Design, synthesis and biological evaluation of (E)-2-(2-arylhydrazinyl) quinoxalines, a promising and potent new class of anticancer agents

Article abstract of DOI:10.1016/j.bmcl.2013.12.074

A series of forty-seven quinoxaline derivatives, 2-(XYZC₆H ₂CHN-NH)-quinoxalines, 1, have been synthesized and evaluated for their activity against four cancer cell lines: potent cytotoxicities were found (IC₅₀ ranging fro

Full text of DOI:10.1016/j.bmcl.2013.12.074

Bioorganic & Medicinal Chemistry Letters 24 (2014) 934–939
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and biological evaluation of
(E)-2-(2-arylhydrazinyl)quinoxalines, a promising and potent new
class of anticancer agents
Felipe A. R. Rodrigues a, Igor da S. Bomfim a, Bruno C. Cavalcanti a, Claudia do Ó. Pessoa a,
James L. Wardell b,c, Solange M. S. V. Wardell d, Alessandra C. Pinheiro b, Carlos Roland Kaiser e,
Thais C. M. Nogueira b,e, John N. Low c, Ligia R. Gomes f, Marcus V. N. de Souza b,
⇑
a Laboratório de Oncologia Experimental, Universidade Federal do Ceará, Fortaleza, CE, Brazil
b FioCruz-Fundação Oswaldo Cruz, Instituto de Tecnologia em Fármacos-Far-Manguinhos, Rua Sizenando Nabuco, 100, Manguinhos, 21041-250 Rio de Janeiro, RJ, Brazil
c Department of Chemistry, University of Aberdeen, Old Aberdeen AB 24 3UE, Scotland, UK
d CHEMSOL, 1 Harcourt Road, Aberdeen AB15 5NY, Scotland, UK
e Programa de Pós-Graduação em Química, Instituto de Química, Universidade Federal do Rio de Janeiro, CP 68563, 21945-970 Rio de Janeiro, Brazil
f REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, Rua Campo Alegre, 687, P-4169-007, Porto P-4200-150, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of forty-seven quinoxaline derivatives, 2-(XYZC6H2CH@N–NH)-quinoxalines, 1, have been syn-
Received 10 October 2013
Revised 16 December 2013
Accepted 17 December 2013
Available online 25 December 2013
thesized and evaluated for their activity against four cancer cell lines: potent cytotoxicities were found
(IC50 ranging from 0.316 to 15.749 lM). The structure–activity relationship (SAR) analysis indicated that
the number, the positions and the type of substituents attached to the aromatic ring are critical for bio-
logical activity. The activities do not depend on the electronic effects of the substituents nor on the lypo-
philicities of the molecules. A common feature of active compounds is an ortho-hydroxy group in the
phenyl ring. A potential role of these ortho-hydroxy derivatives is as N,N,O-tridentate ligands complexing
with a vital metal, such as iron, and thereby preventing proliferation of cells. The most active compound
was (1: X,Y = 2,3-(OH)2, Z = H), which displayed a potent cytotoxicity comparable to that of the reference
drug doxorubicin.
Keywords:
Antitumor activity
Quinoxaline
Hydrazone
Drugs
Ó 2013 Elsevier Ltd. All rights reserved.
As indicated by the estimated costs of cancer in 2007 of $226.8
billion,1 and by the 7.6 million cancer world-wide deaths (13% of
all deaths) in 2008,2 cancer is clearly a major concern. The need
for effective treatment and new anticancer agents is of paramount
importance.
We have previously reported on the biological activities of
hydrazone derivatives of a number of different heteroaromatic
compounds.3–10 Now we wish to report the results of an evaluation
of the anti-cancer activities of a series of hydrazone-quinoxaline
derivatives, 2-XYZC6H4–CH@N–NH)-quinoxaline, 1.
Quinoxaline compounds have a wide range of uses, mainly as bio-
logically active compounds, but also as dyestuffs.11–21 The biological
uses of quinoxaline compounds include as antibacterial, antituber-
cular, antimicrobial, antifungal, antimalarial, anti-inflammatory,
antileishmanial, and antitumor agents, as well as herbicides, insecti-
cides11–21 The biological activities of hydrazone derivatives of het-
eroaromatic compounds have also been well reported,22–31 with
anticancer activities being of significant interest.24–31 Our report,
however, is the first of the use of hydrazinyl-quinoxalines of the type,
2-(XYZC6H3–CH@NANH)-quinoxaline, 1, as anticancer agents. Com-
pounds 1 were obtained by molecular hybridization, see Scheme 1.
The quinoxalinyl-hydrazones, 2-(XYZC6H2CH@NANH)-quinox-
alines, 1, were synthesized from reactions of 2-hydrazinylquinoxa-
line, 3, and arenecarbaldehydes, in 55–96% yields, as shown in
Scheme 1. As can be seen from Table 1, compounds with a wide
range of substituents were prepared and studied.
Characterization of the compounds 1 was generally achieved
from NMR and IR spectral data. In the 1H NMR spectra, the signal
for the N@CH proton was in the region 7.98–8.54 ppm, while the
N–H and N@C stretching vibrations in the IR spectra were at
3400–3491 and 1565–1612 cmÀ1, respectively. The spectral data
are included in the Supplementary section, along with melting
points and yields of the derivatives.
The crystal structures of (1: X = Y = Z = H) and the mono-halo-
derivatives, (1: X = 2-, 3-, 4-Br, 2-, 3-, 4-Cl; Y = Z = H) have been
reported.32 As is discussed below, (1: X = Y = Z = H) and the
mono-halo derivatives are all, at best, poorly active. Thus, the
structure determination of a reasonably active compound was
⇑
Corresponding author. Tel.: +55 2139772404; fax: +55 2125602518.
E-mail address: marcos_souza@far.fiocruz.br (M.V.N. de Souza).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.12.074

F. A. R. Rodrigues et al. / Bioorg. Med. Chem. Lett. 24 (2014) 934–939
935
H
N
N
N
N
Cl
N
NHNH2
N
b
a
N
N
R
2
3
1
Scheme 1. Reagent and conditions (a) N2H4ÁH2O (80%), EtOH, rt, 48 h, 85% (b) ArCHO, EtOH, rt 24 h, 55–96%.
Table 1
Y = Z = H) were the same as that of the 2-OH group in (1:
X,Y = (2,4-(OH)2, Z = H).
Parameters for hydrogen bond and intermolecular interactions (Å, o)
The para-hydroxy group in (1: X,Y = (2,4-(OH)2, Z = H) is also
involved in hydrogen bonding, in this case with the oxygen of
the acetone solvent solvate within the asymmetric unit, see Fig-
ure 1. The arrangement about the C@N bond in (1: X,Y = (2,4-
(OH)2, Z = H) is (E), as was found in (1: X = Y = Z = H) and mono-
halo derivatives, (1: X = 2-, 3-, 4-Br and Cl; Y = Z = H).32 All com-
pounds 1, so far studied, show only small deviations from planarity
and it is clear that there are no significant differences in the molec-
ular conformations adopted by non-active and active compounds
in the solid state.
Electron delocalization in the ANHAN@CHA link between the
quinoxaline and phenyl groups in (1: X,Y = (2,4-(OH)2, Z = H) is
indicated by the bond lengths: C2–N3 = 1.3665(17), N3–
N5 = 1.3729(13), N5–C9 = 1.2895(17) and C9–C10 = 1.4496(16) Å,
respectively. A similar finding was obtained for the other (E)-2-
(2-arylhydrazinyl)quinoxalines recently reported.32
(a) Hydrogen bonds
D–HÁ Á ÁA
D–H
HÁ Á ÁA
1.94
2.54(2)
1.88
2.52
DÁ Á ÁA
D–HÁ Á ÁA
147
165.5(12)
176
172
O1–H1Á Á ÁN5
0.84
0.92(2)
0.84
0.95
0.95
2.6826(13)
3.4323(17)
2.7137(15)
3.4585(18)
3.1836(17)
3.3754(17)
N3–HN3Á Á ÁN4i
O2–H2Á Á ÁO3
C5–H5Á Á ÁN1ii
C15–H15Á Á ÁO1i
C16–H16CÁ Á ÁO2i
2.50
2.51
129
148
0.98
Symmetry operation: i = À1+x,y,z; ii = 1+x,y,z.
(b) C—HÁÁÁCga
C–HÁ Á ÁCga
CÁ Á ÁCg
2.65
Hperp
c
C–HÁ Á ÁCg
140
CÁ Á ÁCg
3.4534(13)
C18–H18BÁ Á ÁCg3i
2.64
2.93
Symmetry operations: i = 1Àx,1Ày,1Àz.
(c) Cg—Cga
Cg(I)Á Á ÁCg(J)a
CgÁ Á ÁCg
4.1140(7)
3.6103(7)
a
b
c
CgIperp
CgJperp
Cg1Á Á ÁCg1i
0
0
36.68
24.45
33.68
24.45
3.2993(4)
3.2864(4)
3.2993(4)
3.2864(4)
The presence of nitrogen and hydroxyl groups in (1: X,Y = (2,4-
(OH)2, Z = H) allows for various intra- and inter-molecular interac-
tions, see Table 2 for details of the symmetry operations and the
geometric parameters. A PLATON analysis34e indicated the presence
of intermolecular N–HÁ Á ÁN and C–HÁ Á ÁO hydrogen bonds as well as
Cg1Á Á ÁCg1ii
Symmetry operations: i = 2Àx,2Ày,Àz; ii = 3Àx,1Ày,Àz.
a
Cg1 and Cg3 are the centroids of the rings defined by [N1,C2,C3,N4,C4A,C8A]
and, [C10–C15], respectively.
p
Á Á Á
p
and C–HÁ Á Á
p interactions, see Table 1. The importance of
weaker intermolecular interactions, such as C–HÁ Á ÁO hydrogen
considered to be a useful exercise as it would indicate any confor-
mational differences between active and non-active compounds.
Suitable crystals of an acetone solvate of (1: X,Y = (2,4-(OH)2,
Z = H), an active compound, were grown from an acetone solution.
The crystal structure of [(1: X,Y = (2,4-(OH)2, Z = H)ÁMe2CO] was
determined from data collected at 100(1) K.33–35 Atomic coordi-
nates, bond lengths, angles and thermal parameters have been
deposited at the Cambridge Crystallographic Data centre, deposi-
tion number 960209. The asymmetric unit of [(1: X,Y = (2,4-
bonds, and C–HÁ Á Á interactions, in crystal structures have
pÁ Á Á
p
p
been well documented.36 Additionally, a theoretical study of
pÁ Á Áp
interactions in the quinoxaline dimer has recently been published37
and has provided a firm understanding of such interactions.
There are two major subsets of intermolecular interactions
present in the crystal of [(1: X,Y = (2,4-(OH)2, Z = H)ÁMe2CO]. The
first subset involves N–HÁ Á ÁN and C–HÁ Á ÁO hydrogen bonds, which
generate columns of linked (1: X,Y = (2,4-(OH)2, Z = H) and acetone
molecules, see Figure 3a. Figure 3b shows the orientation of two of
these columns through the crystallographic cell. The strongest
interaction, the N3–HN3Á Á ÁN4 hydrogen bond, generates C(6)
chains.38 The N3–HN3Á Á ÁN4 hydrogen bond together with
O2–H2Á Á ÁO3 and weaker C5–H5Á Á ÁN1, C15–H15Á Á ÁO1 and C16–
(OH)2, Z = H)ÁMe2CO] consists of
a molecule each of (1:
X = Y = Z = H) and Me2CO. Figure 1 shows the atom arrangements
and numbering scheme of the solvate.
The arrangement of the ortho-hydroxy group in (1: X,Y = (2,4-
(OH)2, Z = H) allows intramolecular hydrogen bonding to occur
with the imino N atom, see Figure 1. The arrangements of the ortho
halogen atoms in (1: X = 2-Br and Cl; Y = Z = H) are different as
shown in Figure 2. Considerable steric hindrance would have re-
sulted if the positions of the halo groups in (1: X = 2-Br and Cl;
H16CÁ Á ÁO2 hydrogen bonds forms
R22(8)R22(16)R33(15).38 The second major subset is formed from
alternating stronger and weaker (pyridine)— (pyridine) stacking
a
series of rings,
p
p
interactions, see Figure 3c. Table 2 contains details of the interac-
tions. There is an additional, but less important, intermolecular
Figure 1. Atom arrangements and numbering scheme for [(1: X,Y = (2,4-(OH)2, Z = H)ÁMe2CO]. Probability ellipsoids are drawn at the 50% level. Hydrogen atoms are drawn as
spheres of arbitrary radius. Hydrogen bonds are drawn as dashed lines.

936
F. A. R. Rodrigues et al. / Bioorg. Med. Chem. Lett. 24 (2014) 934–939
X
H
N
H
N
N
N
OH
N
N
N
H
N
O
Figure 2. Different positions of the ortho substituents in (1: X,Y = (2,4-(OH)2, Z = H) and (1: X = 2-Cl or 2-Br,Y = Z = H).
Table 2
Growth inhibition (%)a for three tumors cells line by the MTT Assay and calculated LogP values
Compound
SF-295
(sd)%
OVCAR-8
(sd)%
HCT-116
(sd)%
LogPb
(1: X = Y = Z = H)
(1: X = 2-F, Y = Z = H)
(1: X = 3-F, Y = Z = H)
(1: X = 2-Cl, Y = Z = H)
(1: X = 3-Cl, Y = Z = H)
(1: X = 4-Cl, Y = Z = H)
(1: X ,Y = 2,3-Cl, Z = H)
(1: X ,Y = 2,6-Cl, Z = H)
(1: X ,Y = 2,4-Cl, Z = H)
(1: X ,Y = 3,4-Cl, Z = H)
(1: X = 2-Br, Y = Z = H)
(1: X = 3-Br, Y = Z = H)
À3.37(12.09)
12.76(5.35)
8.36(2.34)
34.46(1.61)
0.00(0.00)
5.59(5.08)
9.09(0.00)
0.00(0.00)
41.72(7.14)
100.17(1.73)
53.55(2.81)
4.90(7.17)
0.00(0.00)
18.01(5.44)
0.00(0.00)
18.18(5.56)
21.81(4.29)
16.94(5.82)
40.58(14.93)
17.82(7.58)
39.87(0.65)
28.96(9.57)
11.48(3.41)
50.85(2.77)
95.38(0.63)
51.48(7.43)
67.29(16.01)
97.90(0.99)
84.19(0.13)
60.49(2.47)
46.32(1.64)
12.76(7.91)
20.99(6.89)
41.56(3.95)
14.42(11.20)
0.00(0.00)
À18.44(4.62)
7.01(5.99)
10.80(0.76)
12.07(7.66)
0.00(0.00)
À10.20(1.66)
97.53(1.61)
71.19(1.84)
16.48(0.09)
0.50(2.27)
3.15
3.27
3.29
3.78
3.80
3.83
4.41
4.41
4.44
4.44
3.91
3.94
3.96
3.06
3.09
3.11
3.55
3.58
3.60
2.88
2.91
3.09
2.65
2.67
2.39
2.59
2.59
2.18
1.89
3.16
3.18
3.21
2.97
3.19
3.19
3.17
2.80
2.78
3.56
3.58
3.52
3.52
3.12
3.03
3.72
3.54
3.32
31.86(0.17)
33.23(1.79)
44.48(4.24)
84.33(3.66)
52.04(1.60)
28.48(3.35)
37.83(2.91)
35.24(3.56)
14.18(9.76)
33.47(2.76)
21.90(2.55)
23.45(9.25)
26.97(19.00)
46.35(1.86)
39.84(5.06)
18.05(9.78)
44.55(2.20)
16.03(6.70)
61.48(0.89)
27.52(2.55)
39.43(7.05)
81.17(3.46)
85.27(0.68)
50.69(6.10)
47.60(1.95)
37.75(3.46)
21.78(5.69)
35.84(7.76)
0.52(3.53)
22.16(2.13)
46.89(0.42)
44.65(7.52)
44.73(8.03)
16.53(2.96)
60.12(9.32)
7.19(10.55)
12.30(10.67)
59.88(4.57)
38.40(0.51)
66.37(6.49)
78.51(0.10)
36.75(2.44)
46.65(5.84)
32.43(2.03)
23.64(5.86)
0.00(0.00)
(1: X = 4-Br, Y = Z = H)
23.20(5.08)
23.02(5.57)
À20.99(6.04)
À18.64(3.98)
28.69(3.83)
33.40(1.47)
À19.55(10.74)
30.72(1.15)
À31.58(2.35)
77.00(0.55)
29.56(1.33)
20.24(1.18)
83.95(0.19)
85.09).96)
89.23(0.66)
69.36(2.11)
14.88(7.47)
4.45(0.63)
À14.48(2.90)
-43.61(7.96)
0.00(0.00)
12.76(0.48)
0.00(0.00)
19.36(0.38)
22.32(4.03)
20.69(4.98)
1.29(1.96)
À17.56(24.69)
17.37(7.38)
13.67(6.15)
83.13(2.21)
92.11(1,51)
20.56(2.84)
16.22(8.44)
24.51(7.85)
(1: X = 2-NO2, Y = Z = H)
(1: X = 3-NO2, Y = Z = H)
(1: X = 4-NO2, Y = Z = H)
(1: X = 2-Me, Y = Z = H)
(1: X = 3-Me, Y = Z = H)
(1: X = 4-Me, Y = Z = H)
(1: X = 3-CN, Y = Z = H)
(1: X = 4-CN, Y = Z = H)
(1: X = 2-OH, Y = Z = H)
(1: X = 3-OH, Y = Z = H)
(1: X = 4-OH, Y = Z = H)
(1: X ,Y = 2,3-(OH)2, Z = H)
(1: X ,Y = 2,4-(OH)2, Z = H)
(1: X ,Y = 2,5-(OH)2, Z = H)
(1: X ,Y = 3,4-(OH)2, Z = H)
(1: X = Y = Z = 3,4,5-(OH)3
(1: X = 2-OMe, Y = Z = H)
(1: X=3-OMe, Y = Z = H)
(1: X = 4-OMe, Y = Z = H)
(1: X ,Y = 2,3-(OMe)2, Z = H)
(1: X ,Y = 2,4-(OMe)2, Z = H)
(1: X ,Y = 2,5-(OMe)2, Z = H)
(1: X ,Y = 2,6-(OMe)2, Z = H)
(1: X ,Y = 3,4-(OMe)2, Z = H)
(1: X = Y = Z = 3,4,5-(OMe)3)
(1: X = 3-OEt, Y = Z = H)
(1: X = 4-OEt, Y = Z = H)
(1: X = 2-OH, Y = 4-Me, Z = H)
(1: X = 2-OH, Y = 5-Me, Z = H)
(1: X = 2-OH, Y = 4-OMe, Z = H)
(1: X = 2-OH, Y = 5-NO2, Z=H)
(1: X = = 3-NO2, Y = 4-Cl, Z = H)
(1:X = 3-Cl, Y = 4-OH, Z = H)
(1: X = 2-Cl, Y = 3-OH, Z = 4-OMe)
7.19(2.91)
18.53(20.03)
25.09(9.52)
17.64(0.51)
38.81(1.89)
41.17(0.36)
50.28(1.07)
31.49(1.51)
25.44(2.10)
80.93(0,90)
95.63(1.24)
9.00(0.12)
28.81(7.71)
13.81(3.46)
a
Mean of three determinations.
Calculated using Ref. 39.
b
interaction, of the type C–HÁ Á Á
p
, which produces dimeric units of
Compounds showing good activity in this initial evaluation
were (1: X,Y = 2,3-Cl2, Z = H), (1: X = 2-OH, Y = Z = H), (1:
X,Y = 2,3-(OH)2, Z = H), (1: X,Y = 2,4-(OH)2, Z = H), (1: X = 2-OH,
Y = 4-OMe, Z = H) and (1: X = 2-OH, Y = 5-NO2, Z = H). It is striking
that all but one of these compounds, (1: X,Y = 2,3-Cl2, Z = H), con-
tained a 2-hydroxy group, with a range of other substituents, both
electron releasing and withdrawing, also present. The compounds
with 2-hydroxy substituents have low logP values, but so do other
[(1: X,Y = (2,4-(OH)2, Z = H)ÁMe2CO]: this is not drawn. Overall a
three-dimensional array is produced from the combination of all
the molecular interactions.
All compounds 1 were initially tested in vitro against three can-
cer cells, SF-295 (glioblastoma), OVCAR-8 (human ovary) and HCT-
116 (colon) at a concentration of 5
lg/mL using the MTT assay. The
results are shown in Table 2 along with calculated lypophilicities.39

F. A. R. Rodrigues et al. / Bioorg. Med. Chem. Lett. 24 (2014) 934–939
937
Figure 3. (a) Columns of molecules generated from N3–HN3Á Á ÁN4, O2–H2Á Á ÁO3, C5–H5Á Á ÁN1, C15–H15Á Á ÁO1 and C16–H16CÁ Á ÁO2 hydrogen bonds, (b) A view of two of the
columns passing through the cell, (c) a --- stack of molecules. See Table 2 for symmetry operations and geometric parameters.
p
p
compounds, which are poorly active at best, such as methoxy
substituted derivatives and (1: X = 3-OH, Y = Z = H), (1: X = 4-OH,
Y = Z = H), (1: X,Y = 3,4-(OH)2, Z = H) and (1: X = Y = Z = 3,4,5-
(OH)3). Hence a low lypophilicity is not a requirement for activity.
The majority of the active compounds from the first phase were
selected for further evaluation against the four human cancer cell
lines: OVCAR-8 (human ovary), SF-295 (glioblastoma), HCT-116
(colon) and HL-60 (leukemia), using the MTT assay. The concentra-
Compound (1: X,Y = 2,3-(OH)2, Z = H) exhibited the best results
against all four cancer lines in the second phase of the study. Other
active compounds in the second phase were (1: X = 2-OH,
Y = Z = H) and (1: X = 2-OH, Y = 5-NO2, Z = H).
In looking for factors why the 2-hydroxy-substituted com-
pounds are active, the possibility of coordination with vital metal
centres cannot be ignored. There are various reports in the literature
of (pyridin-2-yl)NHN = CH–C6H4OH-2 and related compounds act-
ing as tridentate, N,N,O-ligands to metal centres. Indeed, crystal
structures of complexes with VO(IV),40 VO(V),41 MoO2(VI),42
tions that induce 50% inhibition of cell growth (IC50) in
reported in Table 3.
lg/mL are
Table 3
Cytotoxic activity of selected compounds [IC50 (l
M)] on tumor cell lines(a)
Compound
HCT-116
OVCAR-8
HL-60
SF-295
(1: X ,Y = 2,3-Cl2, Z = H)
7.006
6.189
10.776
6.205
4.795–10.234
4.941
3.169–7.702
1.758
1.549–1.994
14.233
13.255–15.287
4.323
5.054–7.579
3.472
1.654–3.035
1.065
0.909–1.248
7.550
6.513–8.756
2.839
9.455–12.284
0.819
0.745–0.900
0.316
0.245–0.407
4.193
1.816–9.673
1.746
4.486–8.585
2.308
1.700–3.133
1.264
0.931–1.715
15.749
7.659–32.391
2.108
(1: X = 2-OH, Y = Z = H)
(1: X ,Y = 2,3-(OH)2, Z = H)
(1: X = 2-OH, Y = 4-OMe, Z = H)
(1: X = 2-OH, Y = 5-NO2, Z = H)
Doxorubicin
3.961–4.717
0.230
2.291–3.518
0.488
1.403–2.174
0.037
1.629–2.728
0.423
(0.165–0.313)
(0.313–0.561)
0.018–0.037
0.350–0.460
a
Data are presented as IC50 values and 95% confidence intervals obtained by nonlinear regression for all cell lines colon (HCT-116), ovarium (OVCAR-8), (leukemia (HL-60),
glioblastoma (SF-295), from three independent experiments. Doxorubicin (Dox) was used as positive control. Experiments were performed in triplicate. IC50 = concentrations
that induce 50% inhibition of cell growth in lM.

938
F. A. R. Rodrigues et al. / Bioorg. Med. Chem. Lett. 24 (2014) 934–939
X
N
H
N
H
N
N
N
OH
N
H
N
H
N
O
O
M+ / BASE
X
N
H
N
N
N
M
O
Scheme 2. Required conformation for action as a N,N,O tridentate ligand.
Ru(III),43 Fe(III),44 Cu(II),45–49 Zn(II),50 Mn,51–53 mixed Mn-lantha-
nide,53 and tin(IV)54 have all been reported to contain these N,N,O-
ligands. While the emphasis of many of these reports were on
non-biological aspects, some did highlight biological activities, OK
the anticancer activity,49 the antimicrobial activity46 and interac-
tions with DNA44 of Cu(II) complexes were all mentioned. While
most of these complexes involve pyridin-2-yl species, the Russian
work47 shows that quinolin-2-yl derivatives also form these com-
plexes. Whilst quinoxaline is considerably less basic than either
pyridine or quinoline, as shown by the pKa values,55 complexes of
quinoxalines with transition metals have been known for many
years, see Refs. 56–58. The report by Lakshmi et al. on the antimicro-
bial activities of oxovanadium(IV) complexes of 3-hydroxy-2-(2-
hydroxypheny-lCH@NANH)-quinoxaline (or its carbonyl tautomer)
and related ligands is particular pertinent to our work.59 Lakshmi
et al.’s compounds were characterized spectroscopically and by
elemental analyses, however no crystal structure was determined
and so the precise binding mode of the ligand to the metal centre
remains unknown. A theoretical treatment of the bonding between
quinoxaline and copper ions was very recently reported37 and
provides a good basis for the interactions of metals generally with
quinoxaline ligands.
The conformation determined for crystalline [(1: X,Y = 2,3-
(OH)2, Z = H). Me2CO], is not the required one for complexation,
but a simple rotation about the quinoxaline–N bond will provide
the desired form, see Scheme 2. Findings gathered from the struc-
tures of complexes of related N,N,O-ligands indicate that tauto-
meric changes within the ligand can occur on complexation.
Specific mechanisms could be envisaged for (1: X,Y = 2,3-(OH)2,
Z = H), a catechol derivative. Catechol derivatives are well known
to form O,O-chelates and to be involved in radical formation on
oxidation. However as other di-hydroxy derivatives, (1: X,Y = 3,4-
(OH)2, Z = H) and (1: X = Y = Z = 3,4,5-(OH)3), were not particularly
active, such mechanisms appear less likely.
Comparison of the activities of 1 and the bioisosteric series, 4-
(aryl-CH@NANH)-7-chloro-quinoline, 463–65 is of interest. As
shown by the comparison of activities in Scheme 3, the quinoxaline
compounds are more active than the corresponding 7-chloroquin-
oline derivatives.
In this work, we report the cytotoxicity activity of a series of
quinoxaline derivatives, which have been evaluated against four
cancer cell lines. The SAR of these compounds indicated that the
positions and the type of substituents in the phenyl ring are critical
for the biological activity. The compound (1: X = Y = 2,3-(OH)2,
Z = H) displayed a potent cytotoxicity activity compared to the ref-
erence drug doxorubicin.
Of all the metals mentioned above, iron is particularly relevant.
Iron is very important in DNA synthesis, adenosine triphosphate
production and cell replication. Studies have demonstrated that
cancer cells are susceptible to iron complexing ligands.60 Acylhy-
drazones have shown potential as iron chelators.61 Chelation ther-
apy has demonstrated that effective iron regulation is antitumor62
in humans by depriving cancer cells of iron, an essential ingredient
for proliferation.
Acknowledgments
The use of the NCS X-ray crystallographic service at the Univer-
sity of Southampton, UK and the valuable assistance of the staff
N
HN
H
N
N
R
N
N
Cl
N
R
1: X,Y = 2,3-OH, Z = H; IC50 = 0.314 μΜ
1: X,Y = 2,3-Cl, Z = H; IC50 = 6.999 μΜ
1: X = OH, Y,Z = H; IC50 = 0.757 μΜ
4: X,Y = 2,3-OH, Z = H; IC50 = 4.080 μΜ
4: X,Y = 2,3-Cl, Z = H; Inactive
4: X = OH; Y,Z = H; IC50 = > 16.793 μΜ
Scheme 3. Comparison of anti-cancer activities of two series of hydrazones.

F. A. R. Rodrigues et al. / Bioorg. Med. Chem. Lett. 24 (2014) 934–939
939
hydrogen atoms attached to the atom N3 for which the coordinates, along with
isotropic displacement parameters, were fully refined. All other hydrogen
atoms were placed in calculated positions.
there are gratefully acknowledged. J.L.W. and M.V.N. de S. thank
CNPq (Brazil) for financial support.
34. (a) CrystalClear, Rigaku Inc., The Woodlands, Texas, USA, 2010.; (b) Flack, H. D.
Acta Crystallogr., Sect.
Crystallographic Data Centre.; (d) Sheldrick, G. M. Acta Crystallogr., Sect. A
2008, 64, 112; (e) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
A 1983, 39, 876; (c) MERCURY 3.0. Cambridge
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.12.
074.
35. Crystal data collected at 100(2)K, colourless crystal: 0.19 Â 0.04 Â 0.01 mm.
Formula:
C18H18N4O3;
M = 338.36;
triclinic,
P-1;
a = 6.8085(5) Å,
a
= 80.338(5)°,
b = 7.1011(5) Å,
b = 81.011(5)°,
c = 18.5922(13) Å,
c
= 65.055(4)°, Z = 2, V = 799.80(10) Å3, 3654 [R(int) = 0.0440], 3046 observed
reflections [i > 2
r (I)]: parameters refined 3654; number of restraints 0; R(F)
0.038 [i > 2 (I)], Largest diff. peak 0.31 Å_1. Atomic coordinates, bond lengths,
angles and thermal parameters have been deposited at the Cambridge
Crystallographic Data Centre, deposition number 960209.
r
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