Synthesis of a rhazinilam analogue acting as an inhibitor of tubulin assembly

Article abstract of DOI:10.1016/S0040-4039(00)00983-7

The synthesis of a di-nor-secorhazinilam analogue is described. This phenylpyrrole compound has been prepared by a Susuki cross-coupling reaction between 1,2,5 trisubstituted pyrrole halides and 2-N-(tert- butoxycarbonyl)aminophenyl boronic acid. In contrast to rhazinilam, this new phenylpyrrole inhibits the assembly of tubulin into microtubules. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00983-7

Tetrahedron Letters 41 (2000) 5853±5856
Synthesis of a rhazinilam analogue acting as an inhibitor of
tubulin assembly
Christophe Dupont, Daniel Guenard, Claude Thal, Sylviane Thoret
and Francoise Gueritte*
Institut de Chimie des Substances Naturelles, Centre National de la Recherche Scienti®que,
Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
Received 5 May 2000; accepted 9 June 2000
Abstract
The synthesis of a di-nor-secorhazinilam analogue is described. This phenylpyrrole compound has been
prepared by a Susuki cross-coupling reaction between 1,2,5 trisubstituted pyrrole halides and 2-N-(tert-
butoxycarbonyl)aminophenyl boronic acid. In contrast to rhazinilam, this new phenylpyrrole inhibits the
assembly of tubulin into microtubules. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: rhazinilam; phenylpyrroles; coupling reactions; biological activity tubulin.
(^)-Rhazinilam 1 is a natural tetracyclic compound possessing a phenylpyrrole subunit and a
nine-membered lactam ring.1 It was found to act as an inhibitor of microtubules disassembly.2 As
part of a program3 directed towards the synthesis of biaryls mimicking the structure of the anti-
mitotic (^)-rhazinilam 1, we previously reported the synthesis of various ortho-bridged biphenyls
(compound 2).4,5 We showed that the replacement of the lactam (compound 2a) by an urethane
function (compound 2b) increases the binding with tubulin and that the binding interaction is
stereoselective.5 Moreover, C-9-dialkylated lactam 36 was found to be more active than com-
pound 4,7 indicating that the presence of a quaternary carbon at position 9 greatly in¯uences the
biological activity.8 In continuation of this study, we report herein the synthesis of a di-nor-
secorhazinilam analogue 5 possessing a seven-membered lactam ring and an ortho-disubstituted
phenyl±pyrrole subunit.9
*Corresponding author. Fax: 33 (0) 1 69 07 72 47; e-mail: gueritte@icsn.cnrs-gif.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00983-7

5854
Following our synthesis of biphenyls 2,4,5 the approach to produce compound 5 was based on a
Suzuki cross-coupling reaction10 of ortho-substituted 3-halopyrroles 9 (X=Br or I) and 10
(X=Br or I) with a protected aniline derivative 11.11 1,2,3,5-Tetrasubstituted pyrroles 9 and 10
were obtained from commercial 1-methyl-2-pyrroleacetonitrile 6 (Scheme 1). Due to the necessity
of further transformations, pyrrole 6 was ®rst protected at carbon 5 with a trichloroacetyl group.
Pyrrole 7 was then converted into bromide 8a or iodide 8b. Treatment of the halide 8 with sodium
methoxide led to compounds 9a or 9b which were dialkylated with lithium diisopropylamide and
ethyl iodide to give pyrroles 10a and 10b.
Scheme 1. (a) ClCOCCl3, Et2O, 0ꢀC (7 98%); (b) NBS or NIS, (CH3)2CO, rt (8a 98% or 8b 99%); (c) MeONa,
MeOH, 0ꢀC (9a or 9b 98%); (d) LDA, ^78ꢀC, EtI (^78±>0ꢀC) (10a or 10b 54%)
The Suzuki cross-coupling reaction was attempted on compounds 9 (X=Br or I) and 10
(X=Br or I) with 2-N-(tert-butoxycarbonyl)aminophenyl boronic acid 11 (Table 1). The coupling
reactions were performed with benzyl[bis(triphenylphosphine)]palladium(II) chloride (PdBnCl
(PPh3)2). The coupling between arylboronic acid 11 and pyrrole bromide 9a in a DME±water
solution in the presence of sodium carbonate (entry 1) gave the corresponding 3-phenylpyrrole
derivative 12a in a modest yield (35%). The same reaction applied to the pyrrole iodide derivative
9b was more ecient and led to 12a in 80% yield (entry 2a). When the reaction was performed in
Table 1
Coupling of pyrroles 9 or 10 with phenyl boronic acid 11

5855
toluene (entry 2b), the yield of the coupling reaction decreased to 50%. Dialkylation of phenyl-
pyrrole 12a with LDA and ethyl iodide led to diethyl phenylpyrrole 12b in 44% yield.
We then turned our attention to the cross-coupling reactions between phenyl boronic acid 11
and the more hindered pyrrole halides 10a and 10b. The coupling of bromide 10a with phenyl-
boronic acid 11 in the presence of sodium carbonate in DME±H2O led to the expected product
12b (entry 3a, 30%). When potassium phosphate was used as the base, the coupling product was
isolated with 48% yield (entry 3b). A stronger base such as barium hydroxide proved inecient
to perform the coupling reaction. In the case of the dialkylated pyrroles, no improvement was
obtained when the pyrrole iodide was used instead of the bromide (entry 4). In all performed
experiments, the biphenylic homocoupling product as well as the dehalogenated pyrrole derivative
were isolated as secondary products. It should be noted that the synthesis of 2-formyl-3-aryl-
pyrroles has been recently described.12 In particular, the coupling of 2-nitrophenylboronic acid
with 2-formyl-3-iodo-1-tosylpyrrole is mentioned giving excellent yield of the coupling product
when Pd(dppf)2Cl2 was used as the catalyst. In our case, the use of Pd(dppf)2Cl2 as catalyst did
not improve the yield of the coupling product.
Basic hydrolysis of compound 12b followed by treatment of the mixture with a 10% aqueous
solution of hydrochloric acid gave directly the primary amide 13. Finally, compound 13 was
converted to the desired di-nor-secorhazinilam analogue 513 after lactamization (Scheme 2).
Scheme 2. (a) 50% aq NaOH, MeOH, rfx, 3 h then 10% aq. HCl, rt 15 min (13 100%); (b) conc H2SO4, rfx, 10 min (5
100%)
The e€ect of phenylpyrrole 5 was ®rst evaluated on the disassembly of microtubules;14 5 was
found inactive. In contrast, compound (^)-36 which possesses a nine-membered ring, inhibits
the cold disassembly of microtubules.7 This con®rms that the size of the lactam ring, and
consequently the dihedral angle of the biaryl moiety, are playing an essential role in the inhibition
of cold microtubules disassembly. Because many biaryl-type molecules are inhibitors of tubulin
assembly,15 the activity of compound 5 was also evaluated on the assembly of tubulin into
microtubules. Interestingly, we found that phenylpyrrole 5 acts as an inhibitor of tubulin assem-
bly with an IC50 value of 2.7Â10^5 M. This compound was also found to be cytotoxic towards
tumor KB cell lines with an IC50 value of 7Â10^6 M. Although compound 5 is less active than
colchicinoids, this result opens the door to new chemical and biological investigations on this type
of molecule.
Acknowledgements
We are very grateful to Professor P. Potier for his continuous interest and encouragement. We
also thank C. Gaspard for conducting the cytotoxicity assays and RPR as well as CNRS for a
grant to one of us (CD).

5856
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13. Compound 5 was obtained as a white amorphous solid. H NMR (250 MHz, CDCl3) ꢀ 0.86 (6H, 2t, J=7.3 Hz),
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(2H, m), 7.55 (1H, dd, J=7.5 and 1.5 Hz), 7.77 (1H, bs). 13C NMR (62.5 MHz, CDCl3) ꢀ 9.4, 23.0, 39.5, 53.1,
106.8, 119.3, 121.7, 123.9, 126.2, 126.7, 127.3, 127.9, 129.0, 134.3, 171.3. IR (CHCl3) ꢁ 3334, 3004, 1649, 1513,
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