A facile synthesis of deaza-analogues of the bisindole marine alkaloid topsentin

Article abstract of DOI:10.3390/molecules18032518

A series of substituted ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5- (1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)carbonyl] -1H-pyrrole-3-carboxylates were prepared in excellent yields (82-98%) by one-pot reactions between β-dicarbonyl compounds 12a-e and 1,2-diaza-1,3-diene (DD) 13. Derivatives 10a,c-e, deazaanalogues of the bis-indole alkaloid topsentin, screened by the National Cancer Institute (Bethesda, MD, USA) in the in vitro one dose primary anticancer assay against a panel of about 60 human tumor cell lines, showed no significant activity, with the exception of compound 9e, which showed moderate activity against the HOP-92 cell line of the non small cell lung cancer sub-panel and the SNB-75 cell line of the CNS sub-panel.

Full text of DOI:10.3390/molecules18032518

Molecules 2013, 18, 2518-2527; doi:10.3390/molecules18032518
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A Facile Synthesis of Deaza-Analogues of the Bisindole Marine
Alkaloid Topsentin
Anna Carbone 1,*, Virginia Spanò 1, Barbara Parrino 1, Cristina Ciancimino 1,
Orazio A. Attanasi 2 and Gianfranco Favi 2,*
1
Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche,
Università degli Studi di Palermo, Via Archirafi 32, 90123 Palermo, Italy
2
Dipartimento di Scienze Biomolecolari, Università degli Studi di Urbino “Carlo Bo”,
Via I Maggetti 24, 61029 Urbino, Italy
* Authors to whom correspondence should be addressed; E-Mails: anna.carbone@unipa.it (A.C.);
gianfranco.favi@uniurb.it (G.F.); Tel.: +39-091-23896826 (A.C.); Fax: +39-091-23860854 (A.C.);
Tel.: +39-0722-303440 (G.F.).
Received: 21 January 2013; in revised form: 31 January 2013 / Accepted: 7 February 2013 /
Published: 26 February 2013
Abstract: A series of substituted ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-
(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylates
were prepared in excellent yields (82-98%) by one-pot reactions between β-dicarbonyl
compounds 12a–e and 1,2-diaza-1,3-diene (DD) 13. Derivatives 10a,c–e, deazaanalogues
of the bis-indole alkaloid topsentin, screened by the National Cancer Institute (Bethesda,
MD, USA) in the in vitro one dose primary anticancer assay against a panel of about 60
human tumor cell lines, showed no significant activity, with the exception of compound 9e,
which showed moderate activity against the HOP-92 cell line of the non small cell lung
cancer sub-panel and the SNB-75 cell line of the CNS sub-panel.
Keywords: topsentin; bis-indole alkaloids; antitumor activity; ethyl 1-[(tert-
butoxycarbonyl)amino]-2-methyl-5-(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)-
carbonyl]-1H-pyrrole-3-carboxylates

Molecules 2013, 18
2519
1. Introduction
Marine invertebrates represent a very important source of bioactive molecules. In particular,
bis-indole alkaloids (Figure 1), characterized by two indole units bound to a spacer through their 3
position, constitute a class of deep-sea sponge metabolites with potent antiinflammatory, antimicrobial,
antiviral and antitumor biological activity [1–4]. The first isolated four dragmacidins 1, containing the
six membered piperazine heterocyclic ring, were isolated from a large number of deep water sponges
including Dragmacidon, Halicortex, Spongosorites, Hexadella and the tunicate Didemnum candidum
and showed, among other biological properties, modest cytotoxic activity [5–7]. Successively, more
complex components of this family such as dragmacidin D (2), having a pyrazinone moiety as a
spacer, exhibited varied biological properties such as inhibition of serine-threonine protein
phosphatases, antiviral, antimicrobial and anticancer activities [8,9]. Nortopsentins A–C 3, with
imidazole as a five membered ring spacer, were isolated from the marine sponge Spongosorites ruetzleri
and showed in vitro cytotoxicity against P388 cells and antifungal activity against Candida albicans [10,11].
Topsentins 4 [12], having a bis(indolyl)imidazole skeleton (Figure 1), exhibited antiviral and antitumor
activity. In particular compounds 4a and 4b had in vitro activity against HSV-1, vesicular stomatitis
virus (VSV), and corona virus A-59 [13]. Topsentin 4a showed in vitro cytotoxicity against P388
murine tumor cells (IC50 3.0 µg/mL) and human tumor cells (HCT-8, A-549, T47D 20 µg/mL) and
in vivo activity against P388 (T/C 137%, 150 mg/kg) and B16 melanoma (T/C 144%, 37.5 mg/kg) [14].
Compounds 4b and 4c exhibited antiproliferative activity against human broncopulmonary cancer cells
(NSCLC-N6) with IC50 values of 12.0 and 6.3 µg/mL, respectively [2]. Due to these interesting
biological activities, there has been a rapid growth of interest in the synthesis of this class of
compounds and their analogues [15–19].
We have recently reported the synthesis and the antitumor activity of bis-indolyl thiophene [20] 5,
pyrazoles [21] 6, furans [22] 7 and isoxazoles [22] 8 (Figure 1) that showed inhibitory activity against
a wide range of human tumor cell lines, generally in the micro- and submicromolar range. Even more
recently bis-indolyl pyrroles [23] 9 have exhibited concentration-dependent antitumor activity towards
a panel of 42 human tumor cell lines, with mean IC50 values of 1.54 µM and 0.67 µM, respectively.
Moreover, investigating human tumor xenografts in an ex vivo clonogenic assay revealed selective
antitumor activity.
Thus, in our effort to search for novel antitumor compounds, and with the aim to develop more
potent and selective agents and considering the good antiproliferative activity of bis-indolyl pyrroles,
we designed further new analogues of the indole alkaloid topsentin. We report herein a synthesis of
ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-
yl)carbonyl]-1H-pyrrole-3-carboxylates derivatives 10a–e (Figure 1), in which the imidazole unit
moiety of topsentin was replaced by a highly functionalized pyrrole ring and the two indolyl portions
are placed in adjacent positions.

Molecules 2013, 18
2520
Figure 1. Bis-indolyl alkaloids.
H2N
N
H
N
+
N
H
R1
N
H
N
H
Br
R3
Br
N
N
R
Br
N
H
2
O
_
N
R2
H
N
1
CF3CO2
HO
H
Dragmacidin
R=H; R1=Me; R2=Br; R3=OH
Dragmacidin A R=R2=R3=H; R1=Me
Dragmacidin B R=R1=Me; R2=R3=H
Dragmacidin C R=R1=R2=R3=H
Dragmacidin D
O
N
N
R1
N
H
N
H
N
R
R1
R
H
N
N
N
4
H
H
3
H
(a) Topsentin A R=R1=H
(b) Topsentin B1 R=OH, R1=H
(c) Topsentin B2 R=Br; R1=OH
Nortopsentin A R=R1=Br
Nortopsentin B1 R=Br, R1=H
Nortopsentin B2 R=H; R1=Br
R
Y
Z
R
Me
N
X
R
R
O
N
R1
N
R1
NHCO2t-Bu
N
N
5-9
Me
EtO2C
Me
Thiophenes X=S; Y=Z=CH;
R=H,Me,OMe,Cl,Br; R1=H, Me, SO2Ph
Pyrazoles X=CH; Y=N; Z=NH, NMe;
R=H,Me,OMe,Cl,Br; R1=Me
Furans X=O; Y=Z=CH; R=H,Me,OMe;
R1=Me
10a-e
a R=H, b R=Me, c R=OMe, d R=Cl, e R=Br
Isoxazoles X=CH; Y=N; Z=O;
R=H,Me,OMe,Cl,Br; R1=Me
Pyrroles X=NH; Y = Z = CH;
R=H,Me,OMe,Cl,Br; R1=Me
2. Results and Discussion
The key intermediates for the synthesis of ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(1-
methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylates 10a–e are the
β-diketones 12a–e prepared in good yields (45–70%) from N-methylindoles 11a–e by a Friedel-Crafts
reaction using malonyl dichloride in dichloromethane, as previously reported [22] (Scheme 1).

Molecules 2013, 18
2521
Scheme 1. Synthesis of derivatives 10a–e.
O
O
R
R
R
i
N
N
N
Me
Me
Me
12a-e
11a-e
Me
EtO2C
N
CO2t-Bu
N
R
13
R
Me
N
O
ii
NHCO2t-Bu
N
N
Me
EtO2C
Me
a R=H, b R=Me, c R=OMe, d R=Cl, e R=Br
10a-e
Reagents and conditions: (i) malonyl dichloride, DCM, 2 h, rt; (ii) CuCl2·2H2O, THF, 12 h rt.
One-pot reaction of the latter with 1,2-diaza-1,3-diene (DD) 13 in tetrahydrofuran and in the
presence of CuCl2·2H2O as catalyst furnished the corresponding ethyl 1-[(tert-butoxycarbonyl)amino]-
2-methyl-5-(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxyl-ates
10a–e in excellent yields (82–98%).
The mechanism of the latter reaction involves the copper-promoted attack of the active carbon atom
of diketones 12a–e at the terminal carbon of the heterodiene system of DD (13) to give an adduct
intermediate A, followed by an intramolecular cyclization reaction to generate the ethyl
1-[(tert-butoxycarbonyl)amino]-5-hydroxy-2-methyl-5-(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-
3-yl)carbonyl]-4,5-dihydro-1H-pyrrole-3-carboxylate derivative B. Then, the spontaneous loss of a
water molecule provides the related aromatic pyrrole (Scheme 2). This five membered ring closure is
substantiated by several papers previous presented by some of us [24,25].
Scheme 2. Mechanism.
R
R
R
R
O
O
Me
12a-e
N
O
HO
+
H
N
Me
N
NHCO2t-Bu
EtO2C
Me
N
N
N
Me
NHCO2t-Bu
Me
N
Me
EtO2C
EtO2C
N
CO2t-Bu
Me
B
A
13
- H2O
R
R
Me
a R=H, b R=Me, c R=OMe, d R=Cl, e R=Br
N
O
NHCO2t-Bu
N
N
Me
EtO2C
Me
10a-e

Molecules 2013, 18
2522
Four pyrrole derivatives 10a,c–e were selected for a single dose concentration, in vitro
disease-oriented antitumor screenings against the full NCI panel of about 60 human tumor cell lines
that have grouped in disease sub-panels including leukaemia, non-small lung, colon, central nervous
system, melanoma, ovarian, renal, prostate, and breast tumors cell lines [26]. The results obtained take
into consideration the percent growth of the treated cells. The results revealed that compound 10e,
tested at the concentration of 10−4 M, showed a growth of −27.31% against the SNB-75 cell line of the
CNS cancer sub-panel and 4.73% against the HOP-92 of the non-small cell lung cancer sub-panel. The
other tested compounds showed no significant activity.
3. Experimental
3.1. Chemistry
3.1.1. General Procedures
All the commercially available reagents and solvents were used without further purification. 1,2-
Diaza-1,3-diene (DD, 13) was synthesized as a mixture of E/Z isomers as previously reported [27–29].
Column chromatography was performed with Merck silica gel 230–400 mesh ASTM or with a Büchi
Sepacor chromatography module (a prepacked cartridge system). TLC analysis was performed on
pre-loaded (0.25 mm) glass-supported silica gel plates (Kieselgel 60); compounds were visualized by
exposure to UV light and by dipping the plates in 1% Ce(SO4)·4H2O, 2.5% (NH4)6Mo7O24·4H2O in 10%
1
13
sulfuric acid followed by heating on a hot plate. H-NMR and C-NMR spectra were recorded in
DMSO-d6 or CDCl3 solution on 200 (Bruker AC) or 400 (Varian Mercury) MHz instruments. Proton
and carbon spectra were referenced internally to solvent signals, using values of δ = 2.49 ppm for
proton (middle peak) and δ = 39.50 ppm for carbon (middle peak) in DMSO-d6 and δ = 7.26 ppm for
proton and δ = 77.00 ppm for carbon (middle peak) in CDCl3. The following abbreviations are used to
describe peak patterns where appropriate: s = singlet, d = doublet, t = triplet q = quartet and
m = multiplet. All coupling constants (J) are given in Hz. All melting points were taken on a
Büchi-Tottoli capillary apparatus and are uncorrected; IR spectra were determined in bromoform or
nujol with a Jasco FT/IR 5300 spectrophotometer. Mass spectra were recorded in the EI mode (70eV)
on a Shimadzu GC-MS QP5050A spectrometer. Elemental analyses (C, H, N) were within ±0.4% of
the theoretical values.
3.1.1.1. General Procedure for the Preparation of 1,3-bis(Indol-3-yl)propane-1,3-diones 12a–e
A solution of malonyl dichloride (1 mL, 10 mmol) in DCM (10 mL) was added dropwise to a cold
solution of the appropriate N-methylindole 11a–e (20 mmol) in DCM (20 mL). The reaction mixture
was stirred for 2 h at room temperature. The mixture was then added to 5% aqueous sodium carbonate
and shaken for 2 min. This was extracted with DCM, dried and evaporated. The residue was purified
by chromatography eluting with DCM/Ethyl acetate 9/1 to give derivatives 12a–e. Analytical and
spectroscopic data are reported elsewhere [12].

Molecules 2013, 18
2523
3.1.1.2. General Procedure for the Preparation of Ethyl 1-[(tert-Butoxycarbonyl)amino]-2-methyl-5-(1-
methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylates 10a–e
To a solution of the appropriate 1,3-bis(indol-3-yl)propane-1,3-dione 12a–e (1.0 mmol) in THF (5 mL),
1,2-diaza-1,3-diene (DD, 13) as a mixture of E/Z isomers (1.0 mmol) and CuCl2·2H2O (0.5 mmol)
were added. The reaction was stirred under magnetic stirring at room temperature for 12 h, until the
disappearance of the DD (as monitored by TLC). The crude mixture was concentrated under reduced
pressure and the products 10a–e were purified by column chromatography on silica gel (eluent: ethyl
acetate/cyclohexane = 1:1).
Ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(1-methyl-1H-indol-3-yl)-4-[(1-methyl-1H-indol-3-
yl)carbonyl]-1H-pyrrole-3-carboxylate (10a). Pale yellow solid; yield: 98%; mp: 169–173 °C; IR
1
(nujol) ν 3263, 1765, 1682, 1626 cm−1; H-NMR (400 MHz, DMSO-d6) δ 0.75 (t, J = 7.0 Hz, 3H),
1.34 (s, 9H), 2.39 (s, 3H), 3.68 (s, 3H), 3.69 (s, 3H), 3.87 (q, J = 7.0 Hz, 2H), 6.91 (t, J = 7.2 Hz, 1H),
7.08 (t, J = 7.2 Hz, 1H), 7.12 (t, J = 7.2 Hz, 1H), 7.17 (d, J = 7.2 Hz, 1H), 7.20 (s, 1H), 7.33 (d, J = 8.0 Hz,
1H), 7.39 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 8.0 Hz, 1H), 7.52 (s, 1H), 8.01 (d, J = 8.0 Hz, 1H), 10.20 (s,
13
1H); C-NMR (100 MHz, DMSO-d6) δ 10.4 (q), 13.5 (q), 27.8 (q), 32.4 (q), 32.8 (q), 58.9 (t), 80.7
(s), 102.6 (s), 108.9 (s), 109.7 (d), 110.3 (d), 117.6 (s), 119.2 (s), 119.8 (d), 121.3 (2C, d), 121.6 (d),
122.5 (d), 122.8 (d), 125.9 (s), 127.6 (s), 130.6 (d), 135.8 (s), 136.1 (s), 137.1 (s), 137.6 (d), 154.6 (s),
163.8 (s), 186.5 (s). MS: m/z (%) = 554 [M+] (48), 498 (100), 454 (78), 392 (71), 367 (28), 338 (28), 158
(92). Anal. Calcd for C32H34N4O5: C, 69.30; H, 6.18; N, 10.10. Found: C, 69.47; H, 5.93; N, 10.22.
Ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(1,5-dimethyl-1H-indol-3-yl)-4-[(1,5-dimethyl-1H-
indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylate (10b). Pale yellow solid; yield: 94%; mp: 186–190 °C;
IR (nujol) ν 3247, 1744, 1684, 1628 cm−1; 1H-NMR (400 MHz, DMSO-d6) δ 0.79 (t, J = 7.0 Hz, 3H),
1.34 (s, 9H), 2.21 (s, 3H), 2.35 (s, 3H), 2.38 (s, 3H), 3.64 (s, 3H), 3.65 (s, 3H), 3.89 (q, J = 7.0 Hz,
2H), 6.88 (d, J = 8.0 Hz, 1H), 6.99 (d, J = 7.2 Hz, 1H), 7.11 (s, 1H), 7.20 (d, J = 8.0 Hz, 1H), 7.23
13
(s, 1H), 7.26 (d, J = 8.0 Hz, 1H), 7.49 (s, 1H), 7.81 (s, 1H), 10.16 (s, 1H); C NMR (100 MHz,
DMSO-d6) δ 10.4 (q), 13.6 (q), 21.0 (q), 21.1 (q), 27.8 (q), 32.4 (q), 32.7 (q), 58.9 (t), 80.6 (s), 102.1
(s), 108.7 (s), 109.3 (d), 109.9 (d), 117.2 (s), 119.6 (d), 121.1 (d), 122.8 (d), 123,0 (s), 123.9 (d), 125.9
(s), 126.1 (s), 127.6 (s), 128.0 (s), 130.4 (d), 130.6 (s), 134.5 (s), 135.5 (s), 135.7 (s), 137.6 (d), 154.5
(s), 163.8 (s), 186.5 (s). MS: m/z (%) = 582 [M+] (45), 526 (100), 482 (98), 420 (53), 172 (68).
Anal. Calcd for C34H38N4O5: C, 70.08; H, 6.57; N, 9.62. Found: C, 69.82; H, 6.75; N, 9.75.
Ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(5-methoxy-1-methyl-1H-indol-3-yl)-4-[(5-methoxy-
1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylate (10c). White solid; yield: 83%; mp:
1
202–206 °C; IR (nujol) ν 3148, 1746, 1731, 1705 cm−1; H-NMR (400 MHz, DMSO-d6) δ 0.75 (t,
J = 7.0 Hz, 3H), 1.35 (s, 9H), 2.39 (s, 3H), 3.32 (s, 6H), 3.68 (s, 3H), 3.71 (s, 3H), 3.88 (q, J = 7.0 Hz,
2H), 6.65 (d, J = 8.8 Hz, 1H), 6.78–6.83 (m, 2H), 7.14 (s, 1H), 7.22 (d, J = 8.8 Hz, 1H), 7.31 (d,
J = 8.8 Hz, 1H), 7.47 (s, 1H), 7.60 (s, 1H), 10.15 (s, 1H); 13C-NMR (100 MHz, DMSO-d6) δ 10.4 (q),
13.5 (q), 27.8 (q), 32.6 (q), 32.9 (q), 54.5 (q), 55.1 (q), 58.9 (t), 80.7 (s), 101.2 (d), 102.3 (s), 102.8 (d),
108.9 (s), 110.4 (d), 111.2 (d), 111.7 (d), 112.4 (d), 117.5 (s), 122.7 (s), 126.0 (s), 126.7 (s), 128.0 (s),
130.4 (d), 131.2 (s), 132.2 (s), 135.7 (s), 137.5 (d), 153.6 (s), 154.6 (s), 155.4 (s), 163.8 (s), 186.5 (s).

Molecules 2013, 18
2524
MS: m/z (%) = 614 [M+] (40), 558 (84), 514 (100), 499 (20), 452 (31), 188 (52). Anal. Calcd for
C34H38N4O7: C, 66.43; H, 6.23; N, 9.11. Found: C, 66.79; H, 5.97; N, 9.23.
Ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(5-chloro-1-methyl-1H-indol-3-yl)-4-[(5-chloro-1-
methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylate (10d). White solid; yield: 90%; mp:
1
195–199 °C; IR (nujol) ν 3257, 1754, 1678, 1637 cm−1; H-NMR (400 MHz, DMSO-d6) δ 0.76 (t,
J = 7.2 Hz, 3H), 1.33 (s, 9H), 2.38 (s, 3H), 3.70 (s, 6H), 3.89 (q, J = 7.2 Hz, 2H), 7.08 (dd, J = 8.4 Hz,
J = 1.8 Hz, 1H), 7.21 (dd, J = 8.4 Hz, J = 1.8 Hz, 1H), 7.29 (s, 1H), 7.38 (d, J = 8.4 Hz, 1H), 7.41–7.48
(m, 2H), 7.64 (s, 1H), 8.00 (d, J = 1.8 Hz, 1H), 10.26 (s, 1H); 13C-NMR (100 MHz, DMSO-d6) δ 10.3
(q), 13.5 (q), 27.7 (q), 32.7 (q), 33.0 (q), 59.0 (t), 80.8 (s), 102.2 (s), 108.8 (s), 111.4 (d), 112.2 (d),
117.0 (s), 119.0 (d), 120.3 (d), 121.3 (d), 122.6 (d), 122.8 (s), 124.3 (s), 125.4 (s), 126.6 (s), 126.8 (s),
128.6 (s), 132.4 (d), 134.6 (s), 135.7 (s), 136.2 (s), 138.9 (d), 154.5 (s), 163.6 (s), 186.3 (s). MS: m/z
(%) = 622 [M+] (13), 566 (36), 522 (100), 507 (27), 357 (30), 328 (29), 192 (78). Anal. Calcd for
C32H32Cl2N4O5: C, 61.64; H, 5.17; N, 8.99. Found: C, 61.77; H, 5.29; N, 8.91.
Ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(5-bromo-1-methyl-1H-indol-3-yl)-4-[(5-bromo-1-
methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylate (10e). White solid; yield: 82%; mp:
1
201–205 °C; IR (nujol) ν 3261, 1755, 1693, 1637 cm−1; H-NMR (400 MHz, DMSO-d6) δ 0.77 (t,
J = 7.0 Hz, 3H), 1.33 (s, 9H), 2.38 (s, 3H), 3.70 (s, 6H), 3.89 (q, J = 7.0 Hz, 2H), 7.19 (d, J = 8.4 Hz,
1H), 7.27 (s, 1H), 7.31–7.36 (m, 2H), 7.41 (d, J = 8.4 Hz, 1H), 7.56 (s, 1H), 7.63 (s, 1H), 8.17 (s, 1H),
13
10.26 (s, 1H); C-NMR (100 MHz, DMSO-d6) δ 10.3 (q), 13.6 (q), 27.8 (q), 32.6 (q), 33.0 (q), 59.0
(t), 80.8 (s), 102.1 (s), 108.8 (s), 111.9 (s), 112.3 (s), 112.6 (d), 114.7 (d), 116.9 (s), 122.0 (d), 122.8
(s), 123.3 (d), 123.8 (d), 125.1 (d), 125.4 (s), 127.4 (s), 129.2 (s), 132.3 (d), 134.8 (s), 135.9 (s), 136.1
(s), 138.8 (d), 154.5 (s), 163.6 (s), 186.3 (s). MS: m/z (%) = 710 [M+] (26), 612 (100), 566 (20), 471
(23), 401 (25), 252 (60), 236 (35), 181 (35). Anal. Calcd for C32H32Br2N4O5: C, 53.95; H, 4.53; N,
7.86. Found: C, 53.76; H, 4.65; N, 8.01.
3.2. Biology
3.2.1. Methodology of the in Vitro Cancer Screen
The human tumor cell lines of the cancer screening panel are grown in RPMI 1640 medium
containing 5% fetal bovine serum and 2 mM L-glutamine. For a typical screening experiment, cells are
inoculated into 96 well microtiter plates in 100 µL at plating densities ranging from 5,000 to
40,000 cells/well depending on the doubling time of individual cell lines. After cell inoculation, the
microtiter plates are incubated at 37 °C, 5% CO2, 95% air and 100% relative humidity for 24 h prior to
addition of experimental drugs. After 24 h, two plates of each cell line are fixed in situ with TCA, to
represent a measurement of the cell population for each cell line at the time of drug addition (Tz).
Experimental drugs are solubilized in dimethyl sulfoxide at 400-fold the desired final maximum test
concentration and stored frozen prior to use. At the time of drug addition, an aliquot of frozen
concentrate is thawed and diluted to twice the desired final maximum test concentration with complete
medium containing 50 µg/mL gentamicin. Additional four, 10-fold or ½ log serial dilutions are made
to provide a total of five drug concentrations plus control. Aliquots of 100 µL of these different drug

Molecules 2013, 18
2525
dilutions are added to the appropriate microtiter wells already containing 100 µL of medium, resulting
in the required final drug concentrations. Following drug addition, the plates are incubated for an
additional 48 h at 37 °C, 5% CO2, 95% air, and 100% relative humidity. For adherent cells, the assay is
terminated by the addition of cold TCA. Cells are fixed in situ by the gentle addition of 50 µL of cold
50% (w/v) TCA (final concentration, 10% TCA) and incubated for 60 min at 4 °C. The supernatant is
discarded, and the plates are washed five times with tap water and air dried. Sulforhodamine B (SRB)
solution (100 µL) at 0.4% (w/v) in 1% acetic acid is added to each well, and plates are incubated for
10 min at room temperature. After staining, unbound dye is removed by washing five times with 1%
acetic acid and the plates are air dried. Bound stain is subsequently solubilized with 10 mM trizma
base, and the absorbance is read on an automated plate reader at a wavelength of 515 nm. For
suspension cells, the methodology is the same except that the assay is terminated by fixing settled cells
at the bottom of the wells by gently adding 50 µL of 80% TCA (final concentration, 16% TCA). Using
the seven absorbance measurements [time zero, (Tz), control growth, (C), and test growth in the
presence of drug at the five concentration levels (Ti)], the percentage growth is calculated at each of
the drug concentrations levels. Percentage growth inhibition is calculated as:
[(Ti-Tz)/(C-Tz)] × 100 for concentrations for which Ti>/=Tz
[(Ti-Tz)/Tz] × 100 for concentrations for which Ti<Tz.
For further information to see NCI website [30].
4. Conclusions
In conclusion, five ethyl 1-[(tert-butoxycarbonyl)amino]-2-methyl-5-(1-methyl-1H-indol-3-yl)-4-
[(1-methyl-1H-indol-3-yl)carbonyl]-1H-pyrrole-3-carboxylates derivatives 10a–e, analogues of the
marine alkaloid topsentin, in which the imidazole moiety was replaced by a pyrrole ring, were
obtained. Only one of these compounds (compound 10e) showed moderate activity against two cell
lines belonging to the NCI CNS and non-small cell lung cancer sub-panels.
Acknowledgments
This work was financially supported by Ministero dell’Istruzione dell'Università e della Ricerca.
We thank the National Cancer Institute (Bethesda, MD, USA) for the antitumor tests.
References and Notes
1. Shin, J.; Seo, Y.; Cho, K.W.; Rho, J.R.; Sim, C. New Bis(indole)Alkaloids of the Topsentin class
from the sponge Spongosorites genitrix. J. Nat. Prod. 1999, 62, 647–649.
2. Casapullo, A.; Bifulco, G.; Bruno, I.; Riccio, R. Hamacanthin Classes from the Mediterranean
Marine Sponge Raphisia lacazei. J. Nat. Prod. 2000, 63, 447–451.
3. Bao, B.; Sun, Q.; Yao, X.; Hong, J.; Lee, C.; Sim, C.J.; Jung, J.H. Cytotoxic Bisindole Alkaloids
from a Marine Sponge Spongosorites sp. J. Nat. Prod. 2005, 68, 711–715.

Molecules 2013, 18
2526
4. Gul, W.; Hamann, M.T. Indole alkaloid marine natural products: An established source of cancer
drug leads with considerable promise for the control of parasitic, neurological and other diseases.
Life Sci. 2005, 78, 442–453.
5. Kohmoto, S.; Kashman, Y.; McConnell, O.J.; Rinehart, K.L., Jr.; Wright, A.; Koehn, F. The first
total synthesis of Dragmacidin D. J. Org. Chem. 1988, 53, 3116–3118.
6. Morris, S.A.; Andersen, R.J. Brominated bis(indole)alkaloids from the marine sponge Hexadella
sp. Tetrahedron 1990, 46, 715–720.
7. Fahy, E.; Potts, B.C.M.; Faulkner, D.J.; Smith, K. 6-Bromotryptamine Derivatives from the Gulf
of California Tunicate Didemnum candidum. J. Nat. Prod. 1991, 54, 564–569.
8. Wright, A.E.; Pomponi, S.A.; Cross, S.S.; McCarthy, P. A new bis-(indole)alkaloids from a deep-
water marine sponge of the genus Spongosorites. J. Org. Chem. 1992, 57, 4772–4775.
9. Capon, R.J.; Rooney, F.; Murray, L.M.; Collins, E.; Sim, A.T.R.; Rostas, J.A.P.; Butler, M.S.;
Carroll, A.R. Dragmacidins: New protein phosphatase inhibitors from a Southern Australian
deep-water Marine Sponge Spongosorites sp. J. Nat. Prod. 1998, 61, 660–662.
10. Alvarez, M.; Salas, M. Marine, nitrogen-containing heterocyclic natural products-structures and
syntheses of compounds containing indole units. Heterocycles 1991, 32, 1391–1429.
11. Sakem, S.; Sun, H.H. Cytotoxic and antifungal imidazolediylbis[indoles] from the sponge
Spongosorites ruetzleri. J. Org. Chem. 1991, 56, 4304–4307.
12. Bartik, K.; Braekman, J.-C.; Daloze, D.; Stoller, C. Topsentin, new toxic bis-indole alkaloids from
the marine sponge Topsentia genitrix. Can. J. Chem. 1987, 65, 2118–2121.
13. Burres, N.S.; Barber, D.A.; Gunasekera, S.P.; Shen, L.L.; Clement, J. Antitumor activity and
biochemical effects of topsentin. Biochem. Pharmacol. 1991, 42, 745–751.
14. Tsuji, S.; Rinehart, K.L.; Gunasekera, S.P.; Kashman, Y.; Cross, S.S.; Lui, M.S.; Pomponi, S.A.;
Diaz, M.C. Topsentin, Bromotopsentin, and Dihydrodeoxybromotopsentin: Antiviral and
Antitumor bis (indolyl)imidazoles from Caribbean deep-sea sponge of the family Halichondriidae.
Structural and synthetic studies. J. Org. Chem. 1988, 53, 5446–5453.
15. Gu, X.; Wan, X.; Jiang, B. Syntheses and biological activities of bis(3-indolyl)thiazoles,
analogues of marine bis(indole)alkaloid nortopsentin. Bioorg. Med. Chem. Lett. 1999, 9, 569–572.
16. Jiang, B.; Gu, X.H. Syntheses and cytotoxicity evaluation of bis(indolyl)thiazole,
bis(indolyl)pyrazinone and bis(indolyl)pyrazine: analogues of cytotoxic marine bis(indole)
alkaloid. Bioorg. Med. Chem. Lett. 2000, 8, 363–371.
17. Jiang, B.; Xiong, X.; Yang, C. Synthesis and cytotoxicity evaluation of novel indolylpyrimidines
and indolylpyrazines as potential antitumor agents. Bioorg. Med. Chem. 2001, 9, 1149–1154.
18. Jiang, B.; Xiong, X.; Yang, C. Synthesis and antitumor evaluation of novel monoindolyl-4-
trifluoromethylpyridines and bisindolyl-4-trifluoromethylpyridines. Bioorg. Med. Chem. Lett.
2001, 11, 475–477.
19. Xiong, W.; Yang, C.; Jiang, B. Synthesis of novel analogues of marine indole alkaloids:
mono(indolyl)-4-trifluoromethylpyridines and bis(indolyl)-4-trifluoromethylpyridines as potential
anticancer agents. Bioorg. Med. Chem. 2001, 9, 1773–1770.
20. Diana, P.; Carbone, A.; Barraja, P.; Montalbano, A.; Martorana, A.; Dattolo, G.; Gia, O.;
Dalla Via, L.; Cirrincione, G. Synthesis and antitumor properties of 2,5-bis(3’-indolyl)thiophenes:
analogues of marine alkaloid nortopsentin. Bioorg. Med. Chem. Lett. 2007, 17, 2342–2346.

Molecules 2013, 18
2527
21. Diana, P.; Carbone, A.; Barraja, P.; Martorana, A.; Gia, O.; Dalla Via, L.; Cirrincione, G.
3,5-Bis(3’-indolyl)pyrazoles, analogues of marine alkaloid nortopsentin: synthesis and antitumor
properties. Bioorg. Med. Chem. Lett. 2007, 17, 6134–6137.
22. Diana, P.; Carbone, A.; Barraja, P.; Kelter, H.H.; Fiebig, G.; Cirrincione, G. Synthesis and
antitumor activity of 2,5-bis(3’-indolyl)-furans and 3,5-bis(3’-indolyl)-isoxazoles, nortopsentin
analogues. Bioorg. Med. Chem. 2010, 18, 4524–4529.
23. Carbone, A.; Parrino, B.; Barraja, P.; Spanò, V.; Cirrincione, G.; Diana, P.; Maier, A.; Kelter, G.;
Fiebig, H-H. Synthesis and antiproliferative activity of 2,5-bis(3’-Indolyl)pyrroles, analogues of
the marine alkaloid Nortopsentin. Marine Drugs 2013, 11, doi:10.3390/md110x000x.
24. Recent review on the chemistry of 1,2-diaza-1,3-dienes, see: Attanasi, O.A.; De Crescentini, L.;
Favi, G.; Filippone, P.; Mantellini, F.; Perrulli, F.R.; Santeusanio, S. Cultivating the passion to
build heterocycles from 1,2-diaza-1,3-dienes: the Force of immagination. Eur. J. Org. Chem.
2009, 3109–3127.
25. Report on copper-catalyzed synthesis of pyrroles from 1,2-diaza-1,3-dienes, see: (b)
Attanasi, O.A.; Grossi, M.; Serra-Zanetti, F.; Foresti, E. Effect of metal ions in organic synthesis.
Part XXXIII. 2-hydroxy-4-pyrrolines as stable intermediates in the synthesis of 1-aminopyrrole
derivatives. Tetrahedron 1987, 43, 4249–4256.
26. Monks, A.; Scudiero, D.; Skehan, P.; Shoemaker, R.; Paull, K.; Vistica, D.; Hose, C.; Langely, J.;
Cronise, P.; Vaigro-Wolff, A.; et al. Feasibility of a high-flux anticancer drug screen using a
diverse panel of culture human tumor cell lines. J. Natl. Cancer Inst. 1991, 83, 757–766.
27. Sommer, S. Inverse diels-alder-reaktionen von azo-alkenen mit aliene. Tetrahedron Lett. 1977,
18, 117–120.
28. Attanasi, O.A.; Filippone, P.; Mei, A.; Santeusanio, S. Effect of metal ions in organic synthesis;
Part XXIII. Easy and high-yield direct synthesis of 3-aminocarbonyl-1-ureidopyrroles by the
copper(II) chloride-catalyzed reaction of aminocarbonylazoalkenes with 3-oxoalkanamides.
Synthesis 1984, 671–672.
29. Attanasi, O.A.; Filippone, P.; Mei, A.; Santeusanio, S. Effect of metal ions in organic. Synthesis;
Part XXIV. Facile one-flask synthesis of 1-alkoxycarbonylamino by reaction of
alkoxycarbonylamino-3-aminocarbonylpyrroles by reaction of alkoxycarbonylazoalkenes with
3-oxoalkanamides under copper(II) chloride catalysis. Synthesis 1984, 873–874.
30. NCI-60 DTP Human tumor cell line screen. Available online: http://dtp.nci.nih.gov/
branches/btb/ivclsp.html (accessed on 12 March 2012).
Sample Availability: Samples of the compounds 10a–e are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).