An improved synthesis of arylboronates toward twenty novel 1,3-disubstituted 4-amino-1H-pyrazolo[3,4-d]pyrimidine analogs

Article abstract of DOI:10.3987/COM-09-11857

By developing an improved procedure for arylboronates, twenty 1H-pyrazolo[3,4-d]pyrimidine analogs were efficiently synthesized as a source of a potent kinase inhibitor.

Full text of DOI:10.3987/COM-09-11857

HETEROCYCLES, Vol. 81, No. 1, 2010
73
HETEROCYCLES, Vol. 81, No. 1, 2010, pp. 73 - 77. © The Japan Institute of Heterocyclic Chemistry
Received, 20th October, 2009, Accepted, 12th November, 2009, Published online, 12th November, 2009
DOI: 10.3987/COM-09-11857
AN IMPROVED SYNTHESIS OF ARYLBORONATES TOWARD
TWENTY NOVEL 1,3-DISUBSTITUTED
4-AMINO-1H-PYRAZOLO[3,4-d]PYRIMIDINE ANALOGS
Masato Oikawa*
Graduate School of Nanobioscience and University-Industry Cooperative
Research Center, Yokohama City University, 22-2 Seto, Kanazawa-ku,
Yokohama, 236-0027, Japan
Abstract – By developing an improved procedure for arylboronates, twenty
1H-pyrazolo[3,4-d]pyrimidine analogs were efficiently synthesized as a source of
a potent kinase inhibitor.
Protein kinases are among the most important classes of enzymes that phosphorylate some 10000 cellular
protein hydroxy groups, and account for at least 518 genes (2%) of human genes. Reversible protein
phosphorylation, by protein kinases in association with protein phosphatases, is a ubiquitous signaling
mechanism in eukaryotes, and changes in the phosphorylation states are associated with metabolism,
transcription, cell cycle progression, and cytoskeletal rearrangement. Because it is also related to many
diseases including cancer, selective inhibitor for each protein kinase is of significant value not only for
biological research, but also for cancer therapy.1
X
Y
NH2
4
3
N
N
N
N
1
Z
Figure 1. PPs with substituents at C1 and C3. Bold lines denote the PP core. For X, Y, and Z, see Table 1.

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HETEROCYCLES, Vol. 81, No. 1, 2010
Pyrimidine-containing compounds have been studied as a source of a potent inhibitor for a wide range of
protein kinases through interaction with the ATP-binding site.2 In the course of our studies on Src family
kinases, we have explored a rapid and diverted access toward 1H-pyrazolo[3,4-d]pyrimidines (PPs,
Figure 1). Here, we report our preliminary results toward twenty PPs bearing substituents at C1 and C3,
by developing a modified procedure for arylboronates as a synthetic building block for C3-substituent.3
The synthesis outline for PPs is as follows. The PP core skeleton with substituent at C1 (see Figure 1) was
expected to be readily prepared by a well established, single-step procedure.4,5 A sequence of iodination
at C3 followed by Suzuki–Miyaura cross-coupling reaction6 with arylboronate is then applied to
introduce aryl substituent at the C3 position.
First, preparation of arylboronate 4a was attempted. According to a reported procedure,5 o-anisidine 1
was brominated with 2,4,4,6-tetrabromo-2,5-cyclohexanedienone (TBCO) to give rise to 2 followed by
N-sulfonylation (PhSO2Cl, pyridine) to furnish bromide 3 in 79% yield over two steps (Scheme 1). Yield
for the subsequent borylation of bromide 3 under standard conditions (bis(pinacolato)diboron,
PdCl2(dppf), KOAc, DMF, 80 °C, 2 days) was, however, found to be disappointingly low (27%). Careful
analysis of the reaction mixture revealed that the major product was a dimer (35%, structure not shown),
which indicated that 70% of bromide 3 was consumed by the undesired dimerization process. In fact, it
has been reported that, under some conditions with palladium catalyst and inorganic base, homo-coupling
of aryl bromide readily takes place to furnish symmetrical biaryl.7 Miyaura et al. have also reported that
stronger bases such as K2PO4 promote biaryl formation.8 Therefore, we then explored mild reaction
conditions, and after several attempts with different reaction conditions, it was fortunately found that the
undesired side reaction could be largely suppressed simply by changing the solvent to THF. Thus, when
O
O
SO2Ph
OMe
HN
B
B
SO2Ph
OMe
NH2
HN
O
O
PhSO2Cl
pyridine
NH2
OMe
TBCO
(1.2 equiv)
OMe
CH2Cl2
–10 °C  rt
4 h, 87%
CH2Cl2
rt, 1d
91%
B
PdCl2(dppf) (3 mol%)
KOAc (3 equiv), THF
reflux, 16 h, 85%
O
O
Br
Br
1
2
3
(27% yield in DMF)
4a
Ph
O
Ph
O
HN
B
O
HN
Me
B
OMe
OMe
B
O
O
O
O
O
O
4b (74%)
4c (72%)
4d10
Scheme 1

HETEROCYCLES, Vol. 81, No. 1, 2010
75
the reaction was conducted in THF under reflux for 16 h, desired borylation product 4a was obtained in
85% yield as a sole product. In this case, no dimer was detected. The procedure was also effective for
improved preparation of arylboronates 4b (74%) and 4c (72%), for which DMF was again inefficient as a
solvent in terms of the yields (<30%).9
On the other hand, the PP core skeleton was constructed starting from 3-aminopyrazole-4-carbonitrile (5)
according to a reported procedure4,5 including pyrimidine formation, iodination, and N-alkylation, giving
rise to ketone 6 in 39% yield (Scheme 2). Reductive amination of ketone 6 with morpholine (AcOH,
NaBH(OAc)3, 40 °C) provided amine 7 in 76% yield as a diastereomeric mixture (dr = 2.5:1). Finally,
Suzuki–Miyaura cross-coupling reaction of arylboronate 4a and pyrimidinyl iodide 7 was successfully
effected using Pd(PPh3)4 and Na2CO3 at 80 °C to furnish desired PP analog 8 in 94% yield. The
diastereomeric ratio was determined from 1H NMR spectrum to be 3.0:1. From 6, three other pyrimidinyl
iodides 9-11, analogous to 7, were also synthesized.11
SO2Ph
HN
OMe
NH2
N
NH2
N
I
I
NH2
N
3
3
arylboronate 4a (1.2 equiv)
Pd(PPh3)4 (6 mol%)
morpholine
AcOH
NaBH(OAc)3
N
N
N
N
N
NC
4 steps
Na2CO3 (2.5 equiv)
N
N
N
N
1
N
N
H2N
ref. 4
39%
(CH2Cl)2
40 °C, 2 d
76%
MeOCH2CH2OMe
H2O, 80 °C, 6 h
94%
H
5
6
Z
O
7 (2.5:1)
(
N
Z
= morpholino)
8 (3.0:1)
9: Z = N-methylpiperazinyl
10: Z = pyrrolidinyl
O
11 Z
:
= hydroxy
Scheme 2
With four arylboronates 4a–4d (Scheme 1) and five pyrimidinyl iodides 6, 7, and 9–11 (Scheme 2),
mostly unprecedented twenty PP analogs were readily prepared in a parallel fashion combinatorially as
shown in Table 1 (46–98% yield over one or two steps from ketone 6). All PP analogs were obtained as
an inseparable diastereomeric mixture with a ratio of 2.4:1 – 10:1, which was characterized by
1
LC-ESI-MS and H/13C NMR spectra. The successful synthesis of PP analogs in the present study was
apparently owing to the synthetic improvement of arylboronates which eliminates formation of undesired
dimer. Efforts are currently underway to synthesize up to hundreds of PP analogs, as well as to screen the
members for kinase inhibitors with high specificity.12 The results will be reported in due course.

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HETEROCYCLES, Vol. 81, No. 1, 2010
Table 1.
Synthesis of twenty PPs 8, 12–30 by one- or two-step transformation from 6. For
representative structure, see Figure 1.
Yield from 6 / %
PPs
8b
12
X
Y
Z
dr a
-NHSO2Ph
-NHBz
-NHCbz
-Me
-OMe
-OMe
-OMe
-H
morphorino
morphorino
morphorino
morphorino
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
71d
75d
51d
60d
59d
46d
55d
58d
49d
51d
84d
58d
67e
96e
98e
84e
78d
93d
67d
62d
3.0 : 1
2.8 : 1
3.0 : 1
2.4 : 1
10 : 1
13
14
15
16
17
18
19c
20c
21c
22
23
24
25
26
27
28
29
30
-NHSO2Ph
-NHBz
-NHCbz
-Me
-OMe
-OMe
-OMe
-H
f
–
f
–
3.0 : 1
5.1 : 1
5.2 : 1
3.0 : 1
10 : 1
NAg
-NHSO2Ph
-NHBz
-OMe N-methylpiperazinyl
-OMe
N-methylpiperazinyl
-NHCbz
-Me
-OMe
N-methylpiperazinyl
-H
N-methylpiperazinyl
-NHSO2Ph
-NHBz
-OMe
-OMe
-OMe
-H
=O
=O
NAg
-NHCbz
-Me
=O
NAg
=O
NAg
-NHSO2Ph
-NHBz
-OMe
-OMe
-OMe
-H
-OH
-OH
-OH
-OH
4.5 : 1
4.6 : 1
4.0 : 1
5.5 : 1
-NHCbz
-Me
adr: diastereomeric ratio. In all cases, the diastereomers were not separated for characterization. bSee Scheme
2. cKnown compound.4 dTwo-steps yield from 6. eYields for one-step, Suzuki–Miyaura reaction on 6. fCould
not be determined from MS and NMR analyses. gNA: Not applicable.
ACKNOWLEDGEMENTS
The author is grateful to Professors K. Mizuno, K. Ohashi, and M. Sasaki (Tohoku University) for
valuable discussions. The author also thanks Ms. M. Adachi and Mr. R. Hashimoto (Tohoku University)
for preliminary experiments on this study.
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HETEROCYCLES, Vol. 81, No. 1, 2010
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9. In reference 5, the yield for borylation toward the synthesis of 4c was reported to be 47% yield.
10. Arylboronate 4d was purchased from Wako Pure Chemical Industries, Ltd.
11. Pyrimidyl iodides 9 and 10 were prepared by reductive amination of ketone 6 following the
procedure shown in Scheme 2 in 75% and 69% yield, respectively. Compound 11 was synthesized
by reduction of ketone 6 by NaBH4 (94% yield).
12. Our preliminary biological evaluation has already identified 29 as a selective inhibitor for some Src
family tyrosine kinases.