Facile synthesis and biological evaluation of assorted indolyl-3-amides and esters from a single, stable carbonyl nitrile intermediate

Article abstract of DOI:10.1016/j.tetlet.2015.02.090

The synthesis of biologically relevant amides and esters is routinely conducted under complex reaction conditions or requires the use of additional catalysts in order to generate sensitive electrophilic species for attack by a nucleophile. Here we present the synthesis of different indolic esters and amides from indolyl-3-carbonyl nitrile, without the requirement of anhydrous reaction conditions or catalysts. Additionally, we screened these compounds for potential in vitro antimalarial and anticancer activity, revealing 1H-indolyl-3-carboxylic acid 3-(indolyl-3-carboxamide)aminobenzyl ester to have moderate activity against both lines.

Full text of DOI:10.1016/j.tetlet.2015.02.090

Tetrahedron Letters 56 (2015) 1860–1864
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis and biological evaluation of assorted indolyl-3-
amides and esters from a single, stable carbonyl nitrile intermediate
b
b
c
c
Clinton G. L. Veale a, , Adrienne L. Edkins , Jo-Anne de la Mare , Carmen de Kock , Peter J. Smith ,
⇑
Setshaba D. Khanye d
a Faculty of Pharmacy, Rhodes University, Grahamstown 6140, South Africa
b Biomedical Biotechnology Research Unit, Department of Biochemistry and Microbiology, Rhodes University, Grahamstown 6140, South Africa
c Department of Pharmacology, University of Cape Town, Groote Schuur Hospital, Observatory, 7925, South Africa
d Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of biologically relevant amides and esters is routinely conducted under complex reaction
conditions or requires the use of additional catalysts in order to generate sensitive electrophilic species
for attack by a nucleophile. Here we present the synthesis of different indolic esters and amides from
indolyl-3-carbonyl nitrile, without the requirement of anhydrous reaction conditions or catalysts.
Additionally, we screened these compounds for potential in vitro antimalarial and anticancer activity,
revealing 1H-indolyl-3-carboxylic acid 3-(indolyl-3-carboxamide)aminobenzyl ester to have moderate
activity against both lines.
Received 10 December 2014
Revised 30 January 2015
Accepted 18 February 2015
Available online 25 February 2015
Keywords:
Indole
Amide
Ó 2015 Elsevier Ltd. All rights reserved.
Ester
Carbonyl nitrile
Amide and ester bonds have proven to be two of the most abun-
dant and important functional groups in organic chemistry, featur-
ing prominently in many bioactive natural products and
pharmaceuticals.1,2 The importance of amides in particular, from
a medicinal chemistry viewpoint, is highlighted in a survey of
GSK, AstraZeneca and Pfizer by Roughley and Jordan, which
revealed that amide formation is the most common transformation
used in their synthetic medicinal chemistry programmes.3
Amide and ester bonds are most commonly formed through
activation of a carboxylic acid into an unstable acid halide,2 fol-
lowed by coupling with a nucleophile. However, this methodology
often requires strictly adhered anhydrous conditions4,5 in order to
prevent the acid halide from reverting back into the organic acid
through hydrolysis. Sophisticated new methods which enhance
the reactivity of carboxylic acids towards nucleophilic attack are
increasingly common in contemporary literature. While these
methods have proven extremely versatile, they still rely on either
the vigorous exclusion of moisture from the reaction environ-
ment,6,7 or the concomitant use of numerous additional
reagents.8,9 A progressively more popular method of amide synthe-
sis has emerged through the oxidative coupling of alcohols and
amines. However, this requires the use of expensive rare earth
metal catalysts.10–12 Additionally, in all these cases, the complex
reaction conditions have to be repeated for each synthesis of a
desired amide or ester analogue.
Our interest in amide synthesis initially occurred upon the ser-
endipitous formation of dimethyl-1H-indole-3-carboxamide (1)
when we exposed indolyl-3-carbonylnitrile (2) to a DMF/HCl solu-
tion (Scheme 1).13 In that study we, concluded that the DMF was
being hydrolysed by HCl to form dimethylamine, which subse-
quently attacked the electrophilic carbonyl nitrile, thereby forming
1. Examination of the literature revealed that Murahashi et al. had
previously investigated the use of benzoyl cyanides to protect
aliphatic amines,14 which provided us with a sufficient precedent
to investigate the potential of indolyl-amide and ester synthesis.
Consequently, we present here the synthesis of a diverse selec-
tion of indolyl-3-amides and esters (3–13), which were generated
from a single stable activated electrophile 2, along with three fur-
ther analogues (14–16) synthesized from C-6 fluorinated (17) and
brominated (18) analogues of 2, respectively. This versatile reac-
tion proceeds without the requirement of any protection strategy,
additional reagents, or repeated anhydrous conditions. The indole
moiety,15,16 and its amide- and ester-containing analogues feature
in many bioactive natural products17 and are useful tools in drug
discovery.18–20 Accordingly, we evaluated our new compounds
for activity against Plasmodium falciparum and a triple negative
breast cancer cell line, revealing a single compound (13) with
moderate activity against both cell lines.
⇑
Corresponding author. Tel.: +27 46 603 8096; fax: +27 603 7506.
E-mail address: C.Veale@ru.ac.za (C.G.L. Veale).
http://dx.doi.org/10.1016/j.tetlet.2015.02.090
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

C. G. L. Veale et al. / Tetrahedron Letters 56 (2015) 1860–1864
1861
2–4), with a particular interest in acetonitrile due to the reported
stability of nitrile ions in acetonitrile,21 which we hoped would
render the carbonyl more susceptible to nucleophillic attack by
facilitating removal of the leaving group. Reaction in MeCN result-
ed in an improved yield, although our highest yield was obtained
in DMF, while the use of 1,4-dioxane resulted in a significant
reduction in yield. We however decided to proceed with MeCN
due to the potential difficulty in removing solvents with high boil-
ing points.
Scheme 1. Reagents and conditions: HCl saturated DMF, 80 °C, 1 h.
Satisfied that we had adequate reaction conditions to yield our
desired product in high yield, we proceeded with the synthesis of
seven further amides (4–10), utilizing nucleophilic amines (R2-
H), varying in the nature of their respective chain lengths and
attached rings (Table 2). Reactions with non indolic amines
(Table 2, entries 1–5) proceeded smoothly in high yield, seemingly,
with no observable preference for the nature of the respective
nucleophiles. Reactions with indolic amines (Table 2, entries 6
and 7) proceeded in far lower yields, however, the reasons for this
are not currently clear. We followed this study with an experiment
to assess the relative reactivity of an amine or alcohol towards
nucleophillic attack of the carbonyl nitrile (Table 2, entry 8).
Murahashi et al.14 had conducted a similar experiment with ben-
zoyl cyanide and various aliphatic amino alcohols and found no
evidence of ester formation. However, we were pleased to find that
our method did allow the formation of an ester (12), albeit at a sig-
nificantly lower rate than the corresponding amide (11), both of
which were formed in the same reaction. Interestingly, a third
compound from this reaction, the bisindole compound 13 featuring
both amide and ester moieties was isolated in a higher yield than
that of 12.
Due to the low solubility of indolyl-3-carbonylnitrile in non-po-
lar solvents, we restricted our study to polar solvents while avoid-
ing alcohols due to the possible formation of unwanted aliphatic
esters (Table 1). We subsequently reacted 2 and aniline in acetone
at reflux for 6 h (Table 1, entry 1). The reaction occurred smoothly
to generate the N-phenyl carboxamide 3 in good yield. We pro-
ceeded to investigate three further polar solvents (Table 1, entries
Table 1
Optimized reaction conditions for the formation of N-phenyl carboxamide 3
O
O
CN
NH
solvent, reflux, 6 h
N
N
NH2
H
H
2
3
a
Entry
Solvent
Yield (%)
Application of our synthetic method to the fluorinated indolyl-
3-carbonyl nitrile 17 yielded carboxamide 14 in high yield, and
again showed the preferential formation of amides over the corre-
sponding ester (15) which was formed in the same pot (Table 2,
entry 9). Interestingly, in this instance, no evidence of a bisindole
derivative was observed. The brominated indolyl-3-carbonyl
1
2
3
4
Acetone
MeCN
DMF
71
85
95
23
1,4-Dioxane
a
Isolated yield.
Table 2
Synthesis of amides and esters 4–16
O
O
R2
CN
nucleophile(R2-H) (3 eq.)
MeCN, reflux, 6 h
R1
N
R1
N
H
H
R1 = H
R1 = H
2
4-13
17 R1 = F
14, 15 R1 = F
18 R1 = Br
16
R1 = Br
Entry
R2-H
Product
Yielda (%)
O
H2N
NH
1
78
N
H
4
O
H2N
NH
2
97
N
H
5
(continued on next page)

1862
C. G. L. Veale et al. / Tetrahedron Letters 56 (2015) 1860–1864
Table 2 (continued)
Entry
R2-H
Product
Yielda (%)
N
O
N
NH
H2N
3
4
95
N
H
6
O
O
O
H2N
NH
72
N
H
7
O
O
N
N
O
H2N
NH
5
95
N
H
8
HN
N
H
H2N
O
NH
6
33
N
H
9
NH2
H
N
N
H
O
NH
7
50
N
H
10
OH
OH
O
H2N
NH
8
41
N
H
11
OH
OH
O
O
NH2
H2N
11
N
H
12
O
O
O
H2N
NH
N
H
23
N
H
13

C. G. L. Veale et al. / Tetrahedron Letters 56 (2015) 1860–1864
1863
Table 2 (continued)
Entry
R2-H
Product
Yielda (%)
OH
OH
O
NH
H2N
9
71
N
F
H
14
OH
O
O
NH2
H2N
7
N
H
F
15
O
H
O
O
NH2
H2N
10
7
N
Br
H
16
a
Isolated yield.
nitrile analogue 18 was also able to generate an ester (16) in low
yield, but unfortunately we were unable to purify the amide
derivative to an acceptable level in order to accurately quantify
the yield.
malaria, breast cancer and non-malignant cells. Interestingly, the
compound library displayed generally greater cytotoxicity against
non-cancerous MEF-1 cells in comparison to Hs578T cancer cells.
Structure–activity relationship (SAR) analysis suggests that
ester-containing analogues are more cytotoxic than their amide
containing analogues, while C-6 halogenated analogues are more
cytotoxic than their non halogenated counterparts. These observa-
tions have interesting implications in further SAR analysis of bisin-
dole 13.
In this study, we have investigated the synthesis of indole-3-
carboxamides and esters through a single stable intermediate.
Currently, only two literature methods exist for the synthesis of
indolyl-3-carbonylnitriles, both of which require anhydrous condi-
tions.13,23 However, the generation of a stable activated elec-
trophile means that a variety of diverse chemical scaffolds can be
generated under simple reaction conditions.24 This is in contrast
to most other amide synthetic strategies which require complex
reaction conditions involving the addition of expensive catalysts
for the generation of each new compound. Biological screening of
these compounds revealed bisindole 13, the unique structure of
which warrants more extensive investigation into its SAR, to have
moderate in vitro activity against both malarial and cancer cell
lines.
Following synthesis and characterization, our cohort of amides
and esters was subjected to biological assessment against a chloro-
quine-sensitive malarial strain (NF54), a breast cancer cell line
(Hs578T) and a non-malignant murine embryonic fibroblast cell
line (MEF-1) as a control (Table 3). Chloroquine, artesunate and
paclitaxel (PTX) were used as control compounds against the
respective cell lines in order to assist interpreting the IC50 data.
Antiplasmodial testing was performed according to previously
published methods.22 All compounds were considered inactive
against malaria and breast cancer cells with the exception of 13
which displayed IC50 values in the low micromolar range against
Table 3
Growth inhibition assays of compounds 3–16
Compound
NF54 IC50
M) sda
Hs578T IC50
M) sda
MEF-1 IC50
(l
M) sda
(
l
(
l
3
4
5
6
7
8
9
10
11
12
13
14
15
16
>100
>100
97 20
>100
>100
83 1.4
>100
>100
34 1.5
36 1.7
>100
>100
49 1.6
>100
93 1.2
10 1.8
>100
35 1.5
52 1.3
—
8.9 1.4
27 1.4
30 1.5
59 1.5
47 1.7
63 2.4
4.1 1.2
32 1.3
11 2.2
71 1.4
6.7 1.2
57 1.5
5.0 1.4
4.8 1.4
—
Acknowledgments
This research project was funded by Rhodes University and
the South African Medical Research Council (MRC) with funds from
the National Treasury under its Economic Competitiveness and
Support Package. J-A de la Mare gratefully acknowledges funding
through the South African DST/NRF Innovations programme.
>100
>100
72 17
>100
48 9.8
19 5.4
>100
66 14
28 3.2
Supplementary data
Chloroquine 28 0.3b
Artesunate
PTX
8.6 0.5b
—
—
—
Supplementary data (supplementary data relating to
experimental procedures and spectral data) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.tetlet.2015.02.090.
75b
93b
a
sd = standard deviation.
nM values.
b

1864
C. G. L. Veale et al. / Tetrahedron Letters 56 (2015) 1860–1864
A.-M.; Bonnet-Lignon, S.; Millet, L.; Briot, C. F. D.; Hérault, J.-P.; Savi, P.;
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