T. Shimamura et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3751–3754
3753
b-d
a
N
N
N
OH
e, f
N
N
N
HOOC
BocHN
BocHN
A-1
A-2
A-3
S
N
OTBDPS
N
OTBDPS
g, h
SMe
SMe
N
N
H2N
N
N
N
H2N
H
A-4
A-5
N
N
l
m,n
S
N
OH
S
N
OTBS
N
N
SMe
N
SMe
N
SMe
N
N
k
N
N
i, j
N
N
H
N
SEM
N
OEt
C-1
C-2
Br
Cl
OEt
OEt
B-3
B-1
B-2
O
O
N
O
N
N
N
N
N
o, p
q-s
t
S
N
N
S
N
OTBS
S
N
N
N
N
NMe
N
NMe
N
N
N
N
N
N
SEM
SEM
H
C-3
C-4
Compound 4
t
Scheme 1. Synthesis of compound 4. Reagents and conditions: (a) DPPA, Et3N, 3 h, 100 °C, dioxane, BuOH, 88%; (b) NBS, AIBN, 12 h, 100 °C,
CCl4; (c) KOAc, 18-crown-6-ether, 1 h, rt, MeCN; (d) 3 N NaOH aq, 12 h, rt, MeOH, 29% in 3 steps; (e) TBDPSCl, imidazole, 12 h, rt, DMF, 99% ;
(f) TFA, 5 h, rt, CHCl3, 92%; (g) BzNCS, rt, 2 h, THF, 96%; (h) K2CO3, 45 °C, 5.5 h, THF/MeOH/H2O, 99%; (i) ethyl ethynyl ether, BH3 Æ THF, rt,
4 h, THF; (j) 3 N NaOH aq, PPh3, Pd(OAc)2, rt, 6 h, THF, 88% in 2 steps; (k) NBS, rt, 0.5 h, EtOH, 80%; (l) p-TsOH Æ H2O, 90 °C, 12 h, EtOH/H2O;
(m) TBSCl, imidazole, 3 h, rt, DMF; (n) SEMCl, EtNiPr2, 1 h, 0 °C, DMF, 27% in 3 steps; (o) mCPBA, 1.5 h, 0 °C, CHCl3, 99%; (p) NaH, cyclohexyl
alcohol, rt, 0.5 h, DMF; (q) 1 M TBAF in THF, 1 h, rt, THF, 40% in 2 steps; (r) MsCl, EtNiPr2, 1 h, 0 °C, CHCl3; (s) N-methylpiperazine, K2CO3,
1 h, 70 °C, CHCl3; (t) 4 N HCl in dioxane, 12 h, rt, MeOH, 45% in 3 steps.
in vitro. In addition, this compound inhibited pRb phos-
phorylation and BrdU incorporation by 99% and 91%,
respectively, in Eol-1 xenograft tumor models in nude
rats when administered via constant intravenous
infusion for 24 h at a plasma concentration of 510 nM
(manuscript in preparation).
and for further elucidation of CDK biology. Further
SAR analysis of the analogues and molecular modeling
studies to clarify the mechanism of their CDK4, 6 selec-
tive inhibitions and biological profiles of compound 4
will be reported elsewhere in due course.
The synthetic route for preparation of compound 4 is
illustrated in Scheme 1. Reaction of the thiourea A-5,
which was prepared from 5-methylpyrazine-2-carboxylic
acid A-1 in 8 steps, with 2-bromo-diethyl acetal B-3 gave
5-pyrimidinyl-2-aminothiazole C-1. Next, the methyl-
thio group was oxidized with mCPBA, followed by
introduction of a cyclohexyloxy group to give C-3. N-
Methylpiperazine was then introduced via the mesyla-
tion reaction. Finally, deprotection of SEM group
provided compound 4 as a HCl salt.
Acknowledgments
We acknowledge the excellent contribution of the fol-
lowing scientists to this work: for chemistry, Nobuhiko
Kawanishi, Takashi Hashihayata, and Mikako Kawam-
ura; for biology, Ikuko Suzuki-Takahashi and Yoko
Nakatsuru.
References and notes
In conclusion, we used compound screening and medic-
inal chemistry techniques, supported by molecular mod-
eling, to identify potent CDK4 inhibitors 3 and 4 which
show high selectivity over other CDK family kinases.
These compounds are based on a novel 5-pyrimidinyl-
2-aminothiazole scaffold. Introduction of a methyl
group at the C-6 position of the pyrimidine ring in this
series was found to greatly enhance CDK4 selectivity
over other CDK family kinases. Compound 4, which
showed significant selectivity for CDK4, 6 over non-cell
cycle CDKs, showed mechanism-based inhibition of
CDK4 in cultured cells and in vivo. These results suggest
that compound 4 may be useful in drug development
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