Syntheses of novel 2,3-diaryl-substituted 5-cyano-4-azaindoles exhibiting c-Met inhibition activity

Article abstract of DOI:10.1016/j.bmcl.2009.02.069

Herein we report the syntheses of 2,3-diaryl-substituted pyrrolo[3,2-b]pyridine-5-carbonitriles via a one-pot 5-endo-dig-cyclization/protection reaction followed by palladium catalyzed arylation. In addition, a complementary synthesis route employing Laro

Full text of DOI:10.1016/j.bmcl.2009.02.069

Bioorganic & Medicinal Chemistry Letters 19 (2009) 1879–1882
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Syntheses of novel 2,3-diaryl-substituted 5-cyano-4-azaindoles exhibiting
c-Met inhibition activity
a
a
a
b
Hannes Koolman b, , Timo Heinrich , Henning Böttcher , Wilfried Rautenberg , Michael Reggelin
*
a Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany
b Clemens-Schöpf Institute of Organic Chemistry and Biochemistry, Technical University of Darmstadt, Petersenstrasse 22, 64287 Darmstadt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we report the syntheses of 2,3-diaryl-substituted pyrrolo[3,2-b]pyridine-5-carbonitriles via a one-
pot 5-endo-dig-cyclization/protection reaction followed by palladium catalyzed arylation. In addition, a
complementary synthesis route employing Larock methodology is applied to efficiently explore further
aryl moieties in the 2-position. The novel compounds’ expedient c-Met receptor tyrosine kinase inhibi-
tion activity is discussed.
Received 15 January 2009
Revised 18 February 2009
Accepted 18 February 2009
Available online 21 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Kinase inhibitor
c-Met
4-Azaindole
The hepatocyte growth factor (HGF) receptor, also known as
mesenchymal–epithelial transition factor (c-Met), is a receptor
tyrosine kinase (RTK) present in both normal and malignant cells.1
The main biological effects upon its signaling pathway activation
comprise promotion of tissue regeneration, angiogenesis, and en-
hanced cell motility.2 c-Met is known to be over-expressed and
mutated in a variety of human cancer types.3 Thus, signal trans-
duction through the activation of the c-Met receptor is accountable
for proliferation, scattering, invasiveness and metastasis of tumor
cells. On this account, small molecule inhibitors, preventing recep-
tor autophosphorylation and recruitment of the downstream effec-
tors of c-Met, are of current interest.4
Currently, there are several ATP-competitive c-Met kinase inhib-
itors known, based on different scaffolds.5 Recent developments fea-
ture 7-azaindoles,6 aminopyridines6 and triazolopyridazines7 with
high potency and selectivity for c-Met.
As part of our studies towards aromatically substituted
azaindoles as kinase inhibitors, the rarely used core structure of
the 4-azaindoles in combination with diaryl substitution in the
2- and 3-position attracted our attention.8,9 At present, this struc-
ture is unknown as an inhibitor of c-Met and 5-cyano-2,3-diaryl-4-
azaindoles (1) in particular have not been previously described as
kinase inhibitors. By analogy to known compounds,8 these pyrrol-
opyridines are anticipated to offer kinase inhibiting potential.
For the full syntheses of azaindoles
a growing number of
approaches are known, heavily depending on the desired substitu-
tion pattern.10 Recently, Cacchi et al. transferred their highly versa-
tile methodology for the assembly of 2,3-diaryl-substituted
indoles11 onto 4- and 7-azaindoles.9 This methodology comprises
the reaction of 2-alkynyl-3-trifluoroacetamidopyridines with aryl-
bromides and -triflates in an aminopalladation-reductive elimina-
tion procedure. However, in our hands, this approach failed to yield
the desired 5-cyano-substituted azaindoles due to loss of the
trifluoroacetate protecting group under the reaction conditions.12
Therefore
a strategy based upon base-induced cyclization of
o-aminoalkynylpyridines was employed (Scheme 1).
Silver(I)-assisted electrophilic aromatic iodination of commer-
cially available 5-aminopicolinonitrile 2 gave the mono-iodinated
intermediate 3 in 79% yield. The cyclization precursor 4a could
be obtained under Sonogashira-like alkynylation conditions in
moderate yield (66%).13
Azaindoles 5 were synthesized by subsequent base-induced
5-endo-dig cyclization in NMP,14 direct addition of N-iodosuccini-
mide and BOC-anhydride, leading to the protected 3-iodo-4-azain-
doles in good yields of 73–83% (see Ref. 15 for an exemplary
procedure). These compounds are stable if directly purified via
chromatography over neutral aluminum oxide and stored at
ꢀ20 °C.16 Arylation in the 3-position was achieved via Suzuki–
* Corresponding author. Tel.: +49 6151 72 5881.
E-mail address: hannes.koolman@merck.de (H. Koolman).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.02.069

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H. Koolman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1879–1882
Scheme 1. Reagents and conditions: (a) I2, Ag2SO4, EtOH, rt, 79%; (b) 4-ethynylpyridine, Cs2CO3, Pd(dppf)Cl2, THF, 50 °C, 66%; (c) 3-phenylpropiolic acid, Na2CO3, LiCl,
Pd(dppf)Cl2, DMF, 110 °C, 20%; (d) KOtBu, NMP, 90 °C, then NIS, DCM, 0 °C to RT, then (BOC)2O, DMAP, DCM, 0° to RT, 73–83%; (e) R1B(OR)2, K2CO3, Pd(dppf)Cl2, DME/H2O 2:1,
85 °C then TFA or ethan. HCl, 60 °C, 25–86%; (f) DHP, MgBr2, THF, 65 °C, quant.; (g) 11a or 11b, Na2CO3, LiCl, Pd(dppf)Cl2, DMF, 110 °C (h) IPy2BF4, TfOH, DCE, 83 °C, 48–66%
over two steps (i) R2B(OR)2, K2CO3, Pd(dppf)Cl2, DME/H2O 2:1, 85 °C, 9–77% (j) 13, Na2CO3, LiCl, Pd(dppf)Cl2, DMF, 110 °C then 1 N HCl, 110 °C, 39%.
Miyaura-coupling with various boronic acids or -esters. The
protecting group was quantitatively removed by subsequent acid-
ification of the reaction mixture with TFA or hydrochloric acid to
yield the desired 2,3-diaryl-substituted pyrrolo[3,2-b]pyridine-5-
carbonitriles 1a–e and 1g–p. The BOC-protecting group was indis-
pensable in this step, as unprotected 3-iodo-4-azaindoles failed to
undergo efficient Suzuki-type cross-coupling, showing dehalogen-
ation as major side reaction.17 Thus, employing two one-pot
routines we were able to prepare highly decorated 4-azaindoles
from quite simple starting materials.
An alternative approach was based upon Larock’s indole synthe-
sis. Via this route variable aryl substitution in the 2-position could
be efficiently explored. For the key cycloaddition step introduction
Scheme 2. Reagents and conditions: (a) triethylsilylacetylene, Pd(PPh3)4, CuI,
of an electron donating protecting group was indispensable.18
Et2NH, 75%; (b) nBuLi, TESCl, THF, ꢀ78 °C to rt, 86%; (c) 4-ethynylpyridine-
hydrochloride, Pd(PPh3)4, CuI, Et2NH, 53%.
Therefore, iodo-aminopyridine 3 was reacted with dihydropyran
under Lewis-acid catalysis, yielding 6 quantitatively. The 4-azain-
doles 7 were prepared under Larock conditions with triethylsilyl-
protected alkynes 11 (vide infra).19 Thereby, traces of N-deprotec-
ted products were generated. This crude product mixture was trea-
ted with IPy2BF4 in the presence of an excess of triflic acid to
convert the TES-group to the corresponding iodides 8.20 The THP-
group was concurrently removed during this halogenation. Deriv-
atives 1q–y were achieved under standard Suzuki-coupling condi-
tions. It is noteworthy that this coupling works uneventfully
without N-protection on the indole. The 2,3-dipyridinyl-derivative
1f was yielded by Larock-reaction of 6 with symmetrically substi-
tuted ethyne 13 and subsequent acidic THP-group removal.
The alkynes 11a, 11b and 13, employed in the Larock reactions
with pyridine 6 were prepared according to known procedures
(Scheme 2).21
Iodobenzene 9 was converted to 11a via Sonogashira-coupling
with TES-ethyne, whereas 4-ethylnylpyridine and 4-iodopyridine
12 were coupled to ethyne 13, respectively. Ethynyl-benzene 10
was silylated with TES-chloride after deprotonation with n-BuLi,
to yield 11b.
Synthesis of the non-commercially available boronic acids 14a
and 14b employed in the final synthesis step for the derivatives
1u and 1v, respectively, were performed following a procedure
published by Buchwald et al. (Scheme 3).22
First, the structure activity relation of 4-azaindoles 1a–o with
different aryl-substituents in the 3-position is discussed, and their
c-Met-kinase inhibition activity is listed in Table 1. The phenyl-
Scheme 3. Reagents and conditions: (a) NaH, BnCl, DMF, 0 °C to rt, 23%; (b)
Bis(pincolato)diboron, KOAc, Pd(dba)2, XPhos, dioxane, 110 °C, 95% -quant.
derivative 1a shows an IC50-value of 2.09 lM, suggesting that
lipophilic moieties are preferred as substituents in the 3-position
of the 4-azaindole. The activity is maintained with the introduction
of a p-fluoro substitution (1b), but steadily decreases with increas-
ing size of the halogen (1a–d). Similarly, derivative 1e, bearing a
pseudohalogenic cyano substitution, is significantly less active.
The m-chloro-derivative 1g, exhibiting a 50-fold activity increase
compared to the phenyl derivative 1a, indicates that additional
halogen substituents are beneficial in this position. In case of the
m,m0-disubstituted derivatives (1i, 1j), the m-chloro, m0-fluoro-
derivative (1j) is about 7.5-fold more potent than 1i. But remark-
ably, when combining meta- and para-substitution (1k, 1l) the
chloro-substituent (1k) is about ninefold superior to the fluoro-
derivative (1l). However, the m-,p-dichloro-derivative 1h is

H. Koolman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1879–1882
1881
Table 1
Table 2
c-Met inhibition activity of the 3-aryl-substitued 2-(pyridin-4-yl)-1H-pyrrolo[3,2-
b]pyridine-5-carbonitriles 1a–o
c-Met inhibition activity of the 2-aryl-substituted 3-(4-fluorophenyl)-1H-pyrrolo[3,2-
b]pyridine-5-carbonitriles 1p–s
R1
IC50
a
Compounds
1a
2.09
Compounds
R2
IC50
nab
a
1b
1c
1d
1e
1f
1.95b
10
1p
1q
1r
>10c
>10c
0.33
>10c
>10c
nad
1s
a
IC50 in
l
M.
Inactive at 10
Remaining activity >60% at 10
b
c
lM.
lM.
1g
0.04
The potency of 1g became apparent when the optimization of
the 2-aryl-moiety with derivatives of 1b (see Table 2) and 1k
(see Table 3) was already ongoing.
Comparing the inactive phenyl-derivative 1p with the above-
mentioned pyridine-analog 1b the pyridine nitrogen is clearly
1h
1i
>10c
2.55
accountable for the
lM activity. In case of an influencing
o-chloro-substituent (1q) the activity gain of the pyridine nitrogen
Table 3
1j
0.34
0.18
1.59
>10c
c-Met inhibition activity of the 2-aryl-substituted 3-(3-chloro-4-fluorophenyl)-1H-
pyrrolo[3,2-b]pyridine-5-carbonitriles 1t–y
1k
1l
R2
IC50
a
Compounds
1t
>10b
1m
1n
1u
0.13
nad
nad
1v
1w
1x
5.57
>10b
>10b
>10b
1o
a
IC50 values represent average values from 2 to 4 measurements, and are defined
as the concentration (
l
M) resulting in 50% inhibition of activity, for assay condi-
tions see Ref. 23.
b
Exemplary solubility data: (in water at pH 7.4): 1
Remaining activity >60% at 10 lM
l
g/ml.
c
d
Inactive at 10 lM.
1y
significantly less active. The compounds 1n and 1o, bearing an
amino-pyrimidinyl and a p-methylsulfonyl-phenyl substitution,
respectively, are inactive.
a
IC50 in
l
M.
b
Remaining activity >60% at 10
lM.

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H. Koolman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1879–1882
is reduced. Derivative 1r, the pyridine-2-yl-isomer of 1b,24 cannot
sustain the activity, whereas introduction of a second meta-nitro-
gen (1s) obviously compensates this effect. Thus, 1s is about sixfold
more active than the pyridine-4-yl-analog 1b. In light of published
structures8 this effect is surprising. Obviously, this result cannot be
generally extrapolated, because the pyrimidinyl in position 2
together with a m-chloro,p-fluoro-phenyl-moiety in the 3-position
(1t, Table 3) reduces activity drastically, revealing subtle structure
activity relations.
Compounds 1u and 1v are based on 1k. Comparable derivatiza-
tion of the pyridine-moiety is known to be beneficial for similar
p38-kinase inhibitors.8 Introduction of a 2-aminopyridin-4-yl-moi-
ety in the 2-position of the azaindole has no effect on activity (1u)
evidencing that an o-amino-substituent is tolerated. Benzyl-substi-
tution in 1v, however, results in a 30-fold loss of activity compared
to 1k, showing that large lipophilic moieties are not tolerated in
this position.
The 2-aminopyridin-3-yl isomer 1x cannot maintain the activ-
ity (compared to 1u). This observation is consistent with the loss
of activity already noticed for the pyridine-3-yl-isomer 1r. Simi-
larly, the analogs 1w and 1y exhibit only weak inhibition activity.
Employing two different protocols based on identical starting
material various 2,3-diaryl-substituted 5-cyano-4-azaindoles
could be efficiently synthesized. The application of optimized pro-
tocols in one-pot procedures enabled a convenient synthesis of
highly decorated heterocycles. The compounds showed promising
inhibition activity of the c-Met RTK and led to the identification of
an inhibitor with an IC50 of 40 nM (1g).
Elucidation of the compounds’ selectivity profiles, particularly
discrimination of p38-kinases, is currently under investigation:
Preliminary screening of single compounds against a panel of 80
kinases revealed promising selectivities. Further studies compris-
ing improvement of activity and solubility enhancement are
currently ongoing in our group.
J.; Papp, E.; Reuter, D.; Roberts, R.; Saunders, J.; Song, K.; Villasenor, A.; Warren,
S. D.; Welch, M.; Weller, P.; Whiteley, P. E.; Zeng, Lu; Goldstein, D. M. J. Med.
Chem. 2003, 46, 4702.
9. Cacchi, S.; Fabrizi, G.; Parisi, L. M. J. Comb. Chem. 2005, 7, 510.
10. Song, J. J.; Reeves, J. T.; Gallou, F.; Tan, Z.; Yee, N. K.; Senanayake, C. H. Chem.
Soc. Rev. 2007, 36, 1120.
11. Arcadi, A.; Cacchi, S.; Marinelli, F. Tetrahedron Lett. 1992, 33, 3915.
12. Cacchi, S. et al. reported similar instability of o-acetoxyalkynylpyridines
containing strongly electron-withdrawing substituents in the pyridine ring.
Arcadi, A.; Cacchi, S.; Di Giuseppe, S.; Fabrizi, G.; Marinelli, F. Org. Lett. 2002, 4,
2409.
13. The phenyl-derivative 4b was obtained as a side product of a Larock reaction of
iodoaminopyridine 2 and 3-phenylpropiolic acid in 20% yield.
14. Koradin, C.; Dohle, W.; Rodriguez, A. L.; Schmid, B.; Knochel, P. Tetrahedron
2003, 59, 1571.
15. Exemplary synthesis procedure (1g): 331 mg (1.50 mmol) of 4a were Schlenk
tube-dried and were dissolved in NMP (15 ml) under nitrogen. 287 mg
(2.55 mmol) KOtBu were added and the mixture was stirred for 4 h at 90 °C,
then cooled to 0 °C and a solution of 507 mg (2.25 mmol) NIS in NMP (10 ml)
added drop wise. After 1 h at room temperature,
a solution of 1.64 g
(7.52 mmol) (BOC)2O and 183 mg (1.50 mmol) DMAP in NMP (5 ml) was
added at 0 °C. After 1 h at 0 °C the reaction mixture was diluted with water
(250 ml) and extracted with CH2Cl2. The combined organic phases were
washed with brine, dried and evaporated. After flash-chromatography over
neutral Al2O3 (eluent: ethyl acetate/cyclohexane 9:1) 563 mg (1.26 mmol, 83%)
tert-butyl
5-cyano-3-iodo-2-(pyridin-4-yl)-1H-pyrrolo[3,2-b]pyridin-1-
carboxylat 5a was yielded as white solid. Mp 126–129 °C. 1H NMR (400 MHz,
DMSO-d6): d 8.75 (m, 2H), 8.60 (d, J = 8 Hz, 1H), 8.07 (d, J = 8 Hz, 1H), 7.53 (m,
2H), 1.25 (s, 9H). ESI-MS: m/z: 447 ([M+H]+), 347 ([MꢀBOC]+). Anal. Calcd for
C18H15IN4O2: C, 48.45; H, 3.39; N, 12.56. Found: C, 49.00; H, 3.80; N, 12.00.
223 mg (0.50 mmol) of 5a, 117 mg (0.75 mmol) of 3-chlorophenylboronic acid
and 207 mg (1.50 mmol) of K2CO3 were dissolved in 7.5 ml of DME/H2O 2:1 in
a capped vial. After degassing employing ultrasound under nitrogen 20.4 mg
(0.025 mmol) of Pd(dppf)Cl2xCH2Cl2 were added and the reaction mixture
stirred at 80 °C for 6 h. Then 5 ml of TFA were added and the mixture stirred at
60 °C for 15 h, set to pH 12 using dil NaOH and extracted with ethylacetate. The
combined organic phases were dried, evaporated and purified via flash-
chromatography (eluent: ethyl acetate to ethyl acetate/methanol 95:5) to yield
95.0 mg (0.287 mmol) of 3-(3-chlorophenyl)-2-(pyridin-4-yl)-1H-pyrrolo[3,2-
b]pyridine-5-carbonitrile 1g as a beige solid. Mp 267 °C. 1H NMR (300 MHz,
DMSO-d6): d 12.71 (s, 1H), 8.67 (d, J = 5.6 Hz, 2H), 8.00 (d, J = 8.2 Hz, 1H), 7.82
(d, J = 8.2 Hz, 1H), 7.55 (m, 1H), 7.51–7.36 (m, 5H). APCI-MS: m/z (%): 331 (100,
[M+H]+).
16. At room temperature under atmosphere compounds 5 show dehalogenation as
major decomposition reaction.
17. Without BOC-protection, product yields only less than 15% could be achieved
under various reaction conditions. Similar observations for Suzuki-couplings in
the 3-position of pyrroles are reported in the literature: Handy, S. T.; Bregman,
H.; Lewis, J.; Zhang, X.; Zhang, Y. Tetrahedron Lett. 2003, 44, 427. and citations
within.
18. During our studies towards the synthesis of electron-deficient 4- and 7-
azaindoles via Larock reaction the tetrahydropyranyl-moiety was found to be
highly enhancing reactivity of the aminopyridine. Results will be published
elsewhere.
19. The TES-group was used instead of TMS-protection, offering superior stability
during the Larock-reaction.
20. Barluenga, J.; Gonzales, J. M.; Garcia-Martin, M. A.; Campos, P. J.; Asencio, G. J.
Org. Chem. 1993, 58, 2058.
21. Larock, C.; Yum, E. K.; Refvik, M. D. J. Org. Chem. 1998, 63, 7652; Che, C.-M.; Yu,
W.-Y.; Chan, P.-M.; Cheng, W.-C.; Peng, S.-M.; Lau, K.-C.; Li, W.-K. J. Am. Chem.
Soc. 2000, 122, 11380.
22. Billingsley, K. L.; Barder, T. E.; Buchwald, S. L. Angew. Chem., Int. Ed. 2007, 46,
5359.
23. The assay is based on procedures described in: Hays, J. L.; Watowich, S. J. J. Biol.
Chem. 2003, 278, 27456; Wang, X.; Le, P.; Liang, C.; Chan, J.; Kiewlich, D.; Miller,
T. Mol. Cancer Ther. 2003, 2, 1085.
24. Synthesis of the 1-pyridinyl-isomer of 1r failed, leading to a complex reaction
mixture.
References and notes
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Gaglia, P.; Zara, P.; Comoglio, P. M. Oncogene 1996, 6, 1997.
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Schmidt, R. J.; Williams, D. K.; Tokarski, J. S.; An, Y.; Sack, J. S.; Manne, V.;
Kamath, A.; Zhang, Y.; Marathe, P.; Hunt, J. T.; Lombardo, L. J.; Fargnoli, J.;
Borzilleri, R. M. Bioorg. Med. Chem. Lett. 2008, 18, 3224; b Hamakima, T.;
Nakamura, H.; Tang, J. WO Patent 08049855, 2008.
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Chen, A.; Choquette, D.; Dussault, I.; Fridrich, C.; Hirai, S.; Hoffman, D.; Larrow,
J. F.; Kaplan-Lefko, P.; Lin, J.; Lohman, J.; Long, A. M.; Moriguchi, J.; O’Connor, A.;
Potashman, M. H.; Reese, M.; Rex, K.; Siegmund, A.; Shah, K.; Shimanovich, R.;
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51, 2879.
8. 2,3-Diaryl-4-azaindoles are known as p38-MAP-kinase inhibitors: Trejo, A.;
Arzeno, H.; Browner, M.; Chanda, S.; Cheng, S.; Comer, D. D.; Dalrymple, S. A.;
Dunten, P.; Lafargue, J.; Lovejoy, B.; Freire-Moar, J.; Lim, J.; Mcintosh, J.; Miller,