Design and synthesis of 2(1H)-pyrazinones as inhibitors of protein kinases

Article abstract of DOI:10.1016/j.tet.2012.09.039

Kinase enzymes play a key role in the development and progression of cancer. Inhibitors of deregulated kinases are effective small molecule anticancer drugs. The 2(1H)-pyrazinone heterocycle is a previously unexploited motif that can fulfil the structural requirements for ATP-competitive inhibition of kinases. Rapid solution-phase syntheses of novel 3,5- and 3,6-disubstituted-2(1H)-pyrazinones were developed through selective, sequential substitution of 2,5-dihalo-3-benzyloxypyrazine and 3,5-dihalo-2(1H)-pyrazinone intermediates. Palladium-catalysed cross-couplings and S_NAr reactions were used to introduce substituents chosen on the basis of the calculated physicochemical properties of the target pyrazinones. Representative compounds demonstrated good solubility, kinase inhibitory activity and antiproliferative activity in human tumour cells, confirming the suitability of this chemical class as a kinase-focused library.

Full text of DOI:10.1016/j.tet.2012.09.039

Tetrahedron 68 (2012) 9713e9728
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Design and synthesis of 2(1H)-pyrazinones as inhibitors of protein kinases
*
John J. Caldwell, Nicolas Veillard, Ian Collins
Cancer Research UK Cancer Therapeutics Unit, Division of Cancer Therapeutics, The Institute of Cancer Research, Haddow Laboratories, 15 Cotswold Road, Sutton,
Surrey SM2 5NG, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2012
Received in revised form 24 August 2012
Accepted 10 September 2012
Available online 13 September 2012
Kinase enzymes play a key role in the development and progression of cancer. Inhibitors of deregulated
kinases are effective small molecule anticancer drugs. The 2(1H)-pyrazinone heterocycle is a previously
unexploited motif that can fulfil the structural requirements for ATP-competitive inhibition of kinases.
Rapid solution-phase syntheses of novel 3,5- and 3,6-disubstituted-2(1H)-pyrazinones were developed
through selective, sequential substitution of 2,5-dihalo-3-benzyloxypyrazine and 3,5-dihalo-2(1H)-pyr-
azinone intermediates. Palladium-catalysed cross-couplings and SNAr reactions were used to introduce
substituents chosen on the basis of the calculated physicochemical properties of the target pyrazinones.
Representative compounds demonstrated good solubility, kinase inhibitory activity and antiproliferative
activity in human tumour cells, confirming the suitability of this chemical class as a kinase-focused
library.
Keywords:
2(1H)-Pyrazinone
3-Benzyloxypyrazine
Suzuki
Kinase
Ó 2012 Elsevier Ltd. All rights reserved.
Drug-like
1. Introduction
The choice of the 2(1H)-pyrazinone scaffold as the basis of a new
screening library was initially suggested by the structure of the
Cancer is one of the most frequent causes of death in developed
countries and the identification of new therapies is an area of on-
going importance in biomedical research.1,2 The genetic changes
underlying the development of cancer often lead to deregulation of
cell signalling pathways.3,4 Protein kinases are critical components
of many signal transduction cascades controlling cell growth, dif-
ferentiation and survival, thus aberrant signalling by protein ki-
nases can play a key role in cancer progression.4,5 Protein kinases
natural product ma’edamine A 1 (Fig. 1), which shows some kinase
inhibitory activity (c-erbB-2 IC50¼11
m
M),21 and was refined
through considering the requirements for the binding of ATP-
competitive inhibitors to protein kinases.5,6,8 ATP binds within
a deep cleft in the protein kinase catalytic domain, where it forms
two hydrogen bonds to backbone amides in the hinge-region,
a flexible peptide chain linking the N- and C-terminal sub-
domains. Other important contacts occur between the hydroxyls of
the ribose unit of ATP and polar residues in the kinase, and between
the magnesium-chelated triphosphate group and the protein.
While these pharmacophore points are conserved between differ-
ent protein kinases, there are other areas adjacent to the position of
ATP in the binding site, which vary more in composition and
structure but are accessible to small molecule inhibitors. Pharma-
cophores incorporating these features have proved highly suc-
cessful in guiding the discovery of new ATP-competitive kinase
inhibitors.11,22,23 We hypothesised that the 2(1H)-pyrazinone core
would be capable of binding to the hinge-region while providing
suitable vectors for the addition of further substituents (Fig. 2).
While to our knowledge no details of the structural biology of
pyrazinone kinase inhibitors have been reported to date, the qui-
noxalin-2-one 2 (Fig. 1), which contains the 2(1H)-pyrazinone
substructure within the bicycle has been found to be a potent CDK
inhibitor.24 The crystal structure of 2 bound to CDK2 showed that
the donor (NH)dacceptor (C]O) motif of the fused pyrazinone can
function as hypothesised here. Although the disubstituted 2(1H)-
pyrazinone marine natural product ma’edamine A (1) shows some
catalyse the transfer of the
g
-phosphate from adenosine 50-tri-
phosphate (ATP) to protein substrates, with the target for phos-
phorylation usually being the oxygen atom of a serine, threonine or
tyrosine hydroxyl.6 It is now well established that inhibiting the
catalytic activity of deregulated protein kinases can be an effective
treatment for cancer, and much contemporary research is devoted
to the discovery of new small molecule inhibitors.5,7e12 High-
throughput screening (HTS) has been one successful means of
identifying new chemotypes with protein kinase inhibitory
activity.13e15 In particular, the design of screening libraries of
compounds that are ATP-competitive has provided useful starting
points for drug discovery.8,16e20 As part of an anticancer drug dis-
covery program, we have developed methods for the rapid syn-
thesis of a library of novel 3,6- and 3,5-disubstituted 2(1H)-
pyrazinones suitable for screening against kinase targets.
* Corresponding author. Fax: þ44 0208 7224126; e-mail address: ian.collins@
icr.ac.uk (I. Collins).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.039

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J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
Fig. 1. Structures of 2(1H)-pyrazinone containing kinase inhibitors 1, 2 and 3.
A
B
Fig. 2. Schematics of (A) ATP binding to protein kinases22,23 and (B) the proposed alignment of the 2(1H)-pyrazinone scaffold within the binding site.
kinase inhibitory activity,21 the mode of inhibition of the compound
has not been established. A series of trisubstituted 2(1H)-pyr-
azinones, e.g., 3, have been developed as inhibitors of the protein
kinase PKB.25 In this case, the compounds are known to be ATP-
noncompetitive, allosteric inhibitors of the kinase, which bind to
a distinct region away from the ATP-binding site.26,27 Interestingly,
the 2(1H)-pyrazinone motif also occurs in the dragmacidin family
of marine natural products,28e30 some of which are protein phos-
phatase inhibitors.
give asymmetrically substituted pyrazines.42 Interestingly, they ob-
served that the formation of symmetrical 2,5-disubstituted pyrazines
was difficult to control. In the total synthesis of the dragmacidins,
Stoltz and co-workers demonstrated sequential palladium-catalysed
couplings to a 2,5-dihalo-3-methoxypyrazine, followed later in the
synthesis by removal of the 3-methoxy group using trimethylsilyl
iodide to reveal the 2(1H)-pyrazinone.35,37 Reasoning that masking of
the pyrazinone carbonyl as a benzyloxy substituent would provide
moreoptionsforremovalof the protecting group, we initiallychose to
prepare a range of 2,5-dihalosubstituted 3-benzyloxypyrazines and
investigate their suitability as starting materials for the parallel, so-
lution phase synthesis of novel 3,6-di(hetero)aryl-2(1H)-pyr-
azinones. We subsequently extended our study to include previously
unexplored sequential reactions on N-unsubstituted 3,5-dihalo-
2(1H)-pyrazinones.
There are two main synthetic approaches to 2(1H)-pyrazinones:31
making the ring by condensation25,32,33 or preparing a functionalised
ring with subsequent substitutions.31c,34e39 For an example of the
former, libraries of disubstituted pyrazinone PKB inhibitors related to
3
were produced by condensation of 1,2-diketones and 2-
aminoacetamides, requiring chromatographic separation of the
regioisomeric mixture arising from the ring forming reaction.25 For
our purpose, the second general approach was chosen as it was suited
for parallel synthesis using sequential palladium-catalysed coupling
and/ornucleophilicsubstitutionreactions. Hoornaertandco-workers
have published syntheses ofhighlysubstituted pyrazinones involving
decoration of 3,5-dihalogenated pyrazinone scaffolds.38,39 However,
in many examples the pyrazinone N1 was substituted in the final
products, whereas we required a free NH at this position in order to
explore the hypothesised binding mode. Similar methodology has
been successfully applied by Van der Eycken and colleagues to the
solid phase synthesis of 3,5-disubstituted pyrazinones.40,41 In these
cases, N1 of the pyrazinone was used as the attachment point to the
resin, thus releasing the free NH after cleavage in many cases. In ad-
dition, the Van der Eycken group have extended their methodology to
the microwave assisted palladium coupling of 2(1H)-pyrazinones to
2. Results and discussion
Two main factors were considered in selecting the substituents
for the pyrazinone core in the proposed library. First, substituents
were chosen containing a variety of peripheral hydrogen bonding,
basic or lipophilic groups to interact with potential biological tar-
gets. Second, the physicochemical properties of the compounds
were calculated to ensure that these were compatible with sug-
gested criteria for ‘drug-likeness’ and ‘lead-likeness’ in small mol-
ecules.10,43e48 Since medicinal chemistry optimisation of initial
lead compounds often involves the addition of new functionality to
improve potency or selectivity, it is logical that compounds for HTS
libraries should be smaller than the average size of drugs, with
lead-like compounds typically not larger than 350e400 MW and

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9715
with ClogP <3e4, although the exact values suggested for these
cut-offs vary between research groups.10,43e46
selectivity for, the mono-coupling products 15, 16 or 18 was gen-
erally seen with electron-rich coupling partners, such as 4-
methoxyphenyl- or p-tolylboronic acids. In this case, the initial
reaction appears to be fast to give the mono-substituted product
while subsequent reaction is presumably slower as a result of
electron donation from the newly introduced aryl group reducing
the reactivity towards oxidative addition of the palladium catalyst
and reinforcing the selectivity. Despite extensive attempts to opti-
mise the reactions further, general conditions for highly selective
single Suzuki coupling to the templates 7, 8, 12 and 13 were not
observed.
The lack of reliable control of mono-coupling and the de-
pendence of the selectivity on the nature of the boronic acid
building blocks limited the scope for a general combinatorial syn-
thesis. However, the selectivity possible with 3-benzyloxy-5-
bromo-2-chloropyrazine 8 and its regioisomer 13 with neutral
and electron-rich boronic acids did provide an opportunity for
parallel solution-phase synthesis. The two regioisomeric templates
offered access to complementary 3,6-disubstitution patterns in the
final pyrazinones, and thus to some extent overcame the limita-
tions on the selective reaction of electron-poor arylboronic acids.
The templates 8 and 13 were reacted on a multigram scale with 4-
methoxyphenylboronic acid to afford monoarylpyrazines 19 and 20
in 70% and 61% yield, respectively (Scheme 3). Further Suzuki
couplings were carried out on the chloropyrazines 19 and 20 with
Starting from commercially available 2-aminopyrazine 4 we
prepared four different 2,5-dihalo-3-benzyloxypyrazine templates
(7,49 8, 12 and 13) (Scheme 1). The templates were selected to pos-
sess different halide functionality and therefore potentially show
different sequential reactivities, in particular towards palladium-
catalysed cross-coupling reactions. Bromination of 4 using N-bro-
mosuccinimide afforded 3,5-dibromopyrazin-2-amine 5,34,50 which
reacted with the anion of benzyl alcohol to selectively substitute the
bromine atom at the 3-position. The regiochemistry of the sub-
stitution was confirmed by hydrogenation of compound 673 to ob-
tain 3-aminopyrazin-2-one 1451 (Scheme 1). Transformation of the
amino group of 6 into a halide was achieved through diazonium salt
formation and halide substitution35,52 using hydroiodic acid and
sodium nitrite to give 3-(benzyloxy)-5-bromo-2-iodopyrazine 7.49
Alternatively, treatment of 6 with isoamyl nitrite and a mixture of
copper(II)chloride/copper(I) chloridegave 3-(benzyloxy)-5-bromo-
2-chloropyrazine 8. 3,5-Dichloro-2-aminopyrazine 1075 was syn-
thesised from 2-amino-3-chloropyrazine
9
using N-chlor-
osuccinimide. Displacement of the 3-chloro substituent gave 2-
amino-5-chloro-3-benzyloxypyrazine 11, which was converted
into 5-chloro-2-iodo-3-benzyloxypyrazine 12 and 5-chloro-2-
bromo-3-benzyloxypyrazine 13, using a copper(II) bromide/cop-
per(I) bromide mixture for the Sandmeyer reaction52 to give 13.
Scheme 1. Synthesis of 2,5-dihalo-3-benzyloxypyrazines. Reagents and conditions: (a) NBS, DMSO, H2O, 0 ꢀCert, 16 h, 77%; (b) benzyl alcohol, NaH, THF, reflux, 10e24 h, 72% for 6,
57% for 11; (c) isoamyl nitrite, CuCl2/CuCl (3:2), MeCN, rt, 3 h, 61% for 8; (d) NaNO2, HI, MeCN, H2O, 50 ꢀC, 21e24 h, 64% for 7, 50% for 12; (e) NCS, CHCl3, reflux, 4 h, 76%; (f) isoamyl
nitrite, CuBr2/CuBr (3:2), MeCN, rt, 3 h, 48% for 13; (g) H2, Pd/C, EtOH, rt, 7 h, 20%.
With the 2,5-dihalopyrazine templates in hand, the regio- and
chemoselectivity of Suzuki coupling reactions were investigated
(Scheme 2). Tandem HPLCemass spectrometry (LCeMS) of the
crude reaction mixtures was used to monitor the conversion and
product identities in the trial reactions, which although not quan-
titative in output, provided a rapid and direct means of analysis. An
initial catalyst screen using pyrazine 7 and p-tolylboronic acid
showed Pd(PPh3)4 or Pd(PPh3)2Cl2 formed the mono-coupled
product 15 (Table 1), reacting as expected at the more reactive
iodo centre. Absence of reaction or double coupling was observed
with the remaining catalysts employed. Using Pd(PPh3)2Cl2 with
dihalopyrazines 8, 12 and 13 led to formation of the expected mono
addition products, however only pyrazine 12 reacted completely
and selectively. As increasing the temperature was observed to give
substantially increased amounts of diarylpyrazines, the tempera-
ture was reduced to 25 ꢀC for a subsequent boronic acid screen in an
effort to optimise mono adduct formation (Table 2). Of particular
note was the variation in the degree and selectivity of the reaction
with different boronic acid reagents. Greatest conversion to, and
a varied set of (hetero)aromatic boronic acids or boronate esters
(Scheme 3, Table 3). Although the reactions were performed suc-
cessfully by heating at reflux for 18 h, microwave irradiation at
150 ꢀC for 20 min was found to give similar yields in a much re-
duced time using Pd(PPh3)4 as the catalyst. For some less reactive
coupling partners, a more active palladacyclic catalyst proved
effective.53
Several methods were investigated for removal of the benzyloxy
protecting groups. Although an acid-mediated reaction with tri-
fluoroacetic acid at reflux was successful with some substrates, it
proved unreliable and often low-yielding when generally applied.
Using a dilute solution of trifluoroacetic acid (10% in dichloro-
methane) gave no reaction.54 Hydrogenation removed the benzyl
group but also reduced the pyrazinone ring to a piperazine. Use of
the mild Lewis acid BCl.3SMe255 gave no reaction, but treatment with
1 M boron trichloride in dichloromethane at rt for 30 min was found
to cleanly produce the 2(1H)-pyrazinones (38e54) (Scheme 3, Table
4). Theproducts wereisolatedbyconcentration of thecrude reaction
mixture and filtration through a short column of basic ion exchange

9716
J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
Scheme 2. Suzuki reaction of 2,5-dihalo-3-benzyloxypyrazines. Reagents and conditions: (a) RB(OH)2 (5 equiv), 2 M aq Na2CO3, DME, Pd catalyst (10 mol %).
Table 1
Product distribution for Suzuki couplings to 2,5-dihalo-3-benzyloxypyrazines as a function of starting material, catalyst and temperature
Starting material
Catalyst
Product distributiona
Starting material (%)b
15, 16 or 18 (R¼p-tolyl) (%)b
17 (R¼p-tolyl) (%)b
7
7
7
7
7
8
12
13
trans-[(2-tol)3P]2PdCl2
PdCl2(dppf)
trans-[(c-hex)3P]2PdCl2
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
50
66
100
75
8
55
0
60
0
0
0
25
77
45
100
33
50
34
0
0
15
0
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
0
7
a
Percentage of product in crude reaction mixture as determined by HPLC analysis.
Reaction in the presence of 5 equiv p-tolylboronic acid, 10 mol % catalyst, 2 M aq Na2CO3, DME, 50 ꢀC, 24 h.
b
Table 2
Product distribution for Suzuki coupling of varied boronic acids to 2,5-dihalo-3-benzyloxypyrazines
Starting material
Boronic acid
Product distributiona
Starting material (%)b
15, 16, or 18 (%)b
17 (%)b
7
8
12
13
7
p-Tolylboronic acid
p-Tolylboronic acid
p-Tolylboronic acid
p-Tolylboronic acid
5
0
45
35
35
0
32
0
40
0
32
8
100
85
100
100
85
100
55
65
65
100
68
100
30
85
32
37
0
10
0
0
0
0
0
0
0
30
15
36
55
0
0
0
0
4-Methoxyphenylboronic acid
4-Methoxyphenylboronic acid
4-Methoxyphenylboronic acid
4-Methoxyphenylboronic acid
4-Chlorophenylboronic acid
4-Chlorophenylboronic acid
4-Chlorophenylboronic acid
4-Chlorophenylboronic acid
Pyridin-3-ylboronic acid
Pyridin-3-ylboronic acid
Pyridin-3-ylboronic acid
Pyridin-3-ylboronic acid
8
12
13
7
8
12
13
7
8
12
13
15
0
0
a
Percentage of product in crude reaction mixture as determined by HPLC analysis.
Reaction with Pd(PPh3)2Cl2 (10 mol %), 2 M aq Na2CO3, DME, 25 ꢀC, 24 h.
b
resin. Although the 4-methoxyphenyl groups were unaffected by
the deprotection, in the case of the 2-methoxyphenyl derivative 37
the aryl ether was also cleaved to give phenol 54 in high yield.
Thus far, the focus was on 3,6-disubstituted pyrazinones with
a linear relationship between the two aryl substituents. To provide
compounds within the screening library with an alternative ge-
ometry, the 3,5-disubstitution pattern was also investigated.
Starting from commercially available 2-amino-3-chloropyrazine
(9), bromination followed by diazotization of the amine and
hydrolysis of the diazonium salt50 gave 5-bromo-3-chloropyrazin-
2-one 56 (Scheme 4). Although selective couplings of arylstannanes
with N-phenyl- and N-benzyl-3,5-dichloropyrazin-2-one have
been reported,56 Suzuki reactions of 56 failed to provide useful
levels of regioselectivity with a range of catalysts, or resulted in low
conversion of the starting material. However, reaction of the tem-
plate with an excess of boronic acid was shown to give the simple
3,5-diaryl pyrazinones, such as the use of 4-methoxphenyl boronic
acid to give 57 in 47% yield.

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9717
Scheme 3. Parallel synthesis of 2-benzyloxy-3,6-diarylpyrazines and 3,6-diarylpyrazin-2(1H)-ones. Reagents and conditions: (a) 4-methoxyphenylboronic acid, Pd(PPh3)2Cl2, 2 M
aq Na2CO3, DME, 50 ꢀC, 16 h, 70% for 19, 61% for 20; (b) RB(OR)2, Pd(PPh3)4 6 mol %, 2 M aq Na2CO3, MeCN, 150 ꢀC, microwave, 20 min or RB(OR)2, 10 mol % palladacycle,53 2 M aq
Na2CO3, DME, reflux, 4e40 h, 8e93%; (c) BCl3, DCM, rt, 30 min, 8e98%.
Table 3
2-Benzyloxy-3,6-diarylpyrazine products from Suzuki coupling to templates 19 and 20
R
No.
Yielda,b (%)
No.
Yielda,b (%)
4-Methoxyphenyl
4-Methylphenyl
4-(Ethoxycarbonyl)-phenyl
3-Pyridyl
4-Hydroxyphenyl
3-Acetylaminophenyl
4-Carbamoylphenyl
Pyrazol-4-yl
Pyrazol-3-yl
Thien-3-yl
4-(Morpholin-1-ylmethyl)-phenyl
2-Methoxyphenyl
21
22
23
24
25
26
d
55c
58e
53e
60c
49c
48c
d
dd
30
d
d
51e
d
31
32
d
9
42c
d
33
34
35
36
d
8c
d
d
59c
60c
80c
d
27
28
29
d
47c
62c
93c
d
37
88c
a
Isolated yield of purified material.
Purity determined by HPLC >95%.
b
c
Prepared according to general method A using Pd(PPh3)4 as catalyst.
Reaction not performed.
d
e
Prepared according to general method B using [2-[(dimethylamino-kN)methyl]phenyl-kC](tricyclohexylphosphine)(trifluoroacetato-k
O)-palladium53 as catalyst.
Table 4
Yields of 3,6-di(hetero)aryl-2(1H)-pyrazinones
R
No.
Yielda,b (%)
No.
Yielda,b (%)
4-Methoxyphenyl
4-Methylphenyl
4-(Ethoxycarbonyl)phenyl
3-Pyridyl
4-Hydroxyphenyl
3-Acetylaminophenyl
4-Carbamoylphenyl
Pyrazol-4-yl
Pyrazol-3-yl
Thien-3-yl
4-(Morpholin-1-ylmethyl)-phenyl
2-Hydroxyphenyl
38
39
40
41
42
43
d
95
98
8
dc
47
d
d
98
d
39
94
d
96
49
26
94
d
92
47
94
64
d
d
89
94
61
d
48
49
d
50
51
52
53
d
d
44
45
46
d
54d
a
Isolated yield.
Purity determined by HPLC >95%.
Reaction not performed.
Obtained from reaction of the 2-methoxyphenyl derivative 37.
b
c
d

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J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
Scheme 4. Synthesis of 3,5-diaryl-2-(1H)-pyrazinones. Reagents and conditions: (a) NBS, CHCl3, reflux, 16 h, 69%; (b) NaNO2, concd H2SO4, H2O, 0e40 ꢀC, 1 h, 68%; (c) 4-
methoxyphenylboronic acid (5 equiv), 0.5 M Na2CO3, Pd(PPh3)4, MeCN, MW 150 ꢀC, 20 min, 47%.
In a more successful regioselective strategy, the halides of 5-
bromo-3-chloropyrazin-2-one 56 were differentiated by SNAr dis-
placement of the 3-chloro substituent with amines to give the 5-
bromo-3-amino-2(1H)-pyrazinones 58e63 (Scheme 5, Table 5).
Selective SNAr displacements of N-aryl- and N-alkyl-3,5-
dihalopyrazin-2-ones are known,57e60 but these reactions have
not previously been reported on the simpler N-unsubstituted pyr-
azin-2-one substrates. Six-membered cyclic amines were chosen to
give substituents with similar size to the (hetero)aryl groups
present in the rest of the library. Subsequent Suzuki coupling with
a range of boronic acids using bromides 58e63 provided a further
library set for screening. Exemplars of this reaction sequence are
shown in Table 5. Firstly, bromides 58e62 were found to couple
successfully with 4-methoxyphenylboronic acid to give pyr-
azinones 64e68. In addition, morpholine 63 was reacted with
a range of boronic acids leading to the preparation of compounds
69e73 (Scheme 5, Table 5). Microwave heating was used for both
the CeN and CeC bond forming steps, and the final compounds
were purified by automated MPLC on silica gel. Yields from the
Suzuki coupling were generally low to moderate, primarily as
a consequence of the difficult isolation of pure product due to poor
organic solvent solubility. However, the parallel procedure reliably
and rapidly generated sufficient pure product for the purpose of
building the screening library.
During the production of the library, representative pyrazinones
were tested for their kinase inhibitory activity. Compounds 38 and
57 were profiled against a diverse panel of 150 kinases,63 and
compounds 39 and 65 were assessed against a panel of 85 kinases64
using radiometric (33P-ATP) assay formats (Table 7). The analogues
were screened at a concentration of 30
mM. Encouragingly, mod-
erate activity (30e85% inhibition at 30
mM) was seen with these
prototype ligands for a range of kinases, including targets in on-
cology, for example, Aurora-B65 and PKB.66 Although the potency of
these inhibitors is low, the molecular weight is also low
(292e308 Da). Inhibition (IC50) in the 10e30 mM range would lead
to ligand efficiencies of ca. 0.3 kcal molꢁ1 non-H atomꢁ1, which
would indicate the hits to be viable for optimisation.67,68 In-
terestingly, there was a variation in the pattern of inhibition be-
tween the different test compounds despite the simplicity and low
molecular weight of the compounds, and the similar peripheral
substituents in some cases. These results support the use of the
library for kinase inhibitor screening.
To further establish the suitability of the library for screening,
selected compounds were investigated for antiproliferative ef-
fects69 in human tumour cell lines (Table 8). For instance, the 3,6-
disubstituted pyrazinone 39 was cytotoxic to HCT116 colon can-
cer cells. The 3,5-disubstituted analogue 57 had weaker activity.
This promising data suggests that the pyrazinones are cell pene-
Scheme 5. Sequential chemoselective nucleophilic substitution and Suzuki coupling to 5-bromo-3-chloropyrazin-2-one 56. Reagents and conditions: (a) NHR1R2, DIPEA, nBuOH,
microwave 140 ꢀC, 1 h, 53e91%; (b) R3B(OH)2, 0.5 M aq Na2CO3, PdCl2(dppf), MeCN, microwave 150 ꢀC, 20 min, 8e99%.
To demonstrate the applicability of the sequential SNAr and
Suzuki reaction sequence to the preparation of 3,6-disubstituted
pyrazinones, the 6-bromo-3-chloro-2-benzyloxypyrazine tem-
plate 8 was deprotected with boron trichloride to give 6-bromo-3-
chloropyrazin-2-one 74. Reaction with morpholine and sub-
sequently with 4-methoxyphenylboronic acid gave the 3,6-
disubstituted pyrazinone 76 (Scheme 6).
Aqueous solubility studies were carried out on representative
compounds with different substitution patterns (Table 6). Com-
pound 65 was chosen as it contained a moderately basic N-meth-
ylpiperazine, which could enhance aqueous solubility, and indeed
65 was the most soluble analogue in neutral and acidic aqueous
conditions. The 3,6-disubstituted pyrazin-2-ones 38 and 76 were
the least soluble in methanol and pH 5 buffer, while the corre-
sponding 3,5-disubstituted regioisomers 57 and 69 showed gen-
erally higher solubility. To be useful for in vitro biochemical
screening and cellular assays, it was desirable that the library
trant, as would be predicted from their calculated physicochemical
properties and measured solubilities, and validates the consider-
ation of drug-likeness in the design of the library. However the
observed antiproliferative activity cannot be assigned to any spe-
cific targets at this point, although it might be due in part to the
inhibition of kinases in the cell. It is important to note that 3,5-
(bisindolyl)-2(1H)-pyrazinones related to the dragmacidin natural
products have shown a range of antiproliferative activities in hu-
man tumour cells.70,71 Nevertheless, the initial assessment of
physicochemical, biochemical and antiproliferative properties of
the compounds supports their future use in high-throughput
screening against kinase targets.
3. Conclusions
A kinase-focused library of novel 3,5- and 3,6-disubstituted-
2(1H)-pyrazinones for biochemical screening was synthesised by
selective substitution of previously unreported 2,5-dihalo-3-
benzyloxypyrazine and 3,5-dihalo-2(1H)-pyrazinone intermediates.
Palladium-catalysed Suzuki cross-couplings to 2,5-dihalo-3-
benzyloxypyrazines proceeded preferentially at the most reactive
carbonehalogenbond, giving high chemoselectivity inthereaction of
neutral and electron-rich boronic acids, but not with electron-
components should be soluble up to at least 100
mM in appropriate
aqueous media.61 For example, a molecular weight of ca. 300 with
a concentration of 100 mM corresponds to a solubility of approxi-
mately 0.03 mg mLꢁ1. The data for the representative compounds
indicates that even the less soluble reach this threshold and should
therefore be suitable for screening.

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9719
Table 5
of the core heterocycle as an ATP-mimetic, further studies of struc-
tureeactivity relationships for specific kinases, or X-ray crystallo-
graphic studies on kinase-inhibitor complexes, are required to fully
validate this hypothesis. The antiproliferative activity in cells seen
for some compounds, while not necessarily due to kinase inhibition,
gives confidence that the pyrazinones will be cell-penetrant, an
essential property for drug discovery leads or tool compounds for
studying intracellular kinase targets.61 Further profiling of the li-
brary may give more insights on a possible mechanism of action for
these compounds and other cytotoxic disubstituted pyrazinones.
Yields of the SNAr products 58e62 and subsequent Suzuki products 64e73
No.
X
R
Yielda,b (%)
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
eCH2e
Br
Br
Br
Br
Br
Br
91
53
50
87
88
90
99
38
41
17
18
20
8
eN(Me)e
eCH(OH)e
eCH(CO2Me)e
eCH(Ph)e
eOe
4. Experimental
4.1. General experimental
eCH2e
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Carbamoylphenyl
3-Pyridyl
eN(Me)e
eCH(OH)e
eCH(CO2Me)e
eCH(Ph)e
eOe
eOe
eOe
eOe
eOe
Reagents and solvents were used as purchased from commercial
suppliers unless otherwise stated. N-Bromosuccinimide (NBS) was
purified by recrystallisation from water. Organic solutions were dried
over Na2SO4. Reactions were carried out under an atmosphere of ni-
trogen. Microwave reactions were carried out in sealed glass vials
using a Biotage Initiator microwave reactor. Ion exchange chroma-
tography was performed using Isolute NH2 and SCX-II resin car-
tridges. FlashcolumnchromatographywasperformedonMerck silica
gel 60 (0.015e0.040 mm). Automated MPLC was performed on
a Biotage SP1 instrument using prepacked silica cartridges and UV-
triggered fraction collection (254 nm). Melting points were recor-
12
17
23
4-Chlorophenyl
4-Acetylaminophenyl
a
Isolated yield of purified material.
Purity determined by HPLC >95%.
b
Scheme 6. Synthesis of 6-(4-methoxyphenyl)-3-morpholinopyrazin-2(1H)-one 76. Reagents and conditions: (a) BCl3, DCM, RT, 48 h, 68%; (b) morpholine, DIPEA, nBuOH, microwave
140 ꢀC, 1 h, 45%; (c) 4-methoxyphenylboronic acid, 0.5 M aq Na2CO3, MeCN, PdCl2(dppf), microwave 150 ꢀC, 20 min, 25%.
deficient reagents. Although this presents a limitation to the gener-
ality of the synthesis, it has been partly overcome by using two
regioisomeric templates, 3-benzyloxy-5-bromo-2-chloropyrazine
and 3-benzyloxy-2-bromo-5-chloropyrazine, which can compen-
sate for the restriction to electron-rich boronic acids in the first cou-
pling step. Although the synthetic route was mainly demonstrated
here with 4-methoxyphenyl as one of the substituents, the extension
to other groups can be envisaged. The application of a similar se-
quence to 5-bromo-3-chloropyrazin-2-one foundered on the lack of
regioselectivity of the initial Suzuki coupling. However, better dif-
ferentiation of the halogen substituents was achieved by nucleophilic
substitution with amines, and was exploited in the parallel solution
phase synthesis of 3-(aminoalkyl)-5-(hetero)aryl-2(1H)-pyr-
azinones. This sequence allows the alternative 3,5-disubstituted ge-
ometry of the pyrazinones to be quickly accessed, albeit with
a differently constituted substituent set. Arguably, there are benefits
to the incorporation of saturated nitrogen heterocycles into the li-
brary compounds, since these were found to have greater aqueous
solubility than analogous diaryl substituted pyrazinones. We have
previously found that the replacement of rigid aromatic spacers by
conformationally flexible aliphatic rings gave more potent and se-
lective inhibition of PKB by purine derivatives.72
ded on a Reichert Thermovar apparatus. Infrared spectra were
recorded onaPerkineElmerSpectrumRX-1FT-IR spectrophotometer
with the sample prepared as a thin film on NaCl discs.1H NMR spectra
wererecorded onBrukerAC250orAV500aspectrometers at250MHz
or500MHz,respectively, usinganinternallock.13CNMRspectrawere
recorded on a Bruker AV500a spectrometer at 125 MHz using an in-
ternal lock. Chemical shifts (d, parts per million) are reported relative
to Me4Si (
d
¼0) and/or referenced to the solvent in which they were
measured. Coupling constants (J) are reported in hertz. GCeMS
analyses were recorded on a Thermo Quest Finnigan Polaris Q in-
strument, using a Phenomenex Zebron 2.5
m
mꢂ15 mꢂ0.25 mm col-
umn and a temperature gradient from 40 to 300 ꢀC over 3 min.
Combined HPLC-MS analyses were recorded using a Waters Alliance
2795 separations module, a Waters/Micromass LCT mass detector
with electrospray ionisation, and a Waters 2487 Dual
detector (
¼254 nm). HPLC analyses were performed using a Phe-
nomenex Gemini C18 column (5 cmꢂ4.6 mm i.d., 5 m) and gradient
l absorbance
l
m
elution of 10e90% MeOH/0.1% aqueous formic acid at a flow rate of
1 mL/min (run time of 6, 10 or 15 min as indicated) or a Merck
Chromolith SpeedROD RP-18e column (5 cmꢂ4.6 mm i.d.) and gra-
dientelution of 10e90% MeOH/0.1%aqueous formic acidata flow rate
of 2 mL/min (run time of 3.5 min). HRMS analyses were performed
using an Agilent 1200 series HPLC with an Agilent 6210 ToF mass
spectrometer, referenced to caffeine (MHþ¼195.087652), reserpine
(MHþ¼629.280657) and hexakis(1H,1H,3H-tetrafluoropentoxy)
phosphazene (MHþ¼922.009798).
Representative compounds from the library had appropriate
solubility for use in biochemical and cell-based screens. The ability
of such compounds to inhibit kinases provides support for pursuing
2(1H)-pyrazinones for this purpose. The potency was appropriate
for the molecular weight of the compounds, suggesting the library
would be capable of generating ligand efficient hits in screening.
Although the library design was based on one specific binding mode
4.1.1. 3,5-Dibromopyrazin-2-amine
5.50
A
solution
of
2-
aminopyrazine 4 (3.81 g, 40.1 mmol) in DMSO (80 mL) and water

9720
J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
Table 6
gumwas washed with ethyl acetate (200 mL) and water (100 mL) and
Solubility measurements of selected pyrazinones
the mixture was filtered through Celite. The organic layer was sepa-
rated, washedwithbrine(2ꢂ100 mL), dried, filtered and concentrated.
Flashcolumn chromatographyon silica, eluting with 30%ethyl acetate
in hexanes, gave 6 (4.00 g, 72%) as an orange solid. Rf (35% EtOAc/
hexanes) 0.50; nmax (thin film/cmꢁ1) 3411 (NH2), 3019 (Ar CH), 929
(CeO); (found: C, 47.19; H, 3.52; N, 14.78. C11H10N3OBr requires C,
47.16;H,3.60;N,15.00%);dH (500 MHz; CDCl3)4.80(2H,brs,NH2), 5.41
(2H, s, OCH2), 7.37e7.48 (5H, m, Ph), 7.68 (1H, s, 6-H); dC (125 MHz;
CDCl3) 69.0,120.9,128.5,128.6,128.6,134.7,135.7,144.1,147.0; LCeMS
(6 min) m/z 280 (MHþ); HPLC tR 4.03 min; purity 97%; (HRMS found:
MHþ m/z 280.0081; C11H1179BrN3O requires 280.0080).
No.
Structure
Medium
Solubility
(mg mLꢁ1
a
)
OMe
38
MeOH
10% DMSO
in H2O
0.13
1.7
N
H2O pH¼5
0.06
N
H
O
O
H2O pH¼7.4 0.03b
MeO
MeO
H
N
4.1.3. 3-(Benzyloxy)-5-bromo-2-iodopyrazine 7.49 Aqueous 47%
hydroiodic acid (4.80 mL, 27.0 mmol) was added to a stirred solution
of 6 (0.500 g, 1.78 mmol) in acetonitrile (3.6 mL) and water (5.5 mL)
at 0 ꢀC. A solution of sodium nitrite (2.20 g, 31.9 mmol) in water
(3.7 mL) was added dropwise. The reaction mixture was warmed to
rt then heated at 50 ꢀC for 24 h. The solution was basified to pH 12
with 20% aqueous sodium hydroxide (5 mL). The mixture was
extracted with diethyl ether (3ꢂ30 mL) and the organic layer was
washed with saturated aqueous sodium metabisulfite (40 mL) and
brine (40 mL), dried, filtered and concentrated. Flash column chro-
matography on silica, eluting with 1:1 dichloromethane/hexanes,
gave 7 (0.450 g, 64%) as a yellow oil. Rf (50% CH2Cl2/hexanes) 0.39;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 928 (CeO); dH (500 MHz; CDCl3)
5.46 (2H, s, OCH2), 7.37 (1H, t, J 7.0 Hz, ArH), 7.42 (2H, dd, J 7.0 Hz,
ArH), 7.52 (2H, d, J 7.0 Hz, ArH), 8.10 (1H, s, 6-H); dC (125 MHz; CDCl3)
70.3, 104.6, 128.0, 128.4, 128.6, 135.2, 135.8, 139.3, 158.1; LCeMS
(15 min)m/z 393, 391 (MHþ); HPLC tR 10.38 min; purity >99%; HRMS
(found: MHþ m/z 390.8950; C11H979BrIN2O requires 390.8937).
57
MeOH
10% DMSO
in H2O
0.35
2.5
N
H2O pH¼5
0.15
H2O pH¼7.4 0.11b
OMe
H
N
O
N
69
65
76
MeOH
10% DMSO
in H2O
1.8
1.2
N
O
MeO
H2O pH¼5
2.5
H2O pH¼7.4 0.06b
H
N
O
MeOH
10% DMSO
in H2O
2.2
1.3
N
N
N
H2O pH¼5
3.5
MeO
H2O pH¼7.4 0.15b
4.1.4. 3-(Benzyloxy)-5-bromo-2-chloropyrazine 8. A suspension of 6
(7.70 g, 27.5 mmol), copper(II) chloride (11.1 g, 82.6 mmol), copper(I)
chloride (5.45 g, 55.1 mmol) in acetonitrile (77 mL) was stirred at rt
for 10 min. Isoamyl nitrite (11.0 mL, 82.6 mmol) was added. The
solution was stirred at rt for 3 h. Hydrochloric acid solution (1 M aq,
100 mL) was added and the mixturewas extractedwith diethylether
(220 mL). The organic layer was dried, filtered and concentrated.
Flash column chromatography on silica, eluting with 1:1 ethyl ace-
tate/hexanes, gave 8 (5.00 g, 61%) as a yellow solid. Mp 269e272 ꢀC;
Rf (50% EtOAc/hexanes) 0.29; nmax (thin film/cmꢁ1) 3019 (Ar CH), 928
(CeO); dH (500 MHz; CDCl3) 5.49 (2H, s, OCH2), 7.37e7.44 (3H, m,
ArH), 7.51 (2H, d, J 7.0 Hz, ArH), 8.07 (1H, s, 6-H); dC (125 MHz; CDCl3)
70.0, 128.3, 128.6, 128.7, 133.8, 135.0, 136.3, 137.1, 155.2; GCeMS m/z
302, 300, 298 (Mþ); GC tR 4.19 min; purity >99%; HRMS (found: MHþ
m/z 298.9590; C11H9N279BrClO requires 298.9581).
O
MeOH
10% DMSO
in H2O
0.2
2
N
N
H2O pH¼5
0.08
H2O pH¼7.4 n.d.c
N
H
O
MeO
a
Solubility determined by HPLC measurement of the concentration of a micro-
membrane filtered saturated solution.
b
Solubility determined by high-throughput filtration-based assay.62
n.d.¼not determined.
c
(2 mL) was stirred at 0 ꢀC for 10 min. NBS (16.4 g, 92.2 mmol) was
added portionwise to the solution over 50 min, keeping the tem-
perature below 15 ꢀC. The reaction mixture was warmed to rt and
stirred for 16 h. The solution was poured into ice-water (250 mL) and
stirred. The orange solid was collected by filtration and dried. The
filtrate was extracted with ethyl acetate (200 mL). The organic layer
was washed with 5% aqueous sodium carbonate (50 mL) and water
(50 mL), then dried, filtered and concentrated. The combined mate-
rial was recrystallised from water (200 mL) to give 5 (7.80 g, 77%) as
a brown solid. (Found: C, 19.33; H, 1.10; N, 16.68; C4H3N3Br2 requires
C, 19.00; H, 1.20; N, 16.62%); dH (250 MHz; CDCl3) 5.08 (2H, br s, NH2),
8.07 (1H, s, 6-H); dC (125 MHz; CDCl3) 123.7, 123.9, 143.2, 151.9;
LCeMS (15 min) m/z 256, 254, 252 (MHþ); HPLC tR 4.45 min; purity
98%; (HRMS found: MHþ m/z 251.8776; C4H479Br2N3 requires
251.8766).
4.1.5. 3,5-Dichloropyrazin-2-amine 10.75 2-Amino-3-chloropyrazine
9 (10.0 g, 77.2 mmol) was added to a stirred suspension of NCS
(11.6 g, 86.9 mmol) in chloroform (73 mL). The reaction mixture was
heated at reflux for 4 h. The solution was cooled and partitioned
between chloroform (180 mL) and water (2ꢂ50 mL). The organic
layer was dried, filtered and concentrated. Flash chromatography on
silica, eluting with chloroform, gave 10 (9.66 g, 76%) as a pale yellow
solid. Mp 200e202 ꢀC; Rf (CHCl3) 0.32; nmax (thin film/cmꢁ1) 3411
(NH2), 3019 (Ar CH); (found: C, 28.89; H, 1.71; N, 24.99; C4H3N3Cl2:
0.125(H2O) requires C, 28.90; H, 1.97; N, 25.28%); dH (500 MHz;
CDCl3) 4.96 (2H, br s, NH2), 7.90 (1H, s, 6-H); dC (125 MHz; CDCl3)
131.4, 134.6, 140.0, 150.3; GCeMS m/z 167, 165, 163 (Mþ), GC tR
2.51 min; purity 98%; HRMS (found: MHþ m/z 163.9782;
C4H435Cl2N3 requires 163.9777).
4.1.2. 3-(Benzyloxy)-5-bromopyrazin-2-amine 6.73 A suspension of
sodium hydride (60% dispersed in mineral oil) (1.20 g, 29.7 mmol) in
dry THF (50 mL) was stirred at rt for 10 min. Benzyl alcohol (3.10 mL,
29.9 mmol) was added and the mixture was stirred for 30 min, after
which 5 (5.00 g, 19.8 mmol) was added. The reaction mixture was
heated at reflux for 10 h then cooled and concentrated. The residual
4.1.6. 3-(Benzyloxy)-5-chloropyrazin-2-amine 11. Benzyl alcohol
(7.60 mL, 73.2 mmol) was added slowly to a stirred suspension of
sodium hydride (60% dispersed in mineral oil) (2.95 g, 73.2 mmol)

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9721
Table 7
Kinase inhibition by compounds 39 and 57 screened in a panel of 150 kinases, and compounds 38 and 65 screened in a panel of 85 kinases
No.
Structure
Kinase inhibitiona
Kinase inhibitionb
39
d
ABL2/ARG, CAMK2a,
CAMK4, CDK4, CK1ε,
CK2 , CK2 2, DAPK2,
Fgr, FGFR4, HIPK3, IKK
PKC , TSSK2
a
a
b
,
g
H
N
O
57
38
d
DAPK2, FGFR4, MUSK
N
MeO
OMe
OMe
PKB-
SRPK1
b, Aurora-B,
PKB-
PKB-
a
b
,
N
, S6K1, PKCa, PKD1,
CAMK1, Aurora B, MELK,
NEK6, SRPK1, FGF-R1, EPH-B3,
NUAK1, HER4
N
H
O
MeO
H
N
O
N
65
PHK, PIM3
PKC
a, PHK, CHK2, MELK,
DYRK2, DYRK3, PIM1, PIM3, EPH-B3, HER4
N
N
MeO
Inhibition of >60% at 30 M concentration of test compound.
M concentration of test compound.
a
m
m
b
Inhibition of >30% at 30
Table 8
Antiproliferative activity of compounds 39 and 57
10 min. A solution of sodium nitrite (1.56 g, 22.6 mmol) in water
(2.6 mL) was added dropwise to the reaction mixture, which was
stirred at 0 ꢀC for a further 10 min, then heated at 50 ꢀC for 21 h. The
cooled solution was basified with 20% aqueous sodium hydroxide
(11 mL). The mixture was extracted with diethyl ether (3ꢂ50 mL)
and the organic layer was washed with saturated aqueous sodium
metabisulfite (50 mL) and brine (50 mL), dried, filtered and con-
centrated. Flash column chromatography on silica, eluting with 1:1
dichloromethane/hexanes, gave 12 (0.222 g, 50%) as a pale yellow
solid. Mp 265e268 ꢀC; Rf (50% CH2Cl2/hexanes) 0.55; nmax (thin
film/cmꢁ1) 3019 (Ar CH), 1014 (CeO); (found: C, 37.55; H, 2.21; N,
7.76; C11H8N2O1Cl: 0.25(H2O) requires C, 37.64; H, 2.44; N, 7.98%);
dH (500 MHz; CDCl3) 5.38 (2H, s, OCH2), 7.24e7.38 (3H, m, ArH),
7.43 (2H, d, J 7.0 Hz, ArH), 7.93 (1H, s, 6-H); dC (125 MHz; CDCl3)
70.0, 126.9, 128.2, 128.5, 128.7, 134.9, 135.1, 144.2, 156.0; LCeMS
(3.5 min) m/z 349, 347 (MHþ); HPLC tR 2.95 min; purity 99%; HRMS
(found: MHþ m/z 346.9446; C11H935ClIN2O3 requires 346.9443).
No.
Cytotoxicity HCT116 cells
Cytotoxicity MV4-11 cells
(IC50
,
m
M)a
(IC50, m
M)a
39
57
3.2
30
n.d.b
20
a
Single determination in MTT assay format.69
n.d.¼not determined.
b
in dry THF (122 mL). The mixture was stirred at rt for 10 min, after
which 10 (7.30 g, 44.5 mmol) was added and the mixture was
heated at reflux for 24 h. The cooled solution was concentrated, and
ethyl acetate (500 mL) and water (200 mL) were added to the
resulting gum. The mixture was filtered through Celite. The organic
layer was separated, washed with brine (2ꢂ100 mL), dried, filtered
and concentrated. Flash column chromatography on silica, eluting
with 1:1 ethyl acetate/hexanes, gave a solid, which was recrystal-
lised from water (200 mL) to give 11 (6.00 g, 57%) as an orange solid.
Mp 285e287 ꢀC (from water); Rf (50% EtOAc/hexanes) 0.41; nmax
(thin film/cmꢁ1) 3409 (NH2), 3019 (Ar CH), 929 (CeO); (found: C,
56.19; H, 4.26; N, 17.65; C11H10N3OCl requires C, 56.06; H, 4.28; N,
17.83%); dH (500 MHz; CDCl3) 4.77 (2H, br s, NH2), 5.32 (2H, s,
OCH2), 7.30e7.39 (5H, m, Ph), 7.51 (1H, s, 6-H); dC (125 MHz; CDCl3)
69.0, 128.5, 128.6, 128.6, 131.7, 131.9, 135.7, 143.8, 146.7; LCeMS
(15 min) m/z 236 (MHþ) HPLC tR 7.49 min; purity 99% HRMS (found:
MHþ m/z 236.0585; C11H1135ClN3O requires 236.0585).
4.1.8. 3-(Benzyloxy)-2-bromo-5-chloropyrazine 13. Isoamyl nitrite
(6.80 mL, 50.9 mmol) was added to a stirred suspension of 11
(4.00 g, 17.0 mmol), copper(I) bromide (4.87 g, 33.9 mmol) and
copper(II) bromide (11.4 g, 50.9 mmol) in acetonitrile (48 mL) at rt.
After 3 h the mixture was diluted with dilute hydrochloric acid (1 M
aq, 200 mL) and extracted with ethyl acetate (2ꢂ200 mL). The or-
ganic extracts were dried, filtered and concentrated. Flash column
chromatography on silica, eluting with 1:1 ethyl acetate/hexanes,
gave 13 (2.44 g, 48%) as a white solid. Mp 274e278 ꢀC; Rf (50%
EtOAc/hexanes) 0.75; nmax (thin film/cmꢁ1) 3019 (Ar CH), 1057
(CeO); (found: C, 43.94; H, 2.66; N, 9.20; C11H8N2OBrCl requires C,
44.11; H, 2.69; N, 9.35%); dH (500 MHz; CDCl3) 5.48 (2H, s, OCH2),
4.1.7. 3-(Benzyloxy)-5-chloro-2-iodopyrazine 12. A solution of 11
(0.300 g,1.27 mmol) in acetonitrile (2.5 mL), water (3.9 mL) and 47%
aqueous hydroiodic acid (3.40 mL,19.1 mmol) was stirred at 0 ꢀC for

9722
J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
7.36e7.52 (5H, m, Ph), 7.98 (1H, s, 6-H); dC (125 MHz; DMSO-d6)
70.2,127.2, 128.5, 128.8, 129.0, 135.6, 135.8, 143.9, 156.1; GCeMS m/z
302, 300, 298 (Mþ); GC tR 4.35 min; purity >99%; HRMS (found:
MHþ m/z 298.9586; C11H979Br35ClN2O requires 298.9581).
Wherenecessary, furtherpurificationwascarried outby flashcolumn
chromatography on silica to give the coupled products; those which
required chromatographyhaveanassociatedRf forthecolumneluant.
The following compounds were prepared using Method A.
4.1.9. 3-Amino-pyrazin-2(1H)-one 14.51 A solution of 6 (50.0 mg,
0.178 mmol), 10% w/w palladium on carbon (10.0 mg, 0.094 mmol) in
ethanol (3 mL) was stirred at rt for 7.5 h under an atmosphere of hy-
drogen (1 atm). The mixture was filtered through a plug of Celite and
concentrated. Purification by ion exchange chromatography on acidic
resin, eluting with methanol then 1 M ammonia in methanol, gave 14
(4.0 mg, 20%) as a white solid. dH (500 MHz; CD3OD) 6.57 (1H, d, J
4.5 Hz, Ar), 6.64 (1H, d, J 4.5 Hz, Ar); GCeMS m/z 111 (Mþ); GC tR
5.09 min.
4.2.1. 2-(Benzyloxy)-3,6-bis-(4-methoxyphenyl)pyrazine 21. White
solid; mp >300 ꢀC; Rf (CH2Cl2) 0.33; nmax (thin film/cmꢁ1) 3019 (Ar
CH), 1033, 929; dH (500 MHz; CDCl3) 3.88 (3H, s, OCH3), 3.90 (3H, s,
OCH3), 5.65 (2H, s, OCH2), 7.00 (2H, d, J 9.0 Hz, ArH), 7.04 (2H, d, J
9.0 Hz, ArH), 7.34e7.54 (5H, m, Ph), 8.03 (2H, d, J 9.0 Hz, ArH), 8.18
(2H, d, J 9.0 Hz, ArH), 8.65 (1H, s, 5-H); dC (125 MHz; CDCl3) 55.3,
55.4, 67.9, 113.6, 114.3, 127.8, 128.0, 128.5, 128.9, 130.5, 132.2, 137.2,
139.8, 146.3, 156.1, 160.3, 160.8, two signals not observed; LCeMS
(6 min) m/z 399 (MHþ); HPLC tR 5.21 min; purity >99%; HRMS
(found: MHþ m/z 399.1701; C25H23N2O3 requires 399.1703).
4.1.10. 3-(Benzyloxy)-2-chloro-5-(4-methoxyphenyl)pyrazine 19. A
solution of 8 (5.00 g, 16.7 mmol), 4-methoxyphenylboronic acid
(3.85 g, 25.3 mmol), 2 M aq Na2CO3 (25.0 mL, 50.0 mmol), dichloro-
bis-(triphenylphosphine)palladium(II) (1.20 g, 1.71 mmol) in dry
DME (78 mL) was stirred at 50 ꢀC for 16 h. Ethyl acetate (200 mL) was
added and the organic layer washed with water (200 mL). The
aqueous layer was extracted with ethyl acetate (50 mL). The com-
bined organic layers were filtered through a plug of silica gel (6 g).
The filtrate was concentrated. Flash column chromatography on
silica, eluting with 1:1 dichloromethane/hexanes, gave 19 (3.83 g,
70%) as a yellow solid. Mp >300 ꢀC; Rf (50% CH2Cl2/hexanes) 0.15;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 1185 (CeO), 929 (CeO); (found:
C, 65.86; H, 4.65; N, 8.36; C18H15ClN2O2 requires C, 66.16; H, 4.63; N,
8.57%); dH (500 MHz; CDCl3) 3.90 (3H, s, OCH3), 5.60 (2H, s, OCH2),
7.02 (2H, d, J 9.0 Hz, ArHo- toOMe), 7.35e7.51(5H, m, Ph), 7.94 (2H, d,
J 9.0 Hz, ArH), 8.34 (1H, s, 6-H); dC (125 MHz; CDCl3) 55.4, 68.7,114.4,
127.8,127.9,128.2,128.2,128.6,131.4,134.7,136.2,147.9,154.9,161.2;
GCeMS m/z 328, 326 (Mþ); GC tR 5.90 min; purity 99%; HRMS
(found: MHþ m/z 327.0899; C18H1635ClN2O2 requires 327.0895).
4.2.2. 3-(Benzyloxy)-5-(4-methoxyphenyl)-2-(pyridin-3-yl)pyrazine
24. White solid. Mp >300 ꢀC; Rf (50% EtOAc/hexanes) 0.23; nmax (thin
film/cmꢁ1) 3019 (Ar CH), 1029 (CeO), 929 (CeO); 1H NMR dH
(500 MHz; CDCl3) 3.91 (3H, s, OCH3), 5.67 (2H, s, OCH2), 7.06 (2H, d, J
9.0 Hz, ArH), 7.35e7.53 (6H, m, Ph and pyridine 5-H), 8.06 (2H, d, J
9.0Hz, ArH), 8.47(1H, ddd,J8.0,1.0,1.0Hz, pyridine4-H), 8.64(1H,dd, J
5.0,1.0Hz,pyridine6-H), 8.72 (1H, s,6-H), 9.44 (1H, d,J 1.0 Hz, pyridine
2-H); dC (125 MHz; DMSO-d6) 55.3, 67.8, 114.5, 123.3, 127.6, 128.0,
128.0, 128.3, 128.5, 131.3, 132.8, 135.8, 136.4, 136.7, 147.4, 149.4, 149.5,
156.1, 161.0; LCeMS (6 min) m/z 370 (MHþ); HPLC tR 4.32 min; purity
99%; HRMS (found: MHþ m/z 370.1558; C23H20N3O2 requires
370.1550).
4.2.3. 3-(Benzyloxy)-2-(4-hydroxyphenyl)-5-(4-methoxyphenyl)pyr-
azine 25. Off-white solid. Mp >300 ꢀC; Rf (50% EtOAc/CH2Cl2) 0.10;
nmax (thin film/cmꢁ1) 3500e3200 (OH), 3019 (Ar CH), 928 (CeO); dH
(500 MHz; CDCl3) 3.91 (3H, s, OCH3), 5.07 (1H, s, OH), 5.66 (2H, s,
OCH2), 6.92 (2H, d, J 8.0 Hz, ArH), 7.05 (2H, d, J 9.0 Hz, ArH), 7.34e7.54
(5H, m, Ph), 8.04 (2H, d, J 9.0 Hz, ArH), 8.12 (2H, d, J 8.0 Hz, ArH), 8.66
(1H, s, 6-H); dC (125 MHz; DMSO-d6) 55.8, 67.8, 114.9, 115.5, 126.8,
128.3, 128.3, 128.4, 128.5, 128.9, 130.7, 132.6, 137.5, 139.7, 145.7, 155.8,
158.9,161.0; LCeMS (6 min) m/z 385 (MHþ); HPLC tR 4.59 min; purity
99%; HRMS (found: MHþ m/z 385.1559; C24H21N2O3 requires
385.1552).
4.1.11. 3-(Benzyloxy)-5-chloro-2-(4-methoxyphenyl)pyrazine 20. A
solution of 13 (4.10 g, 13.7 mmol), 4-methoxyphenylboronic acid
(3.14 g, 20.7 mmol), 2 M aq Na2CO3 (21.0 mL, 42.0 mmol), dichloro-
bis-(triphenylphosphine)palladium(II) (0.960 g, 1.37 mmol) in dry
DME (64 mL) was stirred at 50 ꢀC for 16 h. Ethyl acetate (200 mL) was
added and the organic layer was washed with water (200 mL). The
aqueous layer was extracted with ethyl acetate (50 mL). The com-
bined organic extracts were filtered through a plug of silica gel. The
filtrate was concentrated. Flash column chromatography on silica,
eluting with 1:1 dichloromethane/hexanes, gave 20 (2.72 g, 61%) as
an off-white solid. Mp >300 ꢀC; Rf (50% CH2Cl2/hexanes) 0.18; nmax
(thin film/cmꢁ1) 3019 (Ar CH),1177 (CeO), 929 (CeO); dH (500 MHz;
CDCl3) 3.79 (3H, s, OCH3), 5.43 (2H, s, OCH2), 6.89 (2H, d, J 9.0 Hz,
ArH), 7.32e7.36 (3H, m, ArH), 7.41 (2H, d, J 7.0 Hz, ArH), 7.99 (2H, d, J
9.0 Hz, ArH), 8.15 (1H, s, 6-H); dC (125 MHz; CDCl3) 52.8, 66.5, 111.2,
124.8, 125.6, 125.7, 126.0, 128.0, 132.4, 133.4, 138.2, 139.5, 153.3,
158.1; GCeMS m/z 328, 326 (Mþ); GC tR 5.72 min; purity 99%; HRMS
(found MHþ: m/z 327.0896; C18H1635ClN2O2 requires 327.0895).
4.2.4. N-(3-(3-Benzyloxy-5-(4-methoxyphenyl)pyrazin-2-yl]phenyl)
acetamide) 26. Yellow solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1
)
3019 (Ar CH), 1684 (C]O), 928 (CeO); dH (500 MHz; CDCl3) 2.20
(3H, s, CH3), 3.91 (3H, s, OCH3), 5.66 (2H, s, OCH2), 7.05 (2H, d, J
9.0 Hz, ArH), 7.18 (1H, br s, NH), 7.32e7.56 (6H, m, ArH), 7.81 (1H, d,
J 8.0 Hz, ArH), 7.96 (1H, d, J 8.0 Hz, ArH), 8.04 (2H, d, J 9.0 Hz, ArH),
8.09 (1H, s, ArH), 8.67 (1H, s, 6-H); LCeMS (6 min) m/z 426 (MHþ);
HPLC tR 4.30 min; purity 96%; HRMS (found: MHþ m/z 426.1822;
C26H24N3O3 requires 426.1812).
4.2.5. 3-(Benzyloxy)-2-(3-pyrazol-3-yl)-5-(4-methoxyphenyl)pyr-
azine 27. Pale yellow solid. Mp >300 ꢀC; Rf (2:1 EtOAc/hexanes)
0.18; nmax (thin film/cmꢁ1) 3019 (Ar CH), 929 (CeO); dH (500 MHz;
CDCl3) 3.91 (3H, s, OCH3), 5.69 (2H, s, OCH2), 7.05 (2H, d, J 9.0 Hz,
ArH), 7.05 (1H, br s, pyrazole 4-H), 7.40e7.57 (5H, m, Ph), 7.69 (1H,
br s, pyrazole 5-H), 8.04 (2H, d, J 9.0 Hz, ArH), 8.64 (1H, s, 6-H);
LCeMS (6 min) m/z 359 (MHþ); HPLC tR 4.25 min; purity 99%;
HRMS (found: MHþ m/z 359.1506; C21H19N4O2 requires 359.1508).
4.2. General procedures for the preparation of 2-(benzyloxy)-
3,6-di(hetero)arylpyrazines 21e37
Method A: A mixture of the halopyrazine 19 or 20 (100 mg,
0.306 mmol), the (hetero)arylboronic acid or boronate ester
(0.459 mmol, 1.5 equiv), sodium carbonate (45.0 mg, 0.425 mmol,
1.4 equiv) and tetrakis(triphenylphosphine)palladium (0) (23.0 mg,
0.019 mmol) inwater (0.9 mL) and acetonitrile (3.4 mL) was heated in
a microwave reactor using variable wattage to 150 ꢀC for 20 min. The
mixture was partitioned between ethyl acetate (50 mL) and water
(40 mL). The organic layer was filtered through a short plug of silica
gel (6 g) washing with ethyl acetate. The filtrate was concentrated.
4.2.6. 3-(Benzyloxy)-2-(thiophen-3-yl)-5-(4-methoxyphenyl)pyr-
azine 28. Pale yellow solid. Mp >300 ꢀC; Rf (CH2Cl2) 0.71; nmax (thin
film/cmꢁ1) 3060, 3034 (Ar CH), 1024, 974; dH (500 MHz; CDCl3) 3.90
(3H, s, OCH3), 5.69 (2H, s, OCH2), 7.04 (2H, d, J 7.0 Hz, ArH),
7.36e7.45 (3H, m, ArH and thienyl 3-H), 7.57 (2H, d, J 9.0 Hz, ArH)
8.00 (1H, d, J 7.5 Hz, thienyl 4-H), 8.00e8.04 (3H, m, ArH and thienyl

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9723
5-H), 8.27 (1H, s, thienyl 2-H), 8.64 (1H, s, 6-H); dC (125 MHz; CDCl3)
55.4, 68.2, 114.4, 124.8, 126.7, 128.0, 128.1, 128.2, 128.2, 128.6, 128.8,
132.0, 136.0, 136.9, 137.4, 146.3, 155.6, 160.9; LCeMS (10 min) m/z
375 (MHþ); HPLC tR 5.33 min; purity 99%; HRMS (found: MHþ m/z
375.1164; C22H19N2O2S requires 375.1162).
nmax (thin film/cmꢁ1) 3019 (Ar CH), 928 (CeO); dH (500 MHz; ace-
tone-d6) 3.87 (3H, s, OCH3), 5.70 (2H, s, OCH2), 7.01e7.03 (3H, m,
ArH and pyrazole 4-H), 7.31e7.43 (3H, m, ArH), 7.61 (2H, d, J 8.5 Hz,
ArH) 7.80 (1H, br s, pyrazole 5-H), 8.23 (2H, d, J 8.5 Hz, ArH), 8.84
(1H, s, 6-H), 12.50 (1H, br s, NH); dC (125 MHz; acetone-d6) 55.6,
67.8, 104.8, 114.3, 124.4, 128.8, 129.0, 129.3, 131.4, 133.4, 138.2, 143.2,
146.4, 149.1, 157.0, 161.4, one signal not observed; GCeMS m/z 358
(Mþ); GCeMS tR 6.86 min; purity 99%; HRMS (found : MHþ m/z
359.1510; C21H19N4O2 requires 359.1508).
4.2.7. 4-(4-(3-Benzyloxy-5-(4-methoxyphenyl)pyrazin-2-yl)benzyl)
morpholine 29. Pale yellow solid. Mp >300 ꢀC; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1034 (CeO), 929 (CeO); dH (500 MHz; CDCl3)
2.50 (4H, br s, morpholine NCH2), 3.70 (2H, s, NCH2Ar), 3.74 (4H, br
s, morpholine OCH2), 3.91 (3H, s, OCH3), 5.66 (2H, s, OCH2), 7.05
(2H, d, J 9.0 Hz, ArH), 7.33e7.71 (7H, m, Ph and ArH), 8.04 (2H, d, J
9.0 Hz, ArH), 8.14 (2H, d, J 8.0 Hz, ArH), 8.68 (1H, s, 6-H); LCeMS
(6 min) m/z 468 (MHþ); HPLC tR 3.41 min; purity 98%; HRMS
(found: MHþ m/z 468.2283; C29H30N3O3 requires 468.2282).
4.2.13. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(3-thiophenyl)pyr-
azine 36. Off-white solid. Mp >300 ꢀC; Rf (CH2Cl2) 0.57; nmax (thin
film/cmꢁ1) 3019 (Ar CH), 1020 (CeO), 929 (CeO); dH (500 MHz;
CDCl3) 3.89 (3H, s, OCH3), 5.64 (2H, s, OCH2), 7.02 (2H, d, J 9.0 Hz,
ArH), 7.36e7.47 (4H, m, ArH and thiophene 4-H), 7.54 (2H, d, J 7.0 Hz,
ArH), 7.71 (1H, dd, J 5.0, 1.0 Hz, thiophene 5-H), 7.98 (1H, d, J 1.0 Hz,
thiophene 2-H), 8.19 (2H, d, J 9.0 Hz, ArH), 8.60 (1H, s, 6-H); dC
(125 MHz; CDCl3) 55.3, 68.0, 113.7, 124.0, 125.8, 126.6, 127.9, 127.9,
128.4, 128.6, 130.6, 132.8, 137.1, 139.1, 140.4, 143.0, 156.2, 160.4;
LCeMS (6 min) m/z 375 (MHþ); HPLC tR 5.02 min; purity 99%; HRMS
(found: MHþ m/z 375.1164; C22H19N2O2S requires 374.1162).
4.2.8. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(pyridin-3-yl)pyrazine
31. Yellow solid. Mp >300 ꢀC; Rf (2:1 EtOAc/hexanes) 0.30; nmax
(thin film/cmꢁ1) 3019 (Ar CH), 929 (CeO); (found: C, 74.39; H, 5.42;
N, 11.12; C23H19N3O2 requires C, 74.78; H, 5.18; N, 11.37%); dH
(500 MHz; CDCl3) 3.79 (3H, s, OCH3), 5.56 (2H, s, OCH2), 6.91 (2H, d,
J 7.0 Hz, ArH), 7.25e7.35 (5H, m, Ph), 7.43 (2H, d, J 7.0 Hz, ArH), 8.12
(1H, dd, J 8.0, 5.0 Hz, pyridine H), 8.22 (1H, ddd, J 8.0, 2.0, 2.0 Hz,
pyridine H), 8.59 (1H, dd, J 5.0, 2.0 Hz, pyridine H), 8.62 (1H, s, 6-H),
9.21 (1H, d, J 2.0 Hz, pyridine H); dC (125 MHz; CDCl3) 55.3, 68.2,
113.7, 123.6, 127.8, 128.0, 128.3, 128.6, 130.8, 132.0, 132.8, 133.8,
136.8, 141.9, 143.8, 148.1, 150.2, 156.4, 160.7; GCeMS m/z 369 (Mþ);
GCeMS tR 6.90 min; purity 99%.
4.2.14. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(2-methoxyphenyl)
pyrazine 37. Colourless oil. Rf (50% EtOAc/hexanes) 0.56; dH
(500 MHz; CDCl3) 3.89 (3H, s, OCH3), 3.95 (3H, s, OCH3), 5.64 (2H, s,
OCH2), 7.01 (2H, d, J 9.0 Hz, ArH), 7.06 (1H, d, J 7.0 Hz, ArH),
7.12e7.15 (1H, m, ArH), 7.34e7.44 (4H, m, ArH), 7.53 (2H, d, J 6.0 Hz,
ArH), 8.0 (1H, dd, J 8.0, 2.0 Hz, ArH), 8.22 (2H, d, J 9.0 Hz, ArH), 8.98
(1H, s, 6-H); dC (125 MHz; CDCl3) 55.3, 55.6, 67.9, 111.6, 113.6, 121.1,
125.5, 127.8, 127.9, 128.5, 130.4, 130.6, 130.9, 137.3, 137.6, 139.8,
144.9, 156.1, 157.6, 160.3, one signal not observed; LCeMS (6 min)
m/z 399 (MHþ); HPLC tR 5.08 min; purity 99%.
4.2.9. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)pyr-
azine 32. Brown solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1
)
3400e3200 (OH), 3019 (Ar CH), 1020 (CeO), 929 (CeO); dH
(500 MHz; CDCl3) 3.88 (3H, s, OCH3), 5.64 (2H, s, OCH2), 5.70e5.80
(1H, br s, OH), 6.95 (2H, d, J 8.0 Hz, ArH), 7.00 (2H, d, J 7.0 Hz, ArH),
7.34e7.42 (3H, m, ArH), 7.53 (2H, d, J 8.0 Hz, ArH), 7.97 (2H, d, J
8.0 Hz, ArH), 8.16 (2H, d, J 7.0 Hz, ArH), 8.63 (1H, s, 6-H); dC
(125 MHz; CDCl3) 55.3, 67.9, 113.7, 115.9, 127.8, 127.9, 128.2, 128.4,
128.5, 128.9, 130.5, 132.1, 137.1, 139.9, 146.4, 156.1, 157.1, 160.3;
LCeMS (6 min) m/z 385 (MHþ); HPLC tR 4.53 min; purity 99%;
HRMS (found: MHþ m/z 385.1549; C24H21N2O3 requires 385.1552).
Method B: A mixture of the halopyrazine 19 or 20 (100 mg,
0.306 mmol), the (hetero)arylboronic acid or boronate ester
(0.459 mmol, 1.5 equiv), aqueous sodium carbonate (2 M, 0.50 mL,
1.00 mmol) and [2-[(dimethylamino-
clohexylphosphine)(trifluoroacetato-
k
k
N)methyl]phenyl-
kC](tricy-
O)-palladium53 (20.0 mg,
0.031 mmol) in dry 1,2-dimethoxyethane (1.4 mL) was heated at
reflux for 4e40 h until starting material was consumed. The mix-
ture was partitioned between ethyl acetate (60 mL) and water
(40 mL). The organic layer was filtered through a short plug of silica
gel (6 g) washing with ethyl acetate. The filtrate was concentrated.
Flash column chromatography on silica, eluting with mixtures of
ethyl acetate/hexanes or dichloromethane/hexanes, gave the cou-
pled products. The following compounds were prepared using
method B.
4.2.10. 4-(6-Benzyloxy-5-(4-methoxyphenyl)pyrazin-2-yl)benzamide
33. Green solid. Mp >300 ꢀC; Rf (EtOAc) 0.46; nmax (thin film/cmꢁ1
)
3300 (NH2), 3019 (Ar CH), 1684 (C]O), 929 (CeO); dH (500 MHz;
DMSO-d6) 3.84 (3H, s, OCH3), 5.68 (2H, s, OCH2), 7.07 (2H, d, J 9.0 Hz,
ArH), 7.35e7.58 (5H, m, Ph), 8.04 (2H, d, J 9.0 Hz, ArH), 8.16 (2H, d, J
9.0 Hz, ArH), 8.27 (2H, d, J 9.0 Hz, ArH), 9.00 (1H, s, 6-H); dC (125 MHz;
DMSO-d6) 55.3, 67.7, 113.7, 126.2, 127.9, 128.0, 128.1, 128.2, 128.5,
130.4, 133.4, 134.5, 136.8, 138.0, 140.4, 144.7, 155.6, 160.2, 167.3;
LCeMS (6 min) m/z 412 (MHþ); HPLC tR 4.36 min; purity 98%; HRMS
(found: MHþ m/z 412.1657; C25H22N3O3 requires 412.1656).
4.2.15. 3-(Benzyloxy)-5-(4-methoxyphenyl)-2-(4-methylphenyl)pyr-
azine 22. White solid. Mp >300 ꢀC; Rf (50% EtOAc/hexanes) 0.36;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 1033 (CeO), 928 (CeO); dH
(500 MHz; CDCl3) 2.42 (3H, s, CH3), 3.90 (3H, s, OCH3), 5.65 (2H, s,
OCH2), 7.04 (2H, d, J 9.0 Hz, ArH), 7.27e7.54 (7H, m, ArH), 8.03 (2H,
d, J 9.0 Hz, ArH), 8.08 (2H, d, J 9.0 Hz, ArH), 8.67 (1H, s, 6-H); dC
(125 MHz; CDCl3) 20.9, 55.3, 67.5, 114.4, 127.8, 128.1, 128.4, 128.6,
128.8, 129.0, 132.3, 132.7, 136.9, 138.5, 140.0, 146.2, 155.7, 160.9, one
signal not observed; LCeMS (10 min) m/z 383 (MHþ); HPLC tR
8.29 min; purity 99%; HRMS (found: MHþ m/z 383.1757;
C25H23N2O2 requires 383.1760).
4.2.11. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(pyrazol-4-yl)pyrazine
34. Off-white solid. Mp >300 ꢀC; Rf (50% EtOAc/CH2Cl2) 0.18; nmax
(thin film/cmꢁ1
) 3019 (Ar CH), 1021 (CeO), 929 (CeO); dH
(500 MHz; DMSO-d6) 3.81 (3H, s, OCH3), 5.59 (2H, s, OCH2), 7.02
(2H, d, J 9.0 Hz, ArH), 7.31e7.41 (3H, m, ArH), 7.54 (2H, d, J 8.0 Hz,
ArH), 8.07 (2H, d, J 9.0 Hz, ArH), 8.15 and 8.45 (2H, 2ꢂ br s, pyrazole
CH), 8.63 (1H, s, 6-H),13.20 (1H, br s, NH); dC (125 MHz; CDCl3) 55.2,
67.4, 113.6, 118.6, 127.8, 128.0, 128.4, 130.0, 131.4, 131.5, 132.3, 137.1,
137.7, 142.3, 155.6, 159.7; LCeMS (6 min) m/z 359 (MHþ); HPLC tR
4.30 min; purity 97%; HRMS (found: MHþ m/z 359.1502;
C21H19N4O2 requires 359.1503).
4.2.16. 4-(3-Benzyloxy-5-(4-methoxyphenyl)pyrazin-2-yl)benzoic
acid ethyl ester 23. White solid. Mp >300 ꢀC; Rf (CH2Cl2) 0.32; nmax
(thin film/cmꢁ1) 3019 (Ar CH), 929 (CeO); dH (500 MHz; CDCl3) 1.43
(3H, t, J 7.0 Hz, OCH2CH3), 3.91 (3H, s, OCH3), 4.42 (2H, q, J 7.0 Hz,
OCH2CH3), 5.66 (2H, s, OCH2), 7.06 (2H, d, J 9.0 Hz, ArH), 7.31e7.54
(5H, m, Ph), 8.06 (2H, d, J 9.0 Hz, ArH), 8.13 (2H, d, J 8.0 Hz, ArH),
4.2.12. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(pyrazol-3-yl)pyr-
azine 35. White solid. Mp >300 ꢀC; Rf (2:1 EtOAc/hexanes) 0.34;

9724
J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
8.26 (2H, d, J 8.0 Hz, ArH), 8.72 (1H, s, 6-H); dC (125 MHz; DMSO-d6)
14.1, 55.4, 60.8, 67.7, 114.5, 127.5, 127.9, 127.9, 128.4, 128.5, 128.8,
128.9, 129.8, 132.7, 136.8, 137.4, 139.9, 147.5, 156.1, 161.0, 165.4;
LCeMS (10 min) m/z 441 (MHþ); HPLC tR 8.44 min; purity 96%;
HRMS (found: MHþ m/z 441.1812; C27H25N2O4 requires 441.1814).
dd, J 8.0, 6.0 Hz, pyridine 5-H), 8.20 (1H, br s, 5-H), 8.87 (1H, br d, J
6.0 Hz, pyridine 4-H), 9.17 (1H, d, J 8.0 Hz, pyridine 6-H), 9.68 (1H, s,
pyridine 2-H), 13.00 (1H, br s, NH); LCeMS (6 min) m/z 280 (MHþ);
HPLC tR 3.04 min; purity 99%; HRMS (found: MHþ m/z 280.1086;
C16H14N3O2 requires 280.1086).
4.2.17. 3-(Benzyloxy)-2-(4-methoxyphenyl)-5-(4-methylphenyl)pyr-
azine 30. White solid. Mp >300 ꢀC; Rf (2:1 CH2Cl2/hexanes) 0.24;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 928 (CeO); (found: C, 78.49; H,
5.86; N, 7.16); C25H22N2O2 requires C, 78.51; H, 5.80; N 7.32%); dH
(500 MHz; CDCl3) 2.36 (3H, s, CH3), 3.80 (3H, s, OCH3), 5.57 (2H, s,
OCH2), 6.91 (2H, d, J 9.0 Hz, ArH), 7.23 (2H, d, J 8.0 Hz, ArH),
7.25e7.42 (5H, m, Ph), 7.88 (2H, d, J 8.0 Hz, ArH), 8.09 (2H, d, J
9.0 Hz, ArH), 8.60 (1H, s, 6-H); dC (125 MHz; DMSO-d6) 20.9, 55.2,
67.5, 113.6, 126.4, 127.7, 127.8, 127.9, 128.5, 129.6, 130.2, 132.6, 132.7,
136.9, 139.3, 139.4, 145.7, 155.5, 160.0; GCeMS m/z 382 (Mþ);
GCeMS tR 6.92 min; purity 99%; (HRMS (found: m/z 383.1766;
C25H23N2O2 requires 383.1754).
4.3.5. 3-(4-Hydroxyphenyl)-6-(4-methoxyphenyl)-1H-pyrazin-2-one
42. Red solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6) 3.83 (3H, s,
OCH3), 6.85 (2H, d, J 9.0 Hz, ArH), 7.08 (2H, d, J 8.0 Hz, ArH), 7.85
(2H, d, J 9.0 Hz, ArH), 7.96 (1H, br s, 5-H), 8.23 (2H, d, J 8.0 Hz, ArH);
dC (125 MHz; DMSO-d6) 55.4, 114.1, 114.4, 114.8, 115.6, 124.0, 127.0,
128.2, 129.9, 143.7, 155.9, 158.7, 160.6; LCeMS (6 min) m/z 295
(MHþ); HPLC tR 3.62 min; purity 99%; HRMS (found: MHþ m/z
295.1078; C17H15N2O3 requires 295.1077).
4.3.6. N-(3-(5-(4-Methoxyphenyl)-3-oxo-3,4-dihydropyrazin-2-yl)
phenyl)acetamide 43. Brown solid. Mp >300 ꢀC; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1684 (C]O), 1653 (C]O), 928 (CeO); dH
(500 MHz; CDCl3) 2.03 (3H, s, CH3), 3.85 (3H, s, OCH3), 6.98 (2H, d, J
9.0 Hz, ArH), 7.30 (1H, br s, NH), 7.54 (2H, d, J 9.0 Hz, ArH), 7.59 (1H,
dd, J 8.5, 9.0 Hz, ArH), 7.74 (1H, d, J 8.5 Hz, ArH), 7.85 (1H, d, J 9.0 Hz,
ArH), 7.91 (1H, s, 5-H), 8.31 (1H, s, ArH), 11.32 (1H, br s, NH); LCeMS
(6 min) m/z 336 (MHþ); HPLC tR 3.61 min; purity 95%; HRMS
(found: m/z 335.1267; C19H18N3O3 requires 335.1270).
4.3. General procedure for the preparation of 3,6-di(hetero)
arylpyrazin-2(1H)-ones 38e54
Boron trichloride in dichloromethane (1 M, 0.600 mL,
0.600 mmol) was added to a stirred solution of the appropriate 2-
benzyloxy-3,6-di(hetero)arylpyrazine (21e37) (0.130 mmol) in
dichloromethane (3.8 mL) at rt. After 30 min the reaction was
quenched by the addition of methanol (30 mL) and stirred for 16 h.
The mixture was concentrated. The crude product was redissolved
in 9:1 dichloromethane/methanol and filtered through basic ion
exchange resin. The filtrate was concentrated to give the 3,6-
di(hetero)arylpyrazin-2(1H)-one product (38e54).
4.3.7. 6-(4-Methoxyphenyl)-3-(pyrazol-3-yl)-1H-pyrazin-2-one
44. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6) 3.88 (3H, s,
OCH3), 7.04 (1H, d, J 8.0 Hz, pyrazole 4-H), 7.09 (2H, d, J 9.0 Hz, ArH),
7.71 (1H, d, J 8.0 Hz, pyrazole 5-H), 7.94 (2H, d, J 9.0 Hz, ArH), 8.20
(1H, s, 5-H); LCeMS (6 min) m/z 269 (MHþ); HPLC tR 3.48 min;
purity 95%; HRMS (found: MHþ m/z 269.1038; C14H13N4O2 requires
269.1039).
4.3.1. 3,6-Bis-(4-methoxyphenyl)-1H-pyrazin-2-one
38. Orange
solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH), 1636 (C]O),
1029 (CeO), 929 (CeO); dH (500 MHz; DMSO-d6) 3.82 (3H, s, OCH3),
3.83 (3H, s, OCH3), 7.00 (2H, d, J 8.0 Hz, ArH), 7.06 (2H, d, J 8.0 Hz,
ArH), 7.84e7.86 (2H, br d, J 7.0 Hz, ArH), 8.00 (1H, br s, 5-H), 8.33
(2H, br d, J 7.0 Hz, ArH), 12.45 (1H, br s, NH); LCeMS (3.5 min) m/z
309 (MHþ); HPLC tR 2.70 min purity 98%; HRMS (found: MHþ m/z
309.1237; C18H17N2O3 requires 309.1234).
4.3.8. 6-(4-Methoxyphenyl)-3-(thiophen-3-yl)-1H-pyrazin-2-one
45. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6) 3.83 (3H, s,
OCH3), 7.08 (2H, d, J 9.0 Hz, ArH), 7.59 (1H, dd, J 5.0, 3.0 Hz, thio-
phene 4-H), 7.85e7.88 (4H, m, ArH, thiophene 2-H, 5-H), 8.60 (1H,
br s, 5-H), 12.60 (1H, br s, NH); LCeMS (6 min) m/z 285 (MHþ);
HPLC tR 4.03 min; purity 90%; HRMS (found: MHþ m/z 285.0700;
C15H13N2O2S requires 285.0698).
4.3.2. 6-(4-Methoxyphenyl)-3-(4-methylphenyl)-1H-pyrazin-2-one
39. Bright yellow solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3446
(NH), 2994 (Ar CH), 1653 (C]O), 925 (CeO); (found: C, 73.04; H,
5.53; N, 9.13; C18H16N2O2.0.25 (H2O) requires C, 72.83; H, 5.60; N,
9.44%); dH (500 MHz; CDCl3) 2.44 (3H, s, CH3), 3.92 (3H, s, OCH3),
7.07 (2H, d, J 9.0 Hz, ArH), 7.29 (2H, d, J 8.5 Hz, ArH), 7.74 (2H, d, J
8.5 Hz, ArH), 7.85 (1H, s, 6-H), 8.36 (2H, d, J 9.0 Hz, ArH); dC
(125 MHz; CDCl3) 20.9, 55.4, 114.4, 123.6, 128.1, 128.3, 128.4, 128.6,
131.0, 133.4, 138.6, 156.0, 160.7, one signal not observed; LCeMS
(6 min) m/z 293 (MHþ); HPLC tR 3.96 min; purity 99%; HRMS
(found: MHþ m/z 293.1290; C18H17N2O2 requires 293.1290).
4.3.9. 6-(4-Methoxyphenyl)-3-(4-(morpholin-1-ylmethyl)phenyl)-
1H-pyrazin-2-one 46. Yellow solid. Mp >300 ꢀC; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1653 (C]O), 1005 (CeN), 929 (CeO); dH
(500 MHz; DMSO-d6) 2.38 (4H, broad s, morpholine CH2), 3.50 (2H,
s, NCH2), 3.58 (4H, br s, morpholine CH2), 3.83 (3H, s, OCH3), 7.06
(2H, d, J 9.0 Hz, ArH), 7.36 (2H, d, J 8.0 Hz, ArH), 7.89 (2H, d, J 9.0 Hz,
ArH), 8.05 (1H, s, 5-H), 8.29 (2H, d, J 8.0 Hz, ArH); LCeMS (6 min) m/
z 378 (MHþ), 291 (MHþ-morpholine); HPLC tR 2.41 min; purity 99%;
HRMS (found: MHþ m/z 378.1822; C22H24N3O3 requires 378.1812).
4.3.3. 4-(5-(4-Methoxyphenyl)-3-oxo-3,4-dihydropyrazin-2-yl)ben-
zoic acid ethyl ester 40. Bright yellow solid. Mp >300 ꢀC; nmax (thin
film/cmꢁ1) 3019 (Ar CH), 1684 (C]O), 1653 (C]O); dH (500 MHz;
DMSO-d6; 400 K) 1.40 (3H, t, J 8.0 Hz, CH3), 3.89 (3H, s, OCH3), 4.40
(2H, q, J 8.0 Hz, CH2), 7.10 (2H, d, J 8.0 Hz, ArH), 7.92 (2H, d, J 9.0 Hz,
ArH), 8.03 (2H, d, J 8.0 Hz, ArH), 8.148 (1H, s, 5-H), 8.46 (2H, d, J 9.0 Hz,
ArH); LCeMS (6 min) m/z 351 (MHþ); HPLC tR 4.24 min; purity 95%;
HRMS (found: MHþ m/z 351.1339; C20H19N2O4 requires 351.1345).
4.3.10. 3-(4-Methoxyphenyl)-6-(4-methylphenyl)-1H-pyrazin-2-one
47. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6) 2.38 (3H, s, CH3),
3.85 (3H, s, OCH3), 7.01 (2H, d, J 9.0 Hz, ArH), 7.32 (2H, d, J 8.0 Hz,
ArH), 7.79 (2H, d, J 8.0 Hz, ArH), 8.08 (1H, s, 5-H), 8.34 (2H, d, J
9.0 Hz, ArH); LCeMS (4 min) m/z 293 (MHþ); HPLC tR 3.09 min;
purity 96%; HRMS (found: MHþ m/z 293.1287; C18H17N2O2 requires
293.1285).
4.3.4. 6-(4-Methoxyphenyl)-3-(pyridin-3-yl)-1H-pyrazin-2-one
41. Yellow solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6); 3.85 (3H, s,
OCH3), 7.12 (2H, d, J 9.0 Hz, ArH), 7.95 (2H, J 9.0 Hz, ArH), 8.02 (1H,
4.3.11. 3-(4-Methoxyphenyl)-6-(pyridin-3-yl)-1H-pyrazin-2-one
48. Orange solid. Mp >300 ꢀC; nmax/cmꢁ1 3019 (ArH), 1653 (C]O);
(500 MHz; DMSO-d6; 400 K) dH 3.88 (3H, s, OCH3), 7.03 (2H, d, J

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9725
9.0 Hz, ArH), 7.52 (1H, dd, J 8.0, 5.0 Hz, pyridine 5-H), 8.20 (1H, s, 5-
H), 8.28 (1H, d, J 8.0 Hz, pyridine 4-H), 8.33 (2H, d, J 9.0 Hz, ArH), 8.68
(1H, dd, J 5.0,1.0 Hz, pyridine 6-H), 9.13 (1H, d, J 1.0 Hz, pyridine 2-H);
LCeMS (6 min) m/z 280 (MHþ); HPLC tR 3.45 min; purity 99%; HRMS
(found: MHþ m/z 280.1083; C16H14N3O2 requires 280.1081).
207 (Mþ); GC tR 2.82 min; purity >99%; HRMS (found: MHþ m/z
207.9280; C4H479Br35ClN3 requires 207.9272).
4.3.19. 5-Bromo-3-chloro-1H-pyrazin-2-one 56. A solution of 55
(9.00 g, 43.2 mmol) in concentrated sulfuric acid (35 mL) was added
dropwise at 0 ꢀC to a solution of sodium nitrite (3.52 g, 51.0 mmol) in
concentrated sulfuric acid (20 mL). The solution was heated at 40 ꢀC
for 1 h, then cooled to rt and poured onto crushed ice (400 mL). The
mixture was extracted with ethyl acetate (200 mL). The extract was
dried, filtered and concentrated to give 56 (6.13 g, 68%) as a brown
solid. Mp 175e177 ꢀC; nmax/cmꢁ1 3019 (ArH), 1653 (C]O); dH
(500 MHz; CD3OD) 7.72 (1H, s, 6-H); dC (125 MHz; CDCl3) 114.2,127.6,
146.4, 154.0; LCeMS (6 min) m/z 213, 211, 209 (MHþ); HPLC tR
2.54 min purity >99%; HRMS (found: MHþ m/z 208.9113;
C4H379Br35ClN2O requires 208.9112).
4.3.12. 6-(4-Hydroxyphenyl)-3-(4-methoxyphenyl)-1H-pyrazin-2-
one 49. Red solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar
CH), 1653 (C]O), 929 (CeO); dH (500 MHz; CDCl3) 3.81 (3H, s,
OCH3), 6.89 (2H, d, J 9.0 Hz, ArH), 7.00 (2H, d, J 8.0 Hz, ArH), 7.73
(2H, d, J 8.0 Hz, ArH), 8.08 (1H, s, 5-H), 8.33 (2H, d, J 9.0 Hz, ArH),
10.0 (1H, br s, NH or OH); LCeMS (6 min) m/z 295 (MHþ); HPLC tR
3.73 min; purity 99%; HRMS (found: m/z 295.1076; C17H15N2O3
requires 295.1077).
4.3.13. 4-(5-(4-Methoxyphenyl)-6-oxo-1,6-dihydropyrazin-2-yl)ben-
zamide 50. Yellow solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3450
(NH2), 3019 (Ar CH), 1684 (C]O), 1653 (C]O), 929 (CeO); dH
(500 MHz; DMSO-d6; 400 K) 3.88 (3H, s, OCH3), 7.05 (2H, br s, NH2),
7.11 (2H, d, J 9.0 Hz, ArH), 7.91 (2H, d, J 9.0 Hz, ArH), 7.94 (2H, d, J
8.0 Hz, ArH), 8.12 (1H, s, 5-H), 8.38 (2H, d, J 8.0 Hz, ArH); LCeMS
(6 min) m/z 322 (MHþ); HPLC tR 3.10 min; purity 90%; HRMS
(found: m/z 322.1190; C18H16N3O3 requires 322.1186).
4.3.20. 3,5-Bis-(4-methoxyphenyl)-1H-pyrazin-2-one 57. A solution
of 56 (50.6 mg, 0.242 mmol), sodium carbonate (35.6 mg,
0.336 mmol), 4-methoxyphenylboronic acid (182 mg, 1.20 mmol)
and
tetrakis(triphenylphosphine)palladium
(0)
(17
mg,
0.015 mmol) in water (0.7 mL) and acetonitrile (2.7 mL) was heated
in a microwave reactor using variable wattage to 150 ꢀC for 20 min.
The mixture was partitioned between ethyl acetate (50 mL) and
water (40 mL). The organic layer was filtered through a plug of silica
gel (6 g). The filtrate was concentrated and purified by flash column
chromatography on silica, eluting with ethyl acetate to give 57
4.3.14. 3-(4-Methoxyphenyl)-6-(pyrazol-4-yl)-1H-pyrazin-2-one
51. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6; 400 K) 3.86 (3H, s,
OCH3), 6.99 (2H, d, J 9.0 Hz, ArH), 7.83 (1H, s, 5-H), 8.21 (2H, s,
pyrazole CH), 8.34 (2H, d, J 9.0 Hz, ArH); LCeMS (6 min) m/z 269
(MHþ); HPLC tR 3.38 min; purity 98%; HRMS (found: m/z 269.1037;
C14H13N4O2 requires 269.1033).
(73 mg, 47%). Mp >300 ꢀC; Rf (EtOAc) 0.16; nmax (thin film/cmꢁ1
)
3019 (Ar CH), 1653 (C]O), 972 (CeO), 929 (C-eO); dH (500 MHz;
CD3OD) 3.85 (3H, s, OCH3), 3.88 (3H, s, OCH3), 7.01 (2H, d, J 8.0 Hz,
ArH), 7.02 (2H, d, J 9.0 Hz, ArH), 7.66 (1H, s, 6-H), 7.86 (2H, d, J
8.0 Hz, ArH), 8.46 (2H, d, J 9.0 Hz, ArH); LCeMS (6 min) m/z 309
(MHþ); HPLC tR 4.11 min; purity 99%; HRMS (found: m/z 309.1243;
C18H17N2O3 requires 309.1234).
4.3.15. 3-(4-Methoxyphenyl)-6-(pyrazol-3-yl)-1H-pyrazin-2-one
52. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6; 400 K) 3.87 (3H, s,
OCH3), 6.92 (1H, d, J 3.0 Hz, pyrazole 4-H), 7.00 (2H, d, J 9.0 Hz, ArH),
7.77 (1H, d, J 3.0 Hz, pyrazole 5-H), 8.04 (1H, s, 5-H), 8.36 (2H, d, J
9.0 Hz, ArH); LCeMS (6 min) m/z 269 (MHþ); HPLC tR 3.46 min;
purity 98%; HRMS (found: m/z 269.1037; C14H13N4O2 requires
269.1033).
4.4. General procedure for preparation of 5-bromo-3-(alky-
lamino)-1H-pyrazin-2-ones 58e63
A solution of 56 (1.50 g, 7.16 mmol), the appropriate secondary
amine (8.59 mmol, 1.20 equiv) and diisopropylethylamine (1.50 mL,
8.59 mmol) in n-butanol (15 mL) was heated in a microwave reactor
using variable wattage to 140 ꢀC for 1 h. The mixture was con-
centrated and purified by flash column chromatography on silica,
eluting with 9:1 dichloromethane/methanol, to give gave 58e63.
4.3.16. 3-(4-Methoxyphenyl)-6-(thiophen-3-yl)-1H-pyrazin-2-one
53. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3019 (Ar CH),
1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-d6) 3.82 (3H, s,
OCH3), 7.01 (2H, d, J 9.0 Hz, ArH), 7.73 (2H, br s, thiophene 4-H, 2-H),
8.05 (1H, br s, 5-H), 8.31e8.36 (3H, m, ArH, thiophene 2-H); LCeMS
(6 min) m/z 285 (MHþ); HPLC tR 3.89 min; purity 98%; HRMS
(found: m/z 285.0694; C15H13N2O2S requires 285.0692).
4.4.1. 5-Bromo-3-(piperidin-1-yl)-1H-pyrazin-2-one
58. Brown
solid. Mp 248e252 ꢀC; Rf (50% EtOAc/hexanes) 0.54; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1647 (C]O); dH (500 MHz; CDCl3) 1.60e1.70
(6H, br m, piperidine CH2), 3.85e3.88 (4H, br m, piperidine NCH2),
6.75 (1H, s, 6-H),11.35 (1H, br s, NH); dC (125 MHz; CDCl3) 24.7, 26.0,
47.7, 114.1, 114.8, 151.2, 152.9; LCeMS (6 min) m/z 260, 258 (MHþ);
HPLC tR 3.83 min; purity 95%; HRMS (found: m/z 258.0236;
C9H1379BrN3O requires 258.0236).
4.3.17. 6-(2-Hydroxyphenyl)-3-(4-methoxyphenyl)-1H-pyrazin-2-
one 54. Orange solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3250
(OH), 3019 (Ar CH), 1653 (C]O), 929 (CeO); dH (500 MHz; DMSO-
d6; 400 K) 3.88 (3H, s, OCH3), 6.96 (1H, dd, J 7.0, 7.0 Hz, ArH),
7.01e7.04 (3H, m, ArH), 7.32 (1H, dd, J 7.0, 7.0 Hz, ArH), 7.74 (1H, d, J
7.0 Hz, ArH), 8.02 (1H, br s, 5-H), 8.33 (2H, d, J 9.0 Hz, ArH); LCeMS
(6 min) m/z 295 (MHþ); HPLC tR 3.94 min; purity 98%; HRMS
(found: m/z 295.1084; C17H15N2O3 requires 295.1077).
4.4.2. 5-Bromo-3-(N-methylpiperazin-1-yl)-1H-pyrazin-2-one
59. Yellow solid; mp 285e288 ꢀC; Rf (10% methanol/CH2Cl2) 0.25;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 1653 (C]O), 1006 (CeN); dH
(500 MHz; CDCl3) 2.34 (3H, s, NCH3), 2.51e2.53 (4H, br s, piperazine
CH2), 3.94e3.96 (4H, br s, piperazine CH2), 6.77 (1H, s, 6-H); dC
(125 MHz; CDCl3) 46.0, 46.2, 114.5, 114.8, 150.9, 152.7, one signal not
observed; LCeMS (6 min) m/z 275 (MHþ, 50%); 273 (MHþ, 50);
HPLC (6 min) tR 0.93 min; purity 99%; HRMS (found: m/z 273.0343;
C9H1479BrN4O requires 273.0345).
4.3.18. 5-Bromo-3-chloro-pyrazin-2-amine 55.74 A suspension of
NBS (31.0 g, 173 mmol) and 9 (20.0 g, 154 mmol) in chloroform
(140 mL) was heated at reflux for 16 h. The solution was cooled and
washed with water (350 mL). The organic layer was filtered
through Celite and concentrated to give 55 (22.0 g, 69%) as a brown
solid. Mp 230e232 ꢀC; nmax (thin film/cmꢁ1) 3412 (NH2), 3019 (Ar
CH); dH (500 MHz; CDCl3) 5.02 (2H, br s, NH2), 8.05 (1H, s, 6-H); dC
(125 MHz; CDCl3) 123.5, 132.0, 142.9, 150.6; GCeMS m/z 211, 209,
4.4.3. 5-Bromo-3-(4-hydroxypiperidin-1-yl)-1H-pyrazin-2-one
60. Brown solid. Mp 295e298 ꢀC; Rf (10% MeOH/CH2Cl2) 0.29; nmax
(thin film/cmꢁ1) 3500e3300 (OH), 3019 (Ar CH), 1653 (C]O), 1265

9726
J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
(CeN); dH (500 MHz; CDCl3) 1.52e1.68 (2H, m, NCH2CH), 1.98e2.03
(2H, m, NCH2CH), 3.43e3.51 (2H, m, NCH), 3.95e3.99 (1H, m,
CHOH), 4.43e4.48 (2H, m, NCH), 6.75 (1H, s, 6-H), 10.80 (1H, br s,
NH); dC (125 MHz; CDCl3) 34.4, 43.9, 67.6, 104.6, 114.3, 151.0, 152.4;
LCeMS (6 min) m/z 276, 274 (MHþ); HPLC tR 2.88 min; purity 99%;
HRMS (found: m/z 274.0189; C9H1379BrN3O2 requires 274.0186).
(CeO); dH (500 MHz; CDCl3) 2.39 (3H, s, NCH3), 2.61e2.63 (4H, br
m, piperazine CH2), 3.85 (3H, s, OCH3), 3.99e4.01 (4H, br m, pi-
perazine NCH2), 6.95 (2H, d, J 9.0 Hz, ArH), 7.12 (1H, s, 6-H), 7.72
(2H, d, J 9.0 Hz, ArH), 11.50 (1H, br s, NH); dC (125 MHz; CDCl3) 46.1,
46.3, 55.0, 55.4, 110.8, 114.1, 126.2, 129.3, 132.4, 151.4, 152.6, 159.4;
LCeMS (3.5 min) m/z 301 (MHþ); HPLC tR 1.49 min; purity 99%;
HRMS (found: m/z 301.1659; C16H21N4O2 requires 301.1659).
4.4.4. Methyl 1-(6-bromo-3-oxo-3,4-dihydropyrazin-2-yl)piperidine-
4-carboxylate 61. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2)
0.67; nmax (thin film/cmꢁ1) 3019 (ArH), 1684 (C]O), 1653 (C]O); dH
(500 MHz; CDCl3) 1.81e1.86 (2H, m, NCH2CH), 1.99e2.02 (2H, m,
NCH2CH), 2.59e2.61 (1H, m, CHCO2Me), 3.08e3.14 (2H, m, NCH), 3.71
(3H, s, CO2Me), 4.72 (2H, br d, J 14.0 Hz, NCH), 6.79 (1H, s, 6-H), 11.40
(1H, br s, NH); dC (125 MHz; CDCl3) 28.1, 41.1, 46.0, 51.8, 114.5, 114.9,
150.9, 152.5, 174.9; LCeMS (6 min) m/z 318, 316 (MHþ); HPLC tR
3.56 min; purity 98; HRMS (found: 316.0283; C11H1579BrN3O3 re-
quires 316.0291).
4.5.3. 3-(4-Hydroxypiperidin-1-yl)-5-(4-methoxyphenyl)-1H-pyr-
azin-2-one 66. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2)
0.40; nmax (thin film/cmꢁ1) 3500e3460 (OH), 3019 (Ar CH), 1621
(C]O), 929 (CeO); dH (500 MHz; CDCl3) 1.67e1.73 (2H, m),
2.04e2.08 (2H, m), 3.39 (2H, ddd, J 13.0, 10.0, 3.0 Hz, NCHax),
3.85 (3H, s, OCH3), 3.97e3.99 (1H, m, CHOH), 4.53 (2H, br d, J
13.0 Hz, NCHeq), 6.94 (2H, d, J 9.0 Hz, ArH), 7.12 (1H, s, 6-H), 7.71
(2H, d, J 9.0 Hz, ArH), 11.65 (1H, br s, NH); LCeMS (3.5 min) m/z
302 (MHþ); dC (125 MHz; CDCl3) 34.4, 44.2, 55.4, 68.2, 110.7,
114.1, 126.2, 129.4, 132.4, 151.6, 152.7, 159.4; HPLC tR 2.20 min;
purity 98%; HRMS (found: m/z 302.1506; C16H20N3O3 requires
302.1499).
4.4.5. 5-Bromo-3-(4-phenylpiperidin-1-yl)-1H-pyrazin-2-one
62. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2) 0.71; nmax
(thin film/cmꢁ1) 3019 (Ar CH), 1653 (C]O); dH (500 MHz; DMSO-
d6) 1.62e1.70 (2H, m, NCH2CH), 1.81e1.84 (2H, m, NCH2CH),
2.77e2.82 (1H, m, CHPh), 2.87e2.91 (2H, m, NCH), 4.91 (2H, br d, J
13.0 Hz, NCH), 6.91 (1H, s, 6-H), 7.17e7.31 (5H, m, Ph), 11.83 (1H, br
s, NH); dC (125 MHz; DMSO-d6) 32.8, 41.9, 46.4, 112.0, 115.8, 126.1,
126.7, 128.4, 145.8, 151.1, 151.3; LCeMS (6 min) m/z 336, 334 (MHþ);
HPLC tR 4.26 min; purity 98%; HRMS (found: m/z 334.0563;
C15H1779BrN3O requires 334.0549).
4.5.4. Methyl 1-(6-(4-methoxyphenyl)-3-oxo-3,4-dihydropyrazin-2-
yl)piperidine-4-carboxylate 67. Brown solid. Mp >300 ꢀC; Rf
(10% MeOH/CH2Cl2) 0.38; nmax (thin film/cmꢁ1) 2950 (CH), 1720
(C]O), 1643 (C]O), 1028; dH (500 MHz; CDCl3) 1.89e1.97 (2H,
m), 2.04e2.09 (2H, m), 2.61e2.68 (1H, m, CHCO2Me), 3.11e3.17
(2H, m), 3.74 (3H, s, CO2Me), 3.87 (3H, s, OCH3), 4.75e4.89 (2H,
m), 6.94e6.96 (2H, m, ArH), 7.16 (1H, s, 6-H), 7.72e7.74 (2H, m,
ArH), 12.00 (1H, br s, NH); dC (125 MHz; CDCl3) 28.0, 41.5, 46.1,
51.8, 55.4, 111.1, 114.0, 126.2, 129.3, 132.5, 151.6, 153.0, 159.3,
175.3; LCeMS (3.5 min); m/z 344 (MHþ); HPLC tR 3.0 min; pu-
rity 99%; HRMS (found: m/z 344.1613; C18H22N3O4 requires
344.1605).
4.4.6. 5-Bromo-3-(4-morpholinyl)-1H-pyrazin-2-one
63. Yellow
solid. Mp 285e288 ꢀC; Rf (10% MeOH/CH2Cl2) 0.67; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1653 (C]O); dH (500 MHz; CDCl3) 3.80e3.81
(4H, m, morpholine CH2), 3.93e3.95 (4H, m, morpholine CH2), 6.80
(1H, s, 6-H), 11.20 (1H, br s, NH); dC (125 MHz; CDCl3) 46.9, 66.8,
114.5, 115.1, 150.8, 152.6; LCeMS (6 min) m/z 260, 258 (MHþ); HPLC
tR 3.10 min; purity 99%; HRMS (found: m/z 260.0035;
C8H1179BrN3O2 requires 260.0029).
4.5.5. 5-(4-Methoxyphenyl)-3-(4-phenylpiperidin-1-yl)-1H-pyrazin-
2-one 68. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2) 0.50;
nmax (thin film/cmꢁ1) 3019 (Ar CH), 1653 (C]O), 1265 (CeN), 929
(CeO); dH (500 MHz; DMSO-d6) 1.69e1.78 (2H, m), 1.85e1.88 (2H,
m), 2.78e2.85 (1H, m, CHPh), 2.88e2.96 (2H, m), 3.77 (3H, s, OCH3),
4.96e4.99 (2H, m), 6.95 (2H, d, J 9.0 Hz, ArH), 7.19e7.31 (5H, m, Ph,
6-H), 7.77 (2H, d, J 9.0 Hz, ArH), 11.86 (1H, br s, NH); dC (125 MHz;
DMSO-d6) 33.3, 42.6, 47.0, 55.5, 112.3, 114.3, 126.1, 126.6, 127.2,
128.9, 129.8, 129.9, 146.5, 151.8, 151.9, 158.9; LCeMS (3.5 min) m/z
362 (MHþ); HPLC tR 3.13 min; purity 99%; HRMS (found: m/z
362.1860; C22H24N3O2 requires 362.1863).
4.5. General procedure for preparation of 5-(hetero)aryl-3-
(alkylamino)-2(1H)-pyrazinones 64e73
A mixture of the appropriate 5-bromo-3-(alkylamino)-2(1H)-
pyrazinone 58e63 (0.366 mmol, 1 equiv), the appropriate (hetero)
arylboronic acid or boronate ester (0.441 mmol, 1.2 equiv), sodium
carbonate (55 mg, 0.509 mmol, 1.4 equiv) and Pd(dppf)Cl2 (18 mg,
0.022 mmol) in water (1 mL) and acetonitrile (4.1 mL) was heated in
a microwave reactor using variable wattage to 150 ꢀC for 20 min.
The mixture was partitioned between ethyl acetate (20 mL) and
water (50 mL). The organic layer was separated and concentrated.
Automated MPLC on silica, eluting with 9:1 dichloromethane/
methanol, gave the products 64e73.
4.5.6. 5-(4-Methoxyphenyl)-3-morpholino-1H-pyrazin-2-one
69. Yellow solid. Mp >300 ꢀC; Rf (EtOAc) 0.31; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1653 (C]O), 928 (CeO); dH (500 MHz;
CDCl3) 3.86 (3H, s, OCH3), 3.87e3.88 (4H, m, morpholine CH2),
3.96e3.98 (4H, m, morpholine CH2), 6.95 (2H, d, J 8.5 Hz, ArH),
7.08 (1H, s, 6-H), 7.69 (2H, d, J 8.5 Hz, ArH), 10.50 (1H, br s, NH);
dC (125 MHz; CDCl3) 47.0, 55.4, 66.9, 110.8, 114.1, 126.2, 129.2,
132.3, 151.4, 152.3, 159.4; LCeMS (6 min) m/z 288 (MHþ); HPLC tR
3.65 min; purity 99%; HRMS (found: m/z 288.1344; C15H18N3O3
requires 288.1343).
4.5.1. 5-(4-Methoxyphenyl)-3-(piperidin-1-yl)-1H-pyrazin-2-one
64. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2) 0.43; nmax
(thin film/cmꢁ1) 3019 (Ar CH), 1653 (C]O), 929 (CeO); dH
(500 MHz; CDCl3) 1.69e1.74 (6H, m, piperidine CH2), 3.83e3.89
(4H, m, piperidine NCH2), 3.85 (3H, s, OCH3), 6.94 (2H, d, J 9.0 Hz,
ArH), 7.12 (1H, s, 6-H), 6.917.73 (2H, d, J 9.0 Hz, ArH), 11.95 (1H, br s,
NH); dC (125 MHz; CDCl3) 25.0, 26.0, 47.8, 55.4, 110.5, 114.0, 126.2,
129.6, 132.4, 152.0, 153.0, 159.3; LCeMS (3.5 min) m/z 286 (MHþ);
HPLC tR 2.78 min; purity 95%; HRMS (found: m/z 286.1557;
C16H20N3O2 requires 286.1550).
4.5.7. 4-(6-Morpholino-5-oxo-4,5-dihydropyrazin-2-yl)benzamide
70. Brown solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2) 0.27; nmax
(thin film/cmꢁ1) 3420 (NH2), 3019 (Ar CH), 1685 (C]O), 1653 (C]
O); dH (500 MHz; DMSO-d6) 3.71e3.73 (4H, m, morpholine CH2),
3.80e3.83 (4H, m, morpholine CH2), 7.32 (1H, br s, NH2), 7.56 (1H, s,
6-H), 7.86 (2H, d, J 8.5 Hz, ArH), 7.90 (2H, d, J 8.5 Hz, ArH), 12.10 (1H,
br s, NH); dC (125 MHz; DMSO-d6) 46.9, 66.5, 115.3, 124.4, 128.2,
128.8, 132.8, 139.8, 151.6, 152.0, 168.1; LCeMS (3.5 min) m/z 301
4.5.2. 5-(4-Methoxyphenyl)-3-(4-methylpiperazin-1-yl)-1H-pyr-
azin-2-one 65. Pale pink solid. Mp >300 ꢀC; Rf (10% methanol/
CH2Cl2) 0.24; nmax (thin film/cmꢁ1) 3019 (Ar CH), 1653 (C]O), 929

J.J. Caldwell et al. / Tetrahedron 68 (2012) 9713e9728
9727
(MHþ); HPLC tR 2.10 min; purity 99%; HRMS (found: m/z 301.1291;
C15H17N4O3 requires 301.1295).
(0.4 mL) was heated at 150 ꢀC in a microwave reactor for 20 min. The
cooled mixture was concentrated. Flash column chromatography on
silica, eluting with 10% MeOH in dichloromethane, followed by
preparative TLC, eluting with 10% MeOH in dichloromethane, gave
120 (10.0 mg, 25%) as a yellow solid; Rf (10% methanol/CH2Cl2) 0.24;
nmax (thin film/cmꢁ1) 2960, 2854 (CH), 1635 (C]O); dH (500 MHz);
(DMSO-d6) 3.66e3.70 (8H, br m, morpholine CH2), 3.79 (3H, s, OMe),
6.99 (2H, d, J 9.0 Hz, ArH), 7.19 (1H, s, 5-H), 7.59 (2H, d, J 9.0 Hz, ArH),
11.9 (1H, br s, NH); dC (125 MHz); (DMSO-d6) 46.6, 55.2, 66.0, 114.2,
117.8,123.8,127.2,129.4,151.0,152.4,159.5; LCeMS (3.5 min) m/z 288
(MHþ); HPLC tR 2.27 min, purity 99%; HRMS (found MHþ: m/z
288.1344; C15H18N3O3 requires 288.1343).
4.5.8. 3-Morpholino-5-(pyridin-3-yl)-1H-pyrazin-2-one 71. Brown
solid. Mp >300 ꢀC; Rf (10% MeOH/CH2Cl2) 0.42; nmax (thin film/
cmꢁ1) 3019 (Ar CH), 1621 (C]O), 1265 (CeN); dH (500 MHz; DMSO-
d6) 3.71e3.73 (4H, m, morpholine CH2), 3.82e3.84 (4H, m, mor-
pholine CH2), 7.40 (1H, dd, J 8.0, 5.0 Hz, pyridine 5-H), 7.58 (1H, s, 6-
H), 8.19 (1H, ddd, J 8.0, 2.0, 1.5 Hz, pyridine 4-H), 8.46 (1H, dd, J 5.0,
1.5 Hz, pyridine 6-H), 9.05 (1H, d, J 2.0 Hz, pyridine 2-H), 12.12 (1H,
br s, NH); dC (125 MHz; DMSO-d6) 46.9, 66.5, 114.9, 123.9, 127.1,
132.1, 132.7, 146.4, 148.2, 151.9, 152.0; LCeMS (3.5 min) m/z 259
(MHþ); HPLC tR 1.40 min; purity 99%; HRMS (found: m/z 258.1199;
C13H15N4O2 requires 258.1190).
Acknowledgements
4.5.9. 3-Morpholino-5-(4-chlorophenyl)-1H-pyrazin-2-one
72. White solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 2960, 2856
(CH), 1641 (C]O); dH (500 MHz; DMSO-d6) 3.71e3.73 (4H, m,
morpholine CH2), 3.80e3.82 (4H, br s, morpholine CH2), 7.42 (2H, d,
J 9.0 Hz, ArH), 7.50 (1H, s, 6-H), 7.87 (2H, d, J 9.0 Hz, ArH), 12.08 (1H,
br s, NH); dC (125 MHz; DMSO-d6) 46.9, 66.5, 114.5, 126.5, 128.4,
128.9, 131.8, 136.0, 151.6, 151.9; LCeMS (3.5 min) m/z 292 (MHþ);
HPLC tR 2.68 min; purity 99%; HRMS (found: m/z 292.0845;
C14H15ClN3O2 requires 292.0847).
We thank Dr. A. Mirza and Mr. M. Richards for assistance in the
spectroscopic characterisation of test compounds. We thank Pro-
fessor K. Jones and Dr. E. McDonald for helpful conversations. This
work was supported by funding from Cancer Research UK [CUK]
grant numbers C309/A11566, C309/A8274 and C309/A8365, and
The Institute of Cancer Research. We acknowledge NHS funding to
the NIHR Biomedical Research Centre.
Supplementary data
4.5.10. 3-Morpholino-5-(4-acetylaminophenyl)-1H-pyrazin-2-one
73. White solid. Mp >300 ꢀC; nmax (thin film/cmꢁ1) 3274 (NH),
3070 (Ar CH), 2976, 2854 (CH), 1652 (C]O); dH (500 MHz; DMSO-
d6) 2.04 (3H, s, Ac), 3.70e3.72 (4H, m, morpholine CH2), 3.78e3.80
(4H, m, morpholine CH2), 7.34 (1H, s, 6-H), 7.57 (2H, d, J 9.0 Hz,
ArH), 7.74 (2H, d, J 9.0 Hz, ArH), 9.95 (1H, br s, AcNH), 11.93 (1H, br s,
NH); dC (125 MHz; DMSO-d6) 24.5, 46.9, 66.5, 113.2, 119.4, 125.1,
129.6, 131.8, 138.7, 151.6, 151.8, 168.6; LCeMS (3.5 min) m/z 315
(MHþ); HPLC tR 1.93 min; purity 99%; HRMS (found: m/z 315.1448;
C16H19N4O3 requires 315.1452).
Kinase panel selectivity data for compounds 38, 39, 57, 65.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2012.09.039.
References and notes
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