Towards the improvement of the synthesis of novel 4(5)-aryl-5(4)- heteroaryl-2-thio-substituted imidazoles and their p38 MAP kinase inhibitory activity

Article abstract of DOI:10.1039/b717605h

A series of 2-alkylsulfanyl-4-(4-fluorophenyl)-5-(2-aminopyridin-4-yl)- substituted imidazoles was prepared and interaction possibilities of the 2-thioether moiety with phosphate/ribose binding pockets of p38 MAP kinase were investigated. Introduction of the alkyl/benzyl amino function at the pyridine moiety was carried out via nucleophilic substitution or via palladium catalyzed aryl-C-N-bond formation. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b717605h

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Towards the improvement of the synthesis of novel 4(5)-aryl-5(4)-heteroaryl-
2-thio-substituted imidazoles and their p38 MAP kinase inhibitory activity†
Stefan Laufer* and Pierre Koch
Received 14th November 2007, Accepted 11th December 2007
First published as an Advance Article on the web 21st December 2007
DOI: 10.1039/b717605h
A series of 2-alkylsulfanyl-4-(4-fluorophenyl)-5-(2-aminopyri-
din-4-yl)-substituted imidazoles was prepared and interaction
possibilities of the 2-thioether moiety with phosphate/ribose
binding pockets of p38 MAP kinase were investigated. In-
troduction of the alkyl/benzyl amino function at the pyridine
moiety was carried out via nucleophilic substitution or via
palladium catalyzed aryl-C–N-bond formation.
interact with the ribose pocket and/or phosphate binding region
of the enzyme. Furthermore, the introduction of a secondary
amino functionalization on the 2-position of the pyridinyl ring
was investigated.
Herein, we wish to report the structure–activity relationship of
a series of 2-alkylsulfanyl-4-(4-fluorophenyl)-5-(2-aminopyridin-
4-yl)-substituted imidazoles (10a–u) and a convenient, straightfor-
ward synthetic route to 4-aryl-5-heteroaryl-substituted imidazole-
1,3-dihydro-2-thiones (9a–h).
The 2-thioether moieties at the imidazole core, exemplified by
2,3-dihydroxypropyl (Fig. 2), has improved the p38 MAP kinase
inhibition, most likely by targeting the ribose pocket and/or
phosphate binding region of the ATP binding site of p38.
Introduction
p38 mitogen-activated protein (MAP) kinase, a serine/threonine
kinase, is required for the biosynthesis and release of the pro-
inflammatory cytokines IL-1 and TNF-a.1 p38 MAP kinases
are activated by infection or cellular stress such as ultraviolet
light, heat or osmotic shock.2 Inhibition of the p38 MAP kinase
could reduce the expression of these cytokines. Therefore this may
be a promising target for the treatment of many inflammatory
disorders such as rheumatoid arthritis and inflammatory bowel
disease. Pyridinyl imidazoles like the prototype inhibitor SB
203580 1 (Fig. 1) are well-known ATP competitive p38 MAP
kinase inhibitors.3,4
Fig. 2 Kinase inhibitor 10g.
Results and discussion
Synthesis
The first attempted synthetic approach to the title compounds
is outlined in Scheme 1. The starting point of the syn-
thesis is 4-(4-fluorophenyl)-5-(2-fluoropyridin-4-yl)-1,3-dihydro-
imidazol-2-thione (11), which was obtained from 2-fluoro-4-
methylpyridine according to a 4-step synthesis described in previ-
ous publications.5,6 Initial steps to 10 were nucleophilic aromatic
substitutions on the pyridinyl moiety with alkyl or benzyl amines
followed by alkylation of the sulfur atom. The critical point was the
isolation of the thiones 9 accompanied by a poor reproducibility
of the approach.
Fig. 1 Trisubstituted pyridinyl imidazoles.
We previously reported a series of trisubstituted imidazoles
that are structurally related to 1, notably 2-alkyl/benzylsulfanyl-4-
fluorophenyl-5-pyridinylimidazoles 2 as potent p38 MAP kinase
inhibitors (Fig. 1).5 These 2-thioimidazoles are superior compared
to the prototype SB-like 2-arylimidazoles because they have fewer
interactions with metabolic enzymes like CYP-450.5
In a continuous effort to develop improved p38 MAP kinase
inhibitors, we focused our attention on the optimization of
substitution on the pyridinyl and imidazole rings. The 2-thioether
moiety of 2 was investigated as a linker region for residues able to
Institute of Pharmacy, Department of Pharmaceutical and Medicinal Chem-
istry, Eberhard-Karls-University Tu¨bingen, Auf der Morgenstelle 8, 72076
Tu¨bingen, Germany. E-mail: stefan.laufer@uni-tuebingen.de; Fax: +49 7071
295037; Tel: +49 7071 2972459
† Electronic supplementary information (ESI) available: Representative
experimental procedures and analytical data. See DOI: 10.1039/b717605h
Scheme 1 Reagents and conditions: (i) R1–NH2 (10 equiv.), 150 ◦C.
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Starting from N-protected 2-(N-alkylamino)-4-methylpyridines
we developed a new synthetic strategy, leading to the key inter-
mediates 4-aryl-5-heteroaryl-substituted imidazole-1,3-dihydro-2-
thiones 9a–h (Schemes 2 and 3).
of Pd2(dba)3 (2 mol%), BINAP (4 mol%) and t-BuONa (1.4
equiv.) in toluene yielded the corresponding 2-(alkylamino)-4-
methylpyridines 5e–h. These compounds could be Boc-protected
without further purification. In some cases, especially when
introducing small aliphatic substituents, a disubstituted 2-(N,N-
dialkylamino)-4-methylpyridine was found as by-product. To react
4-aminotetrahydropyrano hydrochloride 2.4 equiv. of base was
used.
The straightforward synthesis of imidazole-2-thiones 9a–h from
picolines 5a–h is presented in Scheme 3. Picolines 5a–h were
deprotonated with sodium hexamethyldisilazane (NaHMDS) in
THF at 0 ◦C and reacted with ethyl 4-fluorobenzoate to afford
ethanones 6a–h. The reaction was quenched with aqueous NH4Cl
solution and purified by flash chromatography, thus allowing the
recovery of unreacted starting material 5. Upon treatment with
sodium nitrite (3.0 equiv.) in acetic acid at 10 ◦C ethanones
6a–h were converted into the a-hydroxyiminoketones 7a–h. The
oximes 7a–h were obtained as colourless foams and as a mixture
of isomers, indicated by proton NMR. In methanolic hydrogen
chloride, the oxime functionality was reduced in the presence
of Pd/C under a hydrogen atmosphere at room temperature
and atmospheric pressure to an amino group. This step was
accompanied by cleavage of the Boc-group as well as by transfer
of the amines into the corresponding amine hydrochlorides 8a–h.
The hydrochlorides 8a–h could be used in the next step without
further purification. Cyclisation to imidazole thiones was achieved
by heating 8a–h and potassium thiocyanate in DMF to reflux
temperature. After cooling to room temperature and treatment
with water, the imidazole-1,3-dihydro-2-thiones 9a–h precipitated
as yellowish solids in good yield and purity. Finally, the sulfur atom
was functionalized via nucleophilic substitution to compounds
10a–u (Scheme 4). Alkylation of 9a–h with various hydrophilic
alkyl halides proceeded upon treatment with sodium ethoxide or
potassium tert-butoxide in methanol.
Scheme 2 Synthesis of the 2-(N-Boc-N-alkyl/phenylalkylamino)-4-methyl-
pyridines 5a–h.
To protect the amino function, the Boc-protecting group was
chosen because of its stability under strongly basic conditions and
its facile cleavage under acidic conditions. The intermediate Boc-
protected 2-(N-alkyl/phenylalkylamino)-4-methylpyridines 5a–h
were synthesized by two different routes (Scheme 2). In the
case of benzylic and primary halides (5a–d) we started from 2-
(N-Boc-amino)-4-methylpyridine 3 via nucleophilic substitution
(route A). The carbamate 3 was prepared by reacting 2-amino-4-
methylpyridine and di-tert-butyl dicarbonate in tert-butanol. The
carbamate 3 was deprotonated using sodium hydride (1.25 equiv.)
in dry DMF and treated with the alkyl/benzyl halide (1.15 equiv.)
to afford 5a–d.7
For secondary alkyl halides this method failed or gave poor
yields. Therefore, route B was used for the synthesis of picolines
5e–h (Scheme 2). The aryl-C–N-bond formation was achieved
by a Buchwald–Hartwig reaction.8,9 Thus, heating 2-bromo-4-
methylpyridine (4) with alkyl amines (1.2 equiv.) in the presence
Biological studies
The biological test results (p3810 and TNF-arelease) show that the
prepared 4(5)-aryl-5(4)-heteroaryl-2-thio-substituted imidazoles
10a–u are highly potent inhibitors with IC50 values in the low
nanomolar range (Table 1). Compound 10g inhibits the target
enzyme in vitro 13 times more strongly than the prototype inhibitor
SB 203580. These results indicate that substituents at both the
Scheme 3 Synthesis of the key intermediates 5-(2-(alkyl/phenylamino)pyridin-4-yl)-4-(4-fluorophenyl)-1,3-dihydroimidazol-2-thiones 9a–h; reagents
and conditions: (i) NaHMDS, ethyl 4-fluorobenzoate, THF, 0 ◦C–rt; (ii) NaNO2, acetic acid, 10 ◦C–rt, 2.5 h; (iii) Pd/C 10%, MeOH–HCl, H2, rt, 16 h;
(iv) KSCN, DMF, reflux, 3 h; ayield over 3 steps.
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Fig. 3 Suggested binding mode for 10g. Asp168 is labelled with a white
D. After geometric optimization in the MMFF94 force field, the molecule
has been docked in the p38 active centre by using the docking program
FlexX. As the protein model, the X-ray structure 1bl7.pdb11 had been
used.
2-thiones 9a–h and their functionalization to potent p38 MAP
kinase inhibitors 10a–u.
Residues with polar groups at the 2-thioether moiety, which may
interact with the ribose and/or phosphate pocket of the enzyme,
are well tolerated.
Scheme 4 Synthesis of the target compounds 10a–u; reagents: (i) R2–X
(X = Cl, Br, I), t-BuOK or EtONa, MeOH.
Acknowledgements
Table 1 Evaluation of the prepared compounds 10a and 10g for p38
MAP kinase inhibitory activity and TNF-a release in human whole blood
The authors thank the financial support from the EU, part of the
Framework Project 6 “MACROCEPT”. We thank Dr D. Domeyer
for molecular modelling and S. Luik, K. Bauer and M. Go¨ttert
for the assistance in biological testing.
IC50 SEM/lM
IC50 SEM/lM
TNF-aa
Test compound
p38a
10a
0.049 0.009 (2)
0.003 0.001 (3)
0.038 0.011 (14)
0.936 0.21 (2)
0.126 0.0081 (3)
1.79 0.35 (14)
10g
Notes and references
SB 203580
a Number of experiments is given in brackets.
1 Z. Chen, T. B. Gibson, F. Robinson, L. Silvestro, G. Pearson, B. Xu,
A. Wright, C. Vanderbilt and M. H. Cobb, Chem. Rev., 2001, 101,
2449–2476.
2 S. Kumar, J. Boehm and J. C. Lee, Nat. Rev. Drug Discovery, 2003, 2,
2-thioether moiety and the 2-amino position of the pyridine have
effects on the potency of the inhibitory activity of the compounds.
In particular, substitution at the 2-position of the imidazole
core may allow hydrogen bonding with the ribose and phosphate
binding pockets of p38, thus enhancing inhibitory properties of
the compounds.
717–726.
3 G. Wagner and S. Laufer, Med. Res. Rev., 2006, 26, 1–62.
4 B. Kaminska, Biochim. Biophys. Acta, 2005, 1754, 253–262.
5 S. A. Laufer, G. K. Wagner, D. A. Kotschenreuther and W. Albrecht,
J. Med. Chem., 2003, 46, 3230–3244.
6 S. A. Laufer and A. J. Liedtke, Tetrahedron Lett., 2006, 47, 7199–
7203.
In Fig. 3, the suggested binding mode for 10g is reported. Note-
worthy is the interaction possibility of the 2,3-dihydroxypropyl
residue with the p38 conserved residues Asp168-Phe169-Gly170
(DFG), in particular the hydrogen bonding with Asp168.
7 D. M. Krein and T. L. Lowary, J. Org. Chem., 2002, 67, 4965–4967.
8 J. P. Wolfe, S. Wagaw, J. F. Marcoux and S. L. Buchwald, Acc. Chem.
Res., 1998, 31, 805–818.
9 K. C. Rupert, J. R. Henry, J. H. Dodd, S. A. Wadsworth, D. E. Cavender,
G. C. Olini, B. Fahmy and J. J. Siekierka, Bioorg. Med. Chem. Lett.,
2003, 13, 347–350.
10 S. Laufer, S. Thuma, C. Peifer, C. Greim, Y. Herweh, A. Albrecht and
F. Dehner, Anal. Biochem., 2005, 344, 135–137.
Conclusions
11 Z. Wang, B. J. Canagarajah, J. C. Boehm, S. Kassisa, M. H. Cobb, P. R.
Young, S. Abdel-Meguid, J. L. Adams and E. J. Goldsmith, Structure,
1998, 6, 1117–1128.
In summary, we report a straightforward access to the 2-
(alkyl/benzylamino)pyridinyl substituted imidazole-1,3-dihydro-
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