Synthesis and biological evaluation of furano-epothilone C

Article abstract of DOI:10.1055/s-2004-829071

An efficient synthesis of furano-epothilone C (1) is described by the use of an aldol reaction and a ring-closing metathesis (RCM) for the closure of the macrocyclic ring system. This new type of analog contains a furan ring in the C8-C10 region of the macrocycle. The biology of this new class of epothilone C analog has been studied.

Full text of DOI:10.1055/s-2004-829071

LETTER
1375
Synthesis and Biological Evaluation of Furano-Epothilone C
S
ynthesis and Biol
i
ogical
Evaluation
t
of Fura
e
no-Epothilo
r
ne
C
Schinzer,*a Oliver M. Böhm,a Karl-Heinz Altmann,b Markus Wartmannc
a
b
c
Chemisches Institut, Otto-von-Guericke Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany
Fax +49(391)6712223; E-mail: dieter.schinzer@vst.uni-magdeburg.de
ETH Zürich, Department of Chemistry and Applied BioSciences, Institute of Pharmaceutical Sciences, Winterthurerstr. 190,
8057 Zürich, Switzerland
Novartis Institutes for Biomedical Research, DA Oncology, Novartis Pharma AG, 4002 Basel, Switzerland
Received 22 April 2004
Dedicated to Professor Clayton H. Heathcock, an inspiring scientist and person
RCM
Abstract: An efficient synthesis of furano-epothilone C (1) is de-
12
scribed by the use of an aldol reaction and a ring-closing metathesis
(RCM) for the closure of the macrocyclic ring system. This new
type of analog contains a furan ring in the C8-C10 region of the
macrocycle. The biology of this new class of epothilone C analog
has been studied.
S
N
13
O
8
OH
6
O
1
Key words: epothilone, total synthesis, furans, ring closure, macro-
cycles
1
O
OH
O
The secondary metabolites epothilone A and epothilone B
isolated from myxo bacteria1 have been found to act as
biological analogues of paclitaxel.2 They represent a new
and powerful class of microtubule stabilizing compounds
with remarkable ability to inhibit the growth of multidrug-
resistant (MDR) human cancer cell lines at low nanomolar
concentrations.3 In view of that, a large number of syn-
thetic and medicinal chemists, biologists and clinicians
started many efforts along these lines.4 Besides various to-
tal syntheses of the naturally occurring epothilones, a
large variety of partial solutions, and a tremendous
amount of analogues have been synthesized. Finally, sev-
eral pharmaceutical companies took over the project and
started clinical trials in order to replace Taxol® as a thera-
peutic anticancer agent.5
S
N
O
OTBS
aldol reaction
O
2
esterification
O
OTBS O
S
O
N
O
In this paper, we wish to report the synthesis and biologi-
cal evaluation of a new class of epothilone analogues. A
five-membered ring placed in the region of C8-C10
should provide conformationally more stabilized/restrict-
ed analogues. This idea can be invisioned by careful
analysis of the bond angles of the X-ray structure of
epothilone B.6
4
OH
3
O
O
O
5
Scheme 1 Retrosynthetic analysis.
For biological profiling, we needed a quick and robust en-
try to milligram quantities of compound 1. For that pur-
pose, we used an easy and flexible route to epothilones
developed within the group (Scheme 1).7 Disconnection
at the C12-C13 double bond provides diene 2. Further dis-
connection of the ester moiety gives alcohol 3 and subunit
C1-C12, which can be generated in an aldol reaction of
furano-aldehyde 4 and ethyl ketone 5. Acetonide fragment
5, which was developed in our lab, has been used very
successfully in the total syntheses of epothilones A, B and
a large number of analogues.8
Aldehyde 4 was obtained in multigram quantities by allyl-
ation of furan (6) followed by a Vilsmeier–Haack reaction
(Scheme 2).9,10 Reaction with ethyl ketone 5 gave aldol
products 8a and 8b in a combined yield of 83%
(de = 52%).11 The preformed lithium enolate of 5 provid-
ed only a moderate selectivity in the kinetically controlled
SYNLETT 2004, No. 8, pp 1375–1378
0
1.
0
7.
2
0
0
4
Advanced online publication: 22.06.2004
DOI: 10.1055/s-2004-829071; Art ID: Y02804ST
© Georg Thieme Verlag Stuttgart · New York

1376
D. Schinzer et al.
LETTER
aldol reaction, preferring the desired 6(R),7(S)-diastereo-
mer 8a with the natural epothilone configuration. The low
selectivity in the aldol reaction can be explained by the na-
ture of the aldehyde 4: Furano-aldehyde 4 does not con-
tain an a-chiral center, which would increase the
selectivity by matching chirality in favor of the anti-
Cram/Felkin product 8a.12
i
8a
O
O
OH
OH
O
OH
9
ii
The elaboration of 8a into 1 was mostly done by our pre-
viously developed chemistry. However, the incorporation
of the furan ring moiety created several pitfalls, which had
to be investigated before they could be evaded, mostly due
to quite sensitive molecules that could not be treated with
harsh reagents.11
TBSO
OTBS O
OTBS O
OTBS
10
11
12
iii
iv
O
O
HO
OTBS
OTBS
O
O
ii
i
4
HOOC
6
7
OTBS O
v
2
O
O
vi
O
O
O
O
OH
OH
8a
8b
S
N
iii
8a : 8b
3.2 : 1
+
O
OTBS
O
13
O
OTBS O
O
O
vii
Scheme 2 Reagents and conditions: (i) n-BuLi, allyl bromide,
THF, –78 °C (52%); (ii) DMF, POCl3, 0 °C (56%); (iii) LDA, 5, THF,
–78 °C, 50 min, then 4, 5 min (8a, 63%; 8b, 20%).
1
Scheme 3 Reagents and conditions: (i) Cat. PPTs, MeOH, reflux
(82%); (ii) TBSOTf, lutidine, CH2Cl2, –30 °C (71%); (iii) NH4F,
MeOH, reflux (80%); (iv) PDC, DMF (46%); (v) 3, DCC, cat.
DMAP, PhCH3 (64%); (vi) 0.15 PhCH=Ru(PCy3)2Cl2, CH2Cl2
(55% Z and 22% E); (vii) CsF, DMF, separation; repeat (35% after 3
cycles).
After separation of the diastereomeric aldols 8a and 8b by
flash chromatography, the acetonide moiety of 8a was
cleaved in the presence of PPTs in methanol to obtain the
requisite triol 9 in 82% yield (Scheme 3). Trisilylation
with TBS triflate gave the desired protected triol 10 in
71% yield. Selective deprotection to release the primary
alcohol 11 (80%) was obtained by the use of ammonium
fluoride in refluxing methanol. Subsequent oxidation with
PDC in DMF led to the formation of acid 12 in 46% yield.
Esterification of acid 12 and the known allylic alcohol 37b
in the presence of DCC and DMAP in toluene resulted in
the formation of ester 2 in 64% yield.
The final ring closure was achieved by RCM using
Grubbs catalyst. Gratifyingly, the RCM resulted in a 2.5:1
ratio in favor of the required Z-isomer 13. The structure of
13 has been confirmed by X-ray crystallography
(Figure 1).13
Attempts to cleave the TBS groups by means of well-es-
tablished reagents like HF, HF·pyridine, KF or TBAF all
failed or gave decomposed material.11 Finally, compound
1 was obtained using cesium fluoride in DMF. The
Figure 1 Molecular structure of bis-silylether 13 in the crystal
Synlett 2004, No. 8, 1375–1378 © Thieme Stuttgart · New York

LETTER
Synthesis and Biological Evaluation of Furano-Epothilone C
1377
reaction had to be carried out in a glove box iteratively,
giving a small amount of diol 1 after work-up and chroma-
tography in each cycle before the starting material and in-
termediates had to be treated with fresh CsF again. After
three iterations the diol portions were combined to give a
moderate yield of 35%.10,14
References
(1) Höfle, G.; Bedorf, N.; Reichenbach, H. G. B. F. DE-
4138042, 1993; Chem. Abstr. 1993, 120, 52841.
(2) Gerth, K.; Bedorf, N.; Höfle, G.; Irschik, H.; Reichenbach,
H. J. Antibiot. 1996, 49, 560.
(3) (a) Bollag, D. M.; McQueney, P. A.; Zhu, J.; Jensens, O.;
Koupal, L.; Liesch, J.; Goetz, M. E.; Lazarides, C.; Woods,
M. Cancer Res. 1995, 55, 2325. (b) Kowalski, R. J.;
Giannakakou, P.; Hamel, E. J. Biol. Chem. 1997, 272, 2534.
(4) (a) Schinzer, D. Eur. Chem. Chron. 1996, 1, 7. (b) Kalesse,
M. Eur. Chem. Chron. 1997, 2, 7. (c) Wessjohann, L.
Angew. Chem., Int. Ed. Engl. 1997, 36, 739; Angew. Chem.
1997, 109, 739. (d) Nicolaou, K. C.; Roschangar, F.;
Vourloumis, D. Angew. Chem. Int. Ed. 1998, 37, 2014;
Angew. Chem. 1998, 110, 2120.
(5) (a) Wartmann, M.; Altmann, K.-H. Curr. Med. Chem. 2002,
2, 123. (b) Harris, C. R.; Kuduk, S. D.; Danishefsky, S. J.
Chemistry for the 21st Century 2001, 8.
(6) Höfle, G.; Bedorf, N.; Steinmetz, H.; Schomburg, D.; Gerth,
K.; Reichenbach, H. Angew. Chem., Int. Ed. Engl. 1996, 35,
1567; Angew. Chem. 1996, 108, 1671.
Furano-epothilone C was evaluated for its ability to pro-
mote tubulin polymerization in vitro as well as for anti-
proliferative activity in a cell growth inhibition assay
using human epidermoid carcinoma cell lines KB-31 and
KB-8511.
Table 1 Growth Inhibition of Human Carcinoma Cells by Com-
pound 1 in Comparison with Natural Epothilones (IC50-values
[nM])a
Compound
KB-31b
594
KB-8511c
415
1
Deoxyepothilone Ad
Deoxyepothilone Bd
Epothilone Ad
Epothilone Bd
25
64
(7) (a) Schinzer, D.; Limberg, A.; Bauer, A.; Böhm, O. M.;
Cordes, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 523;
Angew. Chem. 1997, 109, 543. (b) Schinzer, D.; Limberg,
A.; Böhm, O. M. Chem.–Eur. J. 1996, 2, 1477.
2.70
2.10
0.19
1.44
1.90
(c) Schinzer, D.; Limberg, A.; Bauer, A.; Böhm, O. M.
Chem.–Eur. J. 1999, 5, 2483. (d) Schinzer, D.; Bauer, A.;
Schieber, J. Chem.–Eur. J. 1999, 9, 2492. (e) Schinzer, D.;
Bauer, A.; Schieber, J. Synlett 1998, 861.
0.19
a Average values from at least two independent experiments.
b Human epidermoid cancer cell line.
c Multidrug-resistant subline of the KB-31 line (overexpression of
P-gp170).
(8) (a) Taylor, R. E.; Chen, Y. Org. Lett. 2001, 3, 2221.
(b) Taylor, R. E.; Galvin, G. M.; Hilfiker, K. A.; Chen, Y. J.
Org. Chem. 1998, 63, 9580. (c) Taylor, R. E.; Haley, J. D.
Tetrahedron Lett. 1997, 38, 2061. (d) Yoshikawa, N.;
Yamada, Y. M. A.; Das, J.; Sasai, H.; Shibasaki, M. J. Am.
Chem. Soc. 1999, 121, 4168. (e) Sawada, D.; Shibasaki, M.
Angew. Chem. Int. Ed. 2000, 39, 209; Angew. Chem. 2000,
112, 215. (f) Sawada, D.; Kanai, M.; Shibasaki, M. J. Am.
Chem. Soc. 2000, 122, 10521. (g) Koch, G.; Loiseleur, O.;
Altmann, K.-H. Synlett 2004, 693. (h) Altmann, K.-H.
Mini-Rev. Med. Chem. 2003, 3, 149. (i) Klar, U.; Skuballa,
W.; Buchmann, W.; Schwede, B.; Bunte, T.; Hoffmann, J.;
Lichtner, R. B. ACS Symposium Series 2001, 796, 131.
(9) Lautens, M.; Kumanovic, S. J. Am. Chem. Soc. 1995, 117,
1954.
(10) All new compounds gave spectral and analytical
spectrometric data consistent with the assigned structure.
(11) Böhm O. M.; planned thesis, University of Magdeburg 2004.
(12) Schinzer, D. In Modern Aldol Reactions, Vol. 1;
Mahrwald, R., Ed.; Wiley-VCH: Weinheim, 311–328.
(13) Crystal data for compound 13: C38H61NO6SSi2, monoclinic,
C2, a = 2260.7 (2), b = 1427.7 (2), c = 1555.6 (2) pm,
a = 90°, b = 108.894 (3)°, g = 90°, U = 4.7503 (10) nm3,
Z = 4, m = 0.155 mm–1, T = 143 (2) K, colorless prism,
0.45 × 0.35 × 0.15 mm, Siemens SMART CCD, 16635
intensities, q = 1.38°–28.28°. Structure refinement: least
squares fit on F2, abs config with x = 0.07 (9), wR2 = 0.1679,
S 1.016, max 689, min –288 enm–3. Crystallographic data of
11 have been deposited at the Cambridge Crystallographic
Data Centre under the number CCDC 236809; copies may
be obtained without charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing
d Data from ref.1
As illustrated by the data summarized in Table 1, com-
pound 1 is a significantly less potent inhibitor of human
cancer cell growth in vitro than epothilones A/B or even
deoxyepothilone A.
It should be kept in mind that 1 is the first biologically ac-
tive epothilone analog where the chiral center at C8 is ab-
sent, which is known to be very sensitive in connection
with biological activity. Current work is in progress to
construct more of this new class of epothilone ana-
logues.16
Acknowledgment
We express our thanks to Dr. A. Fischer (Universität Magdeburg)
for the X-ray crystal structure determination of compound 13. This
work was supported by the Fonds der Chemischen Industrie and the
Novartis Pharma AG, Basel.
data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 (1223)336033.
Synlett 2004, No. 8, 1375–1378 © Thieme Stuttgart · New York

1378
D. Schinzer et al.
LETTER
(14) Analytical data for 1: 1H NMR (400 MHz, C6D6): d = 6.78
(s, 1 H), 6.54 (s, 1 H), 6.17 (br d, 3.0 Hz, 1 H), 5.77 (br d,
J = 3.0 Hz, 1 H), 5.57 (ddd, J = 10.7 Hz, 9.5 Hz, 6.0 Hz,
1 H), 5.44–5.37 (m, 1 H), 5.42 (dd, J = 8.9 Hz, 2.9 Hz, 1 H),
4.93 (br d, J = 6.6 Hz, 1 H), 4.07 (dd, J = 10.8 Hz, 2.6 Hz,
1 H), 3.41 (br dd, J = 15.7 Hz, 9.5 Hz, 1 H), 3.32 (Y-quint,
J = 6.8 Hz, 1 H), 3.08 (br dd, J = 15.7 Hz, 6.0 Hz, 1 H), 2.56–
2.48 (m, 1 H), 2.28 (dd, J = 15.7 Hz, 10.8 Hz, 1 H), 2.26 (s,
3 H), 2.15–2.07 (m, 1 H), 2.14 (d, J = 1.2 Hz, 3 H), 1.91 (dd,
J = 15.7 Hz, 2.6 Hz, 1 H), 1.15 (d, J = 6.8 Hz, 3 H), 0.99 (s,
3 H), 0.91 (s, 3 H). 13C NMR (100 MHz, C6D6): d = 217.48
(s), 170.51 (s), 164.70 (s), 154.07 (s), 153.37 (s), 153.36 (s),
137.62 (s), 128.35 (d), 126.45 (d), 120.26 (d), 116.45 (d),
108.20 (d), 106.70 (d), 78.02 (d), 72.95 (d), 69.44 (d), 52.75
(s), 47.60 (d), 38.95 (t), 31.05 (t), 27.10 (t), 22.61 (q), 19.73
(q), 18.74 (q), 15.69 (q), 14.92 (q). MS (EI): m/z (%) = 487
(37) [M+], 469 (31) [M+ - H2O], 381 (5), 328 (18), 310 (37),
300 (100) [C7 – C23+], 190 (42), 168 (51), 149 (75). HRMS
(EI): m/z calcd for C26H33NO6S: 487.20286; found:
487.2027.
(15) Altmann, K.-H.; Bold, G.; Caravatti, G.; End, N.;
Florsheimer, A.; Guagnano, V.; O’Reilly, T.; Wartmann, M.
Chimia 2000, 54, 612.
(16) Schinzer, D.; Bourguet, E.; Ducki, S. Chem.–Eur. J. 2004,
in press.
Synlett 2004, No. 8, 1375–1378 © Thieme Stuttgart · New York