Synthesis of 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine, a potent cyclin dependent kinase 1 (CDK1) inhibitor

Article abstract of DOI:10.1016/j.bmcl.2006.12.031

The novel compound 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine was discovered to be a potent CDK1 inhibitor. Described here is the chemistry for its synthesis, including Pd(II) catalyzed Stille coupling reaction and sulfur(0) induced benzimidazole formation. Its effects on VEGFR-2 kinase activity and tumour cell proliferation are also described.

Full text of DOI:10.1016/j.bmcl.2006.12.031

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1243–1245
Synthesis of 3-(1H-benzimidazol-2-yl)-5-isoquinolin-
4-ylpyrazolo[1,2-b]pyridine, a potent cyclin
dependent kinase 1 (CDK1) inhibitor
Shenlin Huang,* Ronghui Lin, Yang Yu, Yanhua Lu, Peter J. Connolly,
George Chiu, Shengjian Li, Stuart L. Emanuel and Steven A. Middleton
Johnson & Johnson Pharmaceutical Research & Development L. L. C., 1000 Route 202, Raritan, NJ 08869, USA
Received 15 November 2006; revised 2 December 2006; accepted 4 December 2006
Available online 15 December 2006
Abstract—The novel compound 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine was discovered to be a potent
CDK1 inhibitor. Described here is the chemistry for its synthesis, including Pd(II) catalyzed Stille coupling reaction and sulfur(0)
induced benzimidazole formation. Its effects on VEGFR-2 kinase activity and tumour cell proliferation are also described.
Ó 2006 Elsevier Ltd. All rights reserved.
The cell division cycle is one of the most fundamental
processes in biology that ensures the controlled produc-
tion of subsequent generations of cells with defined bio-
logical function. Under normal growth conditions, cell
proliferation is tightly regulated. This is achieved by a
complex network of proto-oncogenes and tumour-
suppressor genes that are components of various signal
transduction pathways. Activation of a proto-oncogene
and/or a loss of a tumour-suppressor gene can lead to
the unregulated cell proliferation and to the accumula-
tion of genetic errors which ultimately will result in
the development of cancer.1 In the eukaryotic cell cycle
a key role is played by cyclin dependent kinases, which
consist of regulatory subunits (such as CDK1, CDK2,
CDK4, or CDK6) and catalytic subunits (such as cyclin
A, B, D1, D2 or E). The coordinated activation of these
complexes drives the cells through the cell cycle and en-
sures the fidelity of the process.2 Each step in the cell cy-
cle is regulated by a distinct and specific cyclin
dependent kinase. For example, complexes of CDK4
and D-type cyclin govern the early G1 phase of the cell
cycle, while the activity of the CDK2/cyclin E complex is
rate limiting for the G1 to S-phase transition. The
CDK2/cyclin A kinase is required for the progression
through S-phase and the CDK1/cyclin B complex
controls the entry into M-phase.2 A key regulator of
these transitions is CDK1, a universal intracellular fac-
tor which triggers the G2/M transition of the cell cycle
in all organisms. Both biochemical and genetic evidenc-
es have shown that CDK1 is the primary activity re-
quired for a cell to enter mitosis in all eukaryotic cells.
There is now strong evidence that CDKs, their sub-
strates, and regulatory proteins are the target of genetic
alterations in many human cancers.3 Overexpression of
CDK1 and CDK4 has been reported in a large variety
of tumours.4,5
Developing small molecule CDK inhibitors for antican-
cer therapy has been the goal of intensive drug discovery
efforts at many pharmaceutical companies. Several
CDK inhibitors are currently in clinical trials including
the 2-aminothiazole derivative BMS-387032 (SNS-
032)6 and the purine analogue (R)-roscovitine (CYC-
202).7 There are also numerous candidates in preclinical
development.8 Recently, the disubstituted indazole scaf-
fold (1a) (Fig. 1) has emerged as a promising structural
core for enzyme inhibitors with excellent potency
against numerous kinase targets.9 Part of our CDK ef-
fort was directed towards this series. In addition to
focusing on a 3,5-arrangement of the R1 and R2 side
chains, our strategy was to add one additional heteroat-
om to the central indazole ring,10 by which we hoped to
generate a novel scaffold with better potency and desir-
able PK properties. Though all the three unsubstituted
carbons (C-4, C-6, C-7) of the indazole ring could poten-
tially be replaced with nitrogen, only the chemistry for
Keywords: Pyrazolo[1,2-b]pyridine; Cyclin dependent kinase 1 inhibitor.
*
Corresponding author. Tel.: +1 858 332 4684; e-mail:
shenlinhuang@hotmail.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.12.031

1244
S. Huang et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1243–1245
afford intermediate 4, which was then coupled with trib-
utyl(vinyl)tin to give compound 5 in 70% yield.13 Care-
ful temperature control in this step was necessary in
order to avoid the coupling reaction of tributyl(vinyl)tin
with the bromide-bearing pyridine ring. Next, ozonoly-
sis of the vinyl group followed by reduction with
dimethyl sulfide gave aldehyde 6.14 Direct conversion
of iodo compound 4 to compound 6 was also explored
by treating intermediate 4 with tert-butyl lithium, fol-
lowed by addition of DMF. Though various conditions
were studied, the desired product could only be generat-
ed in very low yield with the de-iodinated compound
appearing as the major side product. Then, cyclization
of carboxaldehyde 6 with 1,2-phenylenediamine in the
presence of sulfur(0) generated the benzimidazole
compound 7.15 Coupling of bi-aryl compound 7 with
4-(trimethylstannyl)isoquinoline gave tri-aryl compound
8 in 81% yield.16 Last, the BOC group was removed with
trifluoroacetic acid in dichloromethane to afford the
final product 1b.
HN
O
N
N
N
O
N
S
N
HO
S
N
H
N
H
N
NH
CYC-202 (R-roscovitine)
SNS-032 (BMS-387032)
N
HN
R1
R2
N
N
N
N
N
N
H
H
1a
1b
Figure 1.
Compound 1b was tested for its ability to inhibit CDK1
activity, which was evaluated using CDK1 in complex
with cyclin B to phosphorylate a histone-H1 biotinyla-
ted peptide substrate. Inhibition of CDK1 activity was
measured by observing a reduced amount of 33P-c-
ATP incorporated into the immobilized substrate in a
Flashplate assay format. In the assay, this compound
showed high potency against CDK1 with an IC50 of
23 nM.
pyrazolo[1,2-b]pyridine synthesis was successfully devel-
oped. Described here is the synthesis of our lead com-
pound 1b (Fig. 1). Its potency for CDK1 and other
biological results are also presented.
The synthesis began with iodination of pyrazolo[1,2-
b]pyridine11 as shown in Scheme 1. Treatment of this
intermediate with iodine in 1,4-dioxane afforded iodo
compound 2, which was subsequently reacted with bro-
mine in acetic acid to generate bromide 3.12 Direct cou-
pling of compound 3 with tributyl (vinyl)tin in the
presence of Pd(II) failed to provide the desired product.
Therefore, the N-1 was protected with BOC group to
Compound 1b was also tested for its effect on the prolif-
eration of several cultured human tumour cell lines as
measured by the ability of cells to incorporate [14C]thy-
I
I
I
Br
Br
a
b
c
N
N
N
N
N
H
N
N
H
N
H
N
N
N
N
BOC
2
3
4
O
N
HN
H
N
Br
Br
d
Br
e
f
N
N
N
N
N
N
N
N
BOC
BOC
BOC
5
6
7
N
N
HN
N
HN
N
g
N
N
h
N
N
N
N
BOC
H
8
1b
Scheme 1. Synthesis of 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine. Reagents and conditions: (a) I2 (1 equiv), NaOH
(1 equiv), 1,4-dioxane, 40 °C, 8 h, 90%; (b) Br2 (2 equiv), acetic acid, 100 °C, 12 h, 52%; (c) (BOC)2O (1 equiv), DMAP (cat.), Et3N (1 equiv), THF,
94%; (d) tributyl (vinyl)tin (1 equiv), Pd(II)(PPh3)2Cl2 (cat.), THF, 80 °C, 12 h, 70%; (e) ozone, Me2S, CH2Cl2, À78 °C, 50%; (f) sulfur(0) (1 equiv),
1,2-phenylenediamine (1 equiv), DMF, 90 °C, 12 h, 70%; (g) Pd(II)(PPh3)2Cl2 (cat.), CuI (cat.), 4-(trimethylstannyl)isoquinonine (1 equiv), 80 °C,
12 h, THF, 81%; (h) TFA (5 equiv), DCM, 38%.

S. Huang et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1243–1245
1245
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In summary, a novel heterocyclic compound was discov-
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may also be useful for the treatment of other indications
such as restenosis, psoriasis, retinopathy and allopecia.8
An efficient chemistry was developed for the synthesis,
with key steps including Pd(II) catalyzed Stille coupling
reaction and sulfur(0) induced benzimidazole formation.
A detailed structure–activity relationship (SAR) study
and pharmacokinetic properties for this series will be
reported in due course.
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Acknowledgments
We are grateful to Robert H. Gruninger and Angel
Fuentes-Pesquera for running the kinase and cell prolif-
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