Pyrazolo[1,5-a]pyridines as p38 kinase inhibitors

Article abstract of DOI:10.1021/ol0519745

(Chemical Equation Presented) A convergent synthesis of substituted pyrazolo[1,5-a]pyridines has been achieved either via a regioselective [3 + 2] cycloaddition of N-aminopyridines with alkynes or by thermal cyclization of disubstituted azirines. Subseque

Full text of DOI:10.1021/ol0519745

ORGANIC
LETTERS
2005
Vol. 7, No. 21
4753-4756
Pyrazolo[1,5-a]pyridines as p38 Kinase
Inhibitors
Kirk L. Stevens,* David K. Jung, Michael J. Alberti, Jennifer G. Badiang,
Gregory E. Peckham, Jim M. Veal,† Mui Cheung, Philip A. Harris,
Stanley D. Chamberlain, and Michael R. Peel‡
GlaxoSmithKline, Research and DeVelopment, Department of Medicinal Chemistry,
5 Moore DriVe, Research Triangle Park, North Carolina 27709
Kirk.L.SteVens@gsk.com
Received August 15, 2005
ABSTRACT
A convergent synthesis of substituted pyrazolo[1,5-a]pyridines has been achieved either via a regioselective [3
N-aminopyridines with alkynes or by thermal cyclization of disubstituted azirines. Subsequent palladium-catalyzed introduction of pyridines
or de novo synthesis of pyrimidines affords inhibitors of p38 kinase.
+ 2] cycloaddition of
The serine/threonine kinase p38, a member of the MAP kin-
ase superfamily, is activated by a variety of environmental
Scheme 1. Initial Cyclization Steps
stresses and inflammatory cytokines. A number of inhibitors
of p38 kinase are known, and many have demonstrated signi-
ficant anti-inflammatory activity in standard animal models
of rheumatoid arthritis.1 As a result, the search for potent,
selective p38 inhibitors to be used for the clinical manage-
ment of rheumatoid arthritis remains an active area of
research.
showing that a prototype inhibitor, SB-203580, binds to p38
Our search for novel p38 inhibitors has been aided by the
via a hydrogen bond between the pyridine nitrogen and a
availability of high-resolution X-ray crystal structure data
hinge-region amide, and through the interaction of the fluor-
ophenyl ring deep within the hydrophobic back-pocket.2 1,2-
† Current address: Serenex, Inc., Durham, NC.
Diaryl heterocycles, such as SB-203580, a 4,5-diaryl imi-
dazole, have been a common motif and highly emulated in
the p38 arena. Using this information, we became interested
in the design of novel p38 inhibitors based on the pyrazolo-
[1,5-a]pyridine template. Our efforts led to the preparation
and investigation of a series of heterocyclic kinase inhibitors
of general structure 1 (see Scheme 4).
‡ Current address: Scynexis, Inc., RTP, NC.
(1) (a) Adams, J. L.; Badger, A. M.; Kumar, S.; Lee, J. C. Prog. Med.
Chem. 2001, 38, 1. (b) Dumas, J.; Sibley, R.; Riedl, B.; Monahan, M. K.;
Lee, W.; Lowinger, T. B.; Redman, A. M.; Johnson, J. S.; Kingery-Wood,
J.; Scott, W. J.; Smith, R. A.; Bobko, M.; Schoenleber, R.; Ranges, G. E.;
Housley, T. J.; Bhargava, A.; Wilhelm, S. M.; Shrikhande, A. Bioorg. Med.
Chem. Lett. 2000, 10 (18), 2047. (c) Pargellis, C.; Tong, L.; Churchill, L.;
Cirillo, P. F.; Gilmore, T.; Graham, A. G.; Grob, P. M.; Hickey, E. R.;
Moss, N.; Pav, S.; Regan, J. Nat. Struct. Biol. 2002, 9, 268.
10.1021/ol0519745 CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/23/2005

eroaryl stannanes or boronic acids/esters to provide com-
pounds (such as 1) in good yields (Scheme 1).
Scheme 2. Synthetic Steps to Intermediate 10
Scheme 4. Final Elaboration of Pyrazolo[1,5-a]pyridines
Synthetic approaches to the pyrazolo[1,5-a]pyridine nucleus
are under-represented in the literature.3 The most commonly
employed method for synthesis involves a [3 + 2] cycload-
dition of N-aminopyridines (3) with alkynoic esters (Scheme
1). The use of functionalized arylalkynoic esters (2) would
allow ready access to a variety of 2-arylpyrazolo[1,5-a]pyri-
dines. Chemistry was developed to investigate the SAR of
the pyrazolo[1,5-a]pyridine core. Several iterations of chem-
istry were followed so the routes would allow for a large
range of analogues to be made.
Compounds (1) were evaluated for their p38 enzyme
inhibition. Those compounds with appropriate substitutions
could be elaborated further via displacement of the 2′-
fluoropyridine or the 2′-thiomethylpyrimidine. The 2′-fluoro
group could be displaced directly by amine nucleophiles (neat
in amine, sealed tube). The 2′-thiomethyl group required
activation via mCPBA or oxone oxidation to the sulfone/
sulfoxide followed by displacement with amines (reflux in
amine). A summary of some of the compounds synthesized
Scheme 3. Pyrazolopyridines via Azirine Intermediate
Table 1. Basic Enzyme SAR-Pyridinesa
A variety of groups on the aryl ring (R2), including ortho,
meta, or para alkyl, halo, or alkoxy substituents were
prepared via this method. We have found that the cyclization
can be mediated by a variety of bases (DBU, NaOH, K2-
CO3, etc.) and can be run in organic solvents (CH2Cl2, DMF,
DMSO, etc.) or biphasic mixtures (CH2Cl2/H2O). The
pyrazolo[1,5-a]pyridines (4) were then decarboxylated ef-
ficiently in strong base followed by electrophilic halogenation
with NBS. These compounds (5) were suitable partners for
transition-metal-mediated cross-coupling reactions with het-
(2) (a) Wilson, K. P.; Fitzgibbon, M. J.; Caron, P. R.; Griffith, J. P.;
Chen, W.; McCaffrey, P. G.; Chambers, S. P.; Su, M. S.-S. J. Biol. Chem.
1996, 271, 27696. (b) Wang, Z.; Harkins, P. C.; Ulevitch, R. J.; Han, J.;
Cobb, M. H.; Goldsmith, E. J. Proc. Natl. Acad. Sci. U.S.A. 1997, 94 (6),
2327. (c) Tong, L.; Pav, S.; White, D. M.; Rogers, S.; Crane, K. M.; Cywin,
C. L.; Brown, M. L.; Pargellis, C. A. Nat. Struct. Biol. 1997, 4, 311.
(3) (a) Awano, K.; Suzue, S.; Segawa, M. Chem. Pharm. Bull. 1986,
34, 2828. (b) Miki, Y.; Yagi, S.; Hachiken, H.; Ikeda, M. Heterocycles
1994, 38, 1881. (c) Koeckritz, P.; Riemer, B.; Michler, A.; Hassoun, A.;
Liebscher, J. J. Heterocycl. Chem. 1994, 31, 1157. (d) Bondo, H. J.; Weis,
J.; Suzdak, P. D.; Eskesen, K. Bioorg. Med. Chem. Lett. 1994, 4, 695.
a The p38 IC50 values (µM) are mean values with minimum two tests and
standard deviation of the mean typically less than 0.03.
in this way is shown in Table 1 with their corresponding
p38 enzyme inhibitory profile.
4754
Org. Lett., Vol. 7, No. 21, 2005

All of the compounds shown in Table 1 contain a p-fluoro
substituent on the 2-phenyl ring (R2) since larger groups at
the para position resulted in poor p38 inhibition. In addition,
p-fluoro was the preferred substituent for metabolic stability.
Modification on the pyrazolo[1,5-a]pyridine core (R1) was
well tolerated, and compounds were prepared with substit-
uents at C(4)-C(7). Amine substituents on the pyridine (R1)
were key to the potency of the molecules and improved
physiochemical properties. Tertiary amine substitutions (1a)
resulted in lower p38 enzyme inhibition. Within the pyridine
series, the most promising combination of substitutions on
the pyrazolo[1,5-a]pyridine core was an alkylamine (R3),
para-fluorophenyl (R2), and an electron-withdrawing sub-
stituent at C(6) or an electron-donating group at the 7-posi-
tion (R1) (Table 1, 1e-i).
Table 2. Basic Enzyme SAR-Pyrimidinesa
This general strategy (depicted in Scheme 1) was limiting
in a number of ways. First, aminated pyridines have scarce
substitutional availability from commercial sources. Several
substituted pyridines were aminated (HOSA or MSH)4 with
variable results. Additionally, the requirement to convert the
ester to a cross-coupling partner, usually a halogen, was
efficient, but cost two extra steps. The availability of
metalated heteroaryl species for the cross-coupling reaction
was also a limitation, and the preparation of these cross-
coupling partners often required a number of undesired
additional steps.
Having confirmed the overall value of the template in the
context of potent p38 inhibition, we developed a compli-
mentary route that would allow for ease of synthesis and
improved substituent flexibility.
The improved route constructs the pyrazolo[1,5-a]pyridine
core template with starting materials that are much more
flexible than the initial route. Substituted acetophenones (6)
are deprotonated (NaH) then alkylated via an appropriately
substituted halo pyridine (7) (Scheme 2). Alternatively,
substituted methylpyridines (8) can be deprotonated and
combined with methylbenzoates (9) to yield the identical
product (10), which is a key intermediate. R1 and R2 can
be easily diversified with these two complimentary routes.
Ketones (10) are efficiently converted to pyrazolo[1,5-a]-
pyridines via azirine intermediates (11, Scheme 3). The
ketone is first converted to the oxime with hydroxylamine
followed by treatment with trifluoroacetic anhydride, giving
spontaneous rearrangement to the azirine. The azirines (11)
are converted to pyrazolo[1,5-a]pyridines via heating (170
°C, neat or 75 °C with catalytic FeCl2 in DME).5 Accessing
pyrazolo[1,5-a]pyridines (12) via azirine intermediates still
requires three synthetic steps; however, these starting materi-
als are derived from commercial sources with no limitations
for substitutions at R1 and R2.
Our initial routes to pyrazolo[1,5-a]pyridines (Scheme 1)
employed the use of a 3-halogenated species to cross-couple
with a metalated heteroaryl partner, but as noted previously,
that limited our chemistry to cross-coupling partners that
a The p38 IC50 values (µM) are mean values with minimum two tests
and standard deviation of the mean typically less than 0.03.
were often several steps and required syntheses that had their
own difficulties. We turned our attention to a de novo
pyrimidine synthesis that would take advantage of the
nucleophilic nature of the 3-position of the pyrazolo[1,5-a]-
(4) HOSA ) Hydroxylamine O-Sulfonic Acid; MSH ) Mesityl O-
Hydroxylamine.
(5) Fitzgerald, R. N.; Jung, D. K.; Eaddy, J. F. PCT Int. Appl. 2001,
WO0183479A2.
Org. Lett., Vol. 7, No. 21, 2005
4755

pyridines (similar to indole).6 This chemistry would allow
for future incorporation of diversity on the pyrimidine rings
important for the two-point hydrogen bonding (of the
2-aminopyrimidine) within the ATP binding pocket. Toward
this goal, pyrazolo[1,5-a]pyridines (12) were treated with
acetic anhydride and a drop of acid to yield acylated species
which were converted to dimethyl enamines (13) by heating
in neat DMF/dimethylacetal (Scheme 4). These enamine
species are excellent partners for cyclization with substituted
guanidines in the principal pyrimidine synthesis to give final
compounds (14) in good yields.
Many guanidines are commercially available or can easily
be prepared via condensation of amines with cyanamide in
the presence of nitric acid.7 By the methods described in
Schemes 1-4, compounds were provided, where R1, R2,
and R3 were varied widely, and the resulting compounds
were tested for their p38 kinase inhibition activity. A brief
SAR is shown in Table 2.
again the preferred substituent on the C(2) phenyl ring (R2),
though m-Cl and m-CF3 were also tolerated (Table 2, 14m,
14n). An amine substituent on the pyrimidine (R3) was also
preferred (Table 2, compare 14c and 14k). As observed with
the pyridine series, electron-withdrawing groups at C(6) and
electron-donating groups at C(7) (R1) are strongly preferred
for p38 enzyme inhibition (Table 2, 14q-14v).
In summary, a convergent synthesis of pyridine-substituted
pyrazolo[1,5-a]pyridines was achieved via [3 + 2] cycload-
dition reactions of N-aminopyridines with alkynes followed
by an organometallic cross-coupling reaction to form com-
pounds that are inhibitors of p38 kinase. Alternatively,
pyrimidine-substituted pyrazolo[1,5-a]pyridines were pre-
pared via thermal cyclization of disubstituted azirines fol-
lowed by de novo pyrimidine synthesis to afford inhibitors
of p38 kinase. Some brief SAR of these molecules has been
described.
Supporting Information Available: Complete experi-
mental details for selected compounds from Table 1 (1a, 1b,
1c, 1i) and Table 2 (14c, 14d, 14h, 14j, 14k, 14o, 14s, 14t,
14u, 14v) as well as all intermediates. This material is
available free of charge via the Internet at http://pubs.acs.org.
In general, the pyrimidine series was more potent for p38
enzyme inhibition than the pyridine series. p-Fluoro was
(6) Ryang, H.-S.; Sakurai, H. J. Chem. Soc., Chem. Commun. 1972, 77.
(7) (a) Palat, K.; Celadnik, M.; Danek, J.; Varkonda, S. Collect. Czech.
Chem. Commun. 1992, 57, 1127. (b) Kreutzberger, A.; Tantawy, A. Arch.
Pharm. 1979, 312, 426.
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