Hetero-annulation reaction between 2-acylnaphthoquinones and 2-aminobenzothiazoles. A new synthetic route to antiproliferative benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-quinones

Article abstract of DOI:10.1016/j.tetlet.2015.07.034

A convenient two-step method is developed for the preparation of benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-quinones from 2-acylnaphthohydroquinones and 2-aminobenzothiazoles. The structure of the heterocyclic quinones is supported by X-ray crystallography. This protocol provides an operationally simple strategy to prepare the title compounds and shows good functional flexibility and easily available starting materials. Evidences are reported on the significant in vitro antiproliferative activities of some of the obtained heterocyclic quinones on prostate, bladder, and breast human-derived tumor cell lines.

Full text of DOI:10.1016/j.tetlet.2015.07.034

Tetrahedron Letters 56 (2015) 5103–5105
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Hetero-annulation reaction between 2-acylnaphthoquinones and 2-
aminobenzothiazoles. A new synthetic route to antiproliferative
benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-quinones
b
c
Jaime A. Valderrama a,b, , David Ríos , Giulio G. Muccioli , Pedro Buc Calderon b,d, Iván Brito e,
⇑
Julio Benites a,b
a Instituto de Ciencias Exactas y Naturales, Universidad Arturo Prat, Casilla 121, Iquique, Chile
b Facultad de Ciencias de la Salud, Universidad Arturo Prat, Casilla 121, Iquique, Chile
c Bioanalysis and Pharmacology of Bioactive Lipids Laboratory, Louvain Drug Research Institute, Université Catholique de Louvain, 72 Avenue E. Mounier, BPBL 7201, 1200
Brussels, Belgium
d Toxicology and Cancer Biology Research Group, Louvain Drug Research Institute, Université Catholique de Louvain, 73 Avenue E. Mounier, GTOX 7309, 1200 Brussels, Belgium
e Departamento de Química, Facultad de Ciencias Básicas Universidad de Antofagasta, Casilla 170, Universidad de Antofagasta, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient two-step method is developed for the preparation of benzo[g]benzothiazolo[2,3-b]quinazo-
line-7,12-quinones from 2-acylnaphthohydroquinones and 2-aminobenzothiazoles. The structure of the
heterocyclic quinones is supported by X-ray crystallography. This protocol provides an operationally sim-
ple strategy to prepare the title compounds and shows good functional flexibility and easily available
starting materials. Evidences are reported on the significant in vitro antiproliferative activities of some
of the obtained heterocyclic quinones on prostate, bladder, and breast human-derived tumor cell lines.
Ó 2015 Published by Elsevier Ltd.
Received 17 June 2015
Revised 10 July 2015
Accepted 11 July 2015
Available online 17 July 2015
Keywords:
Acylnaphthoquinones
Aminobenzothiazoles
Hetero-annulation
Antiproliferative activity
MTT assay
Introduction
of isoquinoline-containing polycyclic quinones.11 It is worth men-
tioning that a number of the reported N-heterocyclic quinones
Quinones are ubiquitous in nature and comprise one of the lar-
gest classes of anticancer agents.1–3 Anticancer quinones are cur-
rently the focus of intensive research because of their biological
activity and complex modes of action, which differ depending on
their particular structure. The biological processes involved with
the antitumor activity of quinones are based mainly on DNA inter-
calation, bioreductive alkylation of biomolecules, and generation of
reactive oxygen species (ROS) through redox cycling.4–8 The DNA
intercalative ability of quinonoid antitumor agents, such as
daunorubicine, doxorubicine, mitoxantrone, and mitomycin C, is
due to their large and planar polycyclic structures, which facilitates
the binding between the base pairs through hydrogen bonds and
exhibit inhibition of topoisomerase I and activation of caspase-3
in HL-60 cells.12 Our synthetic strategy to entry into isoquino-
line-containing polycyclic quinones is based on the hetero-annula-
tion reaction of 2-acyl-1,4-quinones with primary acyclic- and
endocyclic enaminones, where the electrophilic
a,b-unsaturated
acyl fragment of the quinone and the ambident nucleophile
H2NACH@CRA group of the enaminone are involved in the N-hete-
rocyclic ring formation.13–15 Taking into account the similar chem-
ical reactivity of enaminones16 and 2-aminobenzothiazoles17 to act
as ambident nucleophiles with
a,b-unsaturated carbonyl com-
pounds to give heterocycles, we decided to explore the synthesis
of benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-quinones from
2-acylnaphthoquinones and 2-aminobenzothiazoles. To the best
of our knowledge, the sole precedent regarding the synthesis
of antiproliferative benzo[g]benzothiazolo[2,3-b]quinazolinequin
ones is the recent report on amberlyst-15 catalyzed three-compo-
nent condensation of 2-aminobenzothiazole, aromatic aldehydes,
and 2-hydroxy-1,4-naphthoquinone.18 Herein, we wish to
p
-stacking interactions.9,10 Our research group has especially
focused on biologically active compounds based on quinone cores
fused to heterocyclic rings. In this context we have reported the
synthesis and antiproliferative activity on cancer cells of a variety
⇑
Corresponding author. Tel.: +56 57 252 6215; fax: +56 57 2252 6395.
E-mail addresses: jvalderr@uc.cl, jaimeadolfov@gmail.com (J.A. Valderrama).
report
that
benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-
http://dx.doi.org/10.1016/j.tetlet.2015.07.034
0040-4039/Ó 2015 Published by Elsevier Ltd.

5104
J. A. Valderrama et al. / Tetrahedron Letters 56 (2015) 5103–5105
quinone derivatives can be conveniently obtained from easily
available 2-acyl-1,4-naphthoquinones and 2-aminobenzothiazoles.
Preliminary antiproliferative evaluation of some members of the
series on cancer cell lines is also described.
Results and discussion
To explore the possibility of heterocyclic annulation reactions,
2-acetyl-1,4-naphthoquinone 2a and aminobenzothiazole 3a were
initially chosen. Quinone 2a, prepared by oxidation of acylhydro-
quinone 1a19 with silver(I) oxide, was reacted with 3a in dichloro-
methane at room temperature to give a red solid product. The IR
spectrum of the product revealed the presence of OAH and C@O
bands at 3421, 1680 and 1644 cmꢀ1. The 1H NMR spectrum
showed hydroxyl and naphthoquinone peri aromatic protons at d
6.37 (br s), 8.10 (dd) and 8.17 (dd). The 13C NMR spectrum dis-
played characteristic signals at d 28.6, 88.5, 182.7, and 185.7 due
to a methyl, an aliphatic quaternary carbon, and two carbonyl
groups. The mass spectrum showed the molecular ion [M+] peak
at m/z = 348.05649. Based on these data, together with the high
electrophilic character of the C-3 in 2-acyl-1,4-naphthoquinones
and the behavior of 2-aminobenzothiazole to act as ambident
Figure 1. X-ray crystal structure of 4b.
Table 1
Synthesis and yields of compounds 4a–ka
nucleophile with a,b-unsaturated carbonyl compounds, two possi-
ble alternative structures derived from benzo[g]benzothiazolo[2,3-
b]quinazoline (4a) and benzo[g]benzo[4,5]-thiazolo[3,2-a]quina-
zoline (4a0), were assigned for the reaction product isolated in
66% yield (Scheme 1).
Substrates
Quinone
R1
R2
Yieldb (%)
1a
1b
1c
1d
1a
1b
1d
1a
1c
1d
1e
3a
3a
3a
3a
3b
3b
3b
3c
3c
3c
3a
4a
4b
4c
4d
4e
4f
4g
4h
4i
Methyl
Propyl
Pentyl
Heptyl
Methyl
Propyl
Heptyl
Methyl
Pentyl
Heptyl
Furyl
H
H
H
H
Me
Me
Me
OMe
OMe
OMe
H
67
78
47
41
61
68
84
43
75
60
55
Encouraged by these results we explored the substrate scope for
the synthesis of dibenzothiazoloquinazolinequinones such as 4a or
4a0. A variety of 2-acyl-1,4-naphthoquinones 2b–2e, generated by
oxidation from their corresponding acylhydroquinones 1b–e19
with silver(I) oxide, were reacted with different 2-aminobenzoth-
iazoles 3a–c, under standard conditions.20 The spectral data of
the new products 4b–4k were closely similar to those described
above for compound 4a or 4a0. In order to unequivocally establish
the structure of the benzothiazoloquinazolinequinones formed in
the reaction of acylquinones 2 with the aminobenzothiazoles 3,
compound 4b was submitted to X-ray diffraction analysis (Fig. 1).
Based on the X-ray crystallographic data for compound 4b it
may be concluded that the products formed by reaction of 2-acyl-
4j
4k
a
Reagents: (1) acylhydroquinone 1a–e (1 equiv), Ag2O (5 equiv); (2)
aminobenzothiazole 3a–c (1.1 equiv).
b
Isolated yield after column chromatography and referred to 1a–e.
naphthoquinones
benzo[g]benzothiazolo[2,3-b]quinazoline-7,12-quinone
2
with aminobenzothiazoles 3, exhibit the
frame-
work (Fig. 1). Table 1 summarizes the results arising from the reac-
tion of acylquinones 2 with aminobenzothiazoles 3 to produce
compounds 4. The possibility to prepare compounds 4 via a one-
pot procedure from acylhydroquinone 1, amine 3, and silver(I)
oxide in dichloromethane was studied using substrates 1a and
3a. The assays indicate that this procedure is unfeasible to prepare
4a due to oxidative decomposition of amine 3a. A plausible mech-
anism of this interesting hetero-annulation reaction is shown in
Scheme 2 for the formation of compound 4a.
The reaction seems to proceed via an initial attack of the NH2
group of 3a at the 3-position of the activated quinone 2a to give
Scheme 2. Plausible mechanism for the formation of quinone 4a.
a Michael intermediate adduct, which by a further 6-exo trig ring
closure, followed by aerobic oxidation, yields the heteropentacyclic
quinone 4a.
Quinones 4b, 4e, 4f, and 4h were evaluated for their in vitro
antiproliferative activity on a panel of three human-derived tumor
cell lines, using the conventional MTT (microculture tetrazolium
reduction) assay.21 The data in Table 2 show that compounds 4e,
Scheme 1. The reaction of 2-acetyl-1,4-naphthoquinone 2a with 3a.

J. A. Valderrama et al. / Tetrahedron Letters 56 (2015) 5103–5105
5105
Table 2
References and notes
In vitro antiproliferative activity of 4b, 4e, 4f, and 4h on T24 (bladder), DU-145
(prostate) and MCF7 (breast) cancer cell lines
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Compound
IC50 SEMa
DU-145
(lM)
T-24
MCF7
4b
4e
4f
4h
4.86 0.50
0.22 0.06
1.36 0.15
1.32 0.11
0.65 0.07
42.2 5.8
32.9 1.5
11.58 2.05
0.11 0.03
0.85 0.10
0.79 0.11
0.42 0.03
14.3 2.6
4.25 0.50
2.98 0.52
1.77 0.26
2.72 0.18
0.33 0.05
16.8 2.9
23.6 1.4
DOXb
MITc
TFd
28.3 1.7
a
b
c
Data represent IC50 mean values SEM of at least three different experiments.
DOX: doxorubicin.
MIT: mitomycin C.
TF: tamoxifen.
d
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Goodman and Gilman’s The Pharmacological Basis of Therapeutics; Hardman, J. G.,
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4f, and 4h exhibit significant antiproliferative activity compared to
doxorubicin, mitomycin C, and tamoxifen, thus supporting our
approach to entry into new potentially antiproliferative agents,
based on the benzo[g]benzothiazolo[2,3-b]quinazoline chrom-
ophore.
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Bioorg. Med. Chem. 2009, 17, 2894; (b) Vásquez, D.; Rodríguez, J. A.; Theoduloz,
C.; Calderon, P. B.; Valderrama, J. A. Eur. J. Med. Chem. 2010, 45, 5234; (c)
Valderrama, J. A.; Ibacache, A.; Rodríguez, J. A.; Theoduloz, C.; Benites, J. Eur. J.
Med. Chem. 2011, 46, 3398; (d) Iribarra, J.; Vásquez, D.; Theoduloz, C.; Benites,
J.; Ríos, D.; Valderrama, J. A. Molecules 2012, 17, 17101.
12. Monsalve, F. A.; Valderrama, J. A.; Vásquez, D.; Ibacache, A.; Rodríguez, J. A.;
González, D. R.; Leiva, E.; González, E. Int. J. Mol. Med. 2012, 30, 151.
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In conclusion, a novel and straightforward way for the construc-
tion of benzo[g]benzothiazolo[2,3-b]quinazoline 4a–k from 2-acyl-
naphthohydroquinones 1a–e and 2-aminobenzothiazoles 3a–c has
been found. The present method accommodates diverse 2-acyl-
naphthohydroquinones, where the acyl group ranges from C2 to
C8, and easily accessible substituted 2-aminobenzothiazoles.
These remarkable advantages make this approach very suitable
for an easy preparation of these highly annulated antiproliferative
heterocyclic quinones. In progress in our laboratory are the scope
of the hetero-annulation reaction between 2-acylnaphthoquinones
and 2-aminobenzothiazoles, directed to the synthesis of a broad
variety of new members of this class of heterocyclic quinones,
and their biological evaluation on representative cancer cell lines.
20. Synthesis
of
13-hydroxy-13-(1-propyl)-7H-benzo[g]benzo[4,5]thiazolo[2,3-
suspension of
b]quinazoline-7,12-quinone (4b): Typical Procedure.
A
acylhydroquinone (1b; 215 mg, 0.93 mmol), Ag2O (1.1 g, 4.65 mmol),
anhydrous MgSO4 (200 mg), and dichloromethane (15 mL) was left at room
temperature with stirring at rt for 1 h. The mixture was filtered and, over the
solution, 2-aminobenzothiazole (3a; 165.22 mg, 1.02 mmol) was added, and
the resulting solution was left with stirring at room temperature after
completion of the reaction, as indicated by TLC (24 h). The solvent was
removed under reduced pressure to give crude quinone 4b. Further column
chromatography of the crude product over silica gel (petroleum ether/CH2Cl2/
EtOAc, 15:5:20) yielded pure 4b (273 mg, 0.73 mmol, 78%) as a red solid; mp
203.5–204.0 °C; IR (KBr, cmꢀ1): 3444 (O–H), 1664, 1641 (C@O); 1H NMR
(400 MHz, CDCl3): d 0.76 (t, 3H, J = 7.2 Hz, CH3), 0.99 (m, 1H, CHH), 1.35 (m, 1H,
CHH), 2.41 (dt, 1H, J = 13.3, 4.2 Hz, CHH), 2.72 (dt, 1H, J = 13.3, 4.7 Hz, CHH),
6.37 (br s, 1H, OH), 7.29 (t, 1H, J = 8.0 Hz, 2- or 3-H), 7.41 (t, 1H, J = 8.0 Hz, 3- or
2-H), 7.56 (d, 1H, J = 7.8 Hz, 4-H), 7.71 (t, 1H, J = 8.0 Hz, 9- or 10-H), 7.75 (t, 1H,
J = 8.0 Hz, 10- or 9-H), 8.07 (d, 1H, J = 7.4 Hz, 8- or 11-H), 8.15 (d, 1H, J = 7.3 Hz,
11- or 8-H), 8.22 (d, 1H, J = 8.4 Hz, 1-H); 13C NMR (100 MHz, CDCl3): d 13.9,
18.2, 42.4, 91.7, 115.0, 118.1, 122.6, 125.0, 125.4, 126.3, 127.2, 127.5, 131.7,
133.3, 133.7, 134.5, 137.7, 146.7, 167.5, 182.4, 185.7; HRMS (APCI) calcd for
Acknowledgements
We thank the Fondo Nacional de Ciencia y Tecnología, Chile
(Grant N° 1100376), the Fonds Spéciaux de Recherche (FSR),
Université Catholique de Louvain, Belgium, and the FNRS (FRFC
Grant 2.4555.08), Belgium, for financial support to this study. The
authors are grateful to Pilar Díaz for her valuable technical
assistance.
C
21H16N2O3S: 376.08816 [M+H]+; found: 376.09454.
21. Mosmann, T. J. Immunol. Methods 1983, 65, 55.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.07.
034.