An optimized and versatile synthesis to pyridinylimidazole-type p38α mitogen activated protein kinase inhibitors

Article abstract of DOI:10.1039/c5ob01505g

An optimized strategy for the synthesis of the potent p38α mitogen-activated protein kinase inhibitors 2-(2-hydroxyethylsulfanyl)-4-(4-fluorophenyl)-5-(2-aminopyridin-4-yl)imidazole (3) and 2-(2,3-dihydroxypropylsulfanyl)-4-(4-fluorophenyl)-5-(2-aminopyridin-4-yl)imidazole (4) starting from 2-fluoro-4-methylpyridine is reported. In contrast to a previously published synthesis starting from 2-bromo-4-methylpyridine, the overall yield could be increased from 3.6% to 29.4%. Moreover, this strategy avoids the use of palladium as a catalyst and is more diverse and versatile. Using this optimized protocol, both enantiomers of potent inhibitor 3 were synthesized. Biological data demonstrated that the (S)-enantiomer is the two times more potent eutomer.

Full text of DOI:10.1039/c5ob01505g

Organic &
Biomolecular Chemistry
View Article Online
View Journal
PAPER
An optimized and versatile synthesis to
pyridinylimidazole-type p38α mitogen activated
protein kinase inhibitors†
Cite this: DOI: 10.1039/c5ob01505g
Ahmed El-Gokha,a,b Stefan A. Lauferc and Pierre Koch*a
An optimized strategy for the synthesis of the potent p38α mitogen-activated protein kinase inhibitors
2-(2-hydroxyethylsulfanyl)-4-(4-fluorophenyl)-5-(2-aminopyridin-4-yl)imidazole
dihydroxypropylsulfanyl)-4-(4-fluorophenyl)-5-(2-aminopyridin-4-yl)imidazole
(3)
(4)
and
2-(2,3-
from
starting
2-fluoro-4-methylpyridine is reported. In contrast to a previously published synthesis starting from
2-bromo-4-methylpyridine, the overall yield could be increased from 3.6% to 29.4%. Moreover, this strat-
egy avoids the use of palladium as a catalyst and is more diverse and versatile. Using this optimized proto-
col, both enantiomers of potent inhibitor 3 were synthesized. Biological data demonstrated that the (S)-
enantiomer is the two times more potent eutomer.
Received 21st July 2015,
Accepted 3rd September 2015
DOI: 10.1039/c5ob01505g
www.rsc.org/obc
Introduction
The p38α mitogen-activated protein (MAP) kinase is a serine/
threonine kinase, which links extracellular signals to the intra-
cellular machinery modulating a plethora of cellular processes,
e.g., the release of pro-inflammatory cytokines like tumor
necrosis factor-α (TNF-α) and interleukin-1β.1 Therefore, the
inhibition of p38α MAP kinase was evaluated as a therapeutic
strategy for the treatment of cytokine-driven diseases like rheu-
matoid arthritis or psoriasis. Other studies suggest an impor-
tant role of p38α MAP kinase in the pathogenesis of
neurodegenerative diseases like Parkinson’s disease, Alzhei-
mer’s disease or multiple sclerosis.2–4 Recently, several clinical
trials investigating inhibitors of p38α MAP kinase for the treat-
ment of chronic obstructive pulmonary disease were termi-
nated.5,6 Trisubstituted pyridinylimidazoles, like the
prototypical inhibitor SB203580 (1) or ML3403 (2), are
amongst the most prominent adenosine triphosphate (ATP)
competitive inhibitors of p38α MAP kinase (Fig. 1).7–9
In 2008, we reported 2-(2-hydroxyethylsulfanyl)-4-(4-fluoro-
phenyl)-5-(2-aminopyridin-4-yl)imidazole (3) and 2-(2,3-
aInstitute of Pharmaceutical Sciences, Department of Medicinal and Pharmaceutical
Chemistry, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 8, 72076
Tübingen, Germany. E-mail: pierre.koch@uni-tuebingen.de
bMenofia University, Chemistry Department, Faculty of Science, Menofia, Egypt
cInstitute of Pharmaceutical Sciences, Chair for Medicinal and Pharmaceutical
Chemistry, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 8, 72076
Fig. 1 Pyridinylimidazole based p38α MAP kinase inhibitors.
Tübingen, Germany
dihydroxypropylsulfanyl)-4-(4-fluorophenyl)-5-(2-aminopyridin-
4-yl)imidazole (4) as optimized p38α MAP kinase inhibitors
derived from ML3403 (Fig. 1).10 The hydroxyl containing moie-
†Electronic supplementary information (ESI) available: Schemes S1–S7, experi-
mental procedures and analytical data, copies of 1H NMR and 13C NMR. See
DOI: 10.1039/c5ob01505g
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
ties at the imidazole C2-position may interact with the ribose
The major limitations of this synthetic strategy for scale-up
and the phosphate binding site of the enzyme. In contrast to the are the introduction of the amino moiety in the first step of
parent compound ML3403, compounds 3 and 4 show improved the synthesis, which requires the addition and removal of a
p38α MAP kinase inhibitory activity and a strong increase in protecting group and its low overall yield (>4%). Moreover, the
inhibition of LPS-stimulated TNF-α release from human whole required early stage use of the palladium catalyst is quite cost
blood (in case of 3, >110-fold increase compared to 2). In order intensive due to the higher amount needed in scale-up.
to profile these inhibitors in further in vitro and in vivo experi-
Another attempted synthetic strategy to the target com-
ments, a high yielding and scalable synthesis is required.
pounds 3 and 4 reported from our group is depicted in
The previously reported synthetic route toward inhibitors 3 Scheme 2.12 The introduction of the 3-methyl-2-butylamino
and 4 starts from 2-bromo-4-methylpyridine (5) and is depicted moiety at the pyridine C2-position as well as the moiety at the
in Scheme 1.10,11 In the first step of this synthesis, the amino imidazole C2–S-position was envisaged in the last two steps of
moiety was introduced at the pyridine C2-position via Buch- the synthesis. The key compound for this route 4-(4-fluoro-
wald–Hartwig arylamination and the obtained secondary phenyl)-5-(2-fluoropyridin-4-yl)-1,3-dihydroimidazole-2-thione (12)
amine was directly protected with the Boc-group. Picoline 6 was prepared according to a protocol from Laufer and co-
was converted into the ethanone 7 by reaction with ethyl workers8,13 in four steps starting from 2-fluoro-4-methyl-
4-fluorobenzoate using NaHMDS as a base. Upon treatment pyridine (11) in an overall yield of 57% (for details, see
with an excess of sodium nitrite in acetic acid, ethanone 7 was Scheme S1 in the ESI†). The alkylsulfanyl moiety was intro-
converted into the α-hydroxyiminoketone 8. Reduction of duced by nucleophilic substitution of thione 12 and the appro-
oxime
8
to
a
corresponding amine hydrochloride
9
priate alkyl halide. In the last step, the fluorine atom in 13a
accompanied by deprotection of the Boc-group was achieved and 13b should be displaced by 3-methyl-2-butylamine (14) to
by hydrogenation in methanolic hydrogen chloride using Pd/C obtain 3 and 4, respectively. By heating 13a or 13b with the
as a catalyst. Cyclization was accomplished by treatment with amine 14, we observed an unexpected transformation of
potassium thiocyanate. Finally, the thione 10 was converted 2-alkylsulfanylimidazoles bearing a 2-hydroxyethyl or a 2,3-
into the target compounds 3 and 4 by nucleophilic substi- dihydroxypropyl moiety at the imidazole C2–S-position into the
tution reactions with 2-bromoethanol and 3-bromopropane- imidazol-2-one derivative 15.
1,2-diol, respectively. The total yield of this synthetic strategy to
pyridinylimidazoles 3 and 4 starting from 2-bromo-4-methyl- prepare pyridinylimidazoles 3 and 4 in higher yields and with
pyridine (5) is 3.6% and 3.9% (7 linear steps), respectively. increased versatility compared to the previously published
Herein, we report a distinct and optimized procedure to
Scheme 1 Reported synthesis of pyridinylimidazoles 3 and 4. Reagents and conditions: (i) 1. 3-methyl-2-butylamine (rac.), Pd2(dba)3, t-BuONa,
BINAP, toluene, reflux temperature; 2. Boc2O, DMAP, CH2Cl2, r.t.; (ii) NaHMDS, ethyl 4-fluorobenzoate, THF, 0 °C – r.t.; (iii) NaNO2, acetic acid, 10 °C
– r.t.; (iv) Pd/C 10%, MeOH/HCl, H2, atmospheric pressure, r.t.; (v) KSCN, DMF, reflux temperature, 3 h; (vi) 2-bromoethanol, t-BuOK, reflux tempera-
ture, 1.5 h in case of compound 3 or 3-bromopropane-1,2-diol, t-BuOK, reflux temperature, 3 h in case of compound 4; ayield over three steps,
compounds 8 and 9 were not isolated.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Paper
Scheme 2 Unexpected reaction of 2-alkylimidazoles 13a and 13b into imidazol-2-ones. Reagents and conditions: (i) EtONa, methanol, 2-bromo-
ethanol, r.t., 4 h (for synthesis of 13a) or EtONa, methanol, 3-bromopropane-1,2-diol, r.t., 16 h (for synthesis of 13b); (ii) 160 °C, 10 h, sealed tube.
protocols. Moreover, the described synthetic strategy avoids azole C2–S moiety prior to this step and its cleavage in the
the use of palladium as a catalyst for introduction of the subsequent step (Scheme 3).
amino moiety at the pyridine C2-position.
Initially, we attempted to make use of an acetyl ester to
protect the hydroxy function (Scheme 4).14 Therefore, imi-
dazole-2-thione 12 was treated with 2-bromoethyl acetate under
mildly basic conditions in order to obtain ester 16, bearing the
acetyl protected hydroxyethyl moiety at the imidazole C2–S-
Results & discussions
In order to avoid the aforementioned conversion of 2-alkylsul- position. Compound 16 was reacted with 3-methyl-2-butyl-
fanylimidazoles bearing a 2-hydroxyethyl or a 2,3-dihydroxy- amine in a nucleophilic aromatic substitution reaction. Under
propyl moiety at the imidazole C2–S-position into the
these harsh and basic conditions (160 °C, excess of amine),
corresponding imidazol-2-one derivatives, we pursued the stra- the acetyl protecting group was cleaved and imidazol-2-one 15
tegic introduction of a hydroxyl protecting group at the imid- was isolated as the major product of this reaction. Decrease of
Scheme 3 Retrosynthetic approach for synthesis of 3 (PG, protecting group).
Scheme 4 Conversion of ester 16 to imidazol-2-one 15. Reagents and conditions: (i) 2-bromoethyl acetate, Cs2CO3, DMF, 55 °C, 30 min; (ii)
3-methyl-2-butylamine (rac.), 160 °C, 16 h.
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
Scheme 5 Optimized synthesis of 3 using THP as the hydroxyl protecting group. Reagents and conditions: (i) t-BuONa, methanol, 2-(2-bromo-
ethoxy)-tetrahydro-2H-pyran, 30 min, 50 °C; (ii) 3-methyl-2-butylamine (rac.), 160 °C, 16 h, sealed tube; (iii) 1.25 M HCl/EtOH, 3 h, r.t.
both, reaction temperature and excess of amine resulted in a
slow or no conversion.
With a total yield of 29.4% in 7 linear steps, the optimized
synthesis of compound 3 starting from 2-fluoro-4-methyl-
As another protecting group for the hydroxy function, we pyridine (11) compares very well to the reported synthesis start-
used the acid labile but base stable tetrahydropyranyl (THP) ing from 2-bromo-4-methylpyridine (5), giving more than eight
group (Scheme 5).15 Imidazole-2-thione 12 was reacted with 2-(2- times higher yields (compare Scheme 5 vs. Scheme 1). Using
bromoethoxy)-tetrahydro-2H-pyran to give compound 17 in good this protocol, 2.3 g of inhibitor 3 were synthesized starting
to moderate yields. In the next step, the amino function at the from 3.0 g imidazole-2-thione 12 in similar yields as reported
pyridine C2-position was introduced via nucleophilic aromatic in Scheme 5 (for details, see ESI†).
substitution under the above mentioned conditions. To our
For optimizing the synthesis of inhibitor 4, we used an
delight, we found the THP-group to be sufficiently stable and acetal group to protect the 1,2-dihydroxy function.16 Acetals
compound 18, the THP-protected derivative of 3, was obtained are generally stable under basic conditions and liberate the
in good yields. Finally, treatment of 18 with ethanolic hydro- diol under acidic conditions. The optimized route to 4 is
chloric acid solution resulted in cleavage of the THP group and depicted in Scheme 6. The dihydroxypropylsulfanylimidazole
the target compound 3 was obtained in quantitative yields.
13b was converted into the corresponding acetonide 19 by
Scheme 6 Optimized synthesis of 4 using acetal as the 1,2-dihydroxyl protecting group. Reagents and conditions: (i) t-BuONa, methanol, 3-bromo-
propane-1,2-diol, 70 °C, 1 h; (ii) acetone, p-TsOH, reflux temperature, 24 h; (iii) 3-methyl-2-butylamine (rac.), 160 °C, 16 h, sealed tube; (iv) 2 M aq.
HCl, r.t., 3 h; ayield over two steps, compound 13b was not isolated.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Paper
Scheme 7 Synthesis of pyridinylimidazoles S2, S4, S6, S8, S10 and S12. For reagents and conditions, see the ESI.†
Table 1 Biological data for selected pyridinylimidazoles
treatment with acetone under p-toluenesulfonic acid (p-TsOH)
catalysis. In the next step, the amino function was introduced
at the pyridine C2-position via nucleophilic aromatic substi-
tution and finally the acetal protecting group was cleaved with Cmpd
diluted hydrochloric acid to yield inhibitor 4. The overall yield
IC50 SEM
p38α MAP kinasea
TNF-α releaseb
3 (LN950)
11 1 nMc
15 2 nMc
19 2 nM
14 2 nM
32 3 nM
41 7 nMc
42 1 nM
22 0.2 nM
37 4 nMc
183 16 nMc
84 3 nM
20 3 nM
ndd
of this protocol starting from 11 is 20.2% (8 linear steps),
which is five times higher than the previously reported syn-
thesis starting from 5 (Scheme 1, total yield: 3.9%).
4 (LN941)
S2 [(R)-3]
S4 [(S)-3]
S6
Both optimized synthetic strategies presented in Schemes 5
and 6 are versatile and diverse since the introduction of both,
the imidazole C2–S moiety and the pyridine C2–amino func-
tion are in the last steps of the sequence. Therefore, these
strategies allow a facile and high-yielding access to further
pyridinylimidazole derivatives (Scheme 7, Schemes S2–S7,
ESI†). Starting from intermediates 17 or 19, both enantiomers of
S8
936 15 nMc
nd
S10 [(R)-6]
S12 [(S)-6]
nd
a Results are from three experiments. b Tests were carried out in
duplicate. c Test data are from Koch et al.10 d nd. not determined.
3
(compounds S2 and S4), the hydroxyethylsulfanyl and
the (S)-enantiomer as the eutomer. Compared to the racemic
inhibitor 3, the LPS-stimulated TNF-α-release is inhibited two
times more by the (S)-enantiomer S4, whereas, the (R)-enantio-
mer S2 is as expected two times less active in this assay.
dihydroxypropylsulfanyl derivatives of ML3403 (S6 and S8) and
both enantiomers of S6 (compounds S10 and S12) were pre-
pared. The synthesized pyridinylimidazoles were tested in an
ELISA-assay for their ability to inhibit the p38α MAP kinase
(Table 1).17 Furthermore, the potential to inhibit the LPS-stimu-
lated TNF-α release from human whole blood was determined
for S2 and S4, the most promising inhibitors in this set, and
compared to the racemic mixture, compound 3. In case of the
chiral inhibitors S2, S4, S10 and S12, the biological data revealed
Conclusion
An optimized and diverse synthetic approach for the prepa-
ration of pyridinylimidazole-based p38α MAP kinase inhibitors
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
is described. Compared to a previously reported method, an
up to eight times higher overall yield was obtained.
A series of eight pyridinylimidazoles was synthesized using
these optimized conditions. In case of the chiral inhibitor 3,
biological evaluation showed that the (S)-enantiomer is the
two times more potent eutomer.
5 W. MacNee, R. J. Allan, I. Jones, M. C. De Salvo and
L. F. Tan, Thorax, 2013, 68, 738–745.
6 J. C. Betts, R. J. Mayer, R. Tal-Singer, L. Warnock,
C. Clayton, S. Bates, B. E. Hoffman, C. Larminie and
D. Singh, Pharmacol. Res. Perspect., 2015, 3, e00094.
7 C. Peifer, G. Wagner and S. Laufer, Curr. Top. Med. Chem.,
2006, 6, 113–149.
8 S. A. Laufer, G. K. Wagner, D. A. Kotschenreuther and
W. Albrecht, J. Med. Chem., 2003, 46, 3230–3244.
9 S. Laufer, D. Hauser, T. Stegmiller, C. Bracht, K. Ruff,
V. Schattel, W. Albrecht and P. Koch, Bioorg. Med. Chem.
Lett., 2010, 20, 6671–6675.
Acknowledgements
S. Bauer, M. Goettert, D. Müller, and K. Bauer are gratefully
acknowledged for their assistance in biology testing.
10 P. Koch, C. Bäuerlein, H. Jank and S. Laufer, J. Med. Chem.,
2008, 51, 5630–5640.
11 S. Laufer and P. Koch, Org. Biomol. Chem., 2008, 6, 437–
439.
12 P. Koch and S. Laufer, J. Med. Chem., 2010, 53, 4798–4802.
13 S. A. Laufer and A. J. Liedtke, Tetrahedron Lett., 2006, 47,
7199–7203.
14 P. G. M. Green and T. W. Wuts, in Protective Groups in
Organic Synthesis, John Wiley & Sons, Inc., New York, 3rd
edn, 1999, pp. 150–160, 712–715.
Notes and references
1 Z. Chen, T. B. Gibson, F. Robinson, L. Silvestro, G. Pearson,
B. E. Xu, A. Wright, C. Vanderbilt and M. H. Cobb, Chem.
Rev., 2001, 101, 2449–2476.
2 L. Munoz and A. J. Ammit, Neuropharmacology, 2010, 58,
561–568.
15 P. G. M. Green and T. W. Wuts, in Protective Groups in
Organic Synthesis, John Wiley & Sons, Inc., New York, 3rd
edn, 1999, pp. 27–33, 708–711.
16 P. G. M. Green and T. W. Wuts, in Protective Groups in
Organic Synthesis, John Wiley & Sons, Inc., New York, 3rd
edn, 1999, pp. 207–215, 716–719.
3 K. Namiki, H. Matsunaga, K. Yoshioka, K. Tanaka,
K. Murata, J. Ishida, A. Sakairi, J. Kim, N. Tokuhara,
N. Shibakawa, M. Shimizu, Y. Wada, Y. Tokunaga,
M. Shigetomi, M. Hagihara, S. Kimura, T. Sudo,
A. Fukamizu and Y. Kasuya, J. Biol. Chem., 2012, 287,
24228–24238.
17 M. Goettert, R. Graeser and S. A. Laufer, Anal. Biochem.,
2010, 406, 233–234.
4 G. Wang, J. Pan and S. D. Chen, Prog. Neurobiol., 2012, 98,
207–221.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2015