Aza-analogues of the marine pyrroloquinoline alkaloids wakayin and tsitsikammamines: Synthesis and topoisomerase inhibition

Article abstract of DOI:10.1016/j.bmcl.2005.09.063

Two aza-analogues of the marine pyrroloquinoline alkaloids wakayin and tsitsikammamines A and B have been synthesized. The strategy used was based on a 1,3-dipolar cycloaddition reaction between indole 4,7-dione and a diazo-aminopropane derivative. One of

Full text of DOI:10.1016/j.bmcl.2005.09.063

Bioorganic & Medicinal Chemistry Letters 16 (2006) 427–429
Aza-analogues of the marine pyrroloquinoline alkaloids
wakayin and tsitsikammamines: Synthesis and
topoisomerase inhibition
Laurent Legentil,a Brigitte Lesurb and Evelyne Delfournea,*
aLaboratoire SPCMIB, UMR-CNRS 5068, Universite´ Paul Sabatier, 118 route de Narbonne, 31062, Toulouse Ce´dex 4, France
bLaboratoire Cephalon France, Centre de Recherches et de De´veloppement, 19 avenue du Pr Cadiot,
B.P. 22, 94701 Maisons-Alfort Cedex, France
Received 20 July 2005; revised 9 September 2005; accepted 22 September 2005
Available online 18 October 2005
Abstract—Two aza-analogues of the marine pyrroloquinoline alkaloids wakayin and tsitsikammamines A and B have been synthe-
sized. The strategy used was based on a 1,3-dipolar cycloaddition reaction between indole 4,7-dione and a diazo-aminopropane
derivative. One of the two analogues partially inhibits human topoisomerase I, whereas synthetic intermediates inhibit the enzyme
DNA cleavage activity at a concentration comparable to that of the control drug camptothecin.
Ó 2005 Elsevier Ltd. All rights reserved.
Marine organisms are among the most promising
sources of new biologically active molecules. Pyrrol-
o[4,3,2-de]quinoline marine alkaloids have received
considerable attention due to their potential antitumor
activities.1 Wakayin 1, isolated in 1991 from the ascid-
ian Clavelina species, has been reported to exhibit both
murine cell line cytotoxicity and topoisomerase I inhi-
bition.2 The close structurally related tsitsikammamines
A 2 and B 3, isolated from a Latrunculid sponge, are
cytotoxic and exhibit topoisomerase I inhibitory activ-
ity similar to that reported for wakayin (Fig. 1).3
products.8 We report herein, the synthesis of com-
pounds 7a and b.
OH
H
N
N
N
N
N
N
N
2 R = H
3 R = Me
H
H
H
R
1
O
O
Three approaches to structures analogous to wakayin
including compounds 4–6 have been reported so far,
with biological activity described only for the latter.4–6
H
H
H
N
N
N
N
O
O
As part of our work on analogues of natural products
with potential pharmacological value,7 we have been
interested in compounds 7a and b in which the
pyrrole-ring of the pyrroloquinoline moiety has been
replaced by a pyrazole-ring. Several aza-analogues of
natural products were reported to have better antitumor
activity compared to those of the corresponding natural
N
N
N
N
N
H
H
H
H
O
O
O
6
5
4
N
N
H
N
N
N
N
H
N
H
N
H
O
O
Keywords: Marine alkaloids; Pyrroloquinoline; Tsitsikammamines;
Wakayin; Topoisomerases inhibition.
7a
7b
*
Corresponding author. Tel.: +33 561 55 62 93; fax: +33 561 55 60
11; e-mail: delfourn@chimie.ups-tlse.fr
Figure 1.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.09.063

428
L. Legentil et al. / Bioorg. Med. Chem. Lett. 16 (2006) 427–429
The retrosynthetic analysis shown in Scheme 1 was de-
rived from that developed by Bakare et al. for the syn-
thesis of an aza-analogue of damirone B.9
The strategy is based on a 1,3-dipolar cycloaddition reac-
tion between indole-4,7-dione 910 and 1-diazo-4-amino-
propane 10a. As described by the authors, 10a was
generated by nitrosation of tetrahydro-2-pyrimidone
11, which was then reacted with potassium hydroxide.
In order to avoid the formation of tetrahydrotriazine
13, the amino-group, was further protected as a meth-
oxy-benzyloxycarbonyl Moz-group (10d) by action of
potassium p-methoxybenzyloxide in diethyl ether
(Scheme 2). This protective group was chosen because
it is more easily removed than methyl or benzylcarba-
mate groups (10b and c) used by Bakare.
NH2
N
O
N
N
N
N
N
H
N
H
H
H
O
O
7a/7b
8a/8b
The cycloaddition reaction involving N-tosylindole-4,7-
dione 9 and diazo-Moz-aminopropane 10d gave a
mixture of two regioisomers 14a/b as a result of tauto-
merization and in situ oxidation by air of the expected
pyrazoline. These products, in a ratio 60/40, were ob-
tained in 70% yield and were not separated (Scheme 3).
O
O
H2N
+
N
+
-
N
N
Ts
10a
9
Scheme 1.
The Moz-protective group was cleaved quantitatively by
TFA in the presence of m-cresol to give the correspond-
ing salts. Treatment of the compounds 14a/b under
O
˚
reflux in ethanol in the presence of 4 A molecular sieve
HN
NH
and sodium hydrogenocarbonate did not give rise to
the expected tetracyclic compounds but rather to mix-
tures of tricyclic products, which were either monotosy-
lated on the ethylamino side chain or ditosylated both
on the ethylamino side chain and on the indole dione
nitrogen. Therefore, the mixture of cycloadducts 14a/b
was tosylated prior to the cleavage of the Moz-protec-
tive group. In this way, compounds 14a/b were convert-
ed to corresponding tosyl derivatives 15a/b in 80% yield
using Kikugawa conditions (KOH in anhydrous
THF).11 The Moz-group was removed from the
conditions described above (80% yield) to give the
trifluoroacetic salt of compounds 8a/b, which were
cyclized into the tetracyclic derivatives 16a and b (30%
yield). The side products of this reaction are the mono-
and ditosylated derivatives identified previously.
Compounds 16a and b were separated by flash chroma-
NH
2
11
N
N
NH
+
N
a
-
b
N
13
O
10a
NO
N
HN
NHR
c
12
10b, R = CO Me
10c, R = CO Bz
10d, R = Moz
2
+
N
2
-
N
Scheme 2. Reagents and conditions: (a) NaNO2, 2 N H2SO4, 0 °C, 1 h
30 min; (b) KOH, Et2O, rt, 30 min; (c) KOH, MeOH (or BzOH or
pMeOBzOH), Et2O, rt, 30 min.
1
1
NHR
NHR
NHMoz
O
O
O
O
O
Ts
N
a
+
+
N
N
N
+
-
N
N
N
N
N
2
2
Ts
R
R
Ts
O
9
10
1
2
14a R = Moz, R = H
14b
15b
b
1
2
15a R = Moz, R = Ts
c
1
2
8b
8a
R = H, R = Ts
N
N
H
R
N
d
N
R
N
R
+
N
N
N
R
H
O
O
16a
7a
R = Ts
R = H
16b
7b
e
˚
Scheme 3. Reagents and conditions: (a) THF, rt, 30 min; (b) pTsCl, KOH, rt, 30 min; (c) TFA, m-cresol, CH2Cl2, rt, 1 h; (d) EtOH, NaHCO3, 4 A
molecular sieve, reflux, 3 h; (e) 1 N NaOH, dioxane, rt, 24 h.

L. Legentil et al. / Bioorg. Med. Chem. Lett. 16 (2006) 427–429
429
Antunes, E. M.; Beukes, D. R.; Kelly, M.; Samai, T.;
Barrows, L. R.; Marshall, K. M.; Sincich, C.; Davies-
Coleman, M. T. J. Nat. Prod. 2004, 67, 1268.
tography. The ratio of the two compounds (70:30)
indicated that the cyclization is slightly facilitated for
one regioisomer compared to the other. The structure
of each isomer was assigned from HMBC experiments.
The minor isomer showed a correlation of the pyrrolic
proton with the carbon of the carbonyl group, whereas
the major isomer showed a correlation between this
same proton and the carbon of the imino group leading
to the structure assignment 16a and b, respectively.
Finally, the two tosyl-protective groups were cleaved
by the action of 1 N aqueous sodium hydroxide in
dioxane to give the target products 7a and b.12
4. Zhang, L.; Cava, M. P.; Rogers, R. D.; Rogers, L. M.
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´ ´
´
6. Beneteau, V.; Pierre, A.; Pfeiffer, B.; Renard, P.; Besson,
T. Bioorg. Med. Chem. Lett. 2000, 10, 2231.
7. (a) Delfourne, E.; Darro, F.; Bontemps-Subielos, N.;
Decaestecker, C.; Bastide, J.; Frydman, A.; Kiss, R. J.
Med. Chem. 2001, 44, 3275; (b) Delfourne, E.; Darro, F.;
Portefaix, P.; Galaup, C.; Bayssade, S.; Bouteille, A.; Le
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A.; Kiss, R. J. Med. Chem. 2002, 45, 3765; (c) Brahic, C.;
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Bioorg. Med. Chem. 2002, 10, 2845; (d) Delfourne, E.;
Kiss, R.; Le Corre, L.; Dujols, F.; Bastide, J.; Collignon,
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F.; Bastide, J.; Collignon, F.; Lesur, B.; Frydman, A.;
Darro, F. Bioorg. Med. Chem. 2004, 12, 3987.
The ability of 7a and b, and synthetic intermediates to
inhibit the DNA cleavage activities of human topo-
isomerases I and II was assayed in a cell-free assay.
Camptothecin and etoposide, two well-known inhibitors
of topoisomerases I and II, respectively, were used as
positive references in these experiments.
8. See for example: Showalter, H. D. H.; Jonhson, J. L.;
Hoftiezer, J. M.; Turner, W. R.; Werbel, L. M.; Leopold,
W. R.; Shillis, J. J.; Jackson, R. C.; Elslager, E. F. J. Med.
Chem. 1987, 30, 121.
9. Bakare, O.; Zalkow, L. H.; Burgess, E. M. Synth.
Commun. 1997, 27, 1569.
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2003, 44, 2473.
11. Kikugawa, Y. Synthesis 1981, 460.
No inhibition of topoisomerase II activity was observed
at the maximum tested concentration, limited by final
DMSO concentration in the assay mixture. For topoiso-
merase I partial activity was observed at 100 lM for 7a,
whereas no inhibition was seen for its isomer 7b. Howev-
er, precursor tricyclic analogue mixture 14a/b inhibited
the catalytic activity of the enzyme, with a potency ob-
served in our assay comparable to that of camptothecin.
12. Compound 7a: yellow solid, mp > 260 °C. MS m/z (%):
212 (100); 185 (28); 155 (20). 1H NMR (400 MHz,
DMSO-d6): 2.92 (t, 2H, J = 8.1 Hz); 4.16 (t, 2H,
J = 8.1 Hz); 6.56 (d, 1H, J = 2.6 Hz); 7.21 (d, 1H,
J = 2.6 Hz). 13C NMR (DMSO-d6): 21.86; 50.67;
111.12; 117.52; 124.81; 126.21; 129.80; 131.12; 148.87;
References and notes
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1999, 6, 1; (b) Urban, S.; Hickford, S. J. H.; Blunt, J. W.;
Munro, M. H. G. Curr. Org. Chem. 2000, 4, 765; (c)
Antunes, E. M.; Copp, B. R.; Davies-Coleman, M. T.;
Samai, T. Nat. Prod. Rep. 2005, 22, 62.
2. Kokoshka, J. M.; Capson, T. L.; Holden, J. A.; Ireland, C.
M.; Barrows, L. R. Anti-Cancer Drugs 1996, 7, 758.
3. (a) Hooper, G. J.; Davies-Coleman, M. T.; Kelly-Borges,
M.; Coetzee, P. S. Tetrahedron Lett. 1996, 37, 7135; (b)
155.98; 166.57. IR (KBr): 3435; 2925; 1688; 1592 cmÀ1
.
Compound 7b: yellow solid, mp > 260 °C. MS m/z (%):
212 (100); 185 (30); 155 (21). 1H NMR (400 MHz,
DMSO-d6): 2.87 (t, 2H, J = 8.1 Hz); 3.96 (t, 2H,
J = 8.1 Hz); 6.47 (d, 1H, J = 2.4 Hz); 7.14 (d, 1H,
J = 2.4 Hz). 13C NMR (DMSO-d6): 21.76; 50.92;
111.36; 118.02; 124.54; 126.47; 129.91; 131.36; 149.43;
156.14; 166.68. IR (KBr): 3435; 2927; 1684; 1598 cmÀ1
.