 (d,
J = 8.52 Hz, 2H), 7.50–7.47 (d, J = 8.52 Hz, 2H), 7.46–7.43 (d, J = 8.79 Hz, 2H),
7.36–7.31 (m, 3H); 13C NMR (CDCl3) d 135.7, 134.8, 132.7, 130.5, 129.3, 129.2,
125.1, 123.8, 123.3, 119.8, 111.2; HRMS m/z (ESI) calcd for C20H14Cl2N3 (M+H+)
366.0565, found 366.0557.
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Acknowledgments
US National Institutes of Health (Grants RR-16480 and CA-
135579) are gratefully acknowledged for financial support of this
work. We are grateful to the reviewer of this letter for pointing
out the similarity of our proposed mechanism to the metal-free
Meerwein arylation processes.
25. Dekker, W. H.; Selling, H. A.; Overeem, J. C. J. Agric. Food Chem. 1975, 23, 785.