Functionalization of the A ring of pyridoacridine as a route toward greater structural diversity. Synthesis of an octacyclic analogue of eilatin

Article abstract of DOI:10.1016/j.bmcl.2009.06.039

In an effort to increase the structural diversity of pyrido[4,3,2-kl]acridines, compounds containing amino substituents on the A ring were synthesized. The key-reactions involve regioselective electrophilic aromatic substitutions. The methodology was appl

Full text of DOI:10.1016/j.bmcl.2009.06.039

Bioorganic & Medicinal Chemistry Letters 19 (2009) 4836–4838
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Functionalization of the A ring of pyridoacridine as a route toward greater
structural diversity. Synthesis of an octacyclic analogue of eilatin
Laurent Bouffier a, , Rodica Dinica b, Julien Debray a, Pascal Dumy a, Martine Demeunynck a,
*
a Département de Chimie Moléculaire, UMR-5250 & ICMG FR-2607, CNRS-Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
b University Dunarea de Jos, Domneasca str 47, Galati 800008, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 February 2009
Revised 9 June 2009
Accepted 10 June 2009
Available online 13 June 2009
In an effort to increase the structural diversity of pyrido[4,3,2-kl]acridines, compounds containing amino
substituents on the A ring were synthesized. The key-reactions involve regioselective electrophilic aro-
matic substitutions. The methodology was applied to the synthesis of the extended angular octacycle
8, which conjugates the physicochemical and spectroscopic properties of the pyridoacridine skeleton
with the ability of [1,10]phenanthroline ring for metal complexation. The 9-aminopyridoacridine 4 dis-
plays significant cytostatic activities against two cancer cell lines, and may be considered as a new lead
in the search of active derivatives.
Keywords:
Acridine
Pyridoacridine
Phenanthroline
Ligand
Ó 2009 Elsevier Ltd. All rights reserved.
Cytostatic activity
Pyrido[2,3,4-kl]acridines constitute a large family of natural
compounds isolated from marine sources.1 During the last
25 years, a large number of pyridoacridine derivatives have been
isolated,2–4 or synthesized.5 Most of them display a wide range of
biological properties and potential medical applications such as
anti-bacterial, antifungal, antiviral, and antitumor activities.6 De-
spite their large structural diversity, all marine alkaloids derived
from the pyrido[2,3,4-kl]acridine skeleton in which the heterocy-
clic nitrogens are located in positions 3 and 7 (Fig. 1). However
in the case of the tetracyclic Necatorone, which was isolated in
1984 from the gilled toadstool Lactarius Necator (Agaricales), nitro-
gens are in positions 1 and 7.7 From this discovery emerged a new
class of pyridoacridines (pyrido[4,3,2-kl]acridines), which are iso-
mers of the main family (Fig. 1). The group of Stevens reported
in 1997 an original route to the pyrido[4,3,2-kl]acridine skeleton.8
The key step of the synthesis is the formation of D ring by Graebe–
Ullmann thermolysis at 259 °C of 9-([1,2,3]triazo-1-yl)acridine in
diphenyl ether.
N
N
N
N
N
H
N
pyrido[2,3,4-kl]acridine
eilatin
2
1
3
N
B
N
N
11
A
D
O
4
5
10
9
C
6
N
H
7
Cl
8
R
R = amine, thioether,
polyamine
pyrido[4,3,2-kl]acridine
Figure 1. Two isomeric pyridoacridine families.
The same year, our group designed a 2-step pathway starting
from commercially available 6,9-dichloro-2-methoxyacridine.9
The key-step involves D ring closure by acid catalyzed electrophilic
substitution.
Unlike what was observed under thermolysis conditions, the
latter reaction is fully regioselective. Biologically active pyr-
ido[4,3,2-kl]acridine derivatives have then been designed either
by adding an extra ring fused to D ring (for example, RHPS4 devel-
oped as G4 binder and telomerase inhibitor10), or by introducing
various substituents on the C ring. We have thus prepared and
tested a series of amino-, polyamino-, thio- or glyco-derivatives
of the pyrido[4,3,2-kl]acridine and its oxidized pyrido[4,3,2-kl]acri-
din-4-one analogue.11,12 Most of the amino derivatives (see Fig. 1
* Corresponding author. Tel: +33 0 476514429; fax: +33 0 476514946.
E-mail address: Martine.Demeunynck@ujf-grenoble.fr (M. Demeunynck).
Present address: Department of Chemistry, University of Liverpool, Crown Street,
Liverpool L69 7ZD, United Kingdom.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.06.039

L. Bouffier et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4836–4838
4837
for general structure) showed significant micromolar cytotoxic
activities against cancer cell lines.
To prepare the octacyclic pyridoacridine analogue 8 containing
a fused phenanthroline unit (Scheme 2), we used the methodology
designed previously for the acridine series, and involving coupling
ortho-diamines to phen-5,6-dione.14 The required ortho-diamino
acridines were synthesized from the corresponding 3-aminoacri-
dines by reaction with p-nitro-phenyldiazonium salt, followed by
the reductive cleavage of the azo intermediates.
Based on this scheme, two pathways were envisioned to pre-
pare the 8,9-diaminopyridoacridine 7. The electrophilic aromatic
substitution with p-nitro-phenyldiazonium salt may be performed
on the amino substituted pyridoacridine 4, but it appeared to us
that the presence of the strong electron-withdrawing phenyl azo
substituent would favour the regioselectivity of D ring formation.
Indeed, as depicted in Scheme 2, the cyclization step performed
on the azo acridine 5, prepared in 86% yield from the acridine 3,
yielded the azo-pyridoacridine 6 in almost quantitative yield and
excellent purity.
Formation of the ortho-diamine 7 by reductive cleavage of the
azo bond of 6, and coupling with phen-5,6-dione gave the octacy-
clic pyridoacridine 8 in reasonable yield (45% two-steps). An alter-
native pathway, in which the introduction of the phenanthroline
unit precedes the formation of the pyridoacridine core, was also
designed. Compound 8 was obtained in three steps (chlorination,
substitution and cyclization) from the already described hydro-
Surprisingly, both in the marine pyrido[2,3,4-kl]acridine alka-
loids and in the synthetic pyrido[4,3,2-kl]acridine regioisomers,
the A ring appears highly conserved. Therefore, as a route to
greater structural diversity, we report here the synthesis and pre-
liminary biological evaluation of 9-amino-4-ethoxy-7H-pyr-
ido[4,3,2-kl]acridine 4, and exemplify its interest as lead
compound by preparing a new octacyclic polyaza pyridoacridine
8 displaying structural similarities to eilatin. The biological interest
of compounds 4 and 8 were evaluated against two human cancer
cell lines.
Our chemistry depicted in Scheme 1 is based on regioselective
aromatic electrophilic substitutions of judiciously substituted acri-
dines. The 6-amino-9-chloro-2-ethoxyacridine 2, easily prepared
in two steps from ethacridine 1,13 was chosen as starting material.
Nucleophilic substitution of the chlorine atom by aminoacetalde-
hyde dimethylacetal afforded the aminoacridine 3 in 89% yield.
Deprotection of the aldehyde and subsequent intramolecular Fri-
edel–Craft type substitution occurred at room temperature in
TFA. The regioselectivity of the cyclization step results from the
strong ortho directing effect of the 2-OEt group of the acridine 3,
superior to the meta directing effect of the 6-amino group. Pyrido-
acridine 4 was thus obtained in 81% yield.
OMe
NH2
OEt
a,b
Cl
HN
OMe
OEt
OEt
c
H2N
N
H2N
N
H2N
N
89%
76%
2 steps
3
Ethacridine 1
2
N
OEt
d
H2N
N
H
81%
4
Scheme 1. Reagents and conditions. (a) NaOH, PhOH, 160 °C, 48 h; (b) POCl3, 90 °C, 2 h; (c) H2NCH2CH(OMe)2, EtOH, reflux, 1 h; (d) TFA, rt, 5 h.
OMe
HN
N
N
N
OMe
OEt
OEt
OEt
a
b
c
3
H2N
H2N
N
H
6
H2N
N
H
7
86%
95%
89%
N
N
NH2
5
N
N
p
NO2Ph
p
NO2Ph
d
51%
OMe
OMe
N
R
HN
N
OEt
OEt
OEt
N
N
N
H
N
N
N
N
N
f
g
N
N
N
90%
90%
9 R = OH
10 R = Cl
8
11
N
N
N
e
75%
Scheme 2. Reagents and conditions: (a) pNO2PhN2+ BF4À, MeOH, 0 °C, 3 h; (b) TFA, rt, 5 h; (c) Na2S2O4, pH 7 phosphate buffer, DMF, rt, 4 h; (d) phen-5,6-dione, EtOH, reflux,
3 h; (e) POCl3, 90 °C, 2 h; (f) H2NCH2CH(OMe)2, PhOH, 85 °C, 10 h; (g) MsOH, rt, overnight.

4838
L. Bouffier et al. / Bioorg. Med. Chem. Lett. 19 (2009) 4836–4838
xy-heptacycle 9.14 However, due to the low solubility of heptacy-
clic compounds harsher conditions were required (MsOH instead
of TFA) for the key-cyclization step.
N
HN
OEt
OEt
Considering that the new octacyclic pyridoacridine 8 is highly
conjugated, we anticipated that this compound would display sig-
nificant spectroscopic properties. We therefore performed a preli-
minary evaluation of its spectral characteristics. The electronic
H2N
N
H
H2N
N
7H
-isomer
1
H
-isomer
Figure 3. Tautomeric equilibrium.
absorption spectrum recorded in EtOH
+
1% DMSO
(c = 6.4.10À5 mol/L) shows a large and low intense band centered
at 475 nm (
e
= 6950 LÁmolÀ1ÁcmÀ1) whereas the region between
As a conclusion, we have designed a simple route to a new
series of pyrido[4,3,2-kl]acridine substituted on the A ring. The
9-aminopyridoacridine 4 displays significant micromolar cyto-
static activities against two cancer cell lines, and may be consid-
ered as a new lead in the search of active derivatives. It also
appears as an intermediate in the synthesis of larger heterocy-
cles such as the octacyclic derivative 8. This new octacycle not
only contains the main features of the pyridoacridine skeleton,
including its UV–vis and fluorescence properties, but also em-
braces a 1,10-phenanthroline unit that opens the way to metal
complexation related biological properties. Octacyclic pyridoacri-
dine 8 may also be considered as a close analogue of eilatin (see
structure in Fig. 1). The [Ru(bpy)2(eilatin)]2+ complex has been
studied as a potential inhibitor of Rev–RRE interaction,15 which
is critical for the activity of AIDS virus, and more recently as a
probe for mismatched DNA.16 Other recent applications include
the chiral recognition of iron complexes17 and Pd complexes as
copolymer catalysts.18 It would be of major interest to compare
the activities of the new octacyclic phenanthroline analogue 8 on
similar targets and applications.
200 and 400 nm is complex and exhibits several local maxima
(387 (e 13,700), 366 (e 12,200), 248 (e 21,450)). The general shape
of the spectra is independent of the solvent used, but molar extinc-
tion coefficients are clearly lower in aqueous solutions (for exam-
ple, eEtOH/eH2O = 2.04 in the case of the most bathochromic band
centered at 472–475 nm).
Emission spectroscopy was also registered, and an interesting
solvatochromic effect was evidenced (Fig. 2). Compound 8 exhibits
a strong emission band (k = 573 nm) in EtOH, but almost no emis-
sion in water (ratio Imax(EtOH)/Imax(H2O) = 64). This strong emis-
sion extinction was accompanied by
a concomitant small
bathochromic shift ( kmax = 3 nm). Such UV–vis and fluorescence
D
properties may be valuable tools for the biological evaluation (for
examples to study the cell distribution or the mode of binding to
macromolecules).
The biological activity of compounds 4 and 8 were evaluated
against two cancer cell lines, HT29 (human colon adenocarcinoma)
and A431 (human epithelial carcinoma). Compound 4 displayed
micromolar cytostatic effects (IC50 = 1.35 and 2.1 lM against
HT29 and A431 respectively). By comparison, the previously de-
scribed 9-chloropyrido[4,3,2-kl]acridine,9 in which the amino
group is replaced by a chlorine atom, did not show any effect when
Acknowledgements
tested at 20 lM. The Cl to NH2 exchange has little influence on the
We thank D. Carrez and A. Croisy for the cytotoxicity
measurements.
charge of the molecule that remains non-protonated at physiolog-
ical pH (pka only increased from 4.5–5.3), but increases the solubil-
ity in water; this effect correlates well with a decrease of log P
values (3.69–2.62) and an increase of PSA values (34.15–60.17).
It is worth noting that the low pka value (pka = 5.3) measured for
the 9-aminopyrido[4,3,2-kl]acridine 4 clearly confirms the exis-
tence of the 7H-tautomer in water as it has been observed in DMSO
by NMR (NOE data not shown). Indeed, a much higher pka value
(calculated pka = 10.3) has been predicted for the 1H-tautomer in
which the exocyclic amino group is conjugated to the basic hetero-
cyclic acridine-like nitrogen (Fig. 3).
Supplementary data
Supplementary data of synthesis and identification of the new
molecules. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.bmcl.2009.06.039.
References and notes
Due to its low solubility in water, the octacycle 8 could only be
tested at low concentration (5 lM). It did not show any activity
against HT29 and 85% survival on A431 cell lines.
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Figure 2. Emission spectra of 8 recorded in EtOH (black line) and H2O (dashed line)
after excitation at k = 475 nm. Inset: Zoom of the spectrum recorded in water.
(c = 6.4.10À5 mol/L).