Regiochemical control in the amination reaction of phenanthridine-7,10-quinones

Article abstract of DOI:10.1016/j.tetlet.2009.05.042

The reaction of substituted phenanthridine-7,10-quinones with amines to construct novel aminophenanthridinequinone derivatives as antitumor agents is described. The regiochemistry of the amination reaction is discussed in terms of inductive and steric eff

Full text of DOI:10.1016/j.tetlet.2009.05.042

Tetrahedron Letters 50 (2009) 4361–4363
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Tetrahedron Letters
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Regiochemical control in the amination reaction of phenanthridine-7,10-
quinones
*
Jaime A. Valderrama , J. Andrea Ibacache
Facultad de Quimica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of substituted phenanthridine-7,10-quinones with amines to construct novel aminophe-
nanthridinequinone derivatives as antitumor agents is described. The regiochemistry of the amination
reaction is discussed in terms of inductive and steric effects of remote substituents to the quinone ring,
which control the direction of the conjugate addition of the amines across the quinone double bond. Evi-
dences on the significant in vitro antitumor activities of some of the obtained aminoquinones, are
reported.
Received 27 April 2009
Revised 14 May 2009
Accepted 14 May 2009
Available online 19 May 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Phenanthridinequinones
Substitution reaction
Regioselectivity
Cytotoxicity
The molecular framework of several naturally occurring antitu-
moral agents contains an aminoquinonoid moiety as the key struc-
tural component, (e.g., mitomycin C 1, cribrostatin 3 2, and
streptonigrin 3).1,2 This structural array has stimulated the synthe-
sis of novel lead compounds that exhibit significant cytotoxicity on
human cancer cell lines.3–6
These aminoquinones expressed in vitro cytotoxic activity against
human lung fibroblasts, gastric adenocarcinoma, lung cancer, and
bladder carcinoma cell lines. We have observed that replacement
of the amino-proton of the parent compound 7-aminoisoquino-
line-5,8-quinone 6 (R1 = R0 = H) by substituted aryl groups is more
significant upon the antitumor activity than by alkyl groups. The
SAR analysis of the members of the phenylaminoisoquinolinequi-
none derivatives reveals that the half wave potential is an impor-
tant parameter determining the antitumoral activity on gastric
adenocarcinoma and bladder carcinoma cells.
The high regiocontrol of the substitution reaction on quinone 4
with amines can be attributed to the C-5 carbonyl group, which is
more electron deficient than the C-8 carbonyl group due to the
electron-acceptor effect of the para nitrogen atom, as shown in
structure 5. This electron deficiency of the C-5 carbonyl is trans-
ferred to C-7 and finally leads to preferential C-7 substitution via
nucleophilic attack of the amines. The regioselectivity of the 7-
position is greatly increased by coordination with CeCl3ꢀ7H2O or
InBr3 to the heterocyclic nitrogen atom and/or the C-5 carbonyl
group.
O
O
OCONH2
H2N
Me
H2N
Me
OMe
NH
N
O
N
O
O
O
1
2
O
MeO
H2N
N
CO2H
N
O
H2N
Me
OH
On the basis of these results and on the antitumor activity of
some benzo- and pyridophenanthridine-7,10-quinones, prepared
from 7 and 8,8 we decided to study their reaction with amines di-
rected toward the synthesis of 8- (or 9)-aminophenanthridine-
7,10-quinones derivatives for biological evaluation on cancer cell
lines. We herein describe the reaction of phenanthridine-7,10-qui-
nones 7 and 8 with amines, the influence of a Lewis acid catalyst on
the substitution reactions. We also show preliminary results on
their in vitro antitumor activity against human gastric adenocarci-
noma and lung cancer cell lines.
3
Me
We have recently developed a high yield synthesis of 7-substituted
aminoisoquinoline-5,8-quinones 6, using acid-induced substitution
reactions of isoquinolinequinone 4 with alkyl- and arylamines.7
* Corresponding author. Tel.: +56 02 6864432; fax: +56 02 6864744.
E-mail address: jvalderr@uc.cl (J.A. Valderrama).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.042

4362
J. A. Valderrama, J. A. Ibacache / Tetrahedron Letters 50 (2009) 4361–4363
O
O
O
OMe
O
O
O
O
O
OMe
O
OMe
H
O
O
O
O
N
Me
N
6
Me
N
Me
N
+
5
8
5
8
N
N
R1
N
7
H
N
Me
R2
O
Me
O
Me
5
4
6
9a. uncatalysed: 60
9b. uncatalysed: 40
catalysed: 27
R = H
catalysed: 73
Phenanthridinequinones 7 and 8 were prepared according to a re-
cent reported procedure.8,9 Firstly, we explored the reaction of phe-
nantridinequinone 7 with aniline in ethanol at room temperature.
The reaction went to completion in 5 h to give a 60:40 mixture of
regioisomers 9a and 9b in 40% yield (Table 1, entry 1). Interestingly,
when quinone 7 was allowed to react in ethanol in the presence of
10 mmol % of CeCl3ꢀ7H2O, the reaction was clean and fast (90 min)
to give a 27:73 mixture of regioisomers 9a and 9b in 88% yield (en-
try 2). These results clearly demonstrate that the catalyst improves
the yields and changes the regiochemistry of the substitution
reaction.
1
O
O
O
O
9
8
R = Me
10
7
N
N
N
H
O
R
O
Me
7. R = H
8. R = Me
13
Scheme 1. Reaction of quinones 7 and 8 with aniline.
Then we analyzed the reactivity of quinone 8 with aniline in
ethanol at room temperature. Surprisingly, in this case the result
was totally different from that observed for quinone 7, since the
reaction proceeds slowly and with poor yield to give aminoqui-
none 13 as the unique regioisomer (entry 7). The addition of
10 mmol % of CeCl3ꢀ7H2O increases the reaction rate and yield of
13, and does not change the regioselectivity (entry 8). Compounds
9a,10 9b,11 and 1312 were purified by column chromatography and
their structures were determined through (HMBC, HMQC) experi-
ments (Scheme 1).
quinone 8 with p-anisidine and cyclohexylamine produced the cor-
responding substitution products 14 and 15 (entries 10 and 11).
The results of the experiments under uncatalyzed conditions
demonstrate that the C-7 carbonyl group in quinone 7 exerts a
greater activation than the C-10 carbonyl group into the corre-
sponding
p-enone systems favoring the attack of the nucleophile
at the 9-position. According to the resonance and FMO theories
the behavior of 7 on the substitution reactions is opposite to that
predicted by both the theories. In fact, the inductive effect of the
heterocyclic nitrogen atom selectively increases the electron-with-
drawing ability of the C-10 carbonyl group so the attack of the
nucleophiles occurs at the 8-position. Concerning the polarization
of the LUMO on the electrophilic quinone double bond, the 8-posi-
tion is the preferred site of nucleophilic attack due to its largest
LUMO coefficient.13,14
Encouraged by these results we examined the reaction of qui-
nones 7 and 8 with p-anisidine, N-methylaniline and cyclohexyl-
amine. The reaction of quinone 7 with p-anisidine in ethanol
produced a 73:27 mixture of isomers 10a and 10b after 6 h (entry
3). In
a parallel assay of this reaction with 10 mmol % of
CeCl3ꢀ7H2O, a 26:74 mixture of regioisomers 10a and 10b was iso-
lated after 90 min (entry 4). N-Methylaniline and cyclohexylamine
reacted with 7 under catalysis conditions giving aminoquinones
11a,b and 12a,b, respectively (entries 5 and 6). The reaction of
On the contrary, in the case of quinone 8, the C-10 carbonyl
group exerts higher activation on the quinone double bond than
the C-7 carbonyl group, favoring the regiospecific attack of the
Table 1
Reaction of phenanthridinequinone 7 and 8 with amines
O
O
R3
N
O
O
O
O
O
O
R3
R2
+
N H
R3
+
N
R1
R2
N
R1
N
R1
N
R2
O
7, 8
9a-12a
9b-12b, 13-15
Entry
R1
R2
R3
Time
Products
Yieldb
Selectivityc
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
Me
Me
Me
Me
Me
Ph–
Ph–
4-MeO–Ph–
4-MeO–Ph–
Ph–
Cyclohexyl–
Ph–
Ph–
H
H
H
H
Me
H
H
H
H
H
H
5.0 h
90 mina
6 h
9a + 9b
9a + 9b
10a + 10b
10a + 10b
11a + 11b
12a+12b
13
13
14
14
15
40
88
15
86
55
42
18
96
25
79
45
60/40
27/73
73/27
26/74
32/68
40/60
100
100
100
100
100
90 mina
8 ha
6 ha
13 h
1ha
4-MeO–Ph–
4-MeO–Ph–
Cyclohexyl–
9 h
10
11
53 mina
70 mina
a
In the presence of 10% molar of CeCl3ꢀ7H2O.
Isolated yields.
b
c
Evaluated by 1H NMR.

J. A. Valderrama, J. A. Ibacache / Tetrahedron Letters 50 (2009) 4361–4363
4363
ridinequinone derivatives. The effect of remote substituents and
the catalysis on the control of the regioselectivity of the studied
reaction, together with the significant anticancer activity observed
on the 8-regioisomers, appears as valuable precedents toward the
development of new aminophenanthridinequinone-containing
antitumor drugs. The scope of the amination reaction on phenanth-
ridinequinone directed to prepare a variety of new members of this
class of N-heterocyclic quinones, and their biological evaluation on
representative cancer cell lines, are in progress in our laboratory.
Acknowledgment
We thank the FONDECYT and CONICYT (Grants No. 1060591;
AT-24080014) for financial support of this study.
References and notes
1. Pettit, G. R.; Knight, J. C.; Collins, J. C.; Herald, D. L.; Pettit, R. K. J. Nat. Prod. 2000,
63, 793–798.
Figure 1. Minimized molecular model of quinone 7.
2. Rao, K. V.; Beach, J. W. J. Med. Chem. 1991, 34, 1871–1879.
3. Ryu, C.-K.; Lee, I.-K.; Jung, S.-H.; Kang, H.-Y.; Lee, C.-O. Med. Chem. Res. 2000, 10,
40–49.
4. Chung, K.-H.; Hong, S.-Y.; You, H.-J.; Park, R.-E.; Ryu, C.-K. Bioorg. Med. Chem.
2006, 14, 5795–5801.
5. Sarma, M. D.; Ghosh, R.; Patra, A.; Hazra, B. Eur. J. Med. Chem. 2008, 43, 1878–1888.
6. Ling, R.; Yoshida, M.; Mariano, P. S. J. Org. Chem. 1996, 61, 4439–4449.
7. Valderrama, J. A.; Ibacache, J. A.; Rodríguez, J. A.; Theoduloz, C. G. Bioorg. Med.
Chem. 2009, 17, 2894–2901.
8. Valderrama, J. A.; Colonelli, P.; Vásquez, D.; González, M. F.; Rodríguez, J.;
Theoduloz, C. Bioorg. Med. Chem. 2008, 16, 10172–10181.
9. Valderrama, J. A.; Colonelli, P.; Vásquez, D.; González, M. F. Synlett 2006, 2777–
2780.
nucleophile at the 8-position, which is in accordance with the the-
oretical predictions.13
Since the electron-withdrawing effect of the heterocyclic nitro-
gen atom in dienophile 7 does not have a significant influence on
the regioselectivity of the substitution reaction apparently, the
control could be determined in part by steric interactions between
the C-1 and C-10 carbonyl groups into the dienophile.
An inspection of the molecular model of 7 (Fig. 1) shows an
inhibition to the coplanarity of the C-10 carbonyl group with re-
spect to the quinone double bond, due to its steric interaction with
the carbonyl groups at C-1. This effect probably influences the elec-
tron-withdrawing ability of the C-10 carbonyl group so the prefer-
ential attack of the nucleophiles occurs at b-position to the C-7
carbonyl group.
10. Compound 9a: purple solid, mp 182–184 °C; IR (KBr): mmax 3447 (N–H), 1703
(C@O), 1607 and 1590 (C@O quinone); 1H NMR (CDCl3, 400 MHz): d 2.22 (t,
J = 6.5 Hz, 2H, 3-H), 2.89 (t, J = 6.5 Hz, 2H, 2-H), 3.18 (t, J = 6.5 Hz, 2H, 4-H), 6.31
(s, 1H, 8-H), 7.23 (m, 3H, arom.), 7.25 (m, 2H, arom.), 7.41 (s, 1H, NH), 9.33 (s,
1H, 6-H); 13C NMR (CDCl3, 100 MHz): d 21.50, 33.39, 39.26, 102.24, 122.67,
124.25 (2C), 126.25, 128.89 (2C), 129.83,136.93, 140.44, 143.75, 150.75,
3
167.65, 180.79, 181.97, 197.62. The HMBC spectrum of 9b shows JC,H
The regioselectivity of the substitution reaction on quinone 8
can be explained assuming steric and electron-donor interactions
between the methyl and C-7 carbonyl groups. These factors prob-
ably affect the electrophilicity of the C-9 atom and the attack of the
nucleophiles occurs at b-position to the C-10 carbonyl group.
With regard to the experiments on the reactions of quinone 7
with the nucleophiles performed with CeCl3ꢀ7H2O catalyst, a rever-
sal of the regioselectivity with respect to the uncatalyzed reaction
is induced. However, in quinone 8 the catalyst does not change the
regioselectivity of the substitution at the 8-position, as in the case
of the uncatalyzed reactions.
The effect of the catalyst to promote the attack of the nucleophile
at the 8-position in both quinones may be ascribed to coordination
of the cerium ion to the heterocyclic nitrogen atom and/or the car-
bonyl group at the C-10 position. The coordination strongly en-
hances the electron-withdrawing capacity of the carbonyl group at
the C-10 position, which is transferred to the 8-position, leading to
preferential C-8 substitution via nucleophilic attack by the amines.
Some members of the aminophenanthridinequinone deriva-
tives were evaluated in vitro against human gastric adenocarci-
noma and lung cancer cell lines to get preliminary information
on their cytotoxic activities. The screening showed that com-
pounds 9a, 9b, 12a, and 12b exhibit significant antitumor activity
coupling of the C-10 carbon (d 180.79 ppm) with the protons at: d 7.41 and
6.31.HRMS (M+): m/z calcd for C19H14N2O3 [M+]: 318.10044; found:
318.10005.
11. Compound 9b: red solid, mp 179–182 °C; IR (KBr): mmax 3441 (N-H),1668
(C@O),1610 and 1566 (C@O quinone); 1H NMR (CDCl3, 400 MHz): d 2.21 (t,
J = 6.5 Hz, 2H, 3-H), 2.89 (t, J = 6.5 Hz, 2H, 2-H), 3.13 (t, J = 6.5 Hz, 2H, 4-H), 6.43
(s, 1H, 9-H), 7.23 (m, 3H, arom.), 7.25 (m, 2H, arom.), 7.41 (s, 1H, NH), 9.24 (s,
1H, 6-H); 13C NMR (CDCl3, 100 MHz): d 21.37, 33.40, 39.13, 104.99, 122.67,
124.25 (2C), 126.09, 128.90 (2C), 129.84, 136.92, 140.45, 143.74, 149.66,
169.79, 180.80, 181.42, 198.00. The HMBC spectrum of 9a shows 3JC,H coupling
of the C-7 carbon (d 181.42 ppm) with the protons at: d 9.24; 7.41 and
6.43 ppm.HRMS (M+): m/z calcd for C19H14N2O3: 318.10044; found: 318.10012.
12. Compound 13: dark red solid, mp 180–183 °C; IR (KBr): mmax 3443 (N-H), 1696
(C@O), 1591 and 1564 (C@O quinone), 1H NMR (CDCl3, 400 MHz): d 2.20 (t,
J = 6.7 Hz, 2H, 3-H), 2.85 (t, J = 6.7 Hz, 2H, 2-H), 2.92 (s, 3H, 6-CH3), 3.05 (t,
J = 6.7 Hz, 2H, 4-H), 6.35 (s, 1H, 9-H), 7.22 (m, 3H, arom.), 7.38 (m, 2H, arom.),
7.74 (s, 1H, NH); 13C NMR (CDCl3, 100 MHz): d 21.39, 26.33, 33.18, 39.12,
103.27, 122.58 (2C), 125.82, 128.07, 129.71 (2C), 137.17, 143.43, 144.74,
146.54, 162.27, 167.64, 181.42, 181.99, 198.64. The HMBC spectrum of 13
3
shows JC,H coupling of the C-7 carbon (d 181.99 ppm) with the protons at d
4
7.74 and 6.35 ppm, and JC,H coupling of the C-7 carbon (d 181.99 ppm) with
the protons of the methyl group at d 2.92 ppm.HRMS (M+): m/z calcd for
C19H14N2O3 [M+]: 332.11609; found: 332.11552.
13. The LUMO eigenvector coefficients were obtained with DFT level in B3LYP/6-
31G using the package GAUSSIAN 03. Compound 7: C8 = ꢁ0.20625,
C9 = 0.19330 eV. Compound 8: C8 = ꢁ0.20483, C9 = 0.19584 eV.
14. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople,
J. A. GAUSSIAN 03, Revision C.02; Gaussian: Wallingford, CT, 2004.
in the range IC50: 0.23–7.5
9b and 12b exhibit higher antitumor activity (IC50 = 0.23 and
0.38 M) than their corresponding regioisomers 9a and 12a
(IC50 = 3.9 and 2.3 M) on gastric cancer cells. The antitumor activ-
ity of compounds 9b and 12b on gastric cancer cells was compara-
ble to that shown by the reference drug etoposide (IC50 = 0.36 M).
lM. It is noteworthy that compounds
l
l
l
In conclusion, we have described the reaction of two
representative phenanthridinequinones with amines that provides
a regioselective access to potentially antitumor aminophenanth-