Design and synthesis by click triazole formation of paclitaxel mimics with simplified core and side-chain structures

Article abstract of DOI:10.1016/j.tetlet.2011.01.081

A library of paclitaxel (taxol) mimics was obtained by a straightforward strategy involving rational design and an efficient synthesis of a simplified taxane core substitute, together with a click-chemistry combinatorial search for phenylisoserine side-chain surrogates.

Full text of DOI:10.1016/j.tetlet.2011.01.081

Tetrahedron Letters 52 (2011) 1462–1465
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Design and synthesis by click triazole formation of paclitaxel mimics
with simplified core and side-chain structures
Claire Le Manach a, Aurélie Baron a, Régis Guillot a, Boris Vauzeilles a, , Jean-Marie Beau a,b,
⇑
⇑
a Univ Paris Sud and CNRS, Glycochimie Moléculaire et Macromoléculaire, Institut de Chimie Moléculaire et des Matériaux d’Orsay, F-91405 Orsay, France
b Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, Avenue de la Terrasse, F-91198 Gif-sur-Yvette, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A library of paclitaxel (taxol) mimics was obtained by a straightforward strategy involving rational design
and an efficient synthesis of a simplified taxane core substitute, together with a click-chemistry combi-
natorial search for phenylisoserine side-chain surrogates.
Received 16 December 2010
Revised 13 January 2011
Accepted 17 January 2011
Available online 21 January 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Taxoid mimic
Click chemistry
Sugar scaffold
L-glucuronolactone
Tubulin depolymerization
The tubulin/microtubule system plays a key role during mitosis
and disturbing its dynamic equilibrium can prevent cell division
and induce apoptosis. Up to now, most of the known microtu-
bule-stabilizing antitubulin agents, such as paclitaxel (taxol), dis-
codermolide, or epothilones, are characterized by a very complex
structure, and are therefore difficult to synthesize, especially on a
large scale. The structure–activity relationship for taxoids has been
extensively studied and the importance of the C-13 side-chain and
the ester group at C-2 have been highlighted.1,2 Moreover, studies
have been carried out in order to determine the bioactive confor-
mation of the taxoids, namely the conformation they adopt when
binding to and stabilizing microtubules.3–5 Using the 3.7 Å struc-
molecular modeling, we propose here the synthesis of a small
library of taxoid analogues with a simplified core structure suppos-
edly mimicking the taxane skeleton. Moreover, we show that click-
chemistry can efficiently be used to rapidly introduce simple
potential side-chain surrogates.
In order to properly define the relative orientation of the two
exit vectors corresponding to the anchoring of the phenylisoserine
and the benzoate side-chains onto the taxane core, we measured
the interatomic distance between C-13 and C-2 carbons, the dis-
tance between the two corresponding oxygens O-13 and O-2, as
well as the dihedral angle between the C-13–O-13 and the C-2–
O-2 bonds (Fig. 2) for T-shaped taxol,5,7 1JFF taxol,4 and docetaxel
(taxotere)8 in the crystal. These values showed a very good consen-
sus on the relative orientation of these two vectors. Searching for
simple structures that could present similar geometrical orienta-
ture of the a,b-tubulin–taxol complex obtained by electron crystal-
lography of zinc-induced sheets3 and electron crystallographic
density, a ‘T-shaped’ taxol was proposed as the bioactive confor-
mation on b-tubulin.5 Numerous analogues of taxol have been syn-
thesized which possess the complex taxane skeleton. Until now, a
few structures exhibiting a simpler core have been elaborated,6
some of which show interesting biological activity.6c In these at-
tempts, the constructs have, however, retained the phenylisoserine
structure (or close analogues) of the side-chain at C-13 as an
important recognition unit, without (structures B–E) or with
(structures F–I) a mimic of the benzoate at C-2 (Fig. 1). Using geo-
metrical parameters from the tetracyclic taxane skeleton and
tions, we found that bicyclic b-L-glucurono-c-lactone J could be a
good mimic for the taxane core in this respect.9,10 Molecular mod-
eling using MacroModel [MM3⁄, GB/SA water, Systematic Pseudo
Monte Carlo (SPMC) torsional sampling] showed a good correspon-
dence for these three parameters with the previously determined
taxane data (Table 1 and Fig. 2b), hence the synthesis of such deriv-
atives was selected. With further structural simplifications in
mind, we chose to conserve the benzoate group, which has been
shown to be essential for activity,1,2 and to replace the taxol
side-chain by diverse aromatic motifs introduced by click triazole
formation (Scheme 2). We decided to focus on side-chain surro-
gates bearing aromatic groups, which bring attractive interaction
with the corresponding hydrophobic pocket since the taxol side-
chain is known to make such hydrophobic interactions. Both aryl
⇑
Corresponding authors. Tel.: +33 (0)1 69 15 68 36; fax: +33 (0)1 69 15 47 15
(B.V.); tel.: +33 (0)1 69 15 79 60; fax: +33 (0)1 69 85 37 15 (J.-M.B.).
E-mail addresses: boris.vauzeilles@u-psud.fr (B. Vauzeilles), jean-marie.beau@
u-psud.fr (J.-M. Beau).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.081

C. L. Manach et al. / Tetrahedron Letters 52 (2011) 1462–1465
1463
HO
OH
(a) PhCHO, cat. HCl
(b) NaIO4
OH
H
O
O
BzHN
O
O
OH
O
O
O
OH
OH
O
OH
13
(c) TFA, H2O
O
HO
OH
H
2
HO
2
OH
phenylisoserine side-chain
1
HO
(d) Me2CO,
cat. H2SO4
BzO
O
Ac
A Paclitaxel (taxol)
O
O
H
H
O
O
O
(e) BzCl
O
O
O
O
NHBocO
OH
O
O
H
3
H
4
HO
BzO
Ph
O
Ph
O
O
HO
(f)
,
O
B
OH
Ph
NHBz
O
cat. H2SO4
OH
C
O
H
H
O
N
N
NH
O
O
O
N
OH
O
O
O
O
BzO
NH
MTr
O
5
O
O
NH
O
Scheme 1. Reagents and conditions: (a) PhCHO, cat. HCl, 20 °C, 2 h; (b) NaIO4, THF/
H2O, 40 °C, 1 h; (c) TFA, H2O, 20–45 °C, 1 h (88%, three steps); (d) acetone, cat.
H2SO4, 20 °C, 3 h (80%); (e) BzCl, DMAP, CH2Cl2, 20 °C, 3 h (80%); (f) propargyl
alcohol, cat. H2SO4, 80 °C, 1 h (70%).
O
O
N
O
O
D
E
OH
O
O
CO2Bn
H
O
N
H
N3
Ar
6-12
or
R1
O
OH
OH
H
H
H
H
O
N
N
O
(a)
O
N
R
O
O
O
O
R1
O
O
O
O
O
H
Ar
13-15
N3
BzO
BzO
F
R2
R2
G
R2
5
O
16-22 R = Ar
23-25 R = CH2Ar
O
O
O
R1
O
Scheme 2. Reagents and conditions: (a) CuSO4 (0.05 equiv), sodium ascorbate
(0.1 equiv), H2O/CH2Cl2 (1:1), 20 °C overnight, (57–92%).
O
R1
O
O
H
I
R2
F-I: R1 = phenylisoserine side-chain or analogue, R2 = Ph or m-OMePh
overall yield, using the large-scale optimization recently published
by Fleet11,12 from a strategy initially described by Sowa.13 Subse-
quent protection of the two contiguous hydroxy groups in the form
of an isopropylidene (3, 70%), followed by benzoylation of the
remaining hydroxy readily led to intermediate 4 (80%). Sulfuric
acid-catalyzed glycosylation of 4 in propargyl alcohol gave the de-
sired propargyl glycoside 5.14
Product 5 crystallized from ethyl acetate/cyclohexane, and the
stereoselectivity of this last glycosylation step, hence the required
b-configuration of the resulting product, was unambiguously as-
sessed by X-ray crystallographic studies (Fig. 3).15
Compound 5 was then engaged into copper-accelerated azide-
alkyne cycloadditions (CuAAC)16 with a series of substituted aryl
(6–12)17 or benzyl (13–15)18 azides which were prepared accord-
ing to literature procedures.19,20 The ten desired triazoles (16–25,
Fig. 4) were obtained using Soo-Kim reaction conditions,21 using
a biphasic dichoromethane/water system (Scheme 2) in yields
ranging from 57% to 92%.22
Figure 1. The structure of paclitaxel (taxol) A, as well as some literature examples
of taxoid mimics with simpler structures replacing the taxane core.
Table 1
Selected geometrical parameters for T-taxol, 1JFF taxol, taxotere, and b-L-glucurono-
c
-lactone J
C13–C2 (tax.)/
C1–C5 (lact.) (Å)
O13–O2 (tax.)/
O1–O5 (lact.) (Å)
Dihedral
anglea (deg.)
T-taxol
1JFF taxol
3.70
3.74
3.68
3.62
3.63
4.86
4.89
4.93
4.89
4.92
ꢀ47.75
ꢀ44.07
ꢀ45.94
ꢀ51.59
ꢀ52.91
Taxotere
Lactone J (aver.)b
Lactone J (min.)c
a
C13–O13/C2–O2 dihedral angle for taxoids, C1–O1/C5–O5 for lactone J.
Average values over the 20 conformations obtained by SPMC (all within
b
5.0 kcal molꢀ1 above global minimum).
None of these triazoles showed an appreciable antitubulin
activity (tubulin depolymerization assay), although nitro phenol
derivative 19 moderately inhibited the tubulin depolymerization
c
Values for the lowest energy conformation obtained by SPMC and used in
Fig. 2b.
(IC50 of 90 l
M).23
In summary, we have developed a very simple and efficient
synthetic route to possible taxol substitutes by a stereoselective
and benzyl groups were selected to probe the influence of side-
chain flexibility.
Since the core we selected is a derivative of non-natural L-glu-
cose, we had to elaborate this structure from another synthetic
b-glycosylation of L-glucurono-c-lactone followed by a click cyclo-
addition of aromatic structures, providing fast access to a small
library of compounds. The molecules obtained in this work will
be further studied for their biological activity, including potential
inhibition of tubulin depolymerization and possible cytotoxic
properties. This oversimplified taxoid core mimic certainly does
not contain some of the key taxoid–tubulin interactions for
precursor. The propargylic derivative 5 was therefore prepared
from commercially available
D-glucoheptonolactone 1 in six steps
(Scheme 1). The first three steps provided
L
-glucurone 2 in a 60%

1464
C. L. Manach et al. / Tetrahedron Letters 52 (2011) 1462–1465
Figure 2. (a) Schematic representation of the overlay of b-L-glucurono-c
-lactone J with the taxane core of taxol A. (b) Overlay of the MM3⁄ conformation of J (white sticks)
with T-taxol, 1JFF-taxol, and taxotere (dark lines). Hydrogens have been omitted for clarity.
X
OH
H
H
N
N
O
N
O
O
R =
O
X = N3 (6)
X = R (16, 57%)
BzO
O N
2
MeO
X
X
O N
X
2
HO
MeO
F
OMe
X = N3 (9)
X = N3 (7)
X = N3 (8)
X = R (19, 77%)
X = R (17, 77%) X = R (18, 87%)
X
X
O N
2
X
NO
F
2
X = N3 (10)
X = N3 (11)
X = N3 (12)
X = R (20, 82%)
X = R (21, 75%) X = R (22, 92%)
X
X
X
O
Ts
Me
O
Figure 3. X-ray structure of glycoside 5, confirming anomeric configuration.
X = N3 (13)
X = N3 (14)
X = N3 (15)
X = R (23, 60%)
X = R (24, 87%) X = R (25, 57%)
Figure 4. Structure of azido precursors 6–15 and triazole derivatives 16–25 with
yields for the cycloaddition reaction.
efficient tubulin binding activity (e.g., C and D rings). In future
work, the elaboration of closer mimics carrying the phenylisoserine
side-chain of taxol as b-glycosyl esters of the L-glucurono-c-lactone
scaffold will notably be elaborated and the results of these studies
will be reported in due course.

C. L. Manach et al. / Tetrahedron Letters 52 (2011) 1462–1465
1465
dried over Na2SO4, and concentrated in vacuo. Filtration on silica gel (DCM/
MeOH 95:5) gave 590 mg (80%) of 4 as a colorless syrup. H2SO4 (57 L) was
Acknowledgment
l
added dropwise to a solution of 4 (133 mg) in 1.7 mL of propargyl alcohol. The
solution was stirred at 80 °C for 1 h. The mixture was neutralized with
saturated aqueous NaHCO3. After dilution with DCM, the organic layer was
washed with NaHCO3 and brine, dried over Na2SO4 and concentrated in vacuo.
Flash chromatography on silica gel (DCM/MeOH 45:1) gave 91.4 mg (69%) of 5
as a brown syrup. Selected analytical data for 5: mp: 129–130 °C (AcOEt/Cy);
We gratefully acknowledge the Institut de Chimie des
Substances Naturelles for financial support of this study.
References and notes
½
a 2D5
ꢁ
= ꢀ14.0 (c 1, CHCl3); 1H NMR (360 MHz, CDCl3), d (ppm): 8.11 (dd, 2H;
4
4
Jo-m = 7.9, Jo-p = 1.4; 2H-o (Bz)); 7.61 (tt, 1H; Jp-m = 7.5; Jp-o = 1.4; H-p (Bz));
1. Guénard, D.; Guéritte-Voegelein, F.; Potier, P. Acc. Chem. Res. 1993, 26, 160–
167; For reviews see: Ojima, I.; Kuduk, S.; Chakravarty, S. Adv. Med. Chem. 1999,
4, 69–124; Nicolaou, K. C.; Dai, W.-M.; Guy, R. K. Angew. Chem., Int. Ed. 2003, 33,
15–44; Kingston, D. G. I. Taxol and Its Analogues. In Anticancer Agents from
Natural Products; Cragg, G. M., Kingston, D. G. I., Newman, D. J., Eds.; CRC Press:
Boca Raton, FL, 2005; pp 89–122.
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Acad. Sci. U.S.A. 2001, 98, 5312–5316.
4
7.47 (ddt, 2H; Jo-m = 7.9, Jm-p = 7.5, Jm-m = 1.4;
2 H-m (Bz)); 5.44 (d, 1H;
J4-5 = 6.9; H-5); 5.33 (dd, 1H; J3-4 = 4.9, J4-5 = 6.9; H-4); 5.30 (s, 1H; H-1); 4.99
(d, 1H; J3-4 = 4.9; H-3); 4.51 (s, 1H; H-2); 4.27 (dd, 1H; Jgem = 15.7, 4J = 2.3;
OCHaHb); 4.20 (dd, 1H; Jgem = 15.7, 4J = 2.3; OCHaHb); 2.40 (t, 1H; 4J = 2.3;
CCH); 2.27 (br s, 1H, OH). 13C NMR (90 MHz, CDCl3), d (ppm): 170.4 C-6; 165.4
CO (Bz); 134.2 C-p (Bz); 130.3 2C-o (Bz); 128.9 2 C-m (Bz); 128.5 C-q (Bz);
106.4 C-1; 83.9 C-3; 78.3 CCH; 78.1 C-2; 76.3 C-4; 75.5 CCH; 69.9 C-5; 54.7
OCH2. MS (ESI+): m/z = 341.1 M+Na+; HRMS (ESI+): calcd for C16H14O7Na:
341.0632, found: 341.0644.
15. CCDC 804513 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
16. Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064;
Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed.
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J.; Kenny, P.; McDonnell, S.; O’Connor, A. Bioorg. Med. Chem. Lett. 2003, 13,
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Med. Chem. Lett. 2004, 14, 3491–3494; (e) Almqvist, F.; Manner, S.; Thornqvist,
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Roussi, F.; Ngo, Q. A.; Thoret, S.; Guéritte, F.; Guénard, D. Eur. J. Org. Chem. 2005,
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Snyder, J. P.; Kingston, D. G. Bioorg. Med. Chem. 2006, 14, 3447–3454; (h)
Zefirova, O. N.; Nurieva, E. V.; Lemcke, H.; Ivanov, A. A.; Shishov, D. V.; Weiss, D.
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7. The coordinates are from Alcaraz, A. A.; Mehta, A. K.; Johnson, S. A.; Snyder, J. P.
J. Med. Chem. 2006, 49, 2478–2488.
8. Gueritte-Voegelein, F.; Guénard, D.; Mangatal, L.; Potier, P.; Guilhem, J.;
Cesario, M.; Pascard, C. Acta Crystallogr., Sect. C 1990, 46, 781–784.
9. For reviews on the use of carbohydrate-derived scaffolds in medicinal
chemistry, see: Meutermans, W.; Le, G. T.; Becker, B. ChemMedChem 2006, 1,
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Shaikh, N.; Russo, L. J. Biotechnol. 2009, 144, 234–241.
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2303–2304; Peri, F.; Bassetti, R.; Caneva, E.; de Gioia, L.; La Ferla, B.; Presta, M.;
Tanghetti, E.; Nicotra, F. J. Chem. Soc., Perkin Trans. 1 2002, 638–644; Peri, F.;
Airoldi, C.; Colombo, S.; Martegani, E.; van Neuren, A. S.; Stein, M.; Marinzi, C.;
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Gennari, C.; Nicotra, F. Bioog. Med. Chem. 2006, 14, 3349–3367.
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Pino-González, M.-S.; Fleet, G. W. J. Tetrahedron Lett. 2009, 50, 6307–6310.
12. The preparation of bicyclic lactone 3 was previously reported in the PhD thesis
of CLM defended on October 17, 2008 at the University of Paris Sud, Orsay.
13. Sowa, W. Can. J. Chem. 1969, 47, 3931–3934.
17. Aryl azides 6–12 were prepared from the corresponding anilines, via the
diazonium salt.19
18. Benzyl azides 14 and 15 were prepared from p-hydroxy and p-methoxylbenzyl
alcohol, via tosylation in situ and nucleophilic substitution with azide anion.20
19. Molteni, G.; Del Buttero, P. Tetrahedron: Asymmetry 2007, 18, 1197–1201.
20. Soltani Rad, M. N.; Behrouz, S.; Khalafi-Nezhad, A. Tetrahedron Lett. 2007, 48,
3445–3449.
21. Lee, B.-Y.; Park, S. R.; Jeon, H. B.; Soo-Kim, K. Tetrahedron Lett. 2006, 47, 5105–
5109.
22. Representative procedure for the synthesis of 22 using click-chemistry.
Lactone 5 (32 mg, 0.10 mmol, 1.0 equiv) and 4-fluorophenyl azide (15 mg,
0.11 mmol, 1.1 equiv) were dissolved in methylene chloride (300
water (210 L). Solutions of sodium ascorbate (40 L, 0.25 M in water, 2.0 mg,
L, 0.10 M in water, 1.2 mg,
lL) and
l
l
0.01 mmol, 0.1 equiv) and CuSO4ꢂ5H2O (50
l
0.005 mmol, 0.05 equiv) were added, and the mixture was stirred overnight
at room temperature. After dilution with methylene chloride, the organic layer
was washed with water, dried over sodium sulfate, and concentrated in vacuo.
After chromatography on silica gel (cyclohexane/ethyl acetate 1:1), the pure
desired products was obtained (42 mg, 92%). Selected analytical data for 22: 1
H
NMR (300 MHz, CDCl3),
d (ppm): 8.04 (s, 1H; H-triazole); 7.95 (dd, 2H;
Jo-m = 8.1, 4Jo-p = 1.4; 2H-o (Bz)); 7.66 (dd, 2H; Jo-m = 9.1, 4Jo-F = 4.6; 2 H-o (Ar));
4
7.52 (tt, 1H; Jp-m = 7.4, Jp-o = 1.2; H-p (Bz)); 7.36 (dd, 2H; Jm-o = 8.1, Jm-p = 7.4;
3
2H-m (Bz)); 7.14 (dd, 2H; Jm-o = 9.1, Jm-F = 8.0; 2H-m (Ar)); 5.58 (d, 1H;
J4-5 = 6.8; H-5); 5.36 (dd, 1H; J4-5 = 6.8, J3-4 = 4.9; H-4); 5.24 (s, 1H; H-1); 5.07
2
(d, 1H; J3-4 = 4.9; H-3); 4.95 (d, 1H; JHa-Hb = 12.0; OCHaHb); 4.61 (d, 1H;
2JHa-Hb = 12.0; OCHaHb); 4.50 (s, 1H; H-2); 13C NMR (75 MHz, CDCl3), d (ppm):
1
171.7 C-6; 165.3 CO (Bz); 162.7 (d, JC-F = 244) Cq-F (Ar); 144.8 Cq (triazole);
4
134.2 CH-p (Bz); 133.2 (d, JC-3F = 3) Cq (Ar); 130.0 2CH-o (Bz); 128.9 2CH-m
(Bz); 128.3 Cq (Bz); 122.7 (d; JC-F = 9) 2CH-o (Ar); 122.0 CH (triazole); 116.9
2
(d; JC-F = 23) 2CH-m (Ar); 108.5 CH-1; 84.5 CH-3; 77.6 CH-2; 76.5 CH-4; 70.2
CH-5; 61.3 OCH2; MS (ESI+): m/z = 455.9 M+H+; 477.9 M+Na+; HRMS (ESI+):
calcd for
22H18FN3O7, 2H2O: C, 53.77; H, 4.51; N, 8.55. Found: C, 54.03; H, 4.11; N, 8.09.
23. In the standard in vitro tubulin depolymerization assay used, taxol showed an
IC50 of 0.5 M under the same conditions (IC50 is the concentration that
C22H20FN3O7: 456.1202; found: 456.1202. Anal. Calcd for
14. 1,2-O-Isopropylidene-
in 23 mL of freshly distilled methylene chloride (DCM). N,N-Dimethyl-
aminopyridine (535 mg) was added, followed by 377 L of benzoyl chloride,
a-L-glucurono-3,6-lactone (500 mg) 3 was dissolved
C
l
l
at 0 °C. After stirring 5 min, the temperature was allowed to reach room
temperature. After 3 h stirring, control by thin layer chromatography showed
no trace of starting compound, and the reaction was quenched by addition of
saturated aqueous NaHCO3. The organic layer was then washed with brine,
inhibits 50% of the rate of microtubule disassembly). These tests were
preformed by Sylviane Thoret at the Institut de Chimie des Substances
Naturelles, CNRS.