Synthesis of a simplified analogue of eleutherobin via a Claisen rearrangement and ring closing metathesis strategy

Article abstract of DOI:10.1039/b413426e

The enantioselective synthesis of a simplified eleutherobin analogue 7 by ring closing metathesis (RCM) of the 2,9-divinyl-substituted tetrahydro-oxonin 5 is described; the analogue 7 and an advanced intermediate 15 revealed microtubule stabilising proper

Full text of DOI:10.1039/b413426e

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Synthesis of a simplified analogue of eleutherobin via a Claisen
rearrangement and ring closing metathesis strategy{
Gary C. H. Chiang,a Andrew D. Bond,a Andrew Ayscough,b Gilles Pain,b Sylvie Duckic and
Andrew B. Holmes*ad
Received (in Cambridge, UK) 2nd September 2004, Accepted 13th December 2004
First published as an Advance Article on the web 11th February 2005
DOI: 10.1039/b413426e
The enantioselective synthesis of a simplified eleutherobin
analogue 7 by ring closing metathesis (RCM) of the 2,9-
divinyl-substituted tetrahydro-oxonin 5 is described; the analo-
gue 7 and an advanced intermediate 15 revealed microtubule
stabilising properties in the micromolar range.
Eleutherobin 1 (Fig. 1) has emerged as an exciting cytotoxic
compound and was shown to possess microtubule stabilising
properties similar to that of paclitaxel.1,2 Its novel molecular
architecture and its scarce availability from natural sources have
already prompted the total synthesis of eleutherobin by both the
Nicolaou3 and Danishefsky groups.4
We demonstrate that a nine-membered medium ring lactone 3
available from our Claisen ring expansion methodology
(Scheme 1{)5,6 can be elaborated to a simplified analogue of
eleutherobin 7 that shows microtubule stabilising activity.
The design of our target was based upon comprehensive SAR
studies on the sarcodictyins,7 a class of compounds structurally
related to eleutherobin. Thus, we elected to keep the crucial
urocanic acid side chain attached to the core bicycle (see
structure 7). A key intermediate in the synthesis would be the
aldehyde 4,6 which can be readily elaborated to a 2,9-divinyl
derivative for ring closing metathesis8,9 to the core bicycle 6.
The synthesis of eleutherobin analogue 7 is depicted in
Scheme 2.{ From aldehyde 4, Wittig methylenation afforded the
alkene 8 (90%), which was then treated with buffered pyridinium
Scheme 1{
hydrofluoride to effect silyl group removal (89%). Our initial
attempts used TBAF as the reagent for silyl deprotection, but this
was found to be non-chemoselective and gave a diol. IBX
oxidation then yielded an aldehyde (94%), which was subsequently
methylenated to afford the triene 5 (93%). RCM of 5 using the first
generation Grubbs catalyst8 10 yielded two compounds, the
required fused bicyclic medium-ring ether 6 (69%) and the isomeric
bis-cyclopentene 9 (22%).
To investigate whether the bis-cyclopentene was formed directly
from the triene 5 or from the bicyclic compound 6, the latter was
resubjected to the RCM conditions. After 3 days, 1H NMR
analysis of the reaction mixture showed a 20% conversion to 9.
This result suggests that equilibration of 6 to the thermodynami-
Fig. 1 Eleutherobin 1.
cally more stable 9 was possible, but the slow conversion suggested
that 9 was formed directly from the triene 5 and not through 6 as
an intermediate.
{ Electronic supplementary information (ESI) available: experimental
procedure for the conversion of 5 into 6 and 9 and spectroscopic data for 6
and 7. See http://www.rsc.org/suppdata/cc/b4/b413426e/
*abh1@cam.ac.uk
An alternative route to the synthesis of 6 was explored in order
to avoid the formation of the bis-cyclopentene 9. The strategy was
to protect the interfering endocyclic ring double bond with an
aholmes@unimelb.edu.au
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Starting from the alcohol 11, epoxidation furnished a 3 : 1
mixture of epoxides (80%), which were separated by flash
chromatography. Only the major isomer was carried forward in
the synthesis. IBX oxidation (73%) gave aldehyde 12, which was
followed by a Wittig methylenation (98%) to give the epoxide
protected RCM precursor 13. RCM of 13 with the first generation
Grubbs catalyst8 10 efficiently gave the desired ring-closed product
14 (86%). Epoxide removal was accomplished by treatment with
WCl6–nBuLi10 to give the previously isolated bicyclic compound 6,
albeit in only a moderate yield (53%). Although the epoxide
protection route was less efficient than our original route, the
sequence served to confirm independently our assignments of the
major and minor products of the RCM of 5.
Completion of the synthesis of the eleutherobin analogue 7 was
accomplished by silyl deprotection (61%) to give the crystalline
alcohol 15 {61%, mp 85–86 uC (from ether)}, followed by
coupling to the mixed anhydride 163 (50%). The X-ray crystal
structure (Fig. 2) of the bicycle 1511,12 provides insight into the
conformation.
The eleutherobin analogue 7 and the advanced intermediate 15
were investigated for tubulin stabilising properties.9 The results
show that the analogue 7 exhibits microtubule stabilising proper-
ties, but with less potency (ED90 5 6 ¡ 1.2 mM) than paclitaxel
(ED90 , 0.5 ¡ 1.2 mM). Nevertheless, this is an important result
since the cyclohexene ring of eleutherobin is believed to be an
important determinant for its antimitotic activity7 and that
fragment is not present in the analogue 7. In addition, even the
alcohol 15 was more potent (ED90 5 3 ¡ 1.2 mM) than 7. This
result could imply that it is possible to design further eleutherobin
analogues without the urocanic ester linkage and still maintain
microtubule stabilising properties.13
Scheme 2 Synthesis of 6. Reagents and conditions: (i) CH3PPh3Br,
nBuLi, THF, 278 uC, 20 min, 90%; (ii) HF?py, THF, pyridine, 16 h, 89%;
(iii) IBX, DMSO, rt, 16 h, 94%; (iv) CH3PPh3Br, nBuLi, THF, 278 uC,
1 h, 93%; (v) [(PCy3)2Cl2Ru(LCHPh)] 10, CH2Cl2, 18 h, 69% 6, 22% 9; (vi)
[(PCy3)2Cl2Ru(LCHPh)]
10,
CH2Cl2,
72
h,
20%
conv.
IBX 5 o-iodoxybenzoic acid.{
epoxide group, then perform the RCM reaction and then
deoxygenate the epoxide back to the alkene 6 (Scheme 3{).
In summary, the synthesis of a simplified eleutherobin analogue
7 has been accomplished. Biological data indicated that 7 exhibits
microtubule stabilising properties in the micromolar range. In
addition, compound 15, which lacks the crucial urocanic ester side
chain shows greater potency than 7. Efforts to synthesise more
potent analogues are currently under way.
We thank the EPSRC for financial support and provision of the
Swansea National MS Service and British Biotech for a CASE
award (GC). We thank the ARC, CSIRO and VESKI
(Fellowships to ABH) and R. Gilmour and S. Y. F. Mak for
their interest in this work.
Scheme 3 Epoxidation strategy and completion of 7. Reagents and
conditions: (i) mCPBA, THF, 0 uC A rt, 80% (major 63%); (ii) IBX,
DMSO, rt, 20 h, 73%; (iii) CH3PPh3Br, nBuLi, THF, 278 uC, 20 min,
98%; (iv) [(PCy3)2Cl2Ru(LCHPh)] 10, CH2Cl2, 18 h, rt, 86%; (v) WCl6,
nBuLi (2 equiv.), 278 uC A rt, 53%; (vi) TBAF, THF, 36 h, 0 uC A rt,
61%; (x) 16, DMAP, NEt3, THF, 20 h, 50%. IBX 5 o-iodoxybenzoic
acid.{
Fig. 2 X-Ray crystal structure of 15.
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Chem. Commun., 2005, 1860–1862 | 1861

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Gary C. H. Chiang,a Andrew D. Bond,a Andrew Ayscough,b Gilles Pain,b
Sylvie Duckic and Andrew B. Holmes*ad
4 X. T. Chen, S. K. Bhattacharya, B. Zhou, C. E. Gutteridge, T. R.
R. Pettus and S. J. Danishefsky, J. Am. Chem. Soc., 1999, 121, 6563.
5 M. S. Congreve, A. B. Holmes, A. B. Hughes and M. G. Looney,
J. Am. Chem. Soc., 1993, 115, 5815.
6 J. E. P. Davidson, R. Gilmour, S. Ducki, J. E. Davies, R. Green,
J. W. Burton and A. B. Holmes, Synlett, 2004, 1434.
7 K. C. Nicolaou, N. Winssinger, D. Vourloumis, T. Ohshima, S. Kim,
J. Y. Xu and T. Li, J. Am. Chem. Soc., 1998, 120, 10814.
8 P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2039.
aDepartment of Chemistry, University of Cambridge, Lensfield Road,
Cambridge, UK CB2 1EW. E-mail: abh1@cam.ac.uk;
aholmes@unimelb.edu.au
bBritish Biotech Pharmaceuticals Ltd., Watlington Road, Oxford, UK
OX4 6LY
cCentre for Molecular Drug Design, Cockroft Building, University of
Salford, Salford, UK M5 4WT
dBio21 Institute, University of Melbourne, Parkville Vic. 3010, Australia
9 For other ring closing metathesis approaches to eunicellin analogues
and the evaluation of their tubulin stabilising properties see: R. Beumer,
P. Bayon, P. Bugada, S. Ducki, N. Mongelli, F. R. Sirtori,
J. Telser and C. Gennari, Tetrahedron, 2003, 59, 8803 and references
cited therein.
10 K. B. Sharpless, M. A. Umbreit, T. H. Nieh and T. C. Flood, J. Am.
Chem. Soc., 1972, 94, 6538.
11 Crystal data for 15: C10H14O2, M 5 166.21, tetragonal, space group
Notes and references
{ All new compounds exhibited satisfactory spectroscopic and analytical
and/or exact mass data.
§ CCDC 249051. See http://www.rsc.org/suppdata/cc/b4/b413426e/ for
crystallographic data in .cif or other electronic format.
˚
P43212, a 5 b 5 8.2007(4), c 5 26.5896(9) A, a 5 b 5 c 5 90 u,
U 5 1788.19(14) A , Z 5 8, m(Mo–Ka) 5 0.084 mm21, 4922 reflections
3
˚
1 T. Lindel, P. R. Jensen, W. Fenical, B. H. Long, A. M. Casazza,
J. Carboni and C. R. Fairchild, J. Am. Chem. Soc., 1997, 119, 8744.
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A. M. Casazza, P. R. Jensen, T. Lindel, W. Fenical and
C. R. Fairchild, Cancer Res., 1998, 58, 1111.
measured at 180(2) K using an Oxford Cryosystems Cryostream cooling
apparatus, 1997 unique (Rint 5 0.066); R1 5 0.0532, wR2 5 0.118 [I .
2s(I)]; goodness-of-fit on F2, S 5 1.187. The structure was solved with
SHELXS-97 and refined with SHELXL-9712.§
3 K. C. Nicolaou, T. Ohshima, S. Hosokawa, F. L. van Delft,
D. Vourloumis, J. Y. Xu, J. Pfefferkorn and S. Kim, J. Am. Chem.
Soc., 1998, 120, 8674.
12 G. M. Sheldrick, SHELXS-97/SHELXL-97, University of Go¨ttingen,
Germany, 1997.
13 Microtubules did not depolymerise at 10 uC.
1862 | Chem. Commun., 2005, 1860–1862
This journal is ß The Royal Society of Chemistry 2005