Synthetic approach to condensed heterocyclic analogues from etoposide revisited. Synthesis of A-ring pyridazine etoposide

Article abstract of DOI:10.1016/j.tetlet.2007.06.105

The synthetic approach to condensed heterocyclic analogues from etoposide was revisited. The described procedure allows the synthesis of A-ring pyridazine etoposide 13, featuring the use of tetrakis(triphenylphosphine)palladium as catalyst under optimized

Full text of DOI:10.1016/j.tetlet.2007.06.105

Tetrahedron Letters 48 (2007) 5781–5784
Synthetic approach to condensed heterocyclic analogues
from etoposide revisited. Synthesis of A-ring pyridazine etoposide
Emmanuel Bertounesque,* Philippe Meresse, Claude Monneret and Jean-Claude Florent
UMR 176 CNRS-Institut Curie, Centre de Recherche, 26 rue d’Ulm, 75248 Paris Cedex 05, France
Received 27 April 2007; revised 13 June 2007; accepted 18 June 2007
Available online 24 June 2007
Abstract—The synthetic approach to condensed heterocyclic analogues from etoposide was revisited. The described procedure
allows the synthesis of A-ring pyridazine etoposide 13, featuring the use of tetrakis(triphenylphosphine)palladium as catalyst under
optimized conditions in the key Stille cross coupling between bistriflate 14 and the vinylstannane without epimerization at C-2. The
TBDMS-protecting group was critical to cleanly obtain the pivotal intermediate 19.
Ó 2007 Elsevier Ltd. All rights reserved.
Etoposide (VP-16, VepesidÒ) 1 is a cancer chemothera-
peutic agent1 widely used in the treatment of small cell
lung cancer (SCLC), testicular carcinoma, lymphoma
and Kaposi’s sarcoma. Topoisomerase II is the target
for 1 that induces cell death by enhancing enzyme-
mediated double-strand DNA scission.2 To overcome
drug resistance and poor water solubility observed in
clinic with 1, intensive research has been devoted to
the synthesis of etoposide analogues3 (Fig. 1).
family, a composite pharmacophore model has been
proposed by MacDonald et al.4,5 for expression of topo-
isomerase II activity, defining three structural domains:
the intercalation-like domain in ternary complex (i.e.,
the planar polycyclic region), the variable substituent
domain (i.e., the C4 molecular region) and the minor
groove binding domain (i.e., the pendant E-ring region).
The comparative molecular force field analysis
(CoMFA), applied by Lee et al.6 to develop QSAR
models for epipodophyllotoxins, further specified by
the nature of the C4 molecular region for topoisomerase
II inhibition (i.e., bulky substituents).
In the absence of the 3D structure of the topoisomerase
II active site for conducting rational drug design in this
R1
O
O
Me
N
O
HO
O
4
R
N
HN
R1
Me
6
7
O
O
O
O
O
O
O
A
B
D
O
C
O
O
O
O
1
O
O
O
O
E
MeO
OMe
MeO
OMe
MeO
OMe
MeO
OMe
OR2
OH
OR2
OH
1 R1 = OH; R2 = H
4
5 R1 = F, R2 = H
8 R = OCNH(CH2)2NMe2
2 R1 = OH; R2 = PO3H
3 R1 = N(Me)2; R2 = H
6 R1 = NO2, R2 = H
7 R1 = NO2, R2 = COR3
O
9 R = NHCO2Et
10 NHSO2
N
N Me
Figure 1. Structures of etoposide 1 and analogues 2–10.
Keywords: Anticancer drugs; Etoposide; Topoisomerase II; Stille reaction.
*
Corresponding author. Tel.: +33 142346659; fax: +33 142346631; e-mail: emmanuel.bertounesque@curie.fr
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.105

5782
E. Bertounesque et al. / Tetrahedron Letters 48 (2007) 5781–5784
More recently renewed interest in drugs belonging to
this class has led to the clinical development of etopo-
phos (Bristol Myers), a 40-phosphate prodrug of etopo-
side 2.7 Several non-glycoside derivatives including
NK611 (Nippon-Kayaku) 3, a 200-dimethylamino-etopo-
side,8 TOP-53 (Taiho) 4,9 NPF 5,10 and GL-331 611 and
its analogues 712 emerged as promising drug candidates
for cancer but led to some unsuccessful phase II clinical
trials. However, new developments of these compounds
are not excluded. Our continuous interest in the synthe-
sis of etoposide analogues prompted us to extend the
evaluation of the variable substituent domain.4,5 Thus,
incorporation of a 4-O-b-carbamate13 (8) or a 4-N-b-
carbamate14 (9), or a 4-sulfonamide15 (10) resulted in
an increase in the cytotoxicity compared to etoposide
1. SAR studies centered on A-ring modification of eto-
poside or non-glycoside analogues turn out to have been
far less investigated. Kadow et al.16 reported the synthe-
sis and in vivo anti-P388 leukemia activity of com-
pounds 11 wherein the 6,7-dimethoxy (R1 = R2 = Me)
and 6,7-diacetoxy (R1 = R2 = COMe) analogues re-
tained the most activity but inferior to etoposide
(Fig. 2). These developments emphasize the importance
of the methylenedioxy group of 1 for optimal antitu-
mor activity. Lee et al.,17 also investigated the DNA
intercalating domain of the pharmacophore model by
introducing various phenazine rings into 40-O-demethyl-
epipodophyllotoxin analogues. Among these com-
pounds, 40-O-demethyl-4b-(4000-nitroanilino)-4-desoxy-
podophenazine 12 displays an activity superior to etopo-
side when evaluated against KB and KB/7d (VP-16-
Resistant cells) in vitro. It is interesting to note that
extension of the phenazine ring to the benzophenazine
ring led to a loss of activity. Unlike etoposide, podo-
phenazine 12 was found to be a weak topoisomerase II
inhibitor in vitro, but exhibits a novel mechanism of
action.
Based on MacDonald’s composite pharmacophore
model, we designed condensed heterocyclic analogues
from etoposide18 with the aim to enhance p stacking
interactions, intercalation-based pathway and improve
the solubility in water. Herein we report the synthesis
of A-ring pyridazine etoposide 13 from revisiting our
synthetic approach based on the Stille coupling reaction.
The synthesis of pyridazine etoposide 13 began with
bistriflate 14 obtained previously18 in four steps from
etoposide 1 (Scheme 1). After screening of the reaction
conditions of the Stille reaction19 in order to avoid the
O2N
O
O
O
O
Me
Me
O
O
HO
R1O
HO
O
O
NH
OH
OH
N
N
R1 = R2 = H
N
N
O
R
R
R
1 = R2 = Me
O
O
R2O
1 = R2 = COMe
1 = R2 = CO(CH2)6CH3
O
O
O
R1
R2
=
=
S
MeO
OMe
MeO
OMe
MeO
OMe
R1= R2 = CO2CH2Ph
OH
OH
OH
11
12
13
Figure 2. A-ring etoposide analogues.
O
O
O
O
O
O
O
O
Me
Me
a-d
Me
e
Me
O
O
O
O
HO
HO
HO
HO
O
O
O
O
OH
OH
OH
OH
TfO
TfO
O
+
O
O
O
O
O
TfO
O
O
O
O
MeO
OMe
MeO
OMe
MeO
OMe
MeO
OMe
OH
OH
OH
OH
1
14
16
15
O
O
Me
O
O
O
PO
OH
O
O
Me
O
Me
O
O
PO
OP
O
HO
O
OP
OH
TfO
HO
HO
TfO
f,g
+
O
O
O
O
Ha
Hb
HO
P = TBDMS
O
O
OH
MeO
OH
MeO
H8
Hc
OMe
OMe
MeO
OMe
OP
OP
OH
17
18
15 (NOE experiments)
Scheme 1. Reagents and conditions: (a–d) See Ref. 18; (e) Pd(PPh3)4 (0.2 equiv), Bu3Sn(C2H3) (10 equiv), LiCl (6 equiv), dioxane, 100 °C, 1.5 h, 15
(18%) and 16 (48%); (f) TBDMSOTf, 2,6-lutidine, CH2Cl2, 0 °C to rt, 88% yield of triprotected monovinyl relative to 15 and 88% yield of triprotected
bisvinyl relative to 16; (g) OsO4 cat., NMO, acetone:H2O (8/1), rt, 5 h.

E. Bertounesque et al. / Tetrahedron Letters 48 (2007) 5781–5784
5783
O
O
Me
O
O
O
PO
OH
O
O
O
Me
Me
O
O
OP
PO
O
PO
O
OP
OP
HO
HO
OHC
a
b
N
N
O
O
O
OHC
O
O
O
OH
MeO
MeO
OMe
OMe
MeO
OMe
OP
OP
OP
20
19
18 P = TBDMS
O
O
Me
O
HO
O
OH
c
N
N
O
O
MeO
OMe
OH
13
Scheme 2. Reagents and conditions: (a) Pb(OAc)4, benzene, rt, 2 h, 69%; (b) NH2NH2.H2O, CH2Cl2:EtOH (1/1), ꢀ55 °C for 1 h, 84%; (c) HFÆNEt3,
acetonitrile, rt, 6 days, 84%.
undesired epimerization of the D-ring18 deleterious to
the cytotoxic activity of this class of compounds,1d,e
we found that Pd(PPh3)4, under optimized conditions,
gave a mixture of trans-monovinyl 15 and trans-bisvinyl
16 in a 27:73 ratio (18% and 48% yields, respectively,
based on NMR analysis), which were very difficult to
separate. The C-7 site of monovinylation has been deter-
mined on the basis of NOE experiments: irradiation of
the Hc signal caused the enhancement of H8 and Hb sig-
nals. Complete silylation of this mixture as TBDMS
ethers followed by osmylation led to a separable mixture
of diols 17 (83%) and tetrols 18 (92%).20
In conclusion, these results confirm the importance of
the methylenedioxy A-ring of etoposide for optimal
activity. The use of the pivotal intermediate 19 should
give access to novel analogues of 1, in particular those
which have, fused to C-ring, either three rings (e.g.,
phthalazine) as in the case of 12 or methylenedioxy-
phenyl-containing four rings, to pursue the investigation
of the unexplored intercalating domain of McDonald’s
composite pharmacophore model.
Acknowledgment
We thank CNRS and Institut Curie for financial sup-
port. Laboratoires Servier are gratefully acknowledged
for a doctoral grant to Philippe Meresse and for biolog-
ical evaluation on A549 cancer cell line and P388 lym-
phocytic leukemia in mice. We also thank Dr. Paola
Arimondo (CNRS UMR 5153, INSERM U565,
Oxidation of this latter with Pb(OAc)4 gave dialdehyde
19 (2,3-trans relationship: J2,3 = 14.1 Hz). Formation
of the A-ring pyridazine occurred when 19 was treated
with hydrazine hydrate. Since, as expected, the basic
TBAF led to pyridazine picroetoposide18 (TBAF,
CH2Cl2:THF (10/2), rt, 24 h, 72%), deprotection of 20
21
´
Museum National d’Histoire Naturelle USM 0503,
was carried out using HFÆNEt3 to furnish the target
pyridazine etoposide 1322 (Scheme 2). The TBDMS pro-
tecting group was critical for obtaining 13. Unlike the
synthesis of pyridazine picroetoposide,18 protection with
the 2,2,2-trichloroethyl chloroformate group failed to
cleanly deliver the key intermediate dialdehyde 19. Syn-
thesis of 13 using phenoxyacetyl chloride for the tripro-
tection or benzyl chloroformate for the monoprotection
at C-40 was also unsuccessful.
Paris) for inhibition assay on toposiomerase II.
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diastereomers.
21. Hunig, S.; Wehner, G. Synthesis 1975, 180–182.
¨
22. All newly-synthesized compounds gave satisfactory ana-
lytical and spectroscopic data. Pyridazine etoposide 13.
20
Mp 210 °C; ½aꢁD ꢀ37 (c 0.1, CHCl3); IR (CDCl3) 3580,
1
3538, 2928 1776, 1620, 1519 cmꢀ1; H NMR: (300 MHz,
CDCl3) d 9.47 (br s, 1H, CH@N); 9.21 (br s, 1H, CH@N);
8.09 (s, 1H, H-5), 7.76 (s, 1H, H-8), 6.14 (s, 2H, H-20,
H-60), 5.48 (br s, 1H, OH-40), 5.27 (d, 1H, J4,3 = 3,5 Hz,
H-4), 5.09 (d, 1H, J1,2 = 5 Hz, H-1), 4.83 (d, 1H,
00
J1 ,2 = 7.5 Hz, H-1 ), 4.79 (q, 1H, J7 ,Me = 5 Hz, H-700),
00 00
00
4.55 (bt, 1H, J11a,11b = J11a,3 = 9 Hz, H-11a), 4.34 (bt, 1H,
00
00
J11b,11a = J11b,3 = 9Hz, H-11b), 4.22 (dd, 1H, J6 eq,6 ax
=
=
00
00
00
00 00
9.9 Hz, J6 eq,5 = 3.8 Hz, H-6 eq), 3.81 (t, 1 H, J3 ,4
00
0
0
00 00
J3 ,2 = 8.7 Hz, H-6 ax), 3.70 (s, 6H, OMe-3 ,5 ), 3.63
(t, 1H, J6 ax,6 eq = J6 ax,5 = 9.9 Hz, H-300), 3.54–3.38
00
00
00
00
(m, 4H, H-2, H-200, H-500, H-400), 3.11 (m, 1H, H-3), 1.43
00
13. Duca, M.; Guianvarc’h, D.; Meresse, P.; Bertounesque,
(d, JMe,7 = 5 Hz, Me); HRMS (DIC/CH4) m/z Calcd for
´
E.; Dauzonne, D.; Kraus-Berthier, L.; Thirot, S.; Leonce,
C30H32N2O11: 597.2084 [M+H]+; found, 597.2081.