Design, synthesis and biological evaluation of a macrocyclic discodermolide/dictyostatin hybrid

Article abstract of DOI:10.1039/b615122a

A 22-membered macrocyclic discodermolide/dictyostatin hybrid has been designed and synthesised; biological evaluation against a range of human cancer cell lines revealed significant levels of growth inhibition. The Royal Society of Chemistry.

Full text of DOI:10.1039/b615122a

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Design, synthesis and biological evaluation of a macrocyclic
discodermolide/dictyostatin hybrid{
Ian Paterson* and Nicola M. Gardner
Received (in Cambridge, UK) 18th October 2006, Accepted 1st November 2006
First published as an Advance Article on the web 23rd November 2006
DOI: 10.1039/b615122a
promising anticancer properties.5,6 Strong structural similarities
exist between discodermolide and dictyostatin, particularly with
regard to stereochemical homology, as determined by our recent
configurational assignment of the latter structure using detailed
NMR analysis,5c suggesting that they interact in a similar fashion
with the Taxol binding site on b-tubulin.5d,6 Thus, building on
initial encouraging findings reported by the Curran group,7a
constraining the conformation of the more flexible open-chain
structure of discodermolide into the macrocyclic ring motif of
dictyostatin might provide active hybrids of these marine-sponge-
derived polyketides. Herein, we report the synthesis of the designed
22-membered macrolide 3, incorporating the full C2–C24 linear
sequence of discodermolide and the (Z)-enoate of dictyostatin, and
that it shows significant growth inhibitory activity against human
cancer cell lines.
A
22-membered macrocyclic discodermolide/dictyostatin
hybrid has been designed and synthesised; biological evaluation
against a range of human cancer cell lines revealed significant
levels of growth inhibition.
Discodermolide (1, Fig. 1), originally isolated from the deep-sea
sponge Discodermia dissoluta, displays potent antiproliferative
activity against a wide range of human cancer cell lines and inhibits
the growth of drug-resistant solid tumours.1 It shares a similar
microtubule-stabilising mechanism to that of Taxol, while having a
greater tubulin binding affinity,2 and has progressed into clinical
development as a novel anticancer agent.3 Furthermore, the
synergistic combination of Taxol and discodermolide induces
tumour regressions and suppresses angiogenesis in animal models
of ovarian carcinoma, supporting their potential use together in
cancer therapeutics.4
At the outset, we considered it essential to achieve a suitable
overlay of the energetically preferred conformation of our designed
hybrid with that of discodermolide. A 10 000 step Monte Carlo
conformational search was performed using Macromodel (Version
8.0) with the MM2* force field and a Born/surface area (GB/SA)
water solvent model. The calculated global minimum for analogue
3 (see the ESI{) correlates well with the X-ray crystal structure of
discodermolide,1a as shown in the overlay in Fig. 1. The match for
the C9–C26 region (corresponding to C7–C24 of discodermolide)
region is particularly striking. Consequently, if the tubulin-bound
conformation of discodermolide resembles its X-ray structure, the
constrained macrocyclic analogue 3 may possess a similar binding
affinity and cytotoxicity to that of the natural product.
Similarly, dictyostatin (2) displays further elevated levels of
growth inhibition across the same wide range of human cancer cell
lines, and has emerged as a new microtubule-stabilising agent with
Our synthetic strategy leading to analogue 3 was adapted from
previous work on the total synthesis of discodermolide.8
A
complex aldol coupling of enone 4 and aldehyde 5, derived from
previously prepared advanced intermediates, would thus be used
to assemble a suitable linear precursor for macrolactonisation
(Scheme 1).
Synthesis of the enone 4 started from bis-PMB ether 6,8a which
incorporates the stereochemistry required for the C11–C26 region
(Scheme 2). Selective removal of the primary PMB ether was
achieved with BCl3?DMS,9 followed by oxidation of the resulting
alcohol with TEMPO/PhI(OAc)2 and Still–Gennari olefination10
to give enone 4 (73%). Synthesis of the aldehyde partner 5 began
with the selective primary oxidation of diol 7,8b,c again making use
of TEMPO/PhI(OAc)2, followed by a Still–Gennari olefination to
provide (Z)-enoate 8 (60%). Removal of the PMB ether,11 bis-
silylation of the corresponding diol with TBSOTf/2,6-lutidine and
DIBALH reduction of the methyl ester generated allylic alcohol 9
(48%). Finally, PMB ether formation with PMBTCA/Sc(OTf)3,
selective primary TBS cleavage and Dess–Martin oxidation
Fig. 1 Structures of discodermolide, dictyostatin and designed hybrid 3.
Overlay of the lowest energy conformation of hybrid 3 (purple) and the
X-ray structure of discodermolide (green).
University Chemical Laboratory, Lensfield Road, Cambridge, UK
CB2 1EW. E-mail: ip100@cam.ac.uk; Fax: +44 (0)1223 336362;
Tel: +44 (0)1223 336407
{ Electronic supplementary information (ESI) available: Experimental and
modelling details, and 1H/13C NMR data for compounds 3 and 10–15. See
DOI: 10.1039/b615122a
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Chem. Commun., 2007, 49–51 | 49

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Scheme 3 Key boron aldol coupling.
alcohol 12 with TEMPO/PhI(OAc)2 then NaClO2/NaH2PO4
provided seco-acid 13 (58%).
Scheme 1 Retrosynthetic analysis of hybrid analogue 3.
Finally, a Yamaguchi macrolactonisation of 13 (65%) and
global deprotection using 3M HCl in MeOH (53%) afforded the
22-membered macrolide 3 after HPLC purification. Full proton
and carbon assignments were carried out using COSY and
HMQC NMR experiments. The close correlation between the
provided aldehyde 5 (68%) in readiness for the pivotal aldol
coupling step.
Enolisation of enone
4 with c-Hex2BCl/Et3N in Et2O,
and subsequent addition of aldehyde 5, gave adduct 10 with
.95 : 5 dr in favour of the desired (7S)-adduct (48%, Scheme 3).
This anti-Felkin–Anh outcome is consistent with our earlier work
on 1,6-induction in similar boron aldol reactions, and can be
rationalised by invoking the favoured transition state model
TS 1.8b,c
3
calculated (by molecular modelling) and observed JH–H coupling
constants of 3 (see the ESI{) suggests that the conformation
adopted is similar to our modelled prediction.
The cell growth inhibitory activity of macrocyclic discodermo-
lide/dictyostatin hybrid 3 was evaluated in vitro against three
cancer cell lines: MDA-MB-231 (breast), A549 (non-small cell
lung) and HT29 (colon) (Table 1). Notably, analogue 3 displayed
significant antiproliferative activity against these human carcinoma
cells, with a cytotoxicity around one-tenth that of discodermolide.
This preliminary data is consistent with the conformation adopted
by 3 closely resembling the X-ray structure of discodermolide,
which itself has now been reported to correlate strongly with
the NMR-derived conformation of discodermolide bound to
tubulin.15
Reduction of b-hydroxyketone 10 proved problematic, afford-
ing mixtures of epimeric alcohols at C9 when subjected to standard
Evans–Saksena conditions (Scheme 4).12 An improvement was
found by following the same protocol as previously employed for
the reduction of similarly troublesome b-hydroxyketones8b using
stoichiometric quantities of (R)-CBS and BH3?THF,13 which
provided the desired isomer 11 in a 75 : 25 dr.14 The 1,3-anti diol
11 was protected as its acetonide; subsequent oxidative PMB ether
cleavage (with DDQ) and sequential oxidation of the resultant
Scheme 2 Synthesis of enone 4 and aldehyde 5.
50 | Chem. Commun., 2007, 49–51
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Madrid) for providing the biological data, Dr. Stuart Mickel
(Novartis) for the gift of chemicals, Rob Paton and Dr. Jonathan
Goodman (Cambridge) for assistance with the modelling studies,
and Dr. John Leonard (AstraZeneca) and Dr. Isabelle Lyothier
(Cambridge) for helpful discussions.
Notes and references
1 (a) S. P. Gunasekera, M. Gunasekera, R. E. Longley and G. K. Schulte,
J. Org. Chem., 1990, 55, 4912. CCDC reference VINTAN for the X-ray
crystal structure of 1; (b) E. ter Haar, R. J. Kowalski, E. Hamel,
C. M. Lin, R. E. Longley, S. P. Gunasekera, H. S. Rosenkranz and
B. W. Day, Biochemistry, 1996, 35, 243.
2 R. J. Kowalski, P. Giannakakou, S. P. Gunasekera, R. E. Longley,
B. W. Day and E. Hamel, Mol. Pharmacol., 1997, 52, 613.
3 A. Mita, A. C. Lockhart, T.-L. Chen, K. Bochinski, J. Curtright,
W. Cooper, L. Hammond, M. Rothenberg, E. Rowinsky and
S. Sharma, J. Clin. Oncol., 2004, 22, 2025.
4 G. S. Huang, L. L. Barcons, B. S. Freeze, A. B. Smith, III,
G. L. Goldberg, S. B. Horwitz and H. M. McDaid, Clin. Cancer
Res., 2006, 12, 298.
5 (a) G. R. Pettit, Z. A. Cichacz, F. Gao, M. R. Boyd and J. M. Schmidt,
J. Chem. Soc., Chem. Commun., 1994, 1111; (b) R. A. Isbrucker,
J. Cummins, S. A. Pomponi, R. E. Longley and A. E. Wright, Biochem.
Pharamacol., 2003, 66, 75; (c) I. Paterson, R. Britton, O. Delgado and
A. E. Wright, Chem. Commun., 2004, 632; (d) I. Paterson, R. Britton,
O. Delgado, A. Meyer and K. G. Poullennec, Angew. Chem., Int. Ed.,
2004, 43, 4629.
6 C. Madiraju, M. C. Edler, E. Hamel, B. S. Raccor, R. Balachandran,
G. Zhu, K. A. Giuliano, A. Vogt, Y. Shin, J. Fournier, Y. Fukui,
A. M. Bru¨ckner, D. P. Curran and B. W. Day, Biochemistry, 2005, 44,
15053; R. M. Buey, I. Barasoain, E. Jackson, A. Meyer,
P. Giannakakou, I. Paterson, S. Mooberry, J. M. Andreu and
J. F. Diaz, Chem. Biol., 2005, 12, 1269.
7 (a) Y. Shin, N. Choy, R. Balachandran, C. Madiraju, B. W. Day and
D. P. Curran, Org. Lett., 2002, 4, 4443; (b) For an excellent review on
natural product hybrids, see: L. F. Tietze, H. P. Bell and
S. Chandrasekhar, Angew. Chem., Int. Ed., 2003, 42, 3996.
8 (a) I. Paterson, G. J. Florence, K. Gerlach, J. P. Scott and N. Sereinig,
J. Am. Chem. Soc., 2001, 123, 9535; (b) I. Paterson, O. Delgado,
G. J. Florence, I. Lyothier, M. O’Brien, J. P. Scott and N. Sereinig,
J. Org. Chem., 2005, 70, 150; (c) I. Paterson, O. Delgado, G. J. Florence,
I. Lyothier, J. P. Scott and N. Sereinig, Org. Lett., 2003, 5, 35.
9 M. S. Congreve, E. C. Davison, M. A. M. Fuhry, A. B. Holmes,
A. N. Payne, R. A. Robinson and S. E. Ward, Synlett, 1993, 663.
10 W. C. Still and C. Gennari, Tetrahedron Lett., 1983, 24, 4405.
11 Attempts to remove the PMB ether using DDQ resulted in both PMP
acetal formation and over-oxidation to the benzoate.
Scheme 4 Completion of the synthesis of hybrid analogue 3.
Table 1 Human cancer cell growth inhibitory properties of macro-
cyclic discodermolide analogue 3 relative to discodermolide (1)
GI50/mMa
12 D. A. Evans, K. T. Chapman and E. M. Carreira, J. Am. Chem. Soc.,
1988, 110, 3560. NaBH(OAc)3 gave 67 : 33 dr in favour of the 1,3-syn-
diol while Me4NBH(OAc)3 gave little or no diastereoselectivity.
13 E. J. Corey, R. K. Bakshi and S. Shibata, J. Am. Chem. Soc., 1987, 109,
5551.
MDA-MB-231 (breast) A549 (non-small cell lung) HT29 (colon)
1
3
a
0.029
0.208
0.020
0.399
0.015
0.170
50% growth inhibitory concentration after 72 h of continuous
incubation.
14 After chromatographic separation, the syn-diol was subsequently used
to synthesise the C9 epimer of 3.
15 V. M. Sanchez-Pedregal, K. Kubicek, J. Meiler, I. Lyothier, I. Paterson
and T. Carlomagno, Angew. Chem., Int. Ed., 2006, 45, 7388.
16 For recent reports of non-macrocyclic discodermolide analogues, see
inter alia: I. Paterson and O. Delgado, Tetrahedron Lett., 2003, 44, 8877;
A. B. Smith, III and M. Xian, Org. Lett., 2005, 7, 5229; A. B. Smith, III,
B. S. Freeze, M. J. LaMarche, T. Hirose, I. Brouard, M. Xian,
K. F. Sundermann, S. J. Shaw, M. A. Burlingame, S. B. Horwitz and
D. Myles, Org. Lett., 2005, 7, 315; S. J. Shaw, K. F. Sundermann,
M. A. Burlingame, D. C. Myles, B. S. Freeze, M. Xian, I. Brouard and
A. B. Smith, III, J. Am. Chem. Soc., 2005, 127, 6532; J. M. Minguez,
S. Y. Kim, K. A. Giuliano, R. Balachandran, C. Madiraju, B. W. Day
and D. P. Curran, Bioorg. Med. Chem., 2003, 11, 3335; S. P.
Gunasekera, R. E. Longley and R. A. Isbrucker, J. Nat. Prod., 2002,
65, 1830.
In conclusion, we have designed and synthesised the most active
macrocyclic discodermolide/dictyostatin hybrid reported to
date.7,16 The encouraging antiproliferative activity of analogue 3
can be attributed to its constrained (dictyostatin-like) macrocyclic
structure, which bears a strong resemblance to the bioactive
conformation of discodermolide.15 In ongoing work, this rational
design approach is being extended to the synthesis of further novel
hybrid analogues of discodermolide and dictyostatin.
Financial support was provided by the EPSRC and
AstraZeneca. We thank Dr. Carmen Cuevas (PharmaMar,
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Chem. Commun., 2007, 49–51 | 51