Synthesis of Camalexin and Related Analogues

Article abstract of DOI:10.1002/jhet.3192

Camalexin is synthesized on a gram scale by coupling of thiazole and indole in an amidoalkylation–oxidation sequence. Several high-yielding implementations of this two-step approach are demonstrated with variation of either the intermediate acyliminium reagent or the oxidation conditions. Benzo-analogues and aza-analogues of the natural product are also obtained by this method. As a side result, a new formylation of indole is demonstrated.

Full text of DOI:10.1002/jhet.3192

Month 2018
Synthesis of Camalexin and Related Analogues
*
*
Yordan Stremski, Stela Statkova-Abeghe, Plamen Angelov,
and Iliyan Ivanov
Department of Organic Chemistry, University of Plovdiv Paisii Hilendarski, 24 Tsar Asen Str., 4000 Plovdiv, Bulgaria
*
E-mail: stab@uni-plovdiv.bg; angelov@uni-plovdiv.bg
Received January 14, 2018
DOI 10.1002/jhet.3192
Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com).
Camalexin is synthesized on a gram scale by coupling of thiazole and indole in an amidoalkylation–
oxidation sequence. Several high-yielding implementations of this two-step approach are demonstrated with
variation of either the intermediate acyliminium reagent or the oxidation conditions. Benzo-analogues and
aza-analogues of the natural product are also obtained by this method. As a side result, a new formylation
of indole is demonstrated.
J. Heterocyclic Chem., 00, 00 (2018).
INTRODUCTION
amidoalkylation of indole with N-acylthiazolium reagent,
followed by removal of the auxiliary alkoxycarbonyl
group, would be an efficient method for coupling of
thiazole and indole and may provide straightforward access
to camalexin and related analogues. Although several
syntheses of camalexin have already been published, our
approach offers significant advantages by avoiding
expensive catalysts and sensitive organometallic reagents.
Camalexin (Fig. 1) is a phytoalexin [1], first isolated
from Camelina sativa infected with Alternaria brassicae
[2] and later found in other Cruciferae plants [3]. In
addition to its important role in plant immunity, this
natural product has interesting biological profile,
showing antibacterial,3a antifungal [4], and cytotoxic
activity [5]. Camalexin has also attracted interest from
purely chemical standpoint and has been synthesized
by
approaches [6], Suzuki–Miyaura coupling [7], Masuda
borylation/Suzuki arylation sequence [8], and
biomimetic approach [9]. Despite the apparent
simplicity of this natural product, its synthesis has
proved challenging, and all of the published syntheses
have their drawbacks with regard to yield, scale-up
potential, or availability of necessary reagents.
The α-amidoalkylation is a well-established method for
C─C bond formation in which an imine is reacted with
acid chloride to generate electrophilic acyliminium
species, reactive enough to alkylate a broad range of
substrates in either intramolecular or intermolecular
manner [10]. In a variation of this method, pioneered by
the Reissert reaction [11], the acyliminium reagent is
generated from aromatic N-heterocycle [12] instead of
simple imine. We have previously used such approach to
obtain various heterocyclic compounds [13]. In view
of this experience, we reckoned that a sequence of
various
methods,
including
organometallic
RESULTS AND DISCUSSION
a
First, we tried activation of thiazole and benzothiazole
with alkyl chloroformates. As illustrated on Scheme 1,
this proved successful and the obtained acyliminium
reagents 2 reacted in situ with indole in the presence of
triethylamine as HCl scavenger to give 3 in excellent yields
(Table 1). The activation was also tried with acid chlorides
such as acetylchloride and benzoylchloride but was
successful only in the case of benzothiazole and gave
significantly lower yields (Table 1, 3bd–3be).
During the optimization of the synthetic procedure, we
observed that products 3b were often accompanied by
triindolylmethane 4. This side product was isolated in
varying amounts, depending mostly on the acidity of the
reaction mixture. In the case of 3be, for example, when
the reaction was tried in the absence of Et3N,
triindolylmethane became the major product (76% yield
based on indole). This led us to a hypothesis that
© 2018 Wiley Periodicals, Inc.

Y. Stremski, S. Statkova-Abeghe, P. Angelov, and I. Ivanov
Vol 000
acyl or alkoxycabonyl group and rearomatize the thiazole
moiety in 3. Similar task, although in different context,
has been accomplished before by Beveridge et al. under
oxidative conditions [14], using o-chloranil in acetonitrile
at 85°C. Following this literature precedent, and after
some experimentation to adjust the reaction conditions,
we successfully oxidized compounds 3 with o-chloranil
and obtained excellent yields of camalexin or
benzocamalexin, respectively. Comparable yields were
obtained with DDQ as the oxidant (Scheme 3, Table 2).
The melting points and spectral data of the obtained
compounds are in complete agreement with those
published earlier [6–9]. The synthesis of camalexin was
repeated on a gram scale via intermediate 3ac, starting
with 7 mmol thiazole and again was found to work with
excellent yield of 88% over two steps (1.232 g).
Figure 1. Structure of the natural product camalexin.
Scheme 1. Amidoalkylation of indole with acylthiazolium reagents.
Reagents and conditions: (a) RCOCl (1.15 equiv.), dichloroethane
(3–7 mL/mmol), see Table 1 for temperature; (b) indole (1 equiv.), then
slowly Et3N (1 equiv.), see Table 1 for duration.
Experiments to obtain dihydro derivatives of camalexin
and benzocamalexin by removing the Troc group from
3ac and 3bc under reductive conditions were not
successful, although they led to a rather interesting result
in the case of 3ac. While the reaction of the benzo
derivative 3bc with Zn/NH4Cl in refluxing MeOH gave
messy outcome, 3ac on the other hand gave indole-3-
carboxaldehyde (7) in 70% yield (Scheme 4). We assume
Table 1
Reaction conditions and yields of amidoalkylated indoles 3.
Thiazole
Product
R
Conditions
Yield, %
1a
3aa
3ab
3ac
3ba
3bb
3bc
3bd
3be
CH3CH2O
CH3O
CCl3CH2O
CH3CH2O
CH3O
CCl3CH2O
CH3
C6H5
0–25°C, 1.5 h
0–25°C, 1.5 h
0°C, 1 h
25°C, 4 h
25°C, 4 h
0°C, 1 h
83
90
96
75
85
92
63
60
that dihydrocamalexin
6
is formed under these
1b
conditions, but the thiazoline ring is then easily
hydrolyzed upon work up to reveal a formyl group (see
ESI for proposed mechanism). This reaction, along with
the aforementioned formation of triindolylmethane,
suggests potential use of the N-alkoxycarbonyl thiazoline
moiety as a masked formyl equivalent.
With camalexin and benzocamalexin in hand, we turned
our attention to the possibility of using imidazole as the
starting material in order to prepare an aza-analogue of
the natural product. The activation of the imidazole ring
required two equivalents of alkylchloroformate and was
carried out in one pot, without isolation of the monoacyl
derivative. Then indole and Et3N were added to the same
reaction vessel, and the mixture was stirred for 30 min at
0°C and for one more hour at r.t. Thus, compounds 10
were obtained with excellent yields in a one-pot
multicomponent manner. The oxidation of 10 with o-
chloranil at r.t. was successful and gave 72–88% yield of
0°C, 2 h
25°C, 2 h
compounds 3b may undergo acid-catalyzed opening of the
thiazoline ring and turn into electrophilic species, which
quickly reacts with the indole still available in the
solution to give 4 eventually (see ESI for proposed
mechanism). To test this hypothesis, we mixed clean 3ba
with indole (2 equiv) in dichloroethane/TFA (10:1 vol.)
and kept the solution for 24 h at 25°C. Indeed, under
these conditions, triindolylmethane was obtained and
isolated in 37% yield (Scheme 2).
After the successful coupling of the heterocyclic rings,
we proceeded with experiments to remove the auxiliary
Scheme 2. Formation of triindolylmethane 4 from 3b and indole in acidic
media. See ESI for proposed mechanism.
Scheme 3. Oxidation of amidoalkylated indoles 3 to (benzo)camalexin 5.
See Table 2 for reagents and conditions.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Synthesis of Camalexin and Related Analogues
Table 2
Reaction conditions and yields of camalexin 5, obtained according to Scheme 3.
o-Chloranil oxidation
DDQ oxidation
Time, h
Starting
material
Product
5a
T, °C
Time, h
Yield 5, %
97
T, °C
Yield 5, %
74
83
92
70
72
83
–
3aa
3ab
3ac
3ba
3bb
3bc
3bd
3be
0
0
0
85
85
25
25
25
1
1
1
4
1
2
2
2
0
0
0
1
5a
5a
5b
5b
5b
5b
5b
95
93
81
85
80
88
96
1
1
2
1
2
–
–
25
25
25
–
–
–
Scheme 4. Formation of indole-3-carboxaldehyde upon removal of the
Troc group from 3ac. Reagents and conditions: Zn (10 equiv.), NH4Cl
(10 equiv.), CH3OH, reflux, 3 h, H2O.
Scheme 6. Competing Diels–Alder cycloaddition of o-chloranil to
compounds 10.
aza-camalexin 11 (Scheme 5, Table 3). At higher
temperature, a [4 + 2]-cycloaddition between 10 and o-
chloranil took precedence over the desired aromatization,
and the Diels–Alder adducts 12 were isolated in 50–60%
yield (Scheme 6). Compounds 12 were obtained as single
diastereoisomers and the lack of any NOE to the proton
at position 2 in the imidazolidine ring suggests trans-
configuration.
Scheme 7. Formation of indole-3-carboxaldehyde upon removal of the
Troc group from 10c. Reagents and conditions: Zn (10 equiv.), NH4Cl
(10 equiv.), CH3OH/H2O (8:2 vol.), 60°C, 2 h.
Similarly to the reaction of the thia-analogue 3ac
described earlier, reductive removal of the Troc groups in
10c gave indole-3-carboxaldehyde again (Scheme 7), but
in lower yield. However, addition of water (20% vol.) to
the reaction mixture boosted the yield of the aldehyde 7
to 82% – a result suggesting that the imidazoline
intermediate 13 here is more stable to hydrolysis than its
thiazoline counterpart 6.
Scheme 5. Reagents and conditions: (a) Et3N (1 equiv), ROCOCl (2.2
equiv), 0°C; (b) indole (1 equiv), then slowly Et3N, 90 min, 0°C to r.t.;
(c) o-chloranil (1 equiv.), CH3CN, r.t., 4 h.
CONCLUSION
In conclusion, a robust and scalable synthesis of
camalexin and related analogues has been developed. The
amidoalkylation–oxidation sequence does not require
expensive catalysts or sensitive organometallic reagents
and offers significant advantages over other methods of
heteroaromatic ring coupling. Also, a new method of
indole formylation has been demonstrated. The potential
of introducing masked formyl group in other substrates
via acyliminium reagents of imidazole or thiazole is
currently under investigation, and the results will be
reported in due course.
Table 3
Yields of compounds 10 and 11, obtained according to Scheme 5.
Entry
R
Yield 10, %
94
95
92
Yield 11, %
88
80
72
a
b
c
CH3CH2O
CH3O
CCl3CH2O
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Y. Stremski, S. Statkova-Abeghe, P. Angelov, and I. Ivanov
Vol 000
EXPERIMENTAL
All reagents and solvents were
(KBr, cmÀ1): 3331, 2953, 1691, 1599, 1544, 1446, 1250,
741; 1Н-NMR (600 MHz, 80°C, DMSO-d6, δ ppm, J
Hz): 3.61 (s, 3H), 6.00 (d, J = 4.7, 1H), 6.65 (d, J = 4.1,
1H), 6.90 (s, 1H), 7.02 (m, 1H), 7.11 (m, 1H), 7.29 (d,
J = 2.4, 1H), 7.38 (d, J = 8.2, 1H), 7.62 (d, J = 8.2, 1H),
10.87 (s, 1H); 13C-NMR (150 MHz, 80°C, DMSO-d6, δ
ppm): 53.1, 60.5, 104.5, 112.1, 116.4, 119.3, 119.6,
121.3, 121.8, 123.6, 124.7, 137.3, 153.1; HRMS m/z
(ESI): calcd for C13H12N2NaO2S+ [M + Na]+ 283.0512,
found 283.0512; calcd for C13H11N2O2SÀ [M À H]À
General information.
obtained from commercial suppliers (Sigma-Aldrich, St.
Louis, MO or Merck, Kenilworth, NJ) and were used
without further purification. Melting points were taken on
Boetius hot stage apparatus and are not corrected. Nuclear
magnetic resonance (NMR) spectra were measured on
Bruker Avance AV600 or DRX250 spectrometers, and
chemical shifts (δ, ppm) are downfield from TMS. To
average out the rotamers observed in compounds 3, 10, and
12, some spectra were measured at 80°C, as indicated in
the succeeding text. Mass spectral measurements were
performed on a Thermo Scientific Q Exactive hybrid
quadrupole-orbitrap mass spectrometer.
259.0547, found 259.0555.
2,2,2-Trichloroethyl 2-(1H-indol-3-yl)thiazole-3(2H)-carboxylate
(3ac).
Yield: 96%; mp = 135–137°C; IR (KBr, cmÀ1):
1
3362, 3061, 2952, 1695, 1546, 1460, 1430, 1248, 744; Н-
NMR (250 MHz, 80°C, DMSO-d6, δ ppm, J Hz): 4.83 (s,
2H), 6.17 (d, J = 4.7, 1H), 6.72 (d, J = 4.7, 1H), 6.99 (s,
1H), 7.00–7.06 (m, 1H), 7.09–7.16 (m, 1H), 7.34 (d,
J = 2.7, 1H), 7.40 (d, J = 8.1, 1H), 7.65 (d, J = 8.1, 1H),
10.90 (br s, 1H, NH); 13C-NMR (62.5 MHz, 80°C,
DMSO-d6, δ ppm): 60.9, 75.1, 95.9, 106.5, 112.3, 115.8,
119.5, 119.7, 120.7, 121.9, 124.1, 124.8, 132.1, 137.4;
HRMS m/z (ESI): calcd for C14H11Cl3N2NaO2S+
Synthesis of amidoalkylated indoles (3). general
procedure.
The corresponding acid chloride or
chloroformate (1.15 mmol) is slowly added with magnetic
stirring to a solution of the corresponding thiazole
(1 mmol) in 1,2-dichloroethane (3–7 mL) at the
temperature indicated in Table 1. Immediately after that,
indole (1 mmol) is added. Then gradually, in the course of
30 min, triethylamine (1 mmol), dissolved in 1,2-
dichloroethane (2 mL), is added. The reaction mixture is
stirred for the time and at the temperature specified in
Table 1. After completion of the reaction, the mixture is
transferred to a separatory funnel with dichloromethane
(10 mL) and is consecutively extracted with equal
volumes of aqueous HCl (1:10), Na2CO3 (3%), and brine.
The combined organic layers are dried (Na2SO4), and the
solvent is removed under reduced pressure. The residue is
then triturated with small amount of hexane to induce
crystallization. The product obtained in this way is usually
sufficiently clean to be taken to the next stage without
further purification. Analytically pure samples can be
obtained by column chromatography on neutral alumina,
[M + Na]+ 398.9499, found 398.9495.
Ethyl 2-(1H-indol-3-yl)benzo[d]thiazole-3(2H)-carboxylate
(3ba).
Chromatographed on neutral alumina with
petroleum/diethyl ether (4:1); Yield: 75%; mp = 151–
152°C; IR (KBr, cmÀ1): 3383, 2983, 1695, 1577, 1548,
1
1466, 1248, 740; Н-NMR (600 MHz, 20°C, DMSO-d6,
δ ppm, J Hz): 1.18 (t, J = 7.0, 3H), 4.12–4.21 (m, 2H),
6.89 (t, J = 7.6, 1H), 7.05–7.09 (m, 2H), 7.16–7.19 (m,
2H), 7.18 (s, 1H), 7.24 (d, J = 2.3, 1H), 7.30 (d, J = 8.2,
1H), 7.35 (d, J = 8.2, 1H), 7.75 (s, 1H), 11.08 (s, 1H);
13C-NMR (150 MHz, 20°C, DMSO-d6, δ ppm): 14.7,
61.9, 62.4, 112.3, 116.3, 117.5, 119.5, 122.0, 122.8,
123.9, 124.5, 124.8, 125.7, 137.2, 152.7; HRMS m/z
(ESI): calcd for C18H16N2NaO2S+ [M + Na]+ 347.0825,
found 347.0826; [2 M + Na]+ calcd. 671.1757, found
using mixtures of diethyl ether/petroleum ether as eluents.
Ethyl 2-(1H-indol-3-yl)thiazole-3(2H)-carboxylate (3aa).
671.1759.
Methyl 2-(1H-indol-3-yl)benzo[d]thiazole-3(2H)-carboxylate
Chromatographed on neutral alumina with petroleum/
diethyl ether (2,1); Yield: 83%; mp = 111–113°C; IR
(KBr, cmÀ1): 3393, 2983, 1690, 1589, 1552, 1257, 1460,
(3bb).
Chromatographed on neutral alumina with
petroleum/diethyl ether (3:1); Yield: 85%; mp = 184–
185°C; IR (KBr, cmÀ1): 3313, 2955, 1683, 1575, 1537,
1
1
741; Н-NMR (600 MHz, 80°C, DMSO-d6, δ ppm, J Hz):
1471, 1252, 749; Н-NMR (600 MHz, 20°C, DMSO-d6,
1.12 (t, J = 7.0, 3H), 4.05 (t, J = 7.0, 2H), 5.99 (d, J = 4.7,
1H), 6.65 (d, J = 4.7, 1H), 6.91 (s, 1H), 7.03 (m, 1H), 7.12
(m, 1H), 7.30 (d, J = 2.4, 1H), 7.39 (d, J = 8.2, 1H), 7.64 (d,
J = 8.2, 1H), 10.88 (s, 1H); 13C-NMR (150 MHz, 80°C,
DMSO-d6, δ ppm): 14.6, 60.5, 61.9, 104.4, 112.2, 116.6,
119.4, 119.7, 121.5, 121.8, 123.8, 124.9, 137.3, 152.7;
HRMS m/z (ESI): calcd for C14H14N2NaO2S+ [M + Na]+
297.0668, found 297.0667; calcd for C14H13N2O2SÀ
[M À H]À 273.0703, found 273.0698.
δ ppm, J Hz): 3.72 (s, 3H), 6.90 (t, J = 7.6, 1H), 7.06–
7.09 (m, 2H), 7.17–7.21 (m, 2H), 7.18 (s, 1H), 7.23 (d,
J = 2.3, 1H), 7.30 (d, J = 8.2, 1H), 7.36 (d, J = 8.2,
1H), 7.75 (s, 1H), 11.08 (s, 1H); 13C-NMR (150 MHz,
20°C, DMSO-d6, δ ppm): 53.7, 62.0, 112.3, 116.3,
117.5, 119.5, 122.0, 122.8, 123.7, 124.4, 124.9, 125.7,
137.2, 153.2; HRMS m/z (ESI): calcd for
C17H14N2NaO2S+ [M + Na]+ 333.0668, found 333.0660;
calcd for C17H13N2O2SÀ [M À H]À 309.0703, found
309.0705.
Methyl 2-(1H-indol-3-yl)thiazole-3(2H)-carboxylate (3ab).
Chromatographed on neutral alumina with petroleum/
diethyl ether (2:1); Yield: 90%; mp = 132–134°C; IR
2,2,2-Trichloroethyl 2-(1H-indol-3-yl)benzo[d]thiazole-3(2H)-
carboxylate (3bc).
Chromatographed on neutral alumina
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Synthesis of Camalexin and Related Analogues
with petroleum/diethyl ether (2:1 increasing polarity to 1:1);
Yield: 92%; Oil; IR (KBr, cmÀ1): 3405, 3060, 2953, 1720,
0.136 mg) and Et3N (2 mmol, 0.28 mL) in 1,2-
dichloroethane (10 mL). Immediately after that, indole
(2 mmol, 0.234 g) is added. In the course of the next
30 min, Et3N (2 mmol) in 1,2-dichloroethane (4 mL) is
gradually added, then the ice bath is removed, and the
reaction mixture is stirred for 60 more minutes at r.t.
After completion of the reaction, the mixture is
transferred to a separatory funnel with dichloromethane
(50 mL) and is successively extracted with equal volumes
of aqueous HCl (1:10), Na2CO3 (3%), and brine. The
combined organic layers are dried (Na2SO4), and the
solvent is removed under reduced pressure. The solid
residue is then triturated and washed with small amount
of petrol/diethyl ether (4:1) to remove any unreacted
indole. Analytically pure samples were obtained by
column chromatography on neutral alumina, using diethyl
ether as the eluent, and the yields are indicated in Table 3.
1
1578, 1546, 1470, 1241, 743; Н-NMR (250 MHz, 80°C,
2
DMSO-d6, δ ppm, J Hz): 4.94 (d, J = 12.2, 1H) 5.03 (d,
2J = 12.2, 1H), 6.90 (m, 1H), 7.05–7.23 (m, 4H), 7.25 (s,
1H), 7.27 (d, J = 2.7, 1H), 7.36 (m, 2H), 7.83 (m, 1H),
10.88 (br s, 1H); 13C-NMR (62.5 MHz, 80°C, DMSO-d6,
δ ppm): 62.5, 75.4, 95.8, 112.3, 115.9, 117.9, 119.4,
119.6, 122.0, 122.9, 124.0, 124.6, 125.5, 125.8, 126.6,
130.1, 137.4, 143.6; HRMS m/z (ESI): calcd for
C18H13Cl3N2NaO2S+ [M
448.9647.
+
Na]+ 448.9655, found
1-(2-(1H-indol-3-yl)benzo[d]thiazol-3(2H)-yl)ethanone
(3bd).
Chromatographed on neutral alumina with
petroleum/diethyl ether (1:1); Yield: 63%; mp = 108–
110°C; IR (KBr, cmÀ1): 3297, 3057, 1651, 1575, 1541,
1
1465, 744; Н-NMR (600 MHz, 80°C, DMSO-d6, δ ppm,
J Hz): 2.25 (s, 3H), 6.92 (m, 1H), 7.09 (m, 2H), 7.15–
7.19 (m, 2H), 7.26 (m, 2H), 7.33 (s, 1H), 7.36 (d,
J = 8.2, 1H), 7.96 (m, 1H), 10.88 (s, 1H); 13C-NMR
(150 MHz, 80°C, DMSO-d6, δ ppm): 23.7, 62.5, 112.2,
116.2, 119.3, 119.5, 122.0, 122.8, 123.3, 125.2, 134.9,
169.1; HRMS m/z (ESI): calcd for C17H14N2NaOS+
Diethyl
dicarboxylate (10a).
2-(1H-indol-3-yl)-1H-imidazole-1,3(2H)-
Yield 94%; mp = 141–143°C; IR
(KBr, cmÀ1): 3334, 3062, 2985, 1713, 1681, 1551, 1480,
1467, 1278; 1Н-NMR (600 MHz, 80°C, DMSO-d6, δ
ppm, J Hz): 1.15 (br s, 6H, 2 × CH3), 4.07 (q, J = 7.0,
4H, 2 × CH2), 6.58 (s, 2H), 6.94 (s, 1H), 7.06 (m, 1H),
7.15 (m, 1H), 7.45 (m, 2H), 7.62 (d, J = 8.2, 1H), 11.00
(s, 1H); 13C-NMR (150 MHz, 80°C, DMSO-d6, δ ppm):
14.5, 61.5, 70.3, 112.1, 112.9, 113.3, 119.4, 119.5, 121.5,
124.8, 125.6, 136.8, 150.6; HRMS m/z (ESI): calcd for
C17H19N3NaO+4 [M + Na]+ 352.1268, found 352.1260.
[M
+
Na]+ 317.0719, found 317.0711; calcd for
C17H13N2OSÀ [M À H]À 293.0754, found 293.0755.
(2-(1H-indol-3-yl)benzo[d]thiazol-3(2H)-yl)(phenyl)
methanone (3be). Chromatographed on neutral alumina
with petroleum/diethyl ether (1:1); Yield: 60%;
mp = 190–193°C; IR (KBr, cmÀ1): 3303, 3057, 1659,
Dimethyl 2-(1H-indol-3-yl)-1H-imidazole-1,3(2H)-dicarboxylate
1
(10b).
Yield 97%; mp = 182–184°C; IR (KBr, cmÀ1):
1627, 1574, 1547, 1463, 748; Н-NMR (600 MHz, 80°C,
3341, 3048, 2958, 1723, 1698, 1577, 1550, 1450, 1265; 1Н-
NMR (600 MHz, 80°C, DMSO-d6, δ ppm, J Hz): 3.58 (s,
6H, 2 × CH3), 6.51 (s, 2H), 6.88 (s, 1H), 7.00 (m, 1H),
7.10 (m, 1H), 7.40 (m, 2H), 7.56 (d, J = 8.2, 1H), 10.96 (br
s, 1H); 13C-NMR (150 MHz, 80°C, DMSO-d6, δ ppm):
52.9, 70.5, 112.3, 113.1, 113.2, 119.5, 119.6, 121.7, 124.9,
125.7, 137.0, 151.2; HRMS m/z (ESI): calcd for
C15H15N3NaO+4 [M + Na]+ 324.0955, found 324.0942.
DMSO-d6, δ ppm, J Hz): 7.02–7.50 (m, 14H) 7.34 (s,
1H), 8.28 (br s, 1H); 13C-NMR (150 MHz, 80°C,
DMSO-d6, δ ppm): 64.1, 111.6, 116.7, 119.3, 120.0,
120.2, 122.6, 122.7, 123.4, 124.1, 124.9, 125.7, 127.1,
127.6, 128.5, 128.9, 131.0, 132.4, 135.3, 136.7, 138.2,
165.1; HRMS m/z (ESI): calcd for C22H16N2NaOS+
[M + Na]+ 379.0876, found 379.0873.
Tris-(3-indolyl)-methane (4). The general procedure for
preparation of 3be, but without any Et3N, gave 4 in 76%
isolated yield (based on indole). Chromatographed on
neutral alumina with petroleum/diethyl ether (1:1);
mp = 232–235°C (El-Sayed et al. [15] 235–240°C); IR
Bis(2,2,2-trichloroethyl)
2-(1H-indol-3-yl)-1H-imidazole-
1,3(2H)-dicarboxylate (10c).
Yield 92%; Oil; IR (KBr,
cmÀ1): 3398, 3061, 2956, 1711, 1577, 1551, 1410, 1276;
1Н-NMR (600 MHz, 80°C, DMSO-d6, δ ppm, J Hz):
4.79 (s, 4H, 2 × CH2), 6.68 (s, 2H), 7.00 (m, 1H), 7.03
(s, 1H), 7.09 (m, 1H, Ar), 7.39 (d, J = 8.2, 1H), 7.48 (d,
J = 2.9, 1H), 7.56 (d, J = 8.2, 1H), 11.01 (s, 1H); 13C-
NMR (150 MHz, 80°C, DMSO-d6, δ ppm): 71.1, 75.0,
95.9, 112.3, 113.4, 115.4, 119.4, 119.7, 121.8, 123.1,
137.1, 147.7, 152.7; HRMS m/z (ESI): calcd for
1
(KBr, cmÀ1): 3402, 3051, 2929, 1578, 1456, 1420; Н-
NMR (600 MHz, 20°C, DMSO-d6, δ ppm, J Hz): 6.06
(s, 1H), 6.86 (t, J = 7.6, 3H), 6.95 (d, J = 1.8, 3H), 7.02
(t, J = 7.6, 3H), 7.34 (d, J = 8.2, 3H), 7.41 (d, J = 8.2,
3H), 10.73 (s, 3H); 13C-NMR (150 MHz, 20°C,
DMSO-d6, δ ppm): 36.1, 116.6, 123.2, 123.5, 124.5,
125.8, 128.4, 132.0, 141.8; HRMS m/z (ESI): calcd for
C17H13Cl6N3NaO+4 [M + Na]+ 555.8929, found 555.8923.
Oxidation of amidoalkylated indoles (3) to camalexin (5a)
and benzocamalexin (5b). The corresponding compound
C25H18NÀ3 [M À H]À 360.1506, found 360.1499.
Synthesis of amidoalkylated indoles (10), general
3 (1 mmol) is dissolved in CH3CN (5–15 mL/mmol),
then the corresponding oxidant (1 mmol for the thiazole
derivatives 3a or 1.5 mmol for benzothiazole derivatives
procedure.
The
corresponding
chloroformate
(4.4 mmol) is slowly added to a cooled (0°C) and
magnetically stirred solution of imidazole (2 mmol,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Y. Stremski, S. Statkova-Abeghe, P. Angelov, and I. Ivanov
Vol 000
3b) is added, and the reaction mixture is magnetically
stirred under the conditions specified in Table 2. After
completion of the reaction, the solvent is evaporated
under reduced pressure, and the mixture is then dry-
loaded onto neutral alumina. Chromatography on a short
alumina column with diethyl ether/petroleum as the
eluent gave camalexin 5a or benzocamalexin 5b in yields
indicated in Table 2.
1H); 3C-NMR (150 MHz, 20°C, DMSO-d6, δ ppm):
112.0, 118.4, 119.4, 121.4, 121.9, 125.2, 126.0, 126.4,
136.4, 141.0, 152.4; 3215, 2995, 1590, 1570, 1460;
HRMS m/z (ESI): calcd for C11H10N+3 [M + H]+
184.0869, found 184.0864.
Diethyl 5,6,7,8-tetrachloro-2-(1H-indol-3-yl)-3a,9a-dihydro-
1H-benzo[5,6][1,4]dioxino[2,3-d]imidazole-1,3(2H)-
dicarboxylate (12a). Chromatographed on neutral alumina
with petroleum/diethyl ether (1:1, increasing polarity to
neat diethyl ether) Yield: 62%; IR (KBr, cmÀ1): 3329,
3062, 2986, 1718, 1599, 1558, 1466, 1278; 1Н-NMR
(600 MHz, 80°C, DMSO-d6, δ ppm, J Hz): 0.87 (br s,
6H, 2 × CH3), 3.87 (br s, 4H, 2 × CH2), 6.31 (s, 1H),
6.78 (s, 2H), 7.00 (m, 1H), 7.07 (m, 1H), 7.35 (d,
J = 7.6, 1H), 7.38 (d, J = 2.3, 1H), 7.48 (d, J = 7.6, 1H),
10.91 (s, 1H); 13C-NMR (150 MHz, 80°C, DMSO-d6, δ
ppm): 14.0, 61.8, 69.8, 84.4, 112.0, 113.2, 118.4, 119.4,
121.4, 121.9, 125.2, 126.0, 126.4, 136.4, 141.0, 152.3;
HRMS m/z (ESI): calcd for C23H19Cl4N3NaO6+
Camalexin,
2-(1H-indol-3-yl)thiazole
(5a).
Chromatographed on neutral alumina with petroleum/
diethyl ether (2:1, increasing polarity to 1:1); Yield: 74–
97%; mp = 137–138°C; IR (KBr, cmÀ1): 3149, 2980,
1587, 1561, 1455, 738; 1Н-NMR (600 MHz, 20°C,
DMSO-d6, δ ppm, J Hz): 7.17–7.23 (m, 2H), 7.48 (d,
J = 8.2, 1H), 7.54 (d, J = 3.5, 1H), 7.81 (d, J = 3.5, 1H),
8.08 (s, 1H), 8.19 (d, J = 8.2, 1H), 11.72 (s, 1H); 13C-
NMR (150 MHz, 20°C, DMSO-d6, δ ppm): 111.1, 112.6,
116.7, 120.7, 121.1, 122.8, 124.7, 126.8, 137.0, 143.0,
163.4; HRMS m/z (ESI): calcd for C11H9N2S+ [M + H]+
calcd 201.0481, found 201.0488, calcd for C11H7N2SÀ
[M À H]À 199.0335, found 199.0330.
[M + Na]+ 597.9896, found 597.9890.
Dimethyl 5,6,7,8-tetrachloro-2-(1H-indol-3-yl)-3a,9a-dihydro-
1H-benzo[5,6][1,4]dioxino[2,3-d]imidazole-1,3(2H)-dicarboxylate
Benzocamalexin, 2-(1H-indol-3-yl)benzo[d]thiazole (5b).
Chromatographed on neutral alumina with petroleum/
diethyl ether (3:1, increasing polarity to 2:1); Yield: 81–
96%; mp = 171–172°C (Harris [16a] 171–172°C,
Dzurilla et al. [16b] 124–127°C from ethanol); IR (KBr,
cmÀ1): 3218, 2963, 1594, 1552, 1443, 749; 1Н-NMR
(600 MHz, 20°C, DMSO-d6, δ ppm, J Hz): 7.27 (m, 2H),
7.36 (m, 1H), 7.48 (m, 1H), 7.53 (m, 1H), 7.97 (d,
J = 8.2, 1H), 8.05 (d, J = 8.2, 1H), 8.28 (s, 1H), 8.39 (m,
1H), 11.97 (s, 1H); 13C-NMR (150 MHz, 20°C,
DMSO-d6, δ ppm): 110.9, 112.8, 121.2, 121.6, 122.1,
122.2, 123.2, 124.7, 125.0, 126.6, 129.4, 133.5, 137.3,
154.2, 163.4; HRMS m/z (ESI): calcd for C15H11N2S+
(12b).
Chromatographed on neutral alumina with
petroleum/diethyl ether (1:1, increasing polarity to neat
diethyl ether); Yield: 54%; IR (KBr, cmÀ1): 3402, 3059,
1
2990, 1699, 1598, 1549, 1450, 1276; Н-NMR (600 MHz,
80°C, DMSO-d6, δ ppm, J Hz): 3.47 (s, 6H, 2 × CH3),
6.30 (s, 1H), 6.79 (s, 2H), 7.00 (m, 1H), 7.08 (m, 1H), 7.35
(d, J = 8.2, 1H), 7.39 (s, 1H), 7.47 (d, J = 8.2, 1H), 10.91
(s, 1H); 13C-NMR (150 MHz, 80°C, DMSO-d6, δ ppm):
52.9, 69.9, 84.6, 112.1, 112.6, 118.4, 119.5, 121.4, 124.9,
125.6, 126.2, 126.7, 136.5, 141.1, 152.6; HRMS m/z (ESI):
calcd for C21H15Cl4N3NaO+6 [M + Na]+ 569.9583, found
569.9590.
Synthesis of indole-3-carboxaldehyde (7) from either 3ac or
10c. Zinc (325 mg, 5 mmol) is added to a solution of
[M
+
H]+ 251.0637, found 251.0634, calcd for
C15H9N2SÀ [M À H]À 249.0492, found 249.0485.
the corresponding Troc-derivative 3ac or 10c (0.5 mmol)
and NH4Cl (265 mg, 5 mmol) in methanol/water (8:2,
10 mL), and the mixture is heated for 2 h at 60°C (10c)
or at reflux temperature (3ac). The solids are then filtered
off, and the filtrate is diluted with water (50–60 mL) and
extracted with CH2Cl2 (2 × 30 mL). The organic layers
are dried with Na2SO4, and the solvent is evaporated
under reduced pressure, dry-loading the crude indole-3-
carboxaldehyde onto neutral alumina. Column
chromatography on neutral alumina with diethyl
ether/petroleum (1:1) as the eluent gave pure indole-3-
carboxaldehyde in 82% yield (from 10c) or 74% yield
(from 3ac). mp = 194–196°C (Guillon et al. and Wang et
al. [17] 195–196°C); IR (KBr, cmÀ1): 3167, 2931, 1634,
Oxidation of amidoalkylated indoles (10) to aza-camalexin
(11).
o-Chloranil (1 mmol, 246 mg) is added to a
solution of the corresponding compound 10 (1 mmol) in
CH3CN (15–20 mL), and the mixture is stirred for 4 h at
r.t. After completion of the reaction, the solvent is
evaporated under reduced pressure, the residue is
redissolved in dichloromethane and is dry-loaded onto
neutral alumina. Chromatography on a short alumina
column with diethyl ether/petroleum (1:1) as the eluent
gave aza-camalexin 11 in yields specified in Table 3. The
Diels–Alder adducts 12 are obtained as competing side
products when the reaction is carried out at 85°C.
Aza-camalexin, 3-(1H-imidazol-2-yl)-1H-indole (11).
1
Chromatographed on neutral alumina with petroleum/
1577, 1521, 1445; Н-NMR (600 MHz, 20°C, DMSO-d6,
1
diethyl ether (1:1); Yield: 80–88%; Н-NMR (600 MHz,
δ ppm, J Hz): 7.22 (m, 1H), 7.26 (m, 1H), 7.51 (d,
J = 8.2, 1H), 8.09 (d, J = 8.2, 1H), 8.29 (s, 1H), 9.93 (s,
1H), 12.14 (s, 1H); 13C-NMR (150 MHz, 20°C,
DMSO-d6, δ ppm): 112.9, 118.6, 121.3, 122.6, 123.9,
20°C, DMSO-d6, δ ppm, J Hz): 6.57 (s, 2H), 6.63 (d,
J = 1.2, 1H) 6.71 (s, 1H), 6.86 (m, 1H), 6.98 (m, 1H),
7.22 (d, J = 8.2, 1H), 7.35 (d, J = 8.2, 1H), 10.78 (br s,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Synthesis of Camalexin and Related Analogues
[8] Tasch, B. O. A.; Antovic, D.; Merkul, E.; Müller, T. J. J. Eur J
Org Chem 2013, 4564.
[9] Dzurilla, M.; Kutschy, P.; Zaletova, J.; Ruzinsky, M.; Kovacik,
V. Molecules 2001, 6, 716.
[10] (a) Speckamp, W. N.; Hiemstra, H. Tetrahedron 1985, 41,
4367; (b) Comins, D. L.; Badawi, M. Heterocycles 1991, 32, 1869; (c)
Wanner, K. T.; Paintner, F. Tetrahedron 1994, 50, 3113; (d) Yazici, A.;
Pyne, S. G. Synthesis 2009 339; (e) Yazici, A.; Pyne, S. G. Synthesis
2009 513.
124.6, 137.5, 139.0, 185.5; HRMS m/z (ESI): calcd for
C9H7NNaO+ [M + Na]+ 168.0425, found 168.0420; calcd
for C9H6NOÀ [M À H]À 144.0449, found 144.0425.
Acknowledgments. We gratefully acknowledge funding from the
National Science Fund of the Bulgarian Ministry of Education
and Science (grant number DN09/15-2016) and the University
of Plovdiv (grant number FP17-HF013). The authors are
grateful to the Faculty of Biology, Department of Plant
Physiology and Molecular Biology, and Dr Mladen Naydenov
for carrying out high-resolution mass spectral measurements on
equipment provided under the EC FP7/REGPOT-2009-1/
BioSupport project.
[11] (a) Reissert, A. Ber Dtsch Chem Ges 1905, 38, 1603. (b)
Reissert, A. Ber Dtsch Chem Ges 1905, 38, 3415.
[12] (a) Dondoni, A.; Dall’Occo, T.; Galliani, G.; Mastellari, A.;
Medici, A. Tetrahedron Lett 1984, 25, 3637; (b) Itoh, T.; Hasegawa,
H.; Nagata, K.; Ohsawa, A. J Org Chem 1994, 59, 1319; (c) Itoh, T.;
Hasegawa, H.; Nagata, K.; Okada, M.; Ohsawa, A. Tetrahedron 1994,
50, 13089; (d) Itoh, T.; Miyazaki, M.; Nagata, K.; Ohsawa, A. Tetrahe-
dron 2000, 56, 4383; (e) Comins, D. L.; Abdullah, A. H. J Org Chem
1982, 47, 4315; (f) Comins, D. L.; Brown, J. D. Tetrahedron Lett 1984,
25, 3297.
[13] (a) Donova, A. K.; Statkova-Abeghe, S. M.; Venkov, A. P.;
Ivanov, I. Synth Commun 2004, 34, 2813; (b) Venkov, A. P.;
Statkova-Abeghe, S. M.; Donova, A. K. Cent Eur J Chem 2004, 2, 234.
[14] Beveridge, R. E.; Black, D. A.; Arndtsen, B. A. Eur J Org
Chem 2010, 3650.
REFERENCES AND NOTES
[1] Pedras, M. S. C.; Yaya, E. E.; Glawischnig, E. Nat Prod Rep
2011, 28, 1381.
[2] Browne, L. M.; Conn, K. L.; Ayer, W. A.; Tewari, J. P. Tetra-
hedron 1991, 47, 3909.
[3] (a) Glawischnig, E. Phytochemistry 2007, 68, 401; (b) Ma,
W.-H.; Quin, L.-P. Chem Nat Compd 2014, 776.
[4] (a) Pedras, M. S. C.; Minic, Z.; Sarma-Mamillapalle, V. K. J
Agric Food Chem 2009, 57, 2429; (b) Pedras, M. S. C.; Abdoli, A.;
Mamillapalle-Sarma, V. K. Molecules 2017, 22, 1345.
[5] (a) Mezencev, R.; Updegrove, T.; Kutschy, P.; Repovská, M.;
McDonald, J. F. J Nat Med 2011, 65, 488; (b) Mezencev, R.; Mojzis,
J.; Pilatova, M.; Kutschy, P. Neoplasma 2003, 50, 239; (c) Chripkova,
M.; Zigo, F.; Mojzis, J. Molecules 2016, 21, 1626; (d) Pilatova, M.;
Ivanova, L.; Kutschy, P.; Varinska, L.; Saxunova, L.; Repovska, M.;
Sarissky, M.; Seliga, R.; Mirossay, L.; Mojzis, J. Toxicol In Vitro 2013,
27, 939; (e) Odero-Marah, V.; Smith, B. Patent US 2013/0096166 A1,
2013.
[15] El-Sayed, M.; Mahmoud, K.; Hilgeroth, A. Curr Chem Lett
2014, 3, 7.
[16] (a) Harris, R. L. N. Aust J Chem 1974, 27, 2635; (b) Dzurilla,
M.; Ruzinsky, M.; Kutschy, P.; Tewari, J. P.; Kovacik, V. Collect
Czechoslov Chem Commun 1999, 64, 1448; (c) Katla, R.; Chowrasia,
R.; Manjari, P. S.; Domingues, N. L. C. RSC Adv 2015, 5, 41716.
[17] (a) Guillon, R.; Loge, C.; Pagniez, F.; Ferchaud-Roucher, V.;
Duflos, M.; Picot, C.; Pape, P. L. J Enzyme Inhib Med Chem 2011, 26,
261; (b) Wang, Q.-D.; Yang, J.-M.; Fang, D.; Ren, J.; Zeng, B.-B.
Tetrahedron Lett 2017, 58, 2877.
SUPPORTING INFORMATION
[6] (a) Ayer, W. A.; Craw, P. A.; Ma, Y.; Miao, S. Tetrahedron
1992, 48, 2919; (b) Fürstner, A.; Ernst, A. Tetrahedron 1995, 51, 773.
(c) Sakamoto, T.; Kondo, Y.; Takazawa, N.; Yamanaka, H. J Chem Soc
Perkin Trans 1927, 1, 1996.
[7] Nishida, A.; Miyashita, N.; Fuwa, M.; Nakagawa, M.
Heterocycles 2003, 59, 473.
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet