Pyrido[2,3-d]pyrimidin-7-ones as specific inhibitors of cyclin-dependent kinase 4

Article abstract of DOI:10.1021/jm049355+

Inhibition of the cell cycle kinase, cyclin-dependent kinase-4 (Cdk4), is expected to provide an effective method for the treatment of proliferative diseases such as cancer. The pyrido[2,3-d]-pyrimidin-7-one template has been identified previously as a privileged structure for the inhibition of ATP-dependent kinases, and good potency against Cdks has been reported for representative examples. Obtaining selectivity for individual Cdk enzymes, particularly Cdk4, has been challenging. Here, we report that the introduction of a methyl substituent at the C-5 position of the pyrido[2,3-d]pvrimidin-7-one template is sufficient to confer excellent selectivity for Cdk4 vs other Cdks and representative tyrosine kinases. Further optimization led to the identification of highly potent and selective inhibitors of Cdk4 that exhibit potent antiproliferative activity against human tumor cells in vitro. The most selective Cdk4 inhibitors were evaluated for antitumor activity against MDA-MB-435 human breast carcinoma xenografts in mice.

Full text of DOI:10.1021/jm049355+

J. Med. Chem. 2005, 48, 2371-2387
2371
Pyrido[2,3-d]pyrimidin-7-ones as Specific Inhibitors of Cyclin-Dependent
Kinase 4
Scott N. VanderWel,*,† Patricia J. Harvey,‡ Dennis J. McNamara,† Joseph T. Repine,† Paul R. Keller,‡
John Quin III,† R. John Booth,† William L. Elliott,‡ Ellen M. Dobrusin,† David W. Fry,‡ and Peter L. Toogood†
Department of Medicinal Chemistry, and Cancer Pharmacology, Pfizer Global Research and Development,
Michigan Laboratories, 2800 Plymouth Road, Ann Arbor, Michigan 48105
Received August 6, 2004
Inhibition of the cell cycle kinase, cyclin-dependent kinase-4 (Cdk4), is expected to provide an
effective method for the treatment of proliferative diseases such as cancer. The pyrido[2,3-d]-
pyrimidin-7-one template has been identified previously as a privileged structure for the
inhibition of ATP-dependent kinases, and good potency against Cdks has been reported for
representative examples. Obtaining selectivity for individual Cdk enzymes, particularly Cdk4,
has been challenging. Here, we report that the introduction of a methyl substituent at the C-5
position of the pyrido[2,3-d]pyrimidin-7-one template is sufficient to confer excellent selectivity
for Cdk4 vs other Cdks and representative tyrosine kinases. Further optimization led to the
identification of highly potent and selective inhibitors of Cdk4 that exhibit potent antiprolif-
erative activity against human tumor cells in vitro. The most selective Cdk4 inhibitors were
evaluated for antitumor activity against MDA-MB-435 human breast carcinoma xenografts in
mice.
Introduction
CIP/KIP (e.g. p21, p27, p57) proteins are endogenous
inhibitors of Cdk/cyclin complexes that provide a mech-
anism for the regulation of Cdk activity.5
A defining characteristic of cancer is cellular prolif-
eration that continues without regard for the regulatory
mechanisms that exist in normal cells.1,2 In healthy
tissue, cell division occurs in the context of a highly
regulated series of events known as the cell cycle, which
is composed of four phases.3 DNA replication occurs in
S phase and cell division occurs in M phase. S phase is
preceded by the gap phase, G1, during which cells are
preparing for DNA synthesis. A second gap phase, G2,
separates the S and M phases, allowing an opportunity
for cells to perform surveillance of the newly prepared
DNA and to prepare for cell division. Transitions
through each of these phases are controlled by serine-
threonine kinases called cyclin dependent kinases (Cdks),
and their partner cyclins that act as regulatory sub-
units.4 The activity of the holoenzymes is controlled by
extracellular and intracellular signals that dictate the
level of cyclins in the cell.
The Cdk/cyclin pairs that appear to be most essential
for the regulation of progression through the cell cycle
are Cdk1/cyclin B (Cdk1/B), Cdk2/A, Cdk2/E, and
Cdk4/D or its orthologue Cdk6/D. Passage through
S-phase is controlled by Cdk2/A, while Cdk1/B regulates
passage through G2 and entry into M-phase. Cdk4/D
and Cdk2/E restrict the passage through G1 and the
commitment to DNA synthesis via phosphorylation of
the retinoblastoma protein (pRb), a transcriptional
regulator. Hyperphosphorylated pRb dissociates from
the transcription factor E2F making this protein avail-
able to direct the expression of proteins essential for
DNA synthesis. The INK4 (e.g. p15, p16, p18, p19) and
Considerable evidence implicates misregulation of the
Cdk4/p16/Rb pathway in diseases of uncontrolled cell
growth.6 For example, up-regulation of this pathway is
associated with more than 90% of all human tumors.
Overexpression of cyclin D1, mutation of Cdk4, mutation
or deletion of pRb, or deletion of p16 have all been
observed. Consequently, it has been postulated that
specific inhibitors of Cdk4 may restore normal cell
activity and could be used for the treatment for cancer
and other diseases of uncontrolled cell growth.7
Since the first characterization of cyclin-dependent
kinases as key modulators of the cell cycle, these
enzymes have been targeted for small molecule inter-
vention in a variety of proliferative diseases.8-15 By far
the largest effort has focused on the discovery of novel
agents for the treatment of cancer, but other diseases
that might potentially be treatable with Cdk inhibitors
include restenosis, malaria, and some neurodegenera-
tive diseases. The first Cdk inhibitors identified were
relatively nonspecific agents such as butyrolactone,
flavopiridol and UCN-01 (Figure 1).6,13 Several of these
compounds were advanced to clinical trials before their
mechanisms of action were thoroughly understood.
Consequently, a great deal of information regarding the
effect of putative Cdk inhibitors in patients may be
misleading due to a lack of specificity of the compounds
employed. A second generation of Cdk inhibitors was
drawn largely from the class of purines,16-27 following
the initial discovery of olomoucine,25 roscovitine26 and
subsequently, purvalanols,22 as potent and moderately
selective Cdk1/2 inhibitors. R-Roscovitine, known as
CYC-202, is currently in phase-II human clinical trials
and is reported to be relatively nontoxic.28 The purine
template has provided a rich source of Cdk inhibitors,
* Corresponding author. Tel: (734) 622 5147. Fax: (734) 622 5165.
E-mail: Scott.VanderWel@pfizer.com.
† Department of Medicinal Chemistry.
‡ Cancer Pharmacology.
10.1021/jm049355+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/02/2005

2372 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
Figure 1. First and second generation Cdk inhibitors.
but the majority of these compounds are more potent
against Cdk1 and Cdk2 than Cdk4. A third generation
of Cdk inhibitors is now appearing in the literature that
encompasses a broad array of structural classes.29-78
Among this generation of inhibitors are compounds that
display quite remarkable levels of selectivity for specific
Cdks. It is still the case that more Cdk1/2 inhibitors
have been reported than Cdk4 inhibitors, but modest
selectivity for Cdk4 has been achieved with a variety of
structurally dissimilar inhibitors. For example, Honma
and co-workers58,59 used a Cdk4 homology model to
design a urea derivative that is 190-fold selective for
Cdk4 versus Cdk2/Cyclin A in vitro. Cdk4 selectivity
also has been reported for pyrimidine derivatives,60-62
and carbazoles.63-68 Preliminary reports have started
to appear of highly selective Cdk4 inhibitors based on
the diaminothiazole template.76
The discovery of pyrido[2,3-d]pyrimidin-7-ones as
inhibitors of Cdks,77-79 followed by optimization of the
substituents at C-2 and N-8, identified compound 1 as
a potent but poorly selective inhibitor of Cdk4/D versus
Cdk2/A, Cdk2/E and Cdk1/B. While related compounds
did achieve improved levels of selectivity, changes to the
C-2 and N-8 substituents alone failed to contribute
significantly to selectivity for one enzyme over another
in any general way. Consequently, a more extensive
investigation of the structure-activity relationships for
pyrido[2,3-d]pyrimidin-7-one inhibition of Cdk4 was
initiated, and the interesting results associated with
modifications at the C-5 and C-6 positions of this
template are detailed here. Remarkable levels of selec-
tivity for Cdk4 vs other kinases have been achieved by
appropriate substitution at these two positions.
Figure 2. Binding mode of purvalanol B and compound 66
from ref 77.
been postulated that space might be limited around the
inhibitor C-5 and C-6 positions because of their proxim-
ity to Phe-80 (Cdk2 numbering). This residue is con-
served in Cdk4 (Phe-93), but without a crystal structure
its precise location in Cdk4 is not known.
Purine-based Cdk ligands adopt a variety of binding
modes in the Cdk2 ATP site, as exemplified by olomou-
cine, NU2058 and ATP itself, each of which display a
different binding orientation.80 The purvalanols are
potent purine-based Cdk2 inhibitors, which adopt a
binding conformation similar to olomoucine.22 A com-
parison of purvalanol B with pyrido[2,3-d]pyrimidin-7-
one Cdk inhibitors bound in the ATP binding site of
Cdk2 revealed that the isopropyl group at N-9 of
purvalanol B occupies the same region of the space in
the ATP binding pocket as the C-5 hydrogen of the
pyrido[2,3-d]pyrimidin-7-ones, suggesting that addi-
tional potency might be realized by substitution at the
C-5 position of pyrido[2,3-d]pyrimidin-7-ones (Figure 2).
Results and Discussion
The initial impetus for focusing on the C-5 position
derived from a comparison of pyrido[2,3-d]pyrimidin-
7-one and purine Cdk inhibitors bound to the ATP
binding site of Cdk2. In the absence of structural data
for Cdk4, crystal structure data obtained using Cdk2
has been used widely to guide Cdk inhibitor design.
From previous crystal structures obtained by this group
of pyrido[2,3-d]pyrimidin-7-ones bound to Cdk2, it had
To explore this hypothesis, a selection of previously
described pyrido[2,3-d]pyrimidin-7-one Cdk inhibitors
were modified to include a methyl substituent at the

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2373
Table 2. Varying the C5 and N8 Substituents
Table 1. Effect of the C5 Methyl Group on Enzyme
Selectivitya
Cdk4/D Cdk1/B Cdk2/A Cdk2/E
Cdk4/D Cdk1/B Cdk2/A Cdk2/E
compd R1
R2
IC50 (µM) IC50 (µM) IC50 (µM) IC50 (µM)
IC50
IC50
IC50
IC50
compd R1
X
(µM)
(µM)
(µM)
(µM)
11 Et c-pentyl
12 CF3 c-pentyl
0.655
2.650
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
1
2
H
H
H
H
H
N-Me
O
0.007
0.010
0.010
NA
0.014
0.028
0.660
NA
0.039
0.085
0.246
4.550
0.080
0.275
0.570
13 Me isopropyl 0.265
14 Me isopentyl 1.075
3
CH2
4
CH(CH2)3OH 0.034 >5
5
NH
0.006
NA
0.024
attention was turned to substituents at the N-8 position,
which are proposed to occupy the ATP ribose-binding
pocket, upon binding to Cdk4. Changes in this position
did not further enhance potency, but also did not appear
to diminish the selectivity for Cdk4. Compound 13 with
the iso-propyl group at N-8 was 20-fold less potent
versus Cdk4/D than the cyclopentyl derivative, com-
pound 10, while opening the cyclo-pentyl ring to an iso-
pentyl group as in compound 14, led to a more substan-
tial loss in activity. In our previous study, substitutions
at N-8 as large as norbornyl were tolerated by the
enzyme and inhibitory potency appeared to increase
with increasing size of the alkyl. In contrast, for C-5-
methyl pyrido[2,3-d]pyrimidin-7-ones, the data indicate
that substitution at the C-5 position places a limitation
on the size of substituent that can be accommodated in
the ribose-binding pocket.
6
Me N-Me
Me
Me CH2
0.018 >5
>5
>5
7
O
0.116
0.180
1.120 >5
NA
>5
NA
>5
>5
8
>5
>5
>5
9
Me CH(CH2)3OH 0.114 >5
10
Me NH
0.014 >5
a NA means data not available.
C-5 position. These compounds were assessed for their
ability to inhibit a set of four cyclin-dependent kinases
including Cdk1/B, Cdk2/A, Cdk2/E, and Cdk4/D (Table
1). Contrary to the hypothesis, the C-5-methyl ana-
logues proved to be less active against all four kinases
than their C-5-hydrogen counterparts. Unexpectedly,
however, the C-5-methyl analogues all exhibited ex-
quisite selectivity for Cdk4/D versus the other Cdks.
This effect appears to be quite general and independent
of the nature of the C-2 and N-8 substituents.
The observation that C-5 methyl substituted pyrido-
[2,3-d]pyrimidin-7-ones do not inhibit Cdk1 and Cdk2
is consistent with the postulate that there is a space
limitation imposed at the back of the ATP binding site
by the side chain of Phe-80 (Cdk2 numbering). However,
it is not clear why a similar substituent protruding into
this region of space is tolerated when it is presented on
a purine template. Intriguingly, in the absence of
detailed information on the three-dimensional structure
of Cdk4, these selective inhibitors reinforce the notion
that the structure of Cdk4 differs in significant ways
from the structures of other cell cycle-associated Cdks.
To further explore what groups might be tolerated by
Cdk4/D at the C-5 position of the pyrido[2,3-d]pyimidin-
7-one template, a small set of closely related substitu-
ents were explored (Table 2). First, the size of the
substituent was increased from methyl to ethyl as in
compound 11. This change diminished the potency for
Cdk4/D nearly 50-fold, indicating that the binding
pocket in the enzyme is quite small and that further
increases in substituent size would be futile. Satisfy-
ingly, however, the selectivity for Cdk4/D appeared to
be retained. A similar result was observed when the C-5-
methyl group was replaced with trifluoromethyl as in
compound 12. In this case, either the size of the
trifluoromethyl group was too great, or the negative
charge associated with the fluorines being forced adja-
cent to the π-electron cloud of Phe-93 was detrimental
to potency.
With the C-5 and N-8 positions fixed as methyl and
cyclo-pentyl, respectively, the heterocycle connected to
the phenylamine at the C-2 position of the pyrido[2,3-
d]pyrimidin-7-one was varied in an attempt to further
improve the potency for Cdk4. Previous work from this
lab has demonstrated the potential for achieving sig-
nificant changes in potency against Cdk4/D purely as a
function of varying the nature of the C-2 side chain. In
addition, Schultz and co-workers have described a
specific hydrogen bonding interaction between a chlo-
rophenyl group in purvalanol and residue Asp-86 in
Cdk2 that contributes to inhibitor binding.22 Honma and
co-workers similarly attributed improvements in po-
tency displayed by urea-based inhibitors to specific
interactions between the ATP-competitive ligand and
residues Thr-102 and Asp-99 in the mouth of the ATP
binding site.58,59
Acylation of the piperazine nitrogen led to a substan-
tial drop in potency for Cdk4/D as seen for the formyl,
acetyl, and tert-butyl carbamoyl derivatives 15, 16, and
17 (Table 3). The racemic 3-hydroxy-pyrrolidine deriva-
tive, 18, also displayed a sizable drop in potency
(Cdk4/D IC50 ) 0.26 µM); in contrast, the racemic
3-aminopyrrolidine, 19, inhibited Cdk4/D with IC50
)
0.064 µM, only 4-5-fold less potent than piperazine 10.
These data suggest the presence of a productive binding
interaction between an ionizable amine and a residue
in the protein, located in the mouth of the ATP binding
pocket, tentatively identified as Asp-99. In general,
analogues closely resembling compound 10 retained
With compound 10 showing the best profile thus far
with respect to potency and selectivity for Cdk4/D,

2374 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
Table 3. Reoptimizing the C2 Substituenta
VanderWel et al.
In an attempt to simulate the electron-withdrawing
character of a halogen while maintaining a relatively
small volume, ketones and carboxyl derivatives were
investigated. This approach identified methyl ketone 31
as the most potent compound of this study, but this
inhibitor lacked the level of selectivity versus Cdk2/A
now known to be achievable. Interestingly, ketone 31
is selective for Cdk4/D vs Cdk1/B and Cdk2/E and
against a wide selection of additional serine/threonine
and tyrosine kinases (Table 5), while retaining substan-
tial potency against Cdk2/A. Carboxylic acid 32 regained
the desired selectivity for Cdk4/D, but was 2-fold less
potent than compound 10 and 16-fold less potent than
compound 31. In contrast, both potency and selectivity
was achieved with the methyl ester 33 (IC50 ) 0.004
µM) and the ethyl ester 34 (IC50 ) 0.006 µM). The ethyl
ester represents one of the most selective Cdk4/D
inhibitors reported to date. This set of compounds (Table
4) was further evaluated for the ability to inhibit tumor
cell proliferation in vitro as measured by the incorpora-
tion of [14C]-thymidine into two cell lines, HCT116
human colon carcinoma and MDA-MB-435 human
breast carcinoma. Potent inhibition of cell proliferation
was observed (IC50 ) 0.032-1.35 µM for active com-
pounds), and pleasingly the most potent Cdk4 inhibitors
were among the most potent inhibitors in cells, with the
exception of carboxylic acid 32, which was inactive in
cells. In general, the HCT116 and MDA-MB-435 cell
lines exhibited comparable sensitivity (within a factor
of 2, except for compound 34) to the Cdk inhibitors
tested, however, HCT116 cells appeared to be less
readily inhibited by the most selective Cdk4 inhibitors
(Table 4 and data not shown).
Compounds, 31, 33, and 34 were tested for antitumor
activity in vivo, using the MDA-MB-435 human tumor
xenograft model in nude mice. The two esters were
strikingly inactive, possibly due to metabolism of the
esters to give 32, which is inactive in cells. Although,
in vitro experiments employing either mouse plasma,
or chemical hydrolysis at room temperature, indicated
only very slow conversion to the acid under these
conditions, related to steric encumbrance imposed by
the adjacent methyl group. In contrast, ketone 31
caused significant reductions in the growth rate of
MDA-MB-435 tumors, giving a tumor growth delay
compared to untreated controls (T - C) of 9.4 days for
a 14 day, once daily dosing regimen of 75 mg/kg.
Cdk4/D
Cdk1/B
Cdk2/A
Cdk2/E
compd R3
Y
IC50 (µM) IC50 (µM) IC50 (µM) IC50 (µM)
15
16
17
18
19
20
21
22
23
24
a
b
c
d
e
f
H
H
H
H
H
H
H
H
H
Cl
0.161
0.103
0.425
0.260
0.064
0.052
0.035
0.240
0.065
0.028
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
>5
NA
>5
>5
>5
>5
>5
>5
>5
g
h
i
j
a NA means data not available.
most of the potency and selectivity of compound 10, as
seen for example with homopiperazine 20, N-(2-hy-
droxyethyl)-piperazine 21, and gem-3,3-dimethylpipera-
zine 23. One exception was the cis-3,5-dimethylpipera-
zine derivative, 22, which displayed a drop in activity
of greater than 1 order of magnitude, suggesting that
the piperazine ring binds quite snugly to the protein
leaving only limited room for substitution, in contrast
to some models that suggest that this terminal exten-
sion of the C-2 side chain projects out into solvent. An
attempt to exploit the hydrogen bond that is proposed
to assist purvalanol binding to Cdk2 by adding a
chlorine to the side chain phenyl ring led to a 2-fold drop
in potency suggesting the absence of any special stabi-
lizing force in this case.
Attention was focused next on the C-6 position. As
for the C-5 position, models of pyrido[2,3-d]pyrimidin-
7-ones bound to Cdk2 suggested that there may be a
limitation on the size of substituent tolerated at C-6 due
to its proximity to Phe-80. The results obtained with
C-5 substituted pyrido[2,3-d]pyrimidin-7-ones suggested
that this hypothesis should be examined closely for
inhibitors of Cdk4. Initial forays in this direction were
discouraging. For example, the C-5, C-6 dimethyl de-
rivative (25) was 10-fold less potent than compound 10
although selectivity for Cdk4/D was maintained (Table
4). However, the C-5 methyl, C-6 ethyl derivative 26
retained good potency for Cdk4/D, but was only 61-fold
selective for Cdk4/D vs Cdk2/A. When alkyl groups were
replaced by halogens at C-6, a similar trend was
observed. As the size of the halogen was increased the
potency for Cdk4/D increased, but the compounds
became less selective for Cdk4/D versus other Cdks. For
example, fluoro analogue 27 is less potent, but more
selective than chloro analogue 28, and iodo analogue
30 is the most potent and least selective member of the
series (Table 4). These unexpected trends indicated a
subtle balance between the two desirable properties of
potency and selectivity and suggested that the optimal
inhibitor may not be the most potent.
Despite success in treating tumors with compound 31,
the question remained whether a truly selective Cdk4
inhibitor, with no activity against other Cdks, could
inhibit tumor growth in vivo. Since the failure to achieve
efficacy with compounds 33 and 34 could be explained
by potential metabolic instability, it seemed likely that
the optimal experiment to address this question had yet
to be performed. A metabolically more stable, highly
selective Cdk4 inhibitor was required to provide a
sufficiently reliable and definitive tool for in vivo stud-
ies. The pursuit of such a tool was ultimately rewarded
as described in the accompanying manuscript.87
Chemistry
The compounds described in this manuscript were
obtained via small modifications and additions to the
chemistry described previously.77 The general synthetic

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2375
Table 4. Investigations of the C6 Substituenta
Cdk4/D
IC50 (µM)
Cdk1B
IC50 (µM)
Cdk2/A
IC50 (µM)
Cdk2/E
IC50 (µM)
HCT116
IC50 (µM)
MDAMB435
IC50 (µM)
compd
R4
25
26
27
28
29
30
31
32
33
34
Me
0.165
0.025
0.030
0.016
0.005
0.005
0.002
0.032
0.004
0.006
>5
NA
1.538
>5
1.625
0.439
0.443
0.230
>5
2.819
>5
>5
NA
0.75
1.35
0.329
0.220
0.104
NA
NA
0.425
NA
Et
4.119
>5
1.650
>5
F
Cl
>5
1.500
0.950
0.365
NA
NA
Br
2.615
1.865
NA
NA
NA
0.032
>3
0.190
0.170
I
COMe
CO2H
CO2Me
CO2Et
>5
>5
>3
>5
>5
0.340
0.830
>5
>5
a NA means data not available.
Table 5. Inhibitory Activity of Compound 31 against a Panel
lowed by oxidation with the Dess-Martin periodinane
(DMP).84 Ketones D were converted to pyrido[2,3-d]-
pyrimidin-7-ones E using Horner-Wadsworth-Em-
mons chemistry. Thus, each ketone was treated with
triethyl phosphonoacetate (or a substituted version
thereof) and sodium hydride in THF with warming until
condensation and elimination were complete. This ap-
proach permitted the introduction of hydrogen, alkyl
groups or fluorine at the C-6 position of the pyrido[2,3-
d]pyrimidin-7-one. Compounds substituted at C-6 with
bromine were obtained by treatment of compounds E
with N-bromosuccinimide. A similar reaction to install
iodine with N-iodosuccinimide was not successful, but
iodo-derivatives were available by treatment of pyrido-
[2,3-d]pyrimidin-7-ones E with iodine and bis(trifluo-
roacetoxy)iodobenzene. Treatment of a representative
compound E with N-chlorosuccinimide resulted in C-6
chlorination but also caused oxidation and chlorination
of the methyl sulfide as shown in Scheme 2. This
sulfoxide was used in subsequent displacement reac-
tions to introduce the C-2 aniline in an identical manner
to sulfoxides G in Scheme 1. For compounds E in which
R1 ) hydrogen, N-8 alkyl substituents were introduced
at this stage by treatment with sodium hydride and an
alkyl halide such as 2-iodopropane.
The methyl sulfides F (including when step v is
omitted and Y ) X) were oxidized to methyl sulfoxides
G using 2-benzenesulfonyl-3-phenyl-oxaziradine85,86 in
preparation for installation of the C-2 aniline. Side chain
anilines were prepared in two steps as shown in Scheme
3. Aromatic nucleophilic substitution of fluorine in
1-nitro-4-fluorobenzene by amines was readily achieved
in acetonitrile under reflux. The resulting nitrobenzenes
were reduced to anilines using Raney nickel and gener-
ally used without further purification. Sulfoxide dis-
placement reactions were typically performed in DMSO
with heating to 80-100 °C. In many cases the product
H precipitated from the reaction mixture upon cooling.
When this was not the case, an aqueous workup and
extraction into ethyl acetate or methylene chloride was
sufficient to isolate the crude product, which was further
of Protein Kinases
protein kinase
IC50 (µM)a
fibroblast growth factor receptor (FGFr)
platelet growth factor receptor (PDGFr)
C-terminal src kinase (CSK)
glycogen synthase kinase-3â (GSK3â)
c-Jun N-terminal kinase (JNK)
mitogen-activated protein kinase (MAPK2/erk2)
MAPK-activated protein kinase-2 (MAPKAP-K2)
MAPK kinase (MKK1)
1.86
2.46
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
>10
mitogen and stress-activated protein kinase-1 (MSK1)
P70 ribosomal protein S6 kinase (p70S6K1)
3-phosphoinositide-dependent protein kinase-1 (PDK-1)
phosphorylase kinase (PHK)
protein kinase A (PKA)
protein kinase B (PKB)
protein kinase C (PKC)
P38-regulated/activated kinase (PRAK)
stress-activated protein kinase-2a (SAPK2a)
stress-activated protein kinase-2b (SAPK2b)
stress-activated protein kinase-3 (SAPK3)
stress-activated protein kinase-4 (SAPK4)
a Concentration of compound 31 necessary to inhibit activity
by 50%. Values represent the mean of at least two separate
determinations.
approach is summarized in Scheme 1. Starting from
commercially available 4-chloro-2-methylthio-5-pyrim-
idinecarboxylic acid ethyl ester (A), pyrimidines B were
obtained by displacement of the chlorine with amines
including ammonia, isopentylamine and cyclopentyl-
amine. The ester functional group then was converted
to an aldehyde via a two-step reduction-oxidation
sequence employing lithium aluminum hydride followed
by manganese(IV) oxide to give aldehydes C. Intro-
duction of alkyl substituents at what will become the
C-5 position of the pyrido[2,3-d]pyrimidin-7-ones was
achieved most commonly using Grignard chemistry.
Aldehydes C were treated with methylmagnesium
bromide or ethylmagnesium bromide to yield secondary
alcohols, which then were oxidized to ketones D using
either N-methylmorpholine N-oxide (NMO) and cata-
lytic tetra-n-propylammonium perruthenate (TPAP),81
or manganese(IV) oxide. The trifluoromethyl group was
installed using Ruppert’s reagent (TMS-CF3)82,83 fol-

2376 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
Scheme 1. General Scheme for the Synthesis of Pyrido[2,3-d]pyrimidine CDK Inhibitorsa
a (i) R1-NH2, Et3N, THF. (ii) a. LiAlH4, THF. b. MnO2, CH2Cl2, or TPAP, NMO, CH2Cl2. (iii) a. R2MgBr, THF or Rupperts reagent,
THF. b. MnO2, CH2Cl2 or DMP. (iv) (EtO)2P(O)CHXCO2Et, NaH, THF. (v) For Y ) Br: NBS, DMF; For Y ) I: I2, (CF3CO2)2I-Ph, DMF;
this step is skipped when Y ) X. (vi) Davis oxaziridine, CH2Cl2. (vii) R3-Ph-NH2, DMSO. (viii) Z-Met, palladium catalysis.
Scheme 2. Chlorination of a Representative Compound
E (X ) H) Using NCS
of methanol or ethanol to give the methyl and ethyl
esters 96 and 97, respectively. The ethyl ester was
hydrolyzed under forcing conditions to give acid 98.
Compounds 96-98 were deprotected with hydrochloric
acid yield final products 33, 34, and 32. Similar pal-
ladium-mediated cross-coupling reactions also could be
performed at an earlier stage in the synthetic route,
notably starting from intermediates F (Y ) Br). Thus,
compound 63 (Scheme 5) was treated with (1-ethoxyvi-
nyl)tributyltin under Stille conditions to give the ketone
65 after aqueous workup (Scheme 5). This compound
then was oxidized at sulfur and coupled to the N-Boc-
piperazinyl-phenylamine side chain to give intermediate
91. Cleavage of the Boc group generated the C-5 methyl,
C-6 methyl ketone analogue 31.
Scheme 3. Preparation of the Side Chain Anilines.
Amines R4R5NH Include N-Boc-piperazine,
3-(N-Boc-amino)-pyrrolidine, Piperidine, Morpholine
Experimental Section
purified by silica gel chromatography. Compounds 2-4,
6-9, 15-18, and 21 required no deprotection after the
aniline side chains were installed and were tested in
their free base form. In contrast, compounds H and I
containing Boc-piperazines, Boc-homopiperazine, or Boc-
aminopyrrolidines were deprotected using a solution
containing either hydrochloric acid or trifluoroacetic acid
(TFA) to give final compounds 5, 10-14, 19, 20, 22-
24, and 25-34, which were tested as their HCl or TFA
salts. Compound 15 was obtained by refluxing com-
pound 10 in ethyl formate with a catalytic amount of
formic acid.
Methods for further elaboration of the C-6 position
in compounds H are represented generically in Scheme
1 and further detailed in Scheme 4. Starting from C-6
bromo-pyrido[2,3-d]pyrimidin-7-ones, a variety of orga-
nometallic coupling reactions may be employed for
replacement of the halogen with other groups. For
example, compound 89 was combined with TMS-
acetylene under Suzuki conditions (CuI, Pd(PPh3) 4, and
butylamine), to give alkyne 93. The ethynyl group was
deprotected to give intermediate 94, then hydrogenated
to give the C-6 ethyl derivative 95. Compound 95 was
deprotected with acid to provide compound 26. The C-6
esters were readily obtained by treating bromide 89
with Pd(OAc)2 under pressurized CO2 in the presence
General Methods. NaH refers to 60 wt % NaH in mineral
oil. All solvents and reagents were used as obtained. Anhy-
drous solvents were obtained commercially and used without
further drying. Melting points were determined with a Thomas-
Hoover capillary melting point apparatus or a MEL-TEMP
melting point apparatus and are uncorrected. 1H NMR spectra
were recorded using a Varian Unity 400-MHz spectrometer.
Chemical shifts are in parts per million (δ) referenced to Me4-
Si (0.00 ppm) or CHCl3 (7.24 ppm). The amount of solvent or
water present in the molecular formula was determined by
1H NMR and microanalysis. Chemical ionization mass spectra
(CI) were recorded on a VG Trio 2 mass spectrometer instru-
ment using a reagent gas of 1% NH3 in CH4. Atmospheric
pressure chemical ionization mass spectra (APCIMS) were
recorded using a VG Trio 2000 mass spectrometer in a matrix
of MeOH/MeCN/DMSO. Combustion analyses (CHN) were
determined by Robertson Microlit Laboratories, Inc., Madison,
NJ.
4-Cyclopentylamino-2-methylsulfanyl-pyrimidine-5-
carboxylic Acid Ethyl Ester (36): 4-Chloro-2-methylsulfa-
nyl-pyrimidine-5-carboxylic acid ethyl ester (50.0 g, 215 mmol)
was dissolved in THF (1.2 L) to which triethylamine (65.2 g,
645 mmol) was added and stirred for 14 h. The precipitated
salts were filtered and the solvent evaporated in vacuo. The
resultant oil was dissolved in EtOAc (600 mL) and washed
with sodium bicarbonate (2 × 200 mL), then dried over MgSO4.
The salts were filtered, and the solvent was evaporated in
vacuo to give 36 as an orange oil (45.7 g, 76%). 1H NMR (400
MHz, CDCl3) δ 8.56 (s, 1H), 8.24 (br s, 1H), 4.43-4.47 (m, 1H),

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2377
Scheme 4. Elaborated at C6 by Pd-Mediated Cross-Coupling Reactionsa
a (i) TMS-acetylene, Pd(PPh3)4, nBuNH2, CuI. (ii) KOH. (iii) H2, Pd/C. (iv) HCl. (v) CO, Pd(OAc)2, ROH, 500 psi, 125 °C. (vi) a. NaOH
(aq), heat.
Scheme 5. Introduction of the C6 Acetyl Groupa
a (i) (1-Ethoxyvinyl)tributyltin, Pd(PPh3)4, HCl. (ii) Davis oxaziridine, CH2Cl2. (iii) 4-(4-amino-phenyl)-piperazine-1-carboxylic acid tert-
butyl ester, DMSO. (iv) HCl.
4.23-4.28 (q, J ) 7.4 Hz, 2H), 2.50 (s, 3H), 1.99-2.07 (m, 2H),
1.45-1.74 (m, 6H), 1.32 (t, J ) 7.1 Hz, 3H).
solution while keeping the reaction temperature below 12 °C.
After stirring for 6 h at room temperature, the reaction was
quenched by the addition of water (18 mL), then 15% NaOH
(18 mL) and then water again (30 mL). The white solid that
precipitated was filtered and the mother liquor evaporated in
vacuo. The resultant solid was triturated with heptane (150
mL) and filtered to give 39 as a pale yellow solid (33.4 g, 86%).
1H NMR (400 MHz, CDCl3) δ 7.53 (s, 1H), 5.86-5.91 (m, 1H),
4.42 (s, 2H), 4.34-4.37 (m, 1H), 2.45 (s, 3H), 1.99-2.07 (m,
2H), 1.39-1.70 (m, 6H).
(4-Amino-2-methylsulfanyl-pyrimidin-5-yl)-methanol
(40): Starting from 37 (40 g, 188 mmol), 31.6 g (98%) of 40
was obtained according to the method described for the
synthesis of 39: 1H NMR (400 MHz, DMSO-d6) δ 7.83 (s, 1H),
6.66 (br s, 2H), 5.04 (br s, 1H), 4.23 (s, 2H), 2.45 (s, 3H); m/z
172.1 (M + 1).
4-Amino-2-methylsulfanyl-pyrimidine-5-carboxylic Acid
Ethyl Ester (37): Starting from 4-chloro-2-methylsulfanyl-
pyrimidine-5-carboxylic acid ethyl ester (50 g, 215 mmol), 41.6
g (91%) of 37 was obtained according to the method described
for the synthesis of 36: 1H NMR (400 MHz, DMSO-d6) δ 8.55
(s, 1H), 7.98 (s, 1H), 7.60 (s, 1H), 4.18-4.24 (q, J ) 7.1 Hz,
2H), 2.41 (s, 3H), 1.22-1.25 (t, J ) 7.1 Hz, 3H).
4-(1-Ethyl-propylamino)-2-methylsulfanyl-pyrimidine-
5-carboxylic Acid Ethyl Ester (38): Starting from 4-chloro-
2-methylsulfanyl-pyrimidine-5-carboxylic acid ethyl ester (39.3
g, 169 mmol), 35.6 g (81%) of 38 was obtained according to
the method described for the synthesis of 36: 1H NMR (400
MHz, CDCl3) δ 8.50 (s, 1H), 8.05 (br s, 1H), 4.22-4.26 (m, 2H),
4.08-4.13 (m, 2H), 2.42 (s, 3H), 1.43-1.65 (m, 4H), 1.22-1.30
(m, 2H), 0.80-0.85 (m, 6H); m/z 284.0 (M + 1).
[4-(1-Ethyl-propylamino)-2-methylsulfanyl-pyrimidin-
5-yl]-methanol (41): Starting from 38 (38.6 g, 136 mmol), 30.0
g (92%) of 41 was obtained according to the method described
for the synthesis of 39: 1H NMR (400 MHz, CDCl3) δ 7.75 (s,
1H), 6.22-6.27 (m, 1H), 5.12-5.15 (m, 1H), 4.29 (s, 2H), 3.29
(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-methanol (39): Under a nitrogen atmosphere, LAH (9.22
g, 243 mmol) was suspended in THF (500 mL) and cooled with
an ice bath. Compound 36 (45.72 g, 162 mmol) was dissolved
in THF (160 mL) and added dropwise to the cooled LAH

2378 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
(s, 1H), 2.35 (s, 3H), 1.26-1.51 (m, 4H), 0.79-0.82 (m, 6H);
m/z 284.0 (M + 1).
stir bar were added 42 (1.5 g, 6.32 mmol), THF (30 mL) and
cesium fluoride (5 mg). (Trifluoromethyl)trimethylsilane (75
mL, 37.9 mmol) was then added via syringe to the reaction
and stirred at room temperature for 48 h. The reaction was
quenched with 0.5 N HCl (10 mL) and partitioned between
EtOAc and water. The layers were separated, the organic layer
washed with brine then dried over MgSO4. The salts were
filtered and the solvent evaporated in vacuo to give a crude
oil that was purified by silica gel chromatography eluting with
EtOAc and hexanes to give 49 as a light oil (160 mg, 25%). 1H
NMR (400 MHz, CDCl3) δ 7.50 (s, 1H), 6.63-6.64 (d, J ) 6.3,
1H), 4.78-4.84 (m, 1H), 4.37-4.43 (m, 1H), 2.48 (s, 3H), 1.98-
2.02 (m, 2H), 1.46-1.70 (m, 6H); m/z 308.0 (M + 1).
4-Cyclopentylamino-2-methylsulfanyl-pyrimidine-5-
carbaldehyde (42): Compound 39 (30.0 g, 125 mmol) was
dissolved in CHCl3 (1.2 L) to which MnO2 (62 g, 713 mmol)
was added and stirred for 16 h. An additional portion of MnO2
(16.6 g, 191 mmol) was added and stirred for 4 h. The solids
were removed by filtration through a Celite pad and washed
with CHCl3 (4 × 200 mL). The CHCl3 was evaporated in vacuo
to give 42 as a pale yellow solid (29 g, 98%). 1H NMR (400
MHz, CDCl3) δ 9.64 (s, 1H), 8.56 (br s, 1H), 8.23 (s, 1H), 4.45-
4.49 (m, 1H), 2.51 (s, 3H), 2.01-2.07 (m, 2H), 1.47-1.76 (m,
6H).
1-(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-ethanone (50): Compound 45 (1.09 g, 4.3 mmol) was
dissolved in CH2Cl2 (15 mL) to which powdered molecular
sieves (4 Å), N-methyl morpholine oxide (NMO, 1.07 g, 8.6
mmol) and tetrapropylammonium perruthenate (TPAP, 0.227
g, 0.645 mmol) were added successively. The reaction was
stirred at ambient temperature for 2 h. The reaction mixture
was then purified using silica gel chromatography (1:1, EtOAc:
Hex) to yield 50 as a light yellow solid (0.74 g, 70%): 1H NMR
(400 MHz, CDCl3) δ 9.21 (s, 1H) 8.53 (s, 1H), 4.47-4.53 (m,
1H), 2.53 (s, 3H), 2.49 (s, 3H), 2.02-2.08 (m, 2H), 1.51-1.78
(m, 6H); m/z 252.2 (M + 1).
1-(4-Amino-2-methylsulfanyl-pyrimidin-5-yl)-etha-
none (51): Starting from 46 (4.75 g, 25.6 mmol), 1.95 g (42%)
of 51 was obtained according to the method described for the
synthesis of 50: 1H NMR (400 MHz, CDCl3) δ 8.72 (s, 1H),
8.39 (br s, 1H), 8.04 (br s, 1H), 2.43 (s, 3H); m/z 184.1 (M + 1).
1-[4-(1-Ethyl-propylamino)-2-methylsulfanyl-pyrimidin-
5-yl]-ethanone (52): Compound 45 (6.10 g, 23.9 mmol) was
dissolved in toluene (150 mL), MnO2 (5.19 g, 59.17 mmol)
added and the reaction mixture heated to reflux for 4 h. The
solid was filtered from the reaction and the solvent evaporated
in vacuo to give 52 as a clear oil (5.85 g, 97%): 1H NMR (400
MHz, CDCl3) δ 9.04-9.06 (d, J ) 8.3 Hz, 1H), 8.69 (s, 1H),
4.07-4.12 (m, 1H), 2.45 (s, 3H), 1.41-1.63 (m, 6H), 0.80-0.83
(m, 6H); m/z 254.0 (M + 1).
1-(4-Cyclopentylamino-2-methyl-pyrimidin-5-yl)-pro-
pan-1-one (53): Starting from 46 (4.57 g, 17.1 mmol), 3.79 g
(84%) of 53 was obtained according to the method described
for the synthesis of 52: 1H NMR (400 MHz, DMSO-d6) δ 9.20
(d, J ) 6.8, 1H), 8.72 (s, 1H), 4.35 (m, 1H), 2.93 (q, J ) 7.1,
2H), 2.45 (s, 3H), 1.93-2.01 (m, 2H), 1.40-1.67 (m, 6H), 0.99
(t, J ) 7.1 Hz, 3H); m/z 266.2 (M + 1).
4-Amino-2-methylsulfanyl-pyrimidine-5-carbalde-
hyde (43): Starting from 40 (31.6 g, 185 mmol), 23.6 g (74%)
of 43 was obtained according to the method described for the
synthesis of 42: 1H NMR (400 MHz, DMSO-d6) δ 9.70 (s, 1H),
8.51 (s, 1H), 8.24 (br s, 1H), 7.97 (br s, 1H), 2.43 (s, 3H); m/z
170.1 (M + 1).
4-(1-Ethyl-propylamino)-2-methylsulfanyl-pyrimidine-
5-carbaldehyde (44): Starting from 41 (28.2 g, 117 mmol),
18.5 g (66%) of 44 was obtained according to the method
described for the synthesis of 42: 1H NMR (400 MHz, CDCl3)
δ 9.72 (s, 1H), 8.49(s, 1H), 8.40-8.42 (d, J ) 8.6 Hz, 1H), 4.08-
4.13 (m, 1H), 2.45 (s, 3H), 1.44-1.64 (m, 4H), 0.80-0.84 (m,
6H); m/z 240.1 (M + 1).
1-(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-ethanol (45): Compound 42 (1.1 g 4.64 mmol) was
dissolved in THF (30 mL) under nitrogen then cooled with an
ice bath, to which MeMgBr was slowly added (4.4 mL, 13.2
mmol, Aldrich 3 M in ether) and stirred for 1 h. The reaction
mixture was quenched with a small amount of saturated
aqueous NH4Cl then partitioned between water and EtOAc.
The layers were separated, and the aqueous layer was
extracted with EtOAc. The combined organic layers were
washed with brine then dried over MgSO4, and after filtration,
the solvent was removed in vacuo to give 45 as an oil (1.09 g,
90%). 1H NMR (400 MHz, CDCl3) δ 7.57 (s, 1H), 6.28-6.30 (d,
J ) 6.2 Hz, 1H), 4.69-4.74 (m, 1H), 4.38-4.43 (m, 1H), 2.49
(s, 3H), 2.04-2.06 (m, 2H), 1.60-1.76 (m, 4H), 1.42-1.59 (m,
5H); m/z 254.1 (M + 1).
1-(4-Amino-2-methylsulfanyl-pyrimidin-5-yl)-ethanol
(46): Starting from 43 (5.00 g, 29.5 mmol), 4.80 g (88%) of 46
was obtained according to the method described for the
synthesis of 45: 1H NMR (400 MHz, CDCl3) δ 7.88 (s, 1H),
6.66 (br s, 1H), 5.22-5.24 (d, J ) 4.4 Hz, 1H), 4.62-4.64 (m,
1H), 2.34 (s, 3H), 1.22-1.25 (d, J ) 6.4, 3H); m/z 185.9 (M +
1).
1-[4-(1-Ethyl-propylamino)-2-methylsulfanyl-pyrimidin-
5-yl]-ethanol (47): Starting from 44 (7.79 g, 32.5 mmol), 6.30
g (76%) of 47 was obtained according to the method described
for the synthesis of 45: 1H NMR (400 MHz, CDCl3) δ 7.62 (s,
1H), 6.22-6.23 (d, J ) 6.4 Hz, 1H), 4.74-4.79 (m, 1H), 4.08-
4.14 (m, 1H), 2.48 (s, 3H), 1.44-1.68 (m, 7H), 0.87-0.93 (m,
6H); m/z 256.1 (M + 1).
1-(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-propan-1-ol (48): Aldehyde 42 (4.07 g 17.1 mmol) was
dissolved in THF (120 mL) under nitrogen then cooled with
an ice bath, to which EtMgBr was slowly added (13.4 mL, 40.3
mmol, Aldrich 3 M in ether) and stirred for 15 min. The
reaction was quenched with a small amount of saturated
aqueous NH4Cl then partitioned between water and EtOAc.
The layers were separated, and the aqueous layer was
extracted with EtOAc. The combined organic layers were
washed with brine then dried over MgSO4, and after filtration,
the solvent was removed in vacuo to give 48 as an oil (4.50 g,
98%). 1H NMR (400 MHz, DMSO-d6) δ 7.77 (s, 1H), 6.66 (d, J
) 7.0 Hz, 1H), 5.49-5.51 (d, J ) 4.1 Hz, 1H), 4.42-4.46 (m,
1H), 4.28-4.33 (m, 1H), 2.40 (s, 3H), 1.89-1.95 (m, 2H), 1.39-
1.69 (m, 8H), 0.77-0.83 (t, J ) 7.3 Hz, 3H); m/z 268.0 (M +
1).
1-(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-2,2,2-trifluoro-ethanone (54): Compound 49 (0.160 g,
0.52 mmol) was dissolved in CH2Cl2 (5 mL) and Dess-Martin
periodinane (0.255 g, 0.60 mmol). After 30 min the reaction
was diluted with CH2Cl2 (5 mL) and 1 N NaOH (2 mL) and
stirred an additional 5 min. The layers were separated and
the organic layer washed with brine and dried over MgSO4,
the salts were filtered, and the solvent was evaporated in vacuo
to give 54 as an oil (0.160 g, 25%): 1H NMR (400 MHz, CDCl3)
δ 8.92 (br s, 1H), 8.61 (s, 1H), 4.52-4.58 (m, 1H), 2.54 (s, 3H),
2.03-2.12 (m, 2H), 1.51-1.79 (m, 6H); m/z 306.0 (M + 1).
8-Cyclopentyl-5-methyl-2-methylsulfanyl-8H-pyrido-
[2,3-d]pyrimidin-7-one (55): Under nitrogen, a cooled flask
containing THF (50 mL) was charged with NaH (1.23 g, 30.7
mmol, 60% dispersion in mineral oil) to which was added
triethyl phosphonoacetate (6.09 mL, 30.7 mmol). The cooling
bath was removed, and a solution of 50 (3.0 g, 11.9 mmol) in
THF (70 mL) was slowly added to the preformed anion. The
reaction was brought to reflux for 60 h. The reaction mixture
was cooled to room temperature and diluted with water and
EtOAc. The layers were separated, and the aqueous layer was
extracted with EtOAc. The combined organics were washed
with brine and dried over MgSO4, the salts were filtered, and
the filtrate was concentrated in vacuo to give 55 as a waxy
1
solid (2.67 g, 66%). H NMR (400 MHz, DMSO-d6) δ 8.88 (s,
1H), 6.41 (s, 1H), 5.75-5.80 (m, 1H), 2.54 (s, 3H), 2.37 (s, 3H),
2.13-2.19 (m, 2H), 1.91-1.95 (m, 2H), 1.71-1.79 (m, 2H),
1.57-1.60 (m, 2H); m/z 276.1 (M + 1).
1-(4-Cyclopentylamino-2-methylsulfanyl-pyrimidin-5-
yl)-2,2,2-trifluoro-ethanol (49): To an oven dried flask and

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2379
5-Methyl-2-methylsulfanyl-8H-pyrido[2,3-d]pyrimidin-
7-one (56): Starting from 51 (1.90 g, 10.3 mmol), 2.13 g (35%)
of 56 was obtained according to the method described for the
synthesis of 55: 1H NMR (400 MHz, DMSO-d6) δ 8.43 (s, 1H),
6.00 (s, 1H), 2.41 (s, 3H), 2.22 (s, 3H); m/z 208.0 (M + 1).
was purified using silica gel chromatography (4:1 to 7:3,
hexanes:EtOAc) to give 62 as a colorless solid (0.61 g, 23%).
1H NMR (400 MHz, DMSO-d6) δ 8.98 (s, 1H), 5.81-5.85 (m,
1H), 2.55 (s, 3H), 2.46 (s, 3H), 2.17 (br s, 2H), 1.95 (br s, 2H),
1.79 (br s, 2H), 1.60 (br s, 2H); m/z 294.1 (M + 1).
8-Isopropyl-5-methyl-2-methylsulfanyl-8H-pyrido[2,3-
d]pyrimidin-7-one (57): Sodium hydride (0.127 g, 5.28 mmol,
60% dispersion in mineral oil) was suspended in DMF (5 mL)
to which compound 56 (0.75 g, 3.61 mmol) was added. This
mixture was heated to 50 °C for 20 min, at which point
iodopropane (0.898 g, 5.28 mmol) was added. Heating was
continued for 2 h, and then the reaction mixture was poured
into water and extracted with EtOAc (2 × 75 mL). The
combined organic layers were dried over MgSO4, filtered and
concentrated in vacuo to give an orange solid which was
further purified using silica gel chromatography (1:1, hexanes:
EtOAc) to give 57 as a light yellow solid (0.210 g, 23%). 1H
NMR (400 MHz, DMSO-d6) δ 8.86 (s, 1H), 6.40 (s, 1H), 5.60-
5.70 (m, 1H), 2.56 (s, 3H), 2.36 (s, 3H), 1.48-1.49 (d, J ) 6.8,
6H); m/z 250.1 (M + 1).
8-(1-Ethyl-propyl)-5-methyl-2-methylsulfanyl-8H-py-
rido[2,3-d]pyrimidin-7-one (58): Starting from 52 (5.85 g,
23.1 mmol), 3.82 g (61%) of 58 was obtained according to the
method described for the synthesis of 55: 1H NMR (400 MHz,
DMSO-d6) - a mixture of rotamers, δ 8.88 (s, 1H), 6.48 (s,
1/2H), 6.41 (s, 1/2H), 5.45-5.55 (m, 1/2H), 5.12-5.18 (m, 1/2H),
2.56 (s, 3H), 2.10 (s, 3H), 2.13-2.38 (m, 2H), 1.76-1.91 (m,
2H), 0.65-0.71 (m, 6H); m/z 278.1 (M + 1).
8-Cyclopentyl-5-ethyl-2-methylsulfanyl-8H-pyrido[2,3-
d]pyrimidin-7-one (59): Starting from 53 (3.70 g, 13.9 mmol),
2.67 g (66%) of 59 was obtained according to the method
described for the synthesis of 55: 1H NMR (400 MHz, DMSO-
d6) δ 8.94 (s, 1H), 6.37 (s, 1H), 5.75-5.79 (m, 1H), 2.77-2.82
(q, J ) 7.1, 2H), 2.55 (s, 3H), 2.05-2.20 (m, 2H), 1.83-1.95
(m, 2H), 1.68-1.78 (m, 2H), 1.50-1.63 (m, 2H), 1.17 (t, J )
7.1 Hz, 3H); m/z 290.1 (M + 1).
8-Cyclopentyl-2-methylsulfanyl-5-trifluoromethyl-8H-
pyrido[2,3-d]pyrimidin-7-one (60): Starting from 54 (0.240
g, 0.786 mmol), 0.255 g (99%) of 60 was obtained according to
the method described for the synthesis of 55: 1H NMR (400
MHz, CDCl3) δ 8.82 (s, 1H), 7.02 (s, 1H), 5.79-5.84 (m, 1H),
2.58 (s, 3H), 2.14-2.19 (m, 2H), 1.59-1.95 (m, 6H); m/z 330.1
(M + 1).
6-Bromo-8-cyclopentyl-5-methyl-2-methylsulfanyl-8H-
pyrido[2,3-d]pyrimidin-7-one (63): Compound 55 (1.0 g,
3.64 mmol) was dissolved in dry DMF (15 mL) and N-
bromosuccinimide (0.97 g, 5.45 mmol) was added followed by
benzoylperoxide (0.13 g, 0.5 mmol). The resulting solution was
stirred overnight at room temperature then partitioned be-
tween EtOAc and water. The organic layer was washed with
water and saturated aqueous NaCl, then dried over MgSO4.
Removal of the drying agent and evaporation of the solvent
gave 63 (0.86 g, 66%) as a white solid. 1H NMR (400 MHz,
CDCl3) δ 8.78 (s, 1H), 6.01-6.06 (m, 1H), 2.61 (s, 6H), 2.24-
2.29 (m, 2H), 2.07-2.11 (m, 2H), 1.85-1.88 (m, 2H), 1.64-
1.68 (m, 2H); m/z 356.1 (M + 1).
8-Cyclopentyl-6-iodo-5-methyl-2-methylsulfanyl-8H-
pyrido[2,3-d]pyrimidin-7-one (64): Compound 55 (7.03 g,
25.51 mmol) and iodine (7.12 g, 28.06 mmol) were dissolved
in CH2Cl2 (210 mL). The apparatus was covered with alumi-
num foil and the solution stirred at room temperature for 30
min. Bis(trifluoroacetoxy)iodobenzene (13.16 g, 30.61 mmol)
was added in one portion and the dark purple solution heated
to 37 °C for 2 h and then cooled to room temperature for 2 h.
50% Aqueous (w/v) sodium thiosulfate (114 mL) was added to
the reaction mixture, and the dark purple mixture became red,
then blue, red and finally yellow within 1 min. The two phases
were stirred for 30 min and then separated. The aqueous phase
was extracted with CH2Cl2 (50 mL), and the combined organic
phases were washed with 50% aqueous (w/v) sodium thiosul-
fate (50 mL) and water (4 × 130 mL). The organic phase was
dried, filtered and concentrated in vacuo to give a crude
product (15.85 g) which was purified using silica gel chroma-
tography (15% heptane/CH2Cl2) to give 64 as a white solid
(5.94 g, 58%). 1H NMR (300 MHz, CDCl3) δ 8.91 (s, 1H), 6.00-
6.12 (m, 1H), 2.70 (s, 3H), 2.62 (s, 3H), 2.24-2.30 (m, 2H),
2.08-2.15 (m, 2H), 1.81-1.93 (m, 2H), 1.57-1.75 (m, 2H); m/z
402 (M + 1).
6-Acetyl-8-cyclopentyl-5-methyl-2-methylsulfanyl-8H-
pyrido[2,3-d]pyrimidin-7-one (65): Under an argon atmo-
sphere, Compound 63 (1.29 g, 3.64 mmol) and tetrakis-
(triphenylphosphine)-palladium(0) (0.5 g, 0.8 mmol) were
added to toluene (8 mL). (1-Ethoxyvinyl)tributyltin (3.5 g, 20
mmol) was added over 5 h at 110 °C. The mixture was taken
up into EtOAc and extracted with 6 N HCl. The pH of the
aqueous phase was adjusted to pH 7 by addition of 50% NaOH
solution then extracted with EtOAc. The organic phase was
washed with brine, dried over MgSO4, filtered and concen-
trated in vacuo to give a solid (1.2 g). This solid was purified
using silica gel chromatography on a Biotage 12m column
eluted with a gradient of hexanes to 10% ethyl acetate in
hexanes. The combined fractions were evaporated to a solid
and crystallized from diethyl ether giving 65 (0.6 g, 52%) as a
8-Cyclopentyl-5,6-dimethyl-2-methylsulfanyl-8H-pyrido-
[2,3-d]pyrimidin-7-one (61): Under nitrogen, NaH (0.63 g,
26.3 mmol, 60% dispersion in mineral oil) was suspended in
THF (25 mL) and triethyl 2-phosphonopropionate (5.64 mL,
26.3 mmol) was added inducing a small exotherm. To this
mixture was added compound 50 (3.0 g, 11.9 mmol) a solid,
and the reaction mixture was heated to reflux for 36 h. The
reaction mixture was cooled to room temperature and diluted
with water and EtOAc. The layers were separated, and the
aqueous layer was extracted with EtOAc. The combined
organics were dried over MgSO4, the salts were filtered and
the filtrate was concentrated in vacuo to give 61 as a dark
1
1
white solid. H NMR (400 MHz, CDCl3) δ 8.78 (s, 1H), 5.90
orange oil (0.425 g, 12%). H NMR (400 MHz, CDCl3) δ 8.71
(m, 1H), 2.61 (s, 3H), 2.53 (s, 3H), 2.35 (s, 3H), 2.28-2.37 (m,
2H), 2.03-2.06 (m, 2H), 1.84-1.87 (m, 2H), 1.64-1.69 (m, 2H);
m/z 318.0 (M + 1).
(s, 1H), 5.96-6.00 (m, 1H), 2.59 (s, 3H), 2.38 (s, 3H), 2.27-
2.32 (m, 2H), 2.18 (s, 3H), 2.04-2.08 (m, 2H), 1.82-1.85 (m,
2H), 1.64-1.68 (m, 2H); m/z 290.0 (M + 1).
8-Cyclopentyl-2-methanesulfinyl-5-methyl-8H-pyrido-
[2,3-d]pyrimidin-7-one (66): Comopund 55 (2.57 g, 8.88
mmol) was dissolved in CH2Cl2 (50 mL), and 2-benzylsulfonyl-
3-phenyl-oxaziradine was added (3.02 g, 11.5). The reaction
mixture was stirred for 16 h. The solution was evaporated in
vacuo to give an orange oil. EtOAc was added and a white
precipitate formed. This precipitate was filtered and washed
with hexanes to yield 66 as a white solid (2.12 g, 78%). 1H
NMR (400 MHz, CDCl3) δ 8.97 (s, 1H), 6.60 (s, 1H), 5.86-
5.95 (m, 1H), 2.94 (s, 3H), 2.48 (s, 3H), 2.02-2.22 (m, 4H),
1.85-1.90 (m, 2H), 1.66-1.73 (m, 2H); m/z 292.1 (M + 1).
8-Cyclopentyl-6-fluoro-5-methyl-2-methylsulfanyl-8H-
pyrido[2,3-d]pyrimidin-7-one (62): Sodium hydride (771
mg, 19.3 mmol) was suspended in dry THF (20 mL), and the
mixture was cooled to 0 °C in an ice bath. Triethyl 2-fluoro-
2-phosphonoacetate (3.9 mL, 19.3 mmol) was added dropwise
with stirring, and the solution was stirred at room temperature
for 30 min. A solution of 50 (2.27 g, 9.05 mmol) in dry THF
(40 mL) was added via a cannula, then the reaction mixture
was left to stir at room-temperature overnight. The reaction
was quenched by the addition of water (0.5 mL), then the THF
was evaporated in vacuo. The residue was partitioned between
EtOAc and saturated aqueous sodium chloride. The aqueous
layer was extracted twice with EtOAc, and the combined
organic layers were dried over MgSO4. After removal of the
drying agent and evaporation of the solvent, the crude product
8-Isopropyl-2-methanesulfinyl-5-methyl-8H-pyrido[2,3-
d]pyrimidin-7-one (67): Starting from 57 (0.200 g, 0.800
mmol), 0.125 g (61%) of 67 was obtained according to the
method described for the synthesis of 66: 1H NMR (400 MHz,

2380 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
DMSO-d6) δ 9.18 (s, 1H), 6.23 (s, 1H), 5.60-5.68 (m, 1H), 2.89
(s, 3H), 2.45(s, 3H), 1.50-1.52 (d, J ) 7.0, 6H); m/z 266.0 (M
+ 1).
The resulting solid was collected by filtration. Chromatography
on silica gel, eluting with 30-50% EtOAc in hexanes, gave 76
(0.58 g, 22%). 1H NMR (400 MHz, CDCl3) δ 9.19 (s, 1H), 5.98-
6.08 (m, 1H), 4.86 (AB q, J ) 44, 14 Hz, 2H), 2.04-2.44 (m,
4H), 1.86-2.00 (m, 2H), 1.62-1.76 (m, 2H); m/z 362 (M + 1),
360 (M + 1).
8-(1-Ethyl-propyl)-2-methanesulfinyl-5-methyl-8H-py-
rido[2,3-d]pyrimidin-7-one (68): Starting from 58 (1.00 g,
3.60 mmol), 0.80 g (75%) of 68 was obtained according to the
method described for the synthesis of 66: 1H NMR (400 MHz,
DMSO-d6)sa mixture of rotamers, δ 9.20 (s, 1H), 6.68 (s, 1/2H),
6.61 (s, 1/2H), 5.48-5.55 (m, 1/2H), 5.08-5.15 (m, 1/2H), 2.86
(s, 3H), 2.46 (s, 3H), 2.05-2.20 (m, 2H), 1.80-1.95 (m, 2H),
0.64-0.67 (m, 6H); m/z 294.0 (M + 1).
8-Cyclopentyl-5-ethyl-2-methanesulfinyl-8H-pyrido-
[2,3-d]pyrimidin-7-one (69): Starting from 59 (2.57 g, 8.88
mmol), 2.12 g (78%) of 69 was obtained according to the
method described for the synthesis of 66: 1H NMR (400 MHz,
DMSO-d6) δ 9.26 (s, 1H), 6.60 (s, 1H), 5.76-5.85 (m, 1H), 2.87-
2.91 (m, 5H), 2.15-2.20 (m, 2H), 1.98-2.10 (m, 2H), 1.77-
1.82 (m, 2H), 1.52-1.60 (m, 2H), 1.19-1.23 (t, J ) 7.3 Hz, 3H);
m/z 306.1 (M + 1).
8-Cyclopentyl-2-methanesulfinyl-5-trifluoromethyl-
8H-pyrido[2,3-d]pyrimidin-7-one (70): Starting from 60
(0.250 g, 0.785 mmol), 0.185 g (68%) of 70 was obtained
according to the method described for the synthesis of 66: 1H
NMR (400 MHz, CDCl3) δ 9.16 (s, 1H), 7.31 (s, 1H), 5.80-
5.88 (m, 1H), 2.92 (s, 3H), 2.17-2.21 (m, 2H), 2.07-2.10 (m,
2H), 1.83-1.85 (m, 2H), 1.60-1.63 (m, 2H); m/z 330.1 (M +
1).
8-Cyclopentyl-2-methanesulfinyl-5,6-dimethyl-8H-py-
rido[2,3-d]pyrimidin-7-one (71): Starting from 61 (0.420 g,
1.45 mmol), 0.267 g (60%) of 71 was obtained according to the
method described for the synthesis of 66: 1H NMR (400 MHz,
CDCl3) δ 9.24 (s, 1H), 5.85-5.93 (m, 1H), 2.87 (s, 3H), 2.45 (s,
3H), 2.13-2.20 (m, 2H), 2.13 (s, 3H), 2.04-2.08 (m, 2H), 1.76-
1.85 (m, 2H), 1.57-1.62 (m, 2H); m/z 306.1 (M + 1).
8-Cyclopentyl-6-fluoro-2-methanesulfinyl-5-methyl-
8H-pyrido[2,3-d]pyrimidin-7-one (72): Starting from 62
(6.00 g, 20.5 mmol), 5.10 g (81%) of 72 was obtained according
to the method described for the synthesis of 66: 1H NMR (400
MHz, CDCl3) δ 9.01 (s, 1H), 5.94-6.03 (1H, m), 2.95 (s, 3H),
2.46 (s, 3H), 2.21-2.24 (m, 2H), 2.12 (br s, 2H), 1.92-1.95 (m,
2H), 1.66-1.70 (m, 2H); m/z 310.0 (M + 1).
6-Bromo-8-cyclopentyl-2-methanesulfinyl-5-methyl-
8H-pyrido[2,3-d]pyrimidin-7-one (73): Starting from 63
(40.3 g, 114 mmol), 39.8 g (94%) of 73 was obtained according
to the method described for the synthesis of 66: 1H NMR (400
MHz, CDCl3) δ 9.09 (s, 1H), 6.04-6.09 (m, 1H), 2.97 (s, 3H),
2.70 (s, 3H), 2.11-2.24 (m, 4H), 1.92-1.96 (m, 2H), 1.67-1.72
(m, 2H); m/z 372.1 (M + 1).
8-Cyclopentyl-6-iodo-2-methanesulfinyl-5-methyl-8H-
pyrido[2,3-d]pyrimidin-7-one (74): Starting from 64 (1.51
g, 3.76 mmol), 1.16 g (74%) of 74 was obtained according to
the method described for the synthesis of 66: 1H NMR (300
MHz, CDCl3) δ 9.13 (s, 1H), 6.02-6.14 (m, 1H), 2.98 (s, 3H),
2.80 (s, 3H), 2.06-2.27 (m, 4H), 1.87-2.00 (m, 2H), 1.63-1.72
(m, 2H); m/z 418 (M + 1).
6-Acetyl-8-cyclopentyl-2-methanesulfinyl-5-methyl-
8H-pyrido[2,3-d]pyrimidin-7-one (75): Starting from 65 (0.6
g, 1.89 mmol), 0.51 g (81%) of 75 was obtained according to
the method described for the synthesis of 66: 1H NMR (400
MHz, CDCl3) δ 9.08 (s, 1H), 5.94 (m, 1H), 2.96 (s, 3H), 2.54
(s, 3H), 2.43(s, 3H), 2.21-2.26 (m, 2H), 2.09-2.20 (m, 2H),
1.79-1.92 (m, 2H), 1.64-1.70 (m, 2H); m/z 333.1 (M + 1).
6-Chloro-2-chloromethanesulfinyl-8-cyclopentyl-5-
methyl-8H-pyrido[2,3-d]pyrimidin-7-one (76). Compound
55 (2.00 g, 7.27 mmol) and N-chlorosuccinimide (1.46 g, 10.9
mmol, 1.5 equivalents) were suspended in DMF (30 mL), and
benzoyl peroxide (0.35 g, 1.45 mmol) was added, resulting in
a color change from white to orange. Additional DMF (20 mL)
was added, and the resulting solution was stirred at room
temperature under nitrogen. Additional N-chlorosuccinimide
was added after 1 day (0.65 g) and after 3 days (1.25 g) along
with additional benzoyl peroxide (∼50 mg after 1 day and 0.29
g after 3 days). After stirring for a total of 5 days, the solvent
was evaporated and replaced by 20% ethyl acetate in hexanes.
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-car-
boxylic Acid tert-Butyl Ester (17): Compound 66 (0.70 g,
2.4 mmol) and 4-(4-amino-phenyl)-piperazine-1-carboxylic acid
tert-butyl ester (0.70 g, 2.52 mmol) were dissolved in DMSO
(8 mL) and heated to 100 °C for 24 h. The reaction mixture
was then cooled to room temperature and partitioned between
EtOAc and water, the layers were separated and the organic
layer was dried over MgSO4. The inorganic salts were filtered,
and the solvent was evaporated in vacuo to yield a crude solid
that was further purified using silica gel chromatography
(EtOAc and hexanes) and/or trituration with MeCN and water
to yield 17 as a yellow solid (0.549 g, 45%): mp 100-106 °C;
1H NMR (400 MHz, CDCl3) δ 8.57 (s, 1H), 7.43-7.45 (d, J )
9.1 Hz, 2H), 7.18 (s, 1H), 6.93-6.95 (d, J ) 8.8 Hz, 2H), 6.21
(s, 1H), 5.78-5.83 (m, 1H), 3.57-3.59 (m, 4H), 3.08-3.13 (m,
4H), 2.33 (s, 3H), 2.24-2.31 (m, 2H), 1.78-1.90 (m, 4H), 1.58-
1.62 (m, 2H), 1.46 (s, 9H); Exact Mass: Calculated (C28H36N6O3
+ H) 505.2927, found 505.2936 (M + 1); HPLC purity 98.5%.
8-Cyclopentyl-2-(4-morpholin-4-yl-phenylamino)-8H-
pyrido[2,3-d]pyrimidin-7-one (2): Starting from 8-cyclo-
pentyl-2-methanesulfinyl-8H-pyrido[2,3-d]pyrimidin-7-one (0.250
g, 1.00 mmol), 0.51 g (32%) of 2 was obtained according to the
method described for the synthesis of 17: mp 239-241 °C; 1H
NMR (400 MHz, DMSO-d6) δ 8.46 (s, 1H), 8.68 (s, 1H), 7.50-
7.54 (d, J ) 8.3 Hz, 2H), 6.96-7.00 (d, J ) 8.6 Hz, 2H), 6.68
(s, 1H), 5.76-5.81 (m, 1H), 3.26-3.35 (m, 4H), 3.10-3.23 (m,
4H), 2.12-2.21 (m, 2H), 1.75-1.92 (m, 4H), 1.47-1.60 (m, 2H);
m/z 392.1 (M + 1); Anal. (C23H28N6O1‚0.25H2O) C, H, N.
8-Cyclopentyl-2-(4-piperidin-1-yl-phenylamino)-8H-
pyrido[2,3-d]pyrimidin-7-one (3): Starting from 8-cyclo-
pentyl-2-methanesulfinyl-8H-pyrido[2,3-d]pyrimidin-7-one (0.171
g, 0.617 mmol), 0.157 g (65%) of 3 was obtained according to
the method described for the synthesis of 17: mp 198-199
1
°C; H NMR (400 MHz, DMSO-d6) δ 9.73 (br s, 1H), 8.68 (s,
1H), 7.71-7.73 (d, J ) 9.2, 1H), 7.47-7.49 (d, J ) 9.2, 2H),
6.90-6.92 (d, J ) 9.2, 2H), 6.26-6.28 (d, J ) 9.2, 1H), 5.78-
5.83 (m, 1H), 3.04-3.11 (m, 4H), 2.20-2.29 (m, 2H), 1.48-
1.95 (m, 12H); m/z 389.9 (M + 1); Anal. (C23H27N5O1) C, H, N.
8-Cyclopentyl-5-methyl-2-[4-(4-methyl-piperazin-1-yl)-
phenylamino]-8H-pyrido[2,3-d]pyrimidin-7-one (6): Start-
ing from 66 (0.200 g, 0.650 mmol), 0.054 g (20%) of 6 was
obtained according to the method described for the synthesis
of 17: mp 211-213 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.68
(br s, 1H), 8.71 (s, 1H), 7.45-7.47 (d, J ) 8.8 Hz, 2H), 6.86-
6.88 (d, J ) 8.8 Hz, 2H), 6.10 (s, 1H), 5.76-5.81 (m, 1H), 3.00-
3.08 (m, 4H), 2.40-2.50 (m, 4H), 2.31 (s, 3H), 2.17 (s, 3H),
2.15-2.22 (m, 2H), 1.78-1.90 (m, 2H), 1.65-1.75 (m, 2H),
1.47-1.60 (m, 2H); Exact Mass: Calculated (C24H30N6O + H)
419.2559, found 419.2569 (M + 1); HPLC purity 96.7%.
8-Cyclopentyl-5-methyl-2-(4-morpholin-4-yl-phenylami-
no)-8H-pyrido[2,3-d]pyrimidin-7-one (7): Starting from 66
(0.200 g, 0.686 mmol), 0.031 g (11%) of 7 was obtained
according to the method described for the synthesis of 17: mp
1
227-229; H NMR (400 MHz, DMSO-d6) δ 9.71 (s, 1H), 8.72
(s, 1H), 7.48-7.51 (d, J ) 8.8 Hz, 2H), 6.88-6.90 (d, J ) 9.1
Hz, 2H), 6.11 (s, 1H), 5.75-5.80 (m, 1H), 3.69-3.72 (m, 4H),
3.00-3.04 (m, 4H), 2.31 (s, 3H), 2.15-2.24 (m, 2H), 1.80-1.92
(m, 2H), 1.65-1.75 (m, 2H), 1.45-1.58 (m, 2H); m/z 406.2 (M
+ 1); Anal. (C23H28N6O1) C, H, N.
8-Cyclopentyl-5-methyl-2-(4-piperidin-1-yl-phenylami-
no)-8H-pyrido[2,3-d]pyrimidin-7-one (8): Starting from 66
(0.161 g, 0.55 mmol), 0.083 g (37%) of 8 was obtained according
to the method described for the synthesis of 17: mp 205-207;
1H NMR (400 MHz, DMSO-d6) δ 8.59 (s, 1H), 7.40-7.51 (m,
2H), 6.90-7.10 (m, 3H), 6.23 (s, 1H), 5.78-5.87 (m, 1H), 3.14-

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2381
3.22 (m, 4H), 2.35 (s, 3H), 2.25-2.33 (m, 2H), 1.50-1.98 (m,
12H); m/z 404.2 (M + 1); Anal. (C25H29N5O1‚0.41C2H3N1) C,
H, N.
(m, 2H), 1.50-1.58 (m, 2H); Exact Mass: Calculated (C25H30-
N6O2 + H) 447.2508, found 447.2496 (M + 1); HPLC purity
99.3%.
8-Cyclopentyl-2-[4-(3-hydroxy-pyrrolidin-1-yl)-phenyl-
amino]-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (18):
Starting from 66 (0.600 g, 2.06 mmol), 0.510 g (54%) of 18 was
obtained according to the method described for the synthesis
of 17: mp 225-226 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.54
(s, 1H), 8.68 (s, 1H), 7.38 (d, J ) 7 Hz, 2H), 6.43-6.45 (d, J )
8 Hz, 2H), 6.07 (s, 1H), 5.75 (br s, 1H), 4.90 (s, 1H), 4.35 (s,
1H), 3.20-3.40 (m, 3H), 2.98-3.00 (d, J ) 10 Hz, 1H), 2.30 (s,
3H), 2.12-2.24 (m, 2H), 1.90-2.10 (m, 2H), 1.60-1.90 (m, 4H),
1.45-1.60 (m, 2H); Exact Mass: (C23H27N5O2 + H) 406.2243,
found 406.2239 (M + 1); HPLC purity 91%.
{1-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-pyrrolidin-3-yl}-car-
bamic Acid tert-Butyl Ester (82): Starting from 66 (0.150
g, 0.515 mmol), 0.100 g (38%) of 82 was obtained according to
the method described for the synthesis of 17: 1H NMR (400
MHz, CDCl3) δ 8.54 (s, 1H), 7.33-7.36 (d, J ) 9.0 Hz, 2H),
7.13 (br s, 1H), 6.53-6.56 (d, J ) 9.1 Hz, 2H), 6.18 (s, 1H),
5.75-5.83 (m, 1H), 4.75-4.80 (br m, 1H), 4.34-4.39 (m, 1H),
3.88 - 3.93 (m, 1H), 3.53-3.56 (m, 2H), 3.41-3.46 (m, 2H),
3.27-3.33 (m, 2H), 3.15-3.20 (m, 2H), 2.31 (s, 3H), 2.21-2.31
(m, 2H), 1.78-1.95 (m, 4H), 1.55-1.60 (m, 2H), 1.43 (s, 9H);
m/z 505.2 (M + 1).
8-Cyclopentyl-2-{4-[4-(3-hydroxy-propyl)-piperidin-1-
yl]-phenylamino}-5-methyl-8H-pyrido[2,3-d]pyrimidin-
7-one (9): Starting from 66 (0.600 g, 2.06 mmol) and 102
(0.483 g, 2.06 mmol), 0.510 g (54%) of 9 was obtained according
to the method described for the synthesis of 17: mp 180-184
°C; 1H NMR (400 MHz, DMSO-d6) δ 9.66 (s, 1H), 8.71 (s, 1H),
7.43-7.45 (d, J ) 9.3 Hz, 2H), 6.86-8.89 (d, J ) 8.5 Hz, 2H),
6.10 (s, 1H), 5.76-5.81 (m, 1H), 4.31-4.35 (m, 1H), 3.55-3.59
(m, 2H), 3.34-3.38 (m, 2H), 2.50-2.56 (m, 2H), 2.31 (s, 3H),
2.15-2.23 (m, 2H), 1.75-1.92 (m, 2H), 1.63-1.85 (m, 4H),
1.13-1.60 (m, 8H); m/z 462.2 (M + 1); Anal. (C27H35N5O2) C,
H, N.
4-[4-(8-Cyclopentyl-7-oxo-7,8-dihydro-pyrido[2,3-d]py-
rimidin-2-ylamino)-phenyl]-piperazine-1-carboxylic Acid
tert-Butyl Ester (77): Starting from 8-cyclopentyl-2-meth-
anesulfinyl-8H-pyrido[2,3-d]pyrimidin-7-one (1.00 g, 3.61 mmol),
1.00 g (56%) of 77 was obtained according to the method
described for the synthesis of 17: 1H NMR (400 MHz, CDCl3)
δ 8.45 (s, 1H), 7.40-7.45 (m, 3H), 7.13 (br s, 1H), 6.93-6.95
(m, 2H), 6.36-6.39 (d, J ) 9.3 Hz, 1H), 5.78-5.83 (m, 1H),
3.56-3.61 (m, 4H), 3.05-3.12 (m, 4H), 2.16-2.22 (m, 2H),
1.82-1.93 (m, 2H), 1.62-1.68 (m, 2H), 1.48-1.53 (m, 2H), 1.47
(s, 9H); m/z 491.1 (M + 1).
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-[1,4]diazepane-1-
carboxylic Acid tert-Butyl Ester (83): Starting from 66
(0.400 g, 1.52 mmol), 0.230 g (29%) of 83 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, CDCl3) δ 8.51 (br s, 1H), 7.29 (br s, 2H), 7.16
(br s, 1H), 6.65 (br s, 2H), 6.16 (br s, 1H), 5.74 (br t, J ) 8 Hz,
1H), 3.52 (br s, 6H), 3.26 (br s, 1H), 3.16 (br s, 1H), 2.28 (s,
3H), 2.24 (br s, 2H), 1.93 (br s, 2H), 1.74-1.80 (m, 4H), 1.51
(br s, 2H), 1.39 (s, 4.5H), 1.34 (s, 4.5H); m/z 519.3 (M + 1).
8-Cyclopentyl-2-{4-[4-(2-hydroxy-ethyl)-piperazin-1-
yl]-phenylamino}-5-methyl-8H-pyrido[2,3-d]pyrimidin-
7-one (21): Starting from 66 (0.200 g, 0.690 mmol), 0.050 g
(31%) of 21 was obtained according to the method described
for the synthesis of 17: 1H NMR (400 MHz, DMSO-d6) δ 9.70
(s, 1H), 8.74 (s, 1H), 7.47-7.49 (d, J ) 9 Hz, 2H), 6.89 (d, J )
Hz, 2H), 6.13 (s, 1H), 5.70-5.84 (m, 1H), 4.44 (s, 1H), 3.50-
3.52 (dd, J ) 6, 12 Hz, 2H), 3.07 (s, 4H), 2.55 (s, 4H), 2.43 (s,
2H), 2.33 (s, 3H), 2.10-2.28 (m, 2H), 1.60-1.90 (m, 4H), 2.42-
2.60 (m, 2H); m/z 449.3 (M + 1); Anal. (C25H32N6O2‚0.26H2O)
C, H, N.
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-2,6-dimethyl-piper-
azine-1-carboxylic Acid tert-Butyl Ester (84): Starting
from 66 (0.169 g, 0.580 mmol), 0.050 g (16%) of 84 was
obtained according to the method described for the synthesis
of 17: m/z 533.4 (M + 1).
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-2,2-dimethyl-piper-
azine-1-carboxylic Acid tert-Butyl Ester (85): Starting
from 66 (0.150 g, 0.515 mmol), 0.100 g (36%) of 85 was
obtained according to the method described for the synthesis
of 17: 1H NMR (400 MHz, DMSO-d6) δ 9.59 (br s, 1H), 8.70
(s, 1H), 7.40-7.43 (d, J ) 7.8 Hz, 2H), 6.71-6.73 (d, J ) 8.5
Hz, 2H), 6.09 (s, 1H), 5.76-5.81 (m, 1H), 3.60-3.65 (m, 2H),
3.20 - 3.28 (m, 4H), 2.30 (s, 3H), 2.13-2.18 (m, 2H), 1.60-
1.90 (m, 4H), 1.45-1.53 (m, 2H), 1.38 (s, 9H), 1.31 (s, 6H); m/z
533.4 (M + 1).
4-[4-(8-Cyclopentyl-5-ethyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-car-
boxylic Acid tert-Butyl Ester (78): Starting from 69 (0.200
g, 0.654 mmol), 0.160 g (47%) of 78 was obtained according to
the method described for the synthesis of 17: 1H NMR (400
MHz, DMSO-d6) δ 9.72 (s, 1H), 8.78 (s, 1H), 7.48-7.50 (d, J )
9.0 Hz, 2H), 6.90-6.92 (d, J ) 8.0 Hz, 2H), 6.09 (s, 1H), 5.78-
5.83 (m, 1H), 3.41-3.46 (m, 4H), 2.98-3.04 (m, 4H), 2.71-
2.76 (q, J ) 7.6 Hz, 2H), 2.16-2.22 (m, 2H), 1.82-1.93 (m,
2H), 1.62-1.68 (m, 2H), 1.48-1.53 (m, 2H), 1.38 (s, 9H), 1.11-
1.18 (m, 3H); m/z 519.3 (M + 1).
4-[4-(8-Cyclopentyl-7-oxo-5-trifluoromethyl-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (79): Starting from 70
(0.065 g, 0.188 mmol), 0.066 g (63%) of 79 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, DMSO-d6) δ 8.66 (s, 1H), 7.46-7.49 (d, J )
9.1 Hz, 2H), 6.92-6.94 (d, J ) 8.8 Hz, 2H), 6.66 (s, 1H), 5.78-
5.83 (m, 1H), 3.41-3.46 (m, 4H), 3.00-3.08 (m, 4H), 2.13-
2.18 (m, 2H), 1.74-1.90 (m, 4H), 1.52-1.58 (m, 2H), 1.38 (s,
9H); m/z 559.3 (M + 1).
4-[4-(8-Isopropyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-car-
boxylic Acid tert-Butyl Ester (80): Starting from 67 (0.120
g, 0.452 mmol), 0.040 g (19%) of 80 was obtained according to
the method described for the synthesis of 17: m/z 479.4 (M +
1).
4-{4-[8-(1-Ethyl-propyl)-5-methyl-7-oxo-7,8-dihydro-py-
rido[2,3-d]pyrimidin-2-ylamino]-phenyl}-piperazine-1-
carboxylic Acid tert-Butyl Ester (81): Starting from 68
(0.425 g, 1.44 mmol), 0.285 g (39%) of 81 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, DMSO-d6)sa mixture of rotamers δ 9.80 (s,
1/2H), 9.58 (s, 1/2H), 8.72 (s, 1H), 7.46-7.59 (m, 2H), 6.86-
6.95 (m, 2H), 6.16 (s, 1/2H), 6.08 (s, 1/2H), 5.38-5.45 (m, 1/2H),
5.04-5.13 (m, 1/2H), 3.40-3.50 (m, 4H), 2.95-3.08 (m, 4H),
2.33 (s, 3H), 2.11-2.16 (m, 2H), 1.78-1.90 (m, 2H), 1.46 (s,
9H), 0.60-0.78 (m, 6H); m/z 507.4 (M + 1).
4-[2-Chloro-4-(8-cyclopentyl-5-methyl-7-oxo-7,8-dihy-
dro-pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-pipera-
zine-1-carboxylic Acid tert-Butyl Ester (86): Starting from
66 (0.150 g, 0.515 mmol), 0.095 g (34%) of 86 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, CDCl3) δ 8.60 (s, 1H), 7.92 (s, 1H), 7.35 (br s,
1H), 7.23 (br s, 1H), 6.96-6.99 (d, J ) 8.6 Hz, 2H), 6.24 (s,
1H), 5.79-5.83 (m, 1H), 3.57-3.61 (m, 4H), 2.93-2.99 (m, 4H),
2.35 (s, 3H), 2.24-2.35 (m, 2H), 1.92-1.99 (m, 2H), 1.80-1.86
(m, 2H), 1.60-1.65 (m, 2H), 1.46 (s, 9H); m/z 539.2 (M + 1).
2-[4-(4-Acetyl-piperazin-1-yl)-phenylamino]-8-cyclo-
pentyl-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (16):
Starting from 66 (0.100 g, 0.343 mmol), 0.049 g (32%) of 16
was obtained according to the method described for the
synthesis of 17: mp 261-263 °C; 1H NMR (400 MHz, DMSO-
d6) δ 9.73 (br s, 1H), 8.73 (s, 1H), 7.50-7.52 (d, J ) 8.5 Hz,
2H), 6.91-6.94 (d, J ) 8.9 Hz, 2H), 6.12 (s, 1H), 5.78-5.83
(m, 1H), 3.73-3.80 (m, 4H), 3.00-3.08 (m, 4H), 2.33 (s, 3H),
2.18-2.23 (m, 2H), 2.01 (s, 3H), 1.80-1.88 (m, 2H), 1.65-1.75

2382 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
4-[4-(8-Cyclopentyl-5,6-dimethyl-7-oxo-7,8-dihydro-py-
rido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-
carboxylic Acid tert-Butyl Ester (87): Starting from 66
(0.060 g, 0.196 mmol), 0.030 g (29%) of 87 was obtained
according to the method described for the synthesis of 17: m/z
519.3 (M + 1).
4-[4-(8-Cyclopentyl-6-fluoro-5-methyl-7-oxo-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (88). Starting from 72
(0.300 g, 0.971 mmol), 0.230 g (45%) of 88 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, CDCl3) δ 8.59 (s, 1H), 7.44-7.46 (d, J ) 8
Hz, 2H), 7.10 (br s, 1H), 6.98 (br s, 2H), 5.84-5.88 (m, 1H),
3.60 (br s, 4H), 3.11 (br s, 4H), 2.34 (s, 3H), 2.27-2.31 (m, 2H),
1.96 (br s, 2H), 1.83 (br s, 2H), 1.60-1.62 (m, 2H), 1.47 (s, 9H);
m/z 523.2 (M + 1).
4-[4-(6-Bromo-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (89): Starting from 73
(0.300 g, 0.809 mmol), 0.410 g (81%) of 89 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, CDCl3) δ 8.66 (s, 1H), 7.55 (br s, 1H), 7.44-
7.46 (d, J ) 8.9 Hz, 2H), 6.91-6.94 (d, J ) 8.6 Hz, 2H), 5.90-
5.96 (m, 1H,), 3.55-3.58 (m, 4H), 3.06-3.13 (m, 4H), 2.53 (s,
3H), 2.20-2.25 (m, 2H), 1.90-1.98 (m, 2H), 1.78-1.84 (m, 2H),
1.55-1.60 (m, 2H),1.44 (s, 9H); m/z 585.0 (M + 1).
4-[4-(8-Cyclopentyl-6-iodo-5-methyl-7-oxo-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (90): Starting from 74
(0.32 g, 0.766 mmol), 0.40 g (83%) of 90 was obtained according
to the method described for the synthesis of 17: 1H NMR (300
MHz, CDCl3) δ 8.72 (s, 1H), 7.46-7.48 (d, J ) 8.9 Hz, 2H),
7.19 (s, 1H), 6.94-6.96 (d, J ) 8.9 Hz, 2H), 5.96-5.98 (p, J )
8.8 Hz, 1H,), 3.58-3.61 (m, 4H), 3.10-3.13 (m, 4H), 2.66 (s,
3H), 2.20-2.32 (m, 2H), 1.94-2.08 (m, 2H), 1.76-1.90 (m, 2H),
1.58-1.68 (m, 2H), 1.49 (s, 9H); m/z 631 (M + 1).
4-[4-(6-Acetyl-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (91): Starting from 75
(0.493 g, 1.48 mmol), 0.294 g (36%) of 91 was obtained
according to the method described for the synthesis of 17: 1H
NMR (400 MHz, CDCl3) δ 8.7 (s, 1H), 7.43-7.46 (br m, 2H),
7.39-7.42 (br m, 1H), 7.03-7.07 (br m, 2H), 5.80-5.85 (m,
1H), 3.63-3.66 (m, 4H), 3.12-3.17 (m, 4H), 2.52 (s, 3H), 2.34
(s, 3H), 2.25-2.30 (m, 2H), 1.88-1.95 (m, 2H), 1.79-1.83 (m,
2H), 1.57-1.64 (m, 2H), 1.47 (s, 9H); m/z 547.1 (M + 1).
4-[4-(6-Chloro-8-cyclopentyl-5-methyl-7-oxo-7,8-dihy-
dro-pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-pipera-
zine-1-carboxylic Acid tert-Butyl Ester (92). Compound
76 (0.2 g, 0.56 mmol) and 4-(4-amino-phenyl)-piperazine-1-
carboxylic acid tert-butyl ester (0.309 g, 1.1 mmol) were
combined in dry DMSO (3 mL) and heated to 80 °C over 1 h.
After cooling to room temperature, EtOAc was added and the
pale green precipitate was removed by filtration. The filtrate
was washed twice with water, then once with saturated
aqueous NaCl solution and dried over MgSO4. Following
removal of the drying agent and evaporation of the solvent,
the crude product was purified by column chromatography on
silica gel eluting with 30-50% ethyl acetate in hexanes to give
92 as a yellow solid (110 mg, 36%); m/z 541.2 (M + 1), 539.2
(M + 1).
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-6-trimethylsilanyl-
ethynyl-7,8-dihydro-pyrido[2,3-d]pyrimidin-2-ylamino)-
phenyl]-piperazine-1-carboxylic Acid tert-Butyl Ester
(93): Compound 89 (1.4 g, 1.78 mmol) was dissolved in THF
(20 mL) to which were successively added CuI (0.032 g, 0.17
mmol), n-butylamine (1.24 g, 17 mmol) and Pd(PPh3)4 (0.098
g, 0.17 mmol). The reaction was sparged with N2 for 10 min
after which TMS-acetylene was added and the reaction
mixture refluxed for 2 h. More TMS-acetylene was added and
again refluxed for 2 h. The solvent was removed in vacuo and
the crude dissolved in EtOAc and washed with water (2 × 50
mL), saturated aqueous NaHCO3 (2 × 50 mL) and saturated
aqueous NaCl. The organic layer was then dried over MgSO4,
the salts were filtered and the solvent was evaporated in vacuo
to give a crude material. This was further purified by silica
gel chromatography eluting with EtOAc and hexanes to give
93 as a yellow foam (0.160 g, 15%). 1H NMR (400 MHz, CDCl3)
δ 8.98 (s, 1H), 7.19-7.21 (d, J ) 8.8 Hz, 2H), 7.08 (br s, 1H),
6.66-6.68 (d, J ) 8.8 Hz, 2H), 5.58-5.61 (m, 1H,), 3.31-3.35
(m, 4H), 2.83-2.87 (m, 4H), 2.29 (s, 3H), 2.00-2.10 (m, 2H),
1.70-1.78 (m, 2H), 1.48-1.56 (m, 2H), 1.26-1.34 (m, 2H), 1.21
(s, 9H), 0.00 (s, 9H); m/z 601.1 (M + 1).
4-[4-(8-Cyclopentyl-6-ethyl-5-methyl-7-oxo-7,8-dihydro-
pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-
1-carboxylic Acid tert-Butyl Ester (95): Compound 93
(0.150 g, 0.25 mmol) was dissolved in MeOH (10 mL) and K2-
CO3 (0.076 g, 0.55 mmol) and stirred at room temprature for
16 h. The solvent was evaporated and the crude material
dissolved in EtOAc, washed with water and saturated aqueous
NaCl, dried over MgSO4, filtered and concentrated in vacuo
to give 4-[4-(8-cyclopentyl-6-ethynyl-5-methyl-7-oxo-7,8-dihy-
dro-pyrido[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-
carboxylic acid tert-butyl ester (94) as a yellow solid (0.130 g,
99%). m/z 529.1 (M + 1). Compound 94 (0.05 g, 0.095 mmol)
was dissolved in EtOH (20 mL) to which 5% Pd/C (0.050 g)
was added. This mixture was treated to 44 PSI under an H2
atmosphere for 3 days. The catalyst was filtered and the
solvent evaporated to give 95 (0.050 g, 38%) as a green-yellow
solid. 1H NMR (400 MHz, DMSO-d6) δ 8.58 (s, 1H), 7.46-7.48
(d, J ) 8.8 Hz, 2H), 6.95-6.97 (d, J ) 8.8 Hz, 2H), 5.80-5.89
(m, 1H), 3.58-3.62 (m, 4H), 3.10-3.14 (m, 4H), 2.60-2.67 (m,
2H), 2.35 (s, 3H), 2.24-2.30 (m, 2H), 1.82-1.92 (m, 2H), 1.75-
1.82 (m, 2H), 1.58-1.63 (m, 2H), 1.03-1.10 (m, 3H); m/z 533.3
(M + 1).
2-[4-(4-tert-Butoxycarbonyl-piperazin-1-yl)-phenylami-
no]-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido[2,3-
d]pyrimidine-6-carboxylic Acid Ethyl Ester (96): Com-
pound 89 (2.0 g, 3.4 mmol) was dissolved in EtOH (75 mL) to
which Hunig’s base (1.5 mL, 8 mmol), Pd(OAc)2 and DPPP
(0.424 g, 1.03 mmol) were added then subjected to 500 psi of
CO at 100 °C for 30 h. The reaction mixture was diluted with
EtOAc and washed with H2O, sat. NaHCO3 and saturated
aqueous NaCl. Then dried over MgSO4, filtered and concen-
trated in vacuo to give a yellow solid which was recrystallized
to give 96 (1.23 g, 63%). 1H NMR (400 MHz, DMSO-d6) δ 8.64
(s, 1H), 7.42-7.46 (d, J ) 8.8 Hz, 2H), 7.26 (br s, 1H), 6.93-
6.95 (d, J ) 7.3 Hz, 2H), 5.78-5.85 (m, 1H), 4.35-4.41 (q, J )
7.1 Hz, 2H), 3.58-3.62 (m, 4H), 3.10-3.14 (m, 4H), 2.34 (s,
3H), 2.24-2.30 (m, 2H), 1.82-1.92 (m, 2H), 1.75-1.82 (m, 2H),
1.58-1.63 (m, 2H), 1.34-1.37 (t, J ) 7.3 Hz, 3H); m/z 577.1
(M + 1).
2-[4-(4-tert-Butoxycarbonyl-piperazin-1-yl)-phenylami-
no]-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido[2,3-
d]pyrimidine-6-carboxylic Acid Methyl Ester (97): Start-
ing from 75 (0.300 g, 0.510 mmol), 0.100 g (35%) of 97 was
obtained according to the method described for the synthesis
of 96: m/z 563.2 (M + 1).
2-[4-(4-tert-Butoxycarbonyl-piperazin-1-yl)-phenylami-
no]-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido[2,3-
d]pyrimidine-6-carboxylic Acid (98). Compound 96 (0.97
g, 1.7 mmol) and 1.0 N NaOH (5.1 mL) were added to EtOH
(5 mL) and THF (5 mL) and refluxed for 16 h. The solvent
was removed in vacuo, the solid washed with ether, then
acidified with 1 N HCl (5.1 mL) and extracted with EtOAc.
This solution was dried over MgSO4, filtered and concentrated
in vacuo to give a solid that was recrystallized from EtOAc
and hexanes to provide 98 (0.41 g, 44%) as an orange solid.
1H NMR (400 MHz, DMSO-d6) δ 15.00 (br s, 1H), 9.03 (s, 1H),
7.46-7.50 (m, 2H), 7.00-7.04 (m, 2H), 5.91-5.99 (m, 1H),
4.58-4.63 (m, 2H), 3.58-3.62 (m, 4H), 3.10-3.14 (m, 4H), 2.99
(s, 3H), 2.24-2.30 (m, 2H), 1.82-2.03 (m, 4H), 1.58-1.63 (m,
2H), 1.47 (s, 9H); m/z 549.2 (M + 1).
8-Cyclopentyl-5-methyl-2-(4-piperazin-1-yl-phenylami-
no)-8H-pyrido[2,3-d]pyrimidin-7-one (10): Method A: Com-
pound 17 was dissolved in dioxane (4 mL) and 6 N HCl (4 mL)
and stirred for 1.5 h. The solvent was evaporated in vacuo to
give 10 as a yellow solid (0.075 g, 58%): 1H NMR (400 MHz,

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2383
DMSO-d6) δ 9.87 (s, 1H), 9.36 (s, 2H), 8.80 (s, 1H), 7.53-7.55
(d, J ) 8.8 Hz, 2H), 6.99-7.02 (d, J ) 9.0 Hz, 2H), 6.14 (s,
1H), 5.75-5.80 (m, 1H), 3.26-3.35 (m, 4H), 3.10-3.23 (m, 4H),
2.32 (s, 3H), 2.15-2.24 (m, 2H), 1.78-1.92 (m, 2H), 1.65-1.75
(m, 2H), 1.47-1.60 (m, 2H); m/z 405.1 (M + 1); Anal.
(C23H28N6O1‚2.75HCl) C, H, N.
8-Cyclopentyl-2-(4-[1,4]diazepan-1-yl-phenylamino)-5-
methyl-8H-pyrido[2,3-d]pyrimidin-7-one (20): Starting
from 83 (0.230 g, 0.444 mmol), 0.090 g (36%) of 20 was
obtained according to Method A described for the synthesis of
1
10: mp 172 °C; H NMR (400 MHz, DMSO-d6) δ 9.74 (br s,
1H), 9.13 (br s, 2H), 8.73 (s, 1H), 7.47 (d, J ) 8 Hz, 2H), 6.82
(d, J ) 8 Hz, 2H), 5.66-5.83 (m, 1H), 3.71 (br s, 2H), 3.50 (br
t, J ) 5 Hz, 2H), 3.22 (br s, 2H), 3.08 (br s, 2H), 2.33 (s, 3H),
2.02-2.28 (m, 4H), 1.60-1.93 (m, 4H), 1.43-1.60 (m, 2H); m/z
419.3 (M + 1); Anal. (C24H30N6O1‚2.22HCl) C, H, N.
8-Cyclopentyl-2-[4-(3,5-dimethyl-piperazin-1-yl)-phen-
ylamino]-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (22):
Starting from 84 (0.050 g, 0.094 mmol), 0.036 g (65%) of 22
was obtained according to Method A described for the synthesis
of 10: 1H NMR (400 MHz, DMSO-d6) δ 9.76 (br s, 1H), 9.40
(br s, 2H), 8.74 (s, 1H), 7.52-7.54 (d, J ) 8.7 Hz, 2H), 6.97-
6.99 (d, J ) 8.2 Hz, 2H), 6.13 (s, 1H), 5.76-5.81 (m, 1H), 3.75-
3.81 (m, 2H), 3.25-3.40 (m, 2H), 2.58-2.66 (m, 2H), 2.33 (s,
3H), 2.15-2.21 (m, 2H), 1.80-1.92 (m, 2H), 1.64-1.76 (m, 2H),
1.48-1.57 (m, 2H), 1.26-1.29 (d, J ) 6.3 Hz, 6H); m/z 433.3
(M + 1); Anal. (C25H32N6O1‚2.25HCl‚0.1C4H8O2‚0.75C4H10O3)
C, H, N.
8-Cyclopentyl-2-[4-(3,3-dimethyl-piperazin-1-yl)-phen-
ylamino]-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (23):
Starting from 85 (0.100 g, 0.188 mmol), 0.065 g (65%) of 23
was obtained according to Method A described for the synthesis
of 10: 1H NMR (400 MHz, DMSO-d6) δ 9.77 (br s, 1H), 9.23
(br s, 2H), 8.74 (s, 1H), 7.52-7.54 (d, J ) 9.0 Hz, 2H), 6.94-
6.96 (d, J ) 8.9 Hz, 2H), 6.14 (s, 1H), 5.76-5.81 (m, 1H), 3.20-
3.31 (m, 4H), 3.10 (s, 2H), 2.33 (s, 3H), 2.15-2.21 (m, 2H),
1.80-1.92 (m, 2H), 1.65-1.80 (m, 2H), 1.47-1.60 (m, 2H), 1.38
(s, 6H); m/z 433.3 (M + 1); Anal. (C25H32N6O1‚2.0HCl‚0.1C4H8O2‚
0.25C4H10O3) C, H, N.
Method B: Compound 17 (0.280 g, 0.555 mmol) was
dissolved in of dichloromethane (10 mL) to which trifluoro-
acetic acid (5 mL) was added and stirred at room temperature
for 15 h. The solvent was evaporated, and the solid isolated
from diethyl ether to give 10 as a fluffy gray solid (0.343 g,
85%): Anal. (C23H28N6O1‚2.8C2H1O2F3‚0.35H2O) C, H, N.
8-Cyclopentyl-2-(4-piperazin-1-yl-phenylamino)-8H-
pyrido[2,3-d]pyrimidin-7-one (5): Starting from 77 (7.95 g,
16.2 mmol), 4.44 g (57%) of 5 was obtained according to Method
A described for the synthesis of 10: 1H NMR (400 MHz,
DMSO-d6) δ 9.89 (br s, 1H), 9.34 (br s, 2H), 8.69 (s, 1H), 7.70-
7.72 (d, J ) 9.3 Hz, 2H), 7.53-7.55 (d, J ) 8.8 Hz, 2H), 6.99-
7.02 (d, J ) 5.9 Hz, 2H), 6.25-6.27 (d, J ) 9.3 Hz, 2H), 5.75-
5.80 (m, 1H), 3.30-3.38 (m, 4H), 3.14-3.23 (m, 4H), 2.15-
2.22 (m, 2H), 1.72-1.92 (m, 4H), 1.47-1.60 (m, 2H); m/z 391.1
(M + 1); Anal. (C22H26N6O1‚2.0HCl‚1.1H2O) C, H, N.
8-Cyclopentyl-5-ethyl-2-(4-piperazin-1-yl-phenylamino)-
8H-pyrido[2,3-d]pyrimidin-7-one (11): Starting from 78
(0.160 g, 0.308 mmol), 0.128 g (75%) of 11 was obtained
according to Method B described for the synthesis of 10: 1H
NMR (400 MHz, DMSO-d6) δ 9.76 (s, 1H), 8.79 (s, 1H), 8.73
(s, 2H), 7.52-7.54 (d, J ) 8.8 Hz, 2H), 6.92-6.96 (d, J ) 9.0
Hz, 2H), 6.10 (s, 1H), 5.76-5.83 (m, 1H), 3.20-3.30 (m, 8H),
2.72-2.77 (q, J ) 7.6 Hz, 2H), 2.19-2.21 (m, 2H), 1.82-1.91
(m, 2H), 1.62-1.72 (m, 2H), 1.45-1.58 (m, 2H), 1.15-1.19 (t,
J ) 7.5 Hz, 3H); m/z 419.3 (M + 1); Anal. (C24H30N6O1‚
1.21C2H1F3O2) C, H, N.
8-Cyclopentyl-2-(4-piperazin-1-yl-phenylamino)-5-tri-
fluoromethyl-8H-pyrido[2,3-d]pyrimidin-7-one (12): Start-
ing from 79 (0.066 g, 0.118 mmol), 0.040 g (65%) of 12 was
obtained according to Method A described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.11 (s, 2H), 8.68 (s, 1H),
7.50-7.54 (d, J ) 8.3 Hz, 2H), 6.96-7.00 (d, J ) 8.6 Hz, 2H),
6.68 (s, 1H), 5.76-5.81 (m, 1H), 3.26-3.35 (m, 4H), 3.10-3.23
(m, 4H), 2.12-2.21 (m, 2H), 1.75-1.92 (m, 4H), 1.47-1.60 (m,
2H); m/z 459.2 (M + 1); Anal. (C23H28N6O1‚1.44HCl‚0.11EtOAc)
C, H, N.
8-Isopropyl-5-methyl-2-(4-piperazin-1-yl-phenylamino)-
8H-pyrido[2,3-d]pyrimidin-7-one (13): Starting from 80
(0.040 g, 0.083 mmol), 0.038 g (86%) of 13 was obtained
according to Method B described for the synthesis of 10: 1H
NMR (400 MHz, DMSO-d6) δ 9.77 (s, 1H), 8.72 (s, 2H), 7.56-
7.58 (d, J ) 8.3 Hz, 2H), 6.94-6.96 (d, J ) 8.3 Hz, 2H), 6.11
(s, 1H), 5.63-5.70 (m, 1H), 3.18-3.29 (m, 8H), 2.31 (s, 3H),
1.44-1.49 (d, J ) 6.4 Hz, 6H); m/z 379.2 (M + 1); Anal.
(C23H28N6O1‚1.33C2H1F3O2) C, H, N.
8-(1-Ethyl-propyl)-5-methyl-2-(4-piperazin-1-yl-phenyl-
amino)-8H-pyrido[2,3-d]pyrimidin-7-one (14): Starting
from 81 (0.260 g, 0.51 mmol), 0.217 g (80%) of 14 was obtained
according to Method A described for the synthesis of 10: 1H
NMR (400 MHz, DMSO-d6)sa mixture of rotamers δ 9.89 (s,
1/2H), 9.67 (s, 1/2H), 9.03 (s, 1H), 8.72-8.74 (m, 1H), 7.52-
7.62 (m, 2H), 6.95-6.99 (m, 2H), 6.18 (s, 1/2H), 6.08 (s, 1/2H),
5.38-5.42 (m, 1/2H), 5.04-5.13 (m, 1/2H), 3.26-3.35 (m, 4H),
3.19-3.24 (m, 4H), 2.34 (s, 3H), 2.09-2.13 (m, 2H), 1.77-1.87
(m, 2H), 0.63-0.69 (m, 6H); m/z 407.2 (M + 1); Anal.
(C23H30N6O1‚2.88HCl‚0.24C4H8O1) C, H, N.
2-[4-(3-Amino-pyrrolidin-1-yl)-phenylamino]-8-cyclo-
pentyl-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (19):
Starting from 82 (0.100 g, 0.200 mmol), 0.062 g (54%) of 19
was obtained according to Method A described for the synthesis
of 10: 1H NMR (400 MHz, DMSO-d6) δ 9.64 (br s, 1H), 8.70
(s, 1H), 8.03 (br s, 3H), 7.44-7.48 (d, J ) 8.7 Hz, 2H), 6.53-
6.56 (d, J ) 9.0 Hz, 2H), 6.09 (s, 1H), 5.70-5.83 (m, 1H), 3.88-
3.93 (m, 1H), 3.40-3.48 (m, 2H), 3.20-3.26 (m, 2H), 2.31 (s,
3H), 1.95-2.02 (m, 2H), 1.80-1.90 (m, 2H), 1.64-1.78 (m, 2H),
1.48-1.60 (m, 2H); m/z 405.2 (M + 1); Anal. (C24H30N6O1‚
1.4C2H1O2F3‚1.2H2O) C, H, N.
2-(3-Chloro-4-piperazin-1-yl-phenylamino)-8-cyclopen-
tyl-5-methyl-8H-pyrido[2,3-d]pyrimidin-7-one (24): Start-
ing from 86 (0.095 g, 0.176 mmol), 0.059 g (58%) of 24 was
obtained according to Method B described for the synthesis of
1
10: mp 234-237 °C; H NMR (400 MHz, DMSO-d6) δ 10.08
(br s, 1H), 8.80 (s, 1H), 8.74 (br s, 1H), 8.07 (s, 1H), 7.44-7.46
(d, J ) 9.3 Hz, 1H), 7.15-7.18 (d, J ) 8.8 Hz, 1H), 6.18 (s,
1H), 5.78-5.85 (m, 1H), 3.18-3.22 (m, 4H), 3.05-3.12 (m, 4H),
2.34 (s, 3H), 2.18-2.21 (m, 2H), 1.85-1.92 (m, 2H), 1.65-1.75
(m, 2H), 1.48-1.56 (m, 2H); m/z 439.2 (M + 1); Anal. (C23H27-
Cl1N6O1‚1.25C2H1O2F3) C, H, N.
8-Cyclopentyl-5,6-dimethyl-2-(4-piperazin-1-yl-phenyl-
amino)-8H-pyrido[2,3-d]pyrimidin-7-one (25): Starting
from 87 (0.030 g, 0.058 mmol), 0.012 g, (50%) of 25 was
obtained according to Method A described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.69 (br s, 1H), 8.89 (br
s, 2H), 8.79 (s, 1H), 7.53-7.55 (d, J ) 9.0 Hz, 2H), 6.93-6.96
(d, J ) 9.1 Hz, 2H), 5.80-5.86 (m, 1H), 3.20-3.26 (m, 8H),
2.32 (s, 3H), 2.01 (s, 3H), 2.15-2.20 (m, 2H), 1.82-1.92 (m,
2H), 1.65-1.75 (m, 2H), 1.47-1.57 (m, 2H); m/z 419.2 (M +
1); Anal. (C23H28N6O1‚2.75HCl) C, H, N.
8-Cyclopentyl-6-ethyl-5-methyl-2-(4-piperazin-1-yl-phen-
ylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (26): Starting
from 95 (0.030 g, 0.058 mmol), 0.012 g, (50%) of 26 was
obtained according to Method A described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.82 (br s, 1H), 9.25 (br
s, 2H), 8.78 (s, 1H), 7.53-7.55 (d, J ) 9.1 Hz, 2H), 6.96-6.99
(d, J ) 9.0 Hz, 2H), 5.80-5.86 (m, 1H), 3.30-3.37 (m, 4H),
3.12-3.20 (m, 4H), 2.56-2.59 (m, 2H), 2.34 (s, 3H), 2.15-2.20
(m, 2H), 1.82-1.92 (m, 2H), 1.65-1.75 (m, 2H), 1.47-1.57 (m,
2H), 0.94-0.99 (m, 3H); m/z 433.2 (M + 1); Anal. (C25H32N6O1‚
3.0HCl‚0.15CH2Cl2‚0.90EtOH) C, H, N.
8-Cyclopentyl-6-fluoro-5-methyl-2-(4-piperazin-1-yl-
phenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (27): Start-
ing from 88 (0.230 g, 0.441 mmol), 0.210 g, (73%) of 27 was
obtained according to Method B described for the synthesis of
10: mp 254-255 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.78 (s,
1H), 8.77 (br s, 2H), 7.52 (d, J ) 9 Hz, 2H), 6.95 (d, J ) 9 Hz,
2H), 5.83 (br s, 1H), 3.22 (br s, 8H), 2.31 (s, 3H), 2.14-2.21

2384 Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7
VanderWel et al.
(m, 2H), 1.86 (br s, 2H), 1.73-1.75 (m, 2H), 1.54-1.56 (m, 2H);
m/z 423.2 (M + 1); Anal. (C23H27N6O1F1‚1.93C2H1O2F3) C, H,
N.
formic acid and heated to 45 °C for 16 h. The reaction mixture
was partitioned between CH2Cl2 and sat. aqueous NaHCO3.
The layers were separated, and then the organic layer was
dried over MgSO4. The salts were filtered and the solvents
evaporated in vacuo to give a solid that was triturated with
EtOAc and hexanes to yield 15 as a yellow solid (0.05 g, 39%).
mp 244-247 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.00 (s, 1H),
8.31 (s, 1H), 7.76-7.79 (d, J ) 8.8 Hz, 2H), 7.19-7.22 (d, J )
8.8 Hz, 2H), 6.39 (s, 1H), 6.00-6.08 (m, 1H), 6.00 (s, 1H), 3.73-
3.80 (m, 4H), 3.26-3.34 (m, 4H), 2.59 (s, 3H), 2.43-2.49 (m,
2H), 2.02-2.16 (m, 2H), 1.89-1.96 (m, 2H), 1.76-1.82 (m, 2H);
m/z 433.2 (M + 1); Anal. (C24H28N6O2‚0.28EtOAc) C, H, N.
General Procedure for Side Chain Preparation: 1-Flu-
oro-4-nitro-benzene (6.11 g, 43.3 mmol) was dissolved in
acetonitrile (100 mL) to which N,N-diisopropylethylamine (8.1
mL, 46.22 mmol) and piperazine-1-carboxylic acid tert-butyl
ester (8.61 g, 46.22 mmol) were added and heated to reflux
for 14 h. After cooling the reaction mixture to room temper-
ature, a precipitate formed and was filtered to give a yellow
solid. The solid was dissolved in THF (120 mL), and RaNi (5
g) was added and placed under a H2 atmosphere at 50 psi for
5 h. The catalyst was removed by filtration through Celite and
the solvent evaporated in vacuo to give 4-(4-amino-phenyl)-
piperazine-1-carboxylic acid tert-butyl ester as a light tan
powder (6.94 g, 59%).
6-Chloro-8-cyclopentyl-5-methyl-2-(4-piperazin-1-yl-
phenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (28): Start-
ing from 92 (0.110 g, 0.205 mmol), 0.057 g, (52%) of 28 was
obtained according to Method A described for the synthesis of
10: mp 188 °C (decomposed); m/z 441.2 (M + 1), 439.2 (M +
1); Anal. (C23H27N6O1Cl1‚2.00HCl‚1.24 H2O) C, H, N.
6-Bromo-8-cyclopentyl-5-methyl-2-(4-piperazin-1-yl-
phenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (29): Start-
ing from 89 (0.400 g, 0.690 mmol), 0.400 g, (83%) of 29 was
obtained according to Method B described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.92 (br s, 1H), 8.89 (s,
1H), 8.71 (br s, 2H), 7.54 (d, J ) 9 Hz, 2H), 6.96 (d, J ) 9 Hz,
2H), 5.88 (br s, 1H), 3.23 (br d, J ) 11 Hz, 4H), 2.53 (s, 3H),
2.14-2.2 (m, 2H), 1.8-1.96 (m, 2H), 1.65-1.8 (m, 2H), 1.48-
1.60 (m, 2H). m/z 485.1 (M + 1); Anal. (C23H27N6O1Br1‚
1.92C2H1O2F3) C, H, N.
8-Cyclopentyl-6-iodo-5-methyl-2-(4-piperazin-1-yl-phen-
ylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (30): Starting
from 90 (0.140 g, 0.222 mmol), 0.110 g, (79%) of 30 was
obtained according to Method A described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.92 (s, 1H), 9.15 (s, 2H),
8.89 (s, 1H), 7.55 (d, J ) 8 Hz, 2H), 6.97 (d, J ) 8 Hz, 2H),
5.89 (br s, 1H), 3.29 (s, 4H), 3.19 (s, 4H), 2.59 (s, 3H), 2.11 (s,
2H), 1.87 (s, 2H), 1.71 (s, 2H), 1.55 (s, 2H). m/z 531.2 (M + 1);
Anal. (C23H27N6O1I1‚2.00 HCl‚1.44H2O) C, H, N.
4-(4-Amino-phenyl)-piperazine-1-carboxylic Acid tert-
Butyl Ester (99): 1H NMR (400 MHz, CDCl3) δ 6.78-6.80 (d,
J ) 8.5 Hz, 2H), 6.62-6.64 (d, J ) 8.6 Hz, 2H), 3.54-3.58 (m,
4H), 3.53 (br s, 2H), 2.95-2.99 (m, 4H), 1.45 (s, 9H); m/z 278.1
(M + 1).
6-Acetyl-8-cyclopentyl-5-methyl-2-(4-piperazin-1-yl-
phenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one(31): Start-
ing from 91 (0.274 g, 0.47 mmol), 0.200 g, (80%) of 31 was
obtained according to Method A described for the synthesis of
10: 1H NMR (400 MHz, DMSO-d6) δ 9.97 (br s, 1H), 9.2 (br s,
2H), 8.9 (s, 1H), 7.53-7.55 (d, 2H), 6.97-6.94 (d, 2H), 5.5-5.9
(br m, 4H), 3.29-3.30 (m, 4H), 3.19-3.22 (m, 4H), 2.38 (s, 3H),
2.26 (s, 3H), 2.15-2.22 (m, 2H), 1.79-1.83 (m, 2H), 1.72-1.84
(m, 2H), 1.53-1.56 (m, 2H); m/z 447.2 (M + 1). (C25H30N6O2‚
2.5 HCl, 0.5H2O) C, H, N.
4-(4-Methyl-piperazin-1-yl)-phenylamine (100): 1H NMR
(400 MHz, DMSO-d6) δ 6.77-6.79 (d, J ) 8.8 Hz, 2H), 6.61-
6.63 (d, J ) 8.8 Hz, 2H), 3.39 (br s, 2H), 3.01-3.06 (m, 4H),
2.53-2.55 (m, 4H), 2.31 (s, 3H); m/z 192.1 (M + 1).
4-Piperidin-1-yl-phenylamine (101): 1H NMR (400 MHz,
DMSO-d6) δ 6.61-6.64 (d, J ) 8.9 Hz, 2H), 6.42-6.44 (d, J )
8.8 Hz, 2H), 4.50 (s, 2H), 2.77-2.84 (m, 4H), 1.53-1.58 (m,
4H), 1.3-1.39 (m, 2H); m/z 177.0 (M + 1).
3-[1-(4-Amino-phenyl)-piperidin-4-yl]-propan-1-ol (102):
1H NMR (400 MHz, DMSO-d6) δ 6.63 (d, J ) 9 Hz, H), 6.42
(d, J ) 9 Hz, 2H), 4.49 (s, 2H), 4.33 (t, J ) 5 Hz, 1H), 3.25-
3.36 (m, 4H), 2.38 (t, J ) 11 Hz, 2H), 1.66 (br d, J ) 9 Hz,
2H), 1.40-1.43 (m, 2H), 1.15-1.31 (m, 4H); m/z 235.1 (M +
1).
1-[4-(4-Amino-phenyl)-piperazin-1-yl]-ethanone (103):
1H NMR (200 MHz, CDCl3) δ 6.6-6.9 (m, 4H), 3.3-3.9 (br s,
2H), 3.7-3.8 (m, 2H), 3.55-3.65 (m, 2H), 2.95-3.05 (m, 4H),
2.1 (s, 3H).
8-Cyclopentyl-5-methyl-7-oxo-2-(4-piperazin-1-yl-phen-
ylamino)-7,8-dihydro-pyrido[2,3-d]pyrimidine-6-carboxylic
Acid (32): Starting from 98 (0.200 g, 0.370 mmol), 0.170 g,
(77%) of 32 was obtained according to Method A described for
the synthesis of 10: 1H NMR (400 MHz, DMSO-d6) δ 9.99 (br
s, 1H), 9.19 (br s, 2H), 8.88 (s, 1H), 7.53-7.55 (d, J ) 8.8 Hz,
2H), 6.99-7.04 (d, J ) 9.5 Hz, 2H), 5.78-5.83 (m, 1H), 3.32-
3.40 (m, 4H), 3.12-3.20 (m, 4H), 2.38 (s, 3H), 2.15-2.20 (m,
2H), 1.75-1.96 (m, 4H), 1.47-1.57 (m, 2H); m/z 459.2 (M +
1); Anal. (C25H32N6O1‚2.0HCl‚0.47CH2Cl2‚0.11C4H8O1‚ 1.35H2O)
C, H, N.
8-Cyclopentyl-5-methyl-7-oxo-2-(4-piperazin-1-yl-phen-
ylamino)-7,8-dihydro-pyrido[2,3-d]pyrimidine-6-carboxylic
Acid Methyl Ester (33): Starting from 97 (0.274 g, 0.47
mmol), 0.200 g, (80%) of 33 was obtained according to Method
A described for the synthesis of 10: 1H NMR (400 MHz,
DMSO-d6) δ 9.97 (br s, 1H), 9.04 (br s, 2H), 8.85 (s, 1H), 7.53
(d, J ) 9 Hz, 2H), 6.96 (d, J ) 9 Hz, 2H), 5.78 (br s, 1H), 3.76
(s, 3H), 3.28 (d, J ) 5 Hz, 4H), 3.18 (br s, 4H), 2.30 (s, 3H),
2.08-2.20 (m, 2H), 1.66-1.90 (m, 4H), 1.46-1.60 (m, 2H); m/z
463.2 (M + 1).
8-Cyclopentyl-5-methyl-7-oxo-2-(4-piperazin-1-yl-phen-
ylamino)-7,8-dihydro-pyrido[2,3-d]pyrimidine-6-carboxylic
Acid Ethyl Ester (34): Starting from 96 (0.320 g, 0.555
mmol), 0.270 g, (77%) of 34 was obtained according to Method
A described for the synthesis of 10: 1H NMR (400 MHz,
DMSO-d6) δ 10.00 (br s, 1H), 9.37 (br s, 2H), 8.86 (s, 1H), 7.53-
7.55 (d, J ) 8.9 Hz, 2H), 7.01-7.04 (d, J ) 9.0 Hz, 2H), 5.78-
5.83 (m, 1H), 3.32-3.40 (m, 4H), 3.12-3.20 (m, 4H), 2.31 (s,
3H), 2.15-2.20 (m, 2H), 1.65-1.90 (m, 4H), 1.47-1.57 (m, 2H),
1.23-1.28 (t, J ) 7.1 Hz, 3H); m/z 477.2 (M + 1); Anal.
(C25H32N6O1‚2.0HCl‚1.08CH2Cl2) C, H, N.
1-(4-Amino-phenyl)-pyrrolidin-3-ol (104): m/z 179.2 (M
+ 1).
[1-(4-Amino-phenyl)-pyrrolidin-3-yl]-carbamic Acid tert-
Butyl Ester (105): 1H NMR (400 MHz, DMSO-d6) δ 7.07-
7.10 (d, J ) 6.6 Hz, 1H), 6.42-6.45 (d, J ) 8.6 Hz, 2H), 6.25-
6.29 (d, J ) 8.6 Hz, 2H), 4.24 (s, 1H), 4.01-4.04 (m, 1H), 3.25-
3.28 (m, 1H), 3.12-3.16 (m, 1H), 3.01-3.05 (m, 1H), 2.82-
2.85 (m, 1H), 2.05-2.08 (m, 1H), 1.74-1.76 (m, 1H), 1.35 (s,
9H); m/z 278.2 (M + 1).
4-(4-Amino-phenyl)-[1,4]diazepane-1-carboxylic Acid
tert-Butyl Ester (106): 1H NMR (400 MHz, CDCl3) δ 6.62
(s, 2H), 6.57 (s, 2H), 3.4-3.6 (m, 6H), 3.29 (t, J ) 6 Hz, 1H),
3.18 (t, J ) 6 Hz, 1H), 1,90-1.96 (m, 2H), 1.42 (s, 5H), 1.37 (s,
4H); m/z 292.1 (M + 1).
2-[4-(4-Amino-phenyl)-piperazin-1-yl]-ethanol (107): 1H
NMR (400 MHz, DMSO-d6) δ 6.72 (d, J ) 8 Hz, 2H), 6.48 (d,
J ) 7 Hz, 2H), 5.27 (s, 1H), 4.69 (s, 2H), 3.84 (s, 2H), 3.43-
3.55 (m, 4H), 3.1-3.3 (m, 4H); m/z 236.1 (M + 1).
4-(4-Amino-phenyl)-2,6-dimethyl-piperazine-1-carbox-
ylic Acid tert-Butyl Ester (108): 1H NMR (400 MHz, DMSO-
d6) δ 6.63-6.66 (d, J ) 8.5 Hz, 2H), 6.44-6.46 (d, J ) 8.6 Hz,
2H), 4.60 (br s, 2H), 3.96-4.04 (m, 2H), 3.03-3.09 (m, 2H),
2.55-2.60 (m, 2H), 1.38 (s, 9H), 1.22-1.25 (d, J ) 6.5 Hz, 6H);
m/z 306.2 (M + 1).
4-[4-(8-Cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido-
[2,3-d]pyrimidin-2-ylamino)-phenyl]-piperazine-1-car-
baldehyde (15): Compound 10 (0.150 g, 0.297 mmol) sus-
pended in ethyl formate (5 mL) was treated with a drop of
4-(4-Amino-phenyl)-2,2-dimethyl-piperazine-1-carbox-
ylic Acid tert-Butyl Ester (109): 1H NMR (400 MHz, CDCl3)

Inhibitors of Cyclin-Dependent Kinase 4
Journal of Medicinal Chemistry, 2005, Vol. 48, No. 7 2385
(14) Mani, S.; Wang, C.; Wu, K.; Francis, R.; Pestell, R. Cyclin-
Dependent Kinase Inhibitors: Novel Anticancer Agents. Exp.
Opin. Invest. Drugs 2000, 9, 1849-1870.
(15) Meijer, L. Roscovitine and other purines as kinase inhibitors.
From starfish Oocytes to Clinical trials. Acc. Chem. Res. 2003,
36, 417-425.
δ 6.55-6.57 (d, J ) 8.3 Hz, 2H), 6.44-6.46 (d, J ) 8.8 Hz,
2H), 4.47 (br s, 2H), 3.44-3.48 (m, 2H), 2.95-2.99 (m, 2H),
2.82 (s, 2H), 1.37 (s, 9H), 1.31 (s, 6H); m/z 306.2 (M + 1).
4-(4-Amino-2-chloro-phenyl)-piperazine-1-carboxylic
Acid tert-Butyl Ester (110): 1H NMR (400 MHz, DMSO-d6)
δ 6.82-6.85 (d, J ) 8.5 Hz, 1H), 6.58 (s, 1H), 6.40-6.44 (d, J
) 8.5 Hz, 1H), 5.04 (s, 2H), 3.38-3.42 (m, 4H), 2.68-2.72 (m,
4H), 1.37 (s, 9H); m/z 312.0 (M + 1).
Cdk Assays (Cdk4/cyclin D1, Cdk2/cyclin E, Cdk2
cyclin A, and Cdc2/cyclin B). All Cdks were human recom-
binant proteins expressed in insect cells through baculovirus
infection. Enzyme assays for IC50 determinations and kinetic
evaluation were performed in 96-well filter plates (Millipore
MADVN6550). The total volume was 0.1 mL containing a final
concentration of 20 mM Tris (tris[hydroxmethyl]aminomethane),
pH 7.4, 50 mM NaCl, 1 mM dithiothreitol, 10 mM MgCl2, 25
µM ATP (for Cdk4) or 12 µM ATP (for Cdk2/E, Cdk2/A, and
Cdc2/B) containing 0.25 µCi of [32P]ATP, 20 ng of enzyme, 1
µg of GST-retinoblastoma and appropriate dilutions of inhibi-
tor. All components except the ATP were added to the wells,
and the plate was placed on a plate mixer for 2 min. The
reaction was started by adding [32P]ATP, and the plate was
incubated at 25 °C for 15 min. The reaction was terminated
by addition of 0.1 mL of 20% trichloroacetic acid (TCA). The
plate was kept at 4 °C for at least 1 h to allow the substrate
to precipitate. The wells were then washed five times with 0.2
mL of 10% TCA and 32P incorporation was determined with a
beta plate counter (Wallac Inc., Gaithersburg, MD).
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Acknowledgment. The authors would like to ac-
knowledge the contributions of Dr. Vladimir Beylin, Mr.
Michael Waldo and Mr. Mark Marlatt for providing
scaled-up quantities of many key intermediates. In
addition, the authors would like to acknowledge the
contributions of Dr. Garrett Hoge, Mr. Norman Colbry
and Mr. Mark Lovdahl for performing all of the high-
pressure reactions and Erli Zhang for Figure 2.
Supporting Information Available: Purity data for
target compounds. This information is available free of charge
via the Internet at http://pubs.acs.org.
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