Synthesis of novel simplified eleutheside analogues with potent microtubule-stabilizing activity, using ring-closing metathesis as the key-step

Article abstract of DOI:10.1016/S0040-4039(02)02646-1

The synthesis of a number of novel simplified eleutheside analogues with potent tubulin-assembling and microtubule-stabilizing properties is described, using ring-closing metathesis as the key-step for obtaining the 6-10 fused bicyclic ring system.

Full text of DOI:10.1016/S0040-4039(02)02646-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 681–684
Synthesis of novel simplified eleutheside analogues with potent
microtubule-stabilizing activity, using ring-closing metathesis
as the key-step
Raphael Beumer,a Pau Bayo´n,a Piergiuliano Bugada,a Sylvie Ducki,b,† Nicola Mongelli,b
Federico Riccardi Sirtori,b Joachim Telsera,‡ and Cesare Gennaria,*
aDipartimento di Chimica Organica e Industriale, Centro di Eccellenza CISI, Universita´ di Milano, Istituto CNR di Scienze e
Tecnologie Molecolari, via Venezian 21, I-20133 Milano, Italy
bPharmacia, viale Pasteur 10, I-20014 Nerviano, Italy
Received 9 November 2002; revised 21 November 2002; accepted 22 November 2002
Abstract—The synthesis of a number of novel simplified eleutheside analogues with potent tubulin-assembling and microtubule-
stabilizing properties is described, using ring-closing metathesis as the key-step for obtaining the 6–10 fused bicyclic ring system.
© 2003 Elsevier Science Ltd. All rights reserved.
Sarcodictyins A and B 1 and eleutherobin 2 (the
‘eleutheside’ family of microtubule-stabilizing agents,
Fig. 1) are active against paclitaxel-resistant tumor cell
lines and therefore hold potential as second generation
microtubule-stabilizing anticancer drugs.1 The scarce
availability of 1 and 2 from natural sources makes their
total syntheses vital for further biological investiga-
tions.1 To date, sarcodictyins A and B have been syn-
thesized successfully by Nicolaou et al.,2 who have also
exploited a similar route for accessing eleutherobin.3 A
subsequent report by Danishefsky and co-workers
detail an elegant alternative access to eleutherobin.4 A
number of partial syntheses and approaches have also
been described.5
The total syntheses of the eleuthesides have generated
very limited diversity in the diterpenoid core, with
major variations reported only in the C-15 functionality
and C-8 side-chain.1–4 In a recent communication,5h we
described the transformation of aldehyde 3 (prepared in
6 steps on a multigram scale from R-(−)-carvone in
30% overall yield)5a,g into the RCM precursor 8 via
multiple stereoselective Brown allylations6 (Scheme 1).
Diene 8 was subjected to ring-closing metathesis
(RCM)7 using the ‘second generation’ RCM-catalysts8
10 and 11 to give the desired ring-closed product 9 as a
single Z stereoisomer in ]80% yield.5h
As part of our ongoing program aimed at the synthesis
of simplified analogues of the eleutheside natural prod-
ucts, ideally showing improved synthetic accessibility
and retaining microtubule-stabilizing properties, we
describe in this Letter the synthesis of a number of
eleutheside analogues with potent tubulin-assembling
and microtubule-stabilizing activity, using RCM as the
key-step for obtaining the 6–10 fused bicyclic ring
system. A first set of eleutheside analogues (12, 14 and
16) was synthesized from compound 9 using standard,
high-yielding transformations (Scheme 2).9
Figure 1. Marine diterpenoids sarcodictyin A (1a), B (1b) and
eleutherobin (2).
Keywords: allylation; antitumour compounds; metathesis; stereocon-
trol.
* Corresponding author. Tel.: +39-025031-4091; fax: +39-025031-
4072; e-mail: cesare.gennari@unimi.it
† Present address: Centre for Molecular Drug Design, University of
Salford, Salford M5 4WT, UK.
‡ Present address: Medicinal Chemistry PH-R EU CR MC6, Bayer
AG, Pharma Research, D-42096 Wuppertal, Germany.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02646-1

682
R. Beumer et al. / Tetrahedron Letters 44 (2003) 681–684
Scheme 1. Reagents and conditions: (a) i. AllMgBr,
lIpc2BOMe, Et2O–THF, 0°C to rt; ii. 3, −78°C to rt, 6 h; iii.
6N NaOH, H2O2, rt, 15 h, 77% (>95% diastereomeric purity).
(b) TBDPS-Cl, excess imidazole, CH2Cl2, rt, 16 h, 99%. (c) i.
AcOH: THF: H2O (3:1:1), rt, 16 h, 99%; ii. NaBH4, EtOH, rt,
15 min, 98%; iii. MsCl, Et3N, CH2Cl2, 0°C to rt, 1 h, 99%; iv.
KCN, 18-crown-6, MeCN, 80°C, 5 h, 95%; v. DIBAl-H,
hexane–toluene (2:1), −78°C, 40 min, 99%. (d) i. AllMgBr,
lIpc2BOMe, Et2O–THF, 0°C to rt; ii. 6, −78°C to rt, 2 h; iii.
6N NaOH, H2O2, rt, 15 h, 55% (>95% diastereomeric purity).
(e) Ac2O, cat. DMAP, Py, rt, 94%. (f) 10 (30% mol), CH2Cl2,
rt, 24 h, 80% (100% Z), or 11 (7% mol), CH2Cl2, rt, 168 h,
88% (95% after recovering starting material, 100% Z).
Scheme 2. Reagents and conditions: (a) TBAF, THF, rt, 94–
100%. (b) 17 (Ref. 2b), Et3N, DMAP, CH2Cl2, 50–59%. (c)
K2CO3, MeOH, 94%. (d) (COCl)2, DMSO, Et3N, CH2Cl2,
−60 to 0°C, 90%. (e) Ph3P=CH2, THF, 50°C, 92%.
Aldehyde 6 was oxyallylated using Brown’s methodol-
ogy [(Z)-g-(methoxymethoxy)allyldiisopinocampheyl-
borane from (−)-a-pinene]6d in high yield (77%) and
with good stereoselectivity (3S,4S:3R,4R=91:9,
Scheme 3). The major diastereomer 18 was isolated by
flash chromatography and transformed into the allylic
alcohol 19 via
a
simple protection/deprotection
sequence. ‘Second generation’ Grubbs’ catalyst 11 gave
the desired ring-closed product 20 as a single Z
stereoisomer in 73% yield. As expected, entropic sup-
port (by virtue of the cis fusion to the cyclohexyl ring)
made ring closure of diene 19 extremely smooth. Luck-
ily, and delightfully, the stereochemistry of the double
bond created by the RCM reaction was fully controlled
in the desired sense (100% Z) by the structure of the
new ten-membered ring.10 The stereochemical course
likely reflects thermodynamic control.11 The crucial role
of the protecting groups in the cyclization precursor 19
is noteworthy: (a) the large TBDPS group in position 8
helps to suppress the undesired dimerization reaction;12
(b) a free allylic alcohol13 in position 4 is necessary to
promote the cyclization (the RCM reaction did not
occur on dienes with variously protected allylic alcohols
in position 4).14
l
Scheme 3. Reagents and conditions: (a) i. Ipc2BOMe, AllO-
MOM, s-BuLi, BF3·Et2O, THF, −78°C; ii. 6N NaOH, H2O2,
rt, 15 h, 77% (91% diastereomeric purity). (b) t-BuCOCl
(PivCl), cat. DMAP, Py, rt, 80%. (c) BF3·Et2O, PhSH,
CH2Cl2, −78 to −10°C, 64%. (d) 11 (10% mol), CH2Cl2, rt,
120 h, 73% (100% Z).
Aldehyde 6 was also oxyallylated using Brown’s enan-
tiomeric reagent [(Z)-g-(methoxymethoxy)allyldiiso-
pinocampheylborane from (+)-a-pinene]6d in high yield
(76%) and with excellent stereoselectivity (3R,4R:
3S,4S=97.4:2.6, Scheme 5). The major diastereomer 24
was isolated by flash chromatography and transformed
into the allylic alcohol 25 via a simple protection/
deprotection sequence (in this case Me2S, BF3·Et2O
proved more reliable than PhSH, BF3·Et2O for depro-
tecting the allylic alcohol from the MOM group).15
Compound 20 was transformed into a second set of
eleutheside analogues 21–23, using standard, high-yield-
ing transformations (Scheme 4).9

R. Beumer et al. / Tetrahedron Letters 44 (2003) 681–684
683
Scheme 5. Reagents and conditions: (a) i. dIpc2BOMe, AllO-
MOM, s-BuLi, BF3·Et2O, THF, −78°C; ii. H2O2, 6N NaOH,
rt, 15 h, 76% (97.4% diastereomeric purity). (b) t-BuCOCl
(PivCl), cat. DMAP, Py, rt, 94%. (c) BF3·Et2O, Me2S,
CH2Cl2, −20°C, 78%. (d) 11 (10% mol), CH2Cl2, rt, 120 h,
60% (100% Z). (e) MeOTf, 2,6-di-t-Bu-Py, CH2Cl2, 40°C,
99%. (f) TBAF, THF, rt, 67%. (g) 17 (Ref. 2b), Et3N, DMAP,
ClCH2CH2Cl, 79%.
Scheme 4. Reagents and conditions: (a) MeOTf, 2,6-di-t-Bu-
Py, CH2Cl2, 40°C, 96%. (b) TBAF, THF, rt, 89–100%. (c) 17
(Ref. 2b), Et3N, DMAP, CH2Cl2, 61–80%. (d) Ac2O, cat.
DMAP, Py, rt, 71%. (e) DHP, PPTS, CH2Cl2, 77%. (f) PTSA,
EtOH: H2O (8:2), 82%.
Treatment with catalyst 11 gave the desired ring-closed
product 26 as a single Z stereoisomer in 60% yield.
Finally, a standard sequence of reactions transformed
compound 26 into the eleutheside analogue 27.9
that some of these compounds retain activity compara-
ble to paclitaxel in the tubulin polymerization assay is
remarkable. Work is in progress to synthesize more
potent eleutheside analogues, investigate the interaction
with tubulin and establish their cytotoxicity.
The effect of these new eleutheside analogues on the
assembly of tubulin was assessed at Pharmacia (Nervi-
ano, Italy) and at Salford (UK), using paclitaxel as a
reference (Table 1).
Acknowledgements
Eleutheside analogue 14 was shown to be at least as
potent as paclitaxel. Microtubules were generated in the
presence of CaCl2 and were stable (did not depolymer-
ize) at 10°C. Although there is a general agreement that
the (E)-N-methylurocanic side chain, the C-4/C-7 ether
bridge, and the cyclohexene ring are important determi-
nants of antimitotic activity,1 it is interesting to note
that these simplified analogues of the natural product
(lacking inter alia the C-4/C-7 ether bridge) retain
potent microtubule stabilizing activity. Given the dra-
matic impact that the furanose oxygen deletion is likely
to have on the conformation of the ring system, the fact
We thank the European Commission for financial sup-
port (IHP Network grant ‘Design and synthesis of
microtubule stabilizing anticancer agents’ HPRN-CT-
2000-00018) and for postdoctoral fellowships to R.
Beumer (HPRN-CT-2000-00018), S. Ducki (HPRN-
CT-2000-00018), P. Bayo´n (‘Marie Curie’ HPMF-CT-
2000-00838)
and
J.
Telser
(‘Marie
Curie’
HPMF-CT-1999-00001). We also like to thank Merck
(Merck’s Academic Development Program Award to C.
Gennari, 2001–02) and MURST COFIN 2000
(MM03155477) for financial support.
Table 1. Tubulin polymerizing activitiesa
Compound
ED50 (mM)
ED90 (mM)
Paclitaxel ED50 (mM)
Paclitaxel ED90 (mM)
12
14
16
21
22
23
27
2.0
0.2
5.0
3.0
1.0
1.0
n.d.b
10.0
1.2
16.0
7.0
B0.5
0.5
B0.5
0.5
B0.5
B0.5
–
0.5
3.0
0.5
3.0
0.5
0.5
–
1.7
1.8
n.d.b
a ED50=effective dose that induces 50% tubulin polymerization; ED90=effective dose that induces 90% tubulin polymerization (see: Battistini, C.;
Ciomei, M.; Pietra, F.; D’Ambrosio, M.; Guerriero, A. (Pharmacia), PCT Int. Appl. WO 96 36,335, 1996 [Chem. Abstr. 1997, 126, P54863x]).
ED values may vary depending on the tubulin batch (from pig brain): the same batch is used for the paclitaxel reference assay.
b Not determined: the compound precipitates at 1.0 mM concentration, under the ED50 value.

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R. Beumer et al. / Tetrahedron Letters 44 (2003) 681–684
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