Pyridinylquinoxalines and pyridinylpyridopyrazines as lead compounds for novel p38α mitogen-activated protein kinase inhibitors

Article abstract of DOI:10.1021/jm901392x

Various substituted 2(3)-(4-fluorophenyl)-3(2)-(pyridin-4-yl)quinoxalines and 2(3)-(4-fluorophenyl)-3(2)-(pyridin-4-yl)pyridopyrazines were synthesized as novel p38α MAP kinase inhibitors via different short synthetic strategies with high variation possibilities. The formation of the quinoxaline/ pyridopyrazine core was achieved from α-diketones and o-phenylenediamines/ α-diaminopyridines under microwave irradiation. Introduction of an amino moiety at the pyridine C2 position of the 2(3)-(4-fluorophenyl)-3(2)-(pyridin-4- yl)quinoxalines led to compounds showing potent enzyme inhibition down to the double-digit nanomolar range (6f; IC₅₀ = 81 nM). Replacement of the quinoxaline core with pyrido[2,3-b]pyrazine gave compound 9e with superior p38α MAP kinase inhibition (IC₅₀= 38 nM).

Full text of DOI:10.1021/jm901392x

1128 J. Med. Chem. 2010, 53, 1128–1137
DOI: 10.1021/jm901392x
Pyridinylquinoxalines and Pyridinylpyridopyrazines as Lead Compounds for Novel p38r
Mitogen-Activated Protein Kinase Inhibitors
Pierre Koch, Hartmut Jahns, Verena Schattel, Marcia Goettert, and Stefan Laufer*
Department of Pharmaceutical and Medicinal Chemistry, Institute of Pharmacy, Eberhard-Karls-University of Tu€bingen, Auf der Morgenstelle 8,
72076 Tu€bingen, Germany
Received September 18, 2009
Various substituted 2(3)-(4-fluorophenyl)-3(2)-(pyridin-4-yl)quinoxalines and 2(3)-(4-fluorophenyl)-
3(2)-(pyridin-4-yl)pyridopyrazines were synthesized as novel p38R MAP kinase inhibitors via different
short synthetic strategies with high variation possibilities. The formation of the quinoxaline/pyrido-
pyrazine core was achieved from R-diketones and o-phenylenediamines/R-diaminopyridines under
microwave irradiation. Introduction of an amino moiety at the pyridine C2 position of the 2(3)-(4-
fluorophenyl)-3(2)-(pyridin-4-yl)quinoxalines led to compounds showing potent enzyme inhibition down
to the double-digit nanomolar range (6f; IC50 = 81 nM). Replacement of the quinoxaline core with
pyrido[2,3-b]pyrazine gave compound 9e with superior p38R MAP kinase inhibition (IC50 = 38 nM).
Introduction
The p38R mitogen-activated protein (MAP)a kinase, a
serine/threonine kinase, is a key component of the cascade
leading to pro-inflammatory cytokines such as tumor necrosis
factor-R and interleukin-1β.1 This kinase is activated by infec-
tion or cellular stressors such as mechanical wear, heat, or
osmotic shock.2 Inhibition of p38R MAP kinase is therefore a
promising therapeutic strategy for the treatment of cytokine-
driven disorders like inflammatory bowel disease or rheuma-
toid arthritis. Pyridinylimidazoles like the prototype inhibitor
SB203580 or the recently reported 2-alkylsulfanyl-4-(4-
fluorophenyl)-5-(2-aminopyridin-4-yl)-substituted imidazoles
1 (Figure 1) are potent adenosine triphosphate (ATP) compe-
titive p38R MAP kinase inhibitors.3-6 The central pharmaco-
phore of these pyridinylimidazoles consists of a vicinal
4-fluorophenyl/pyridin-4-yl system.7 The nitrogen atom of
the pyridine ring is accepting a hydrogen bond from the
backbone NH group of Met109. The 4-fluorophenyl ring
occupies hydrophobic region I, mainly causing selectivity.
In a continuing effort to develop improved p38R MAP
kinase inhibitors, we focused our attention on the optimiza-
tion of the core structure. Hence, the 4-fluorophenyl/pyridin-
4-yl pharmacophore was maintained, and the five-membered
imidazole core was replaced with six-membered quinoxaline
and pyridopyrazine rings (Figure 1). These geometrical differ-
ences between the six- and five-membered heterocyclic cores
mayimprovetheselectivity of the quinoxaline/pyridopyrazine
inhibitors toward other kinases, for example, c-Jun N-term-
inal kinase 3 (JNK3), as compared to the five-membered core
inhibitors.7
Figure 1. From five to six-membered rings. The 4-fluorophenyl/
pyridin-4-yl pharmacophore is shown in a gray box.
Herein, we report different short syntheses for a series of
2(3)-aryl-3(2)-heteroarylquinoxalines 6a-y and 2(3)-aryl-
3(2)-heteroarylpyridopyrazines 9a-h as well as their struc-
ture-activity relationships (SAR).
The main synthetic step in the preparation of the quino-
xaline derivatives is the formation of heterocyclic core
via click chemistry8 starting from R-diketones and o-phe-
nylenediamines. The facile introduction of substituents
into the quinoxaline core is accomplished by the use of
differently substituted o-phenylenediamines. As previously
demonstrated, the reaction of R-diketones and o-phenyle-
nediamines proceeded rapidly and in excellent yields when
the reactants were stirred at room temperature with ami-
dosulfonic acid (1 h),9 iodine (3 min),10 or cerium(IV)
ammonium nitrate (10 min)11 as a catalyst or under micro-
wave irradiation of the reactants for 5 min in a methanol/
acetic acid mixture (9:1) at 160 °C.12 The multitude of
commercially available, differently substituted o-pheny-
lenediamines leads to numerous variations. The exchange of
*To whom correspondence should be addressed. Telephone: þ497071-
2972459. Fax: þ497071-295037. E-mail: stefan.laufer@uni-tuebingen.de.
a Abbreviations: MAP, mitogen-activated protein; NaHMDS,
sodium hexamethyldisilazane; HB, hydrogen bond; SAR, structure-
activity relationship(s); JNK3, c-Jun N-terminal kinase 3; SEM, “stan-
dard error of the mean”.
r
pubs.acs.org/jmc
Published on Web 01/15/2010
2010 American Chemical Society

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3 1129
Scheme 1. Synthesis of 2-(Fluorophenyl)-3-(pyridin-4-yl)quinoxalines 6a-ea
a Reagents and conditions: (i) NaHMDS, ethyl 4-fluorobenzoate, THF, 0 °C, 1.5 h; (ii) SeO2, AcOH, reflux temperature, 1.5 h; (iii) MeOH/AcOH
(9:1), 160 °C, 5 min, 250 W, microwave irradiation.
Scheme 2. Synthesis of 3-(2-Alkyl/phenylalkylaminopyridin-4-yl)-2-(4-fluorophenyl)quinoxalines 6f-k (route A)a
a Reagents and conditions: (i) R1-NH2, tBuONa, Pd2(dba)3, BINAP, toluene; (ii) Boc2O, DMAP, DCM; (iii) NaHMDS, ethyl 4-fluorobenzoate,
THF, 0 °C to rt; (iv) SeO2, AcOH, reflux temperature, 1.5 h for 2b and 4.5 h for 2c; (v) TFA, DCM, rt, 16 h; (vi) MeOH/AcOH (9:1), 160 °C, 5 min, 250 W,
microwave irradiation.
o-phenylenediamines in this reaction for R-diaminopyridines
results in the pyridopyrazine derivatives.
Earlier analysis of the SAR of the five-membered imida-
zole derivatives 1 indicated that the introduction of an amino
function at the pyridine C2 position resulted in another
possible hydrogen bond interaction with the hinge region.4
The introduction of this amino function was accomplished
by two different synthetic methods.
Results and Discussion
Chemistry. (i) Pyridinylquinoxalines. The “unsubstituted”
pyridinylquinoxaline, 2-(fluorophenyl)-3-(pyridin-4-yl)quino-
xaline (Scheme 1, where R2 and R3 = H, 6a), was prepared in
three steps starting from 4-picoline in a straightforward
synthesis (Scheme 1). 4-Picoline was deprotonated under an
argon atmosphere with NaHMDS in THF at 0 °C and treated
with ethyl 4-fluorobenzoate to yield 1-(4-fluorophenyl)-
2-(pyridin-4-yl)ethanone (2a). This ethanone was oxidized
with selenium dioxide in glacial acetic acid to 1-(4-fluoro-
phenyl)-2-(pyridin-4-yl)ethane-1,2-dione (3a). The R-diketone
3a and o-phenylenediamine (5a) were heated in a methanol/
acetic acid mixture for 5 min at 160 °C in a microwave reactor
according to the protocol of Zhao et al.12 to yield the quinoxa-
line derivative 6a.
The facile introduction of substituents (R2 and R3) at
positions 6 and 7 of the quinoxaline core (quinoxalines 6b-e)
was accomplished by exchanging o-phenylenediamine with
substituted o-phenylenediamines. Click chemistry of dike-
tone 3a and different substituted o-phenylenediamines 5a-d
led to pyridinylquinoxalines 6b-e in good yields (Scheme 1).
The reaction of the unsymmetrically substituted 4-methoxy-
o-phenylenediamine 5d resulted in two regioisomers, 6d and
6e, in a ratio of almost 1:1. These isomers were separated by
flash chromatography. Slow evaporation at room tempera-
ture (rt) of a solution of the first eluted isomer 6d in ethyl
acetate resulted in crystals suitable for X-ray analysis. Com-
pound 6d was identified as 3-(4-fluorophenyl)-6-methoxy-
2-(pyridin-4-yl)quinoxaline.13
The substituted 3-(2-alkyl/phenylalkylaminopyridin-4-yl)-
2-(4-fluorophenyl)quinoxalines 6f-k can be prepared via
click chemistry from 1-(4-fluorophenyl)-2-[2-(alkyl/phenyl-
alkyl)pyridin-4-yl]ethane-1,2-diones 4b and 4c and diverse
o-phenylenediamine derivatives (5a-d) in a microwave-assisted
condensation reaction in excellent yield (route A, Scheme 2).
Compounds 4b and 4c were prepared starting from the recently
published 2-{2-[boc(alkyl/phenylalkyl)amino]pyridin-4-yl}-
1-(4-fluorophenyl)ethanones 2b and 2c5 (Scheme 2). Ethanone
2b was oxidized with selenium dioxide in acetic acid under
reflux conditions for 1.5 h to diketone 3b without cleavage of
the boc protecting group. To remove the protecting group,
diketone 3b was treated with TFA in DCM to yield diketone 4b.
Extending the reaction time of the oxidation with selenium
dioxide, for example, for compound 2c, from 1.5 to 4.5 h
permitted both the oxidation and the cleavage of the boc
protecting group to compound 4c to occur.
Finally, click chemistry of 1-(4-fluorophenyl)-2-[2-(alkyl/
benzylamino)pyridin-4-yl]ethane-1,2-diones 4b and 4c and
o-phenylenediamines 5a-c led to quinoxalines 6f-k. The
limitation of route A is the introduction of the amino moiety
in the first steps of the synthesis, which requires the addition
and removal of a protecting group.
To overcome this drawback, we developed route B
(Scheme 3) which introduced the amino function at the
pyridine C2 position in the final step of the synthesis.
The key intermediates for this synthetic pathway toward

1130 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3
Koch et al.
Scheme 3. Synthesis of 3-(2-Alkyl/phenylalkylaminopyridin-4-yl)-2-(4-fluorophenyl)quinoxalines 6l-y (route B)a
a Reagents and conditions: (i) SeO2, AcOH, reflux temperature or 95 °C, 1.5 h; (ii) MeOH/AcOH (9:1), 160 °C, 5 min, 250 W, microwave irradiation;
(iii) Pd2(dba)3, BINAP, tBuONa, 3-methylbut-2-ylamine, toluene, 120 °C, 250 W, microwave irradiation; (iv) R1-NH2 (excess), 160 °C, sealed glass tube
(6l-v) or R1-NH2 (8 equiv), 135 °C, 1 h, 250 W, microwave irradiation (6w-y).
Scheme 4. Synthesis of Pyridopyrazine Derivatives 9a-ha
a Reagents and conditions: (i) MeOH/AcOH (9:1), 160 °C, 5 min, 250 W, microwave irradiation.
substituted 3-(2-alkyl/phenylalkylaminopyridin-4-yl)-2-(4-
fluorophenyl)quinoxalines 6l-y are 2-(4-fluorophenyl)-
3-(halogenopyridin-4-yl)quinoxalines 7a-d (Scheme 3),
which were prepared starting from the corresponding 1-(4-
fluorophenyl)-2-(2-halogenopyridin-4-yl)ethanones 2d and
2e followed by oxidation with selenium dioxide to diketones
3d and 3e followed by click chemistry with o-phenylenedia-
mines 5a-c.
The amino moiety was introduced at the pyridine C2
position via palladium-catalyzed aryl-C-N bond forma-
tion (Buchwald-Hartwig reaction) of bromo compound 7a
or via nucleophilic aromatic substitution of fluoro deriva-
tives 7b-d. Attempts to introduce the amino function via
Buchwald-Hartwig reaction14 under reflux conditions gave
no conversion, while conducting the same reaction under
microwave irradiation led to only poor yields (5%).
Hence, we endeavored to introduce the amino moieties by
nucleophilic aromatic substitution (route B). In a sealed glass
tube or in a microwave reactor, we heated 2-(4-fluorophenyl)-
3-(2-fluoropyridin-4-yl)quinoxaline derivatives 7b-d with an
excess of the appropriate amine. After the samples had cooled
to room temperature, the unreacted amine was removed and
the residue was purified by flash chromatography to yield
quinoxalines 6l-y in good yields. Using route B, various
substituted 3-(2-alkyl/phenylalkyl-aminopyridin-4-yl)-2-(4-
fluorophenyl)quinoxalines could be prepared starting from
1-(4-fluorophenyl)-2-(2-fluoropyridin-4-yl)ethanone (2e) via
oxidation to diketone 3e, click chemistry to quinoxalines
7b-d, and, finally, nucleophilic aromatic displacement of
the fluoro atom with amines in three steps.
(ii) Pyridinylpyridopyrazines. Replacing the o-phenylene-
diamines with R-diaminopyridines led to pyridopyrazine
derivatives 9a-h. The two regioisomers 9a and 9b were
obtained by reaction of diketone 3a and 2,3-diaminopyridine
(8a) (Scheme 4). These isomers were separated by flash
chromatography and characterized by X-ray analysis.15,16
Isomers 9c and 9d obtained by reaction of diketone 3a and
3,4-diaminopyridine (8b) could not be separated by flash
chromatography.
Click chemistry of diketone 4b, which bears an isopropyla-
mino moiety at the pyridine C2 position, with R-diaminopyri-
dines 8a and 8b yielded pyridinylpyridopyrazines 9e-h
(Scheme 4). Isomers 9e and 9f could be separated by flash
chromatography. Crystals suitable for X-ray analysis were
obtained by slow evaporation at rt of a solution of the first
eluted isomer 9e in diethyl ether and n-hexane. Compound 9e
is 3-(4-fluorophenyl)-2-(2-isopropylaminopyridin-4-yl)pyrido-
[2,3-b]pyrazine.17 As observed for compounds 9c and 9d which
bore no substituent at the pyridine C2 position, isomers 9g and
9h could not be separated by flash chromatography.
Biological Data. The inhibitory potency (as IC50) of the
title compounds was evaluated using a nonradioactive p38R

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3 1131
Table 1. Effect of Quinoxaline Core Substitution on the Biological
Activity of 2(3)-(4-Fluorophenyl)-3(2)-(pyridin-4-yl)quinoxalines 6a-e
Table 2. Biological Activity of 3(2)-(2-Alkyl/phenylalkylpyridin-4-yl)-
2(3)-(4-fluorophenyl)quinoxalines 6f, 6g, 6l, 6o-q, and 6s-w (variation
of moieties at amino function R1)
compound
R2
R3
IC50a (μM) for p38R
6a
6b
6c
6d
6e
H
H
3.15 ( 0.33
3.70 ( 0.35
39% at 10 μM
6.14 ( 0.77
CH3
Cl
CH3
Cl
OCH3
H
H
OCH3
33% at 10 μM
a Mean values ( the standard error of the mean (SEM) of three
experiments.
MAP kinase assay,18 wherein SB203580 (IC50 = 0.043 (
0.001 μM; n = 81) is used as a reference. The ability of test
compounds to compete with ATP (100 μM) for the ATP
binding site of the kinase correlates with the capacity of the
kinase to phosphorylate ATF-2, when incubated with ATP
and the test compound. Inhibition of JNK3 was assessed
using a nonradioactive JNK3 assay.19
(i) Pyridinylquinoxalines. The effect of introducing simple
substituents such as methyl, chloro, and methoxy at positions 6
and 7 of the quinoxaline core is shown in Table 1. The unsub-
stituted pyridinylquinoxaline, 2-(fluorophenyl)-3-(pyridin-4-yl)-
quinoxaline 6a, shows an IC50 value in the low micromolar
range. Introduction of small substituents like a methyl group is
tolerated. On the other hand, introduction of chloro (6c) or
bulkier substituents like methoxy (6d and 6e) led to inactive or
less active compounds as compared to unsubstituted quinoxaline
derivative 6a.
A broad survey of amino moieties introduced at the
2 position of the pyridine ring of the pyridinylquinoxaline
derivatives indicated that introduction of phenylethyl-
amino (6t), aliphatic amino (6f, 6l, 6q, and 6s), and 4-hydro-
xycyclohexylamino (6w) moieties at the pyridine C2 position
increased the inhibitory potency with respect to unsub-
stituted compound 6a (Table 2). Removal of the R-methyl
group resulted in significantly increased IC50 values, and
in a decline in the level of enzyme inhibition (compare 6t-v
vs 6g).
We attempted to optimize the p38R MAP kinase inhibitory
activity of aliphatic analogues 6f, 6l, 6q, and 6s, starting
from 6l, by the systemic, successive removal of a methyl
group (Figure 2). The removal of the R-methyl group shows
only a slight improvement in inhibitory activity (compare
6l vs 6s). Removal of the β-methyl group(s) from amino
substitutents decreased the IC50 value from 794 nM for
3-methylbut-2-yl derivative 6l to 114 nM for sec-butyl 6q, a
value that was then exceeded by that of isopropyl derivative
6f (IC50 = 81 nM).
Compounds with an S-configuration were more potent
than their counterparts with the R-configuration (compare
6l, 6o, 6p, and 6t-v). Benzylic compound 6g was not effective
(see Table 2).
Compounds listed in Table 3 have substituents both at the
pyridine C2 position and at positions 6 and 7 of the quinoxa-
line core. For only compounds with a benzylamino moiety at
the pyridine C2 position, the introduction of substituents at
positions 6 and 7 led to an increased inhibitory activity. For
all other compounds, the introduction of substituents at
a Mean values ( SEM of three experiments.
positions 6 and 7 of the heterocyclic core structure resulted
in a loss of inhibitory activity.
The SAR of the pyridinylquinoxaline derivatives 6a-y are
outlined in Figure 3. The introduction of an isopropylamino
moiety at the pyridine C2 position results in a 39-fold
increase in inhibitory activity compared to that of unsub-
stituted pyridinylquinoxaline 6a. In contrast to pyridinyli-
midazole derivatives 1, substitution of the core of the
pyridinylquinoxalines (R2 and R3) was not tolerated. For
example, for compounds bearing an isopropylamino moiety
at the pyridine C2 position, the introduction of two methyl

1132 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3
Koch et al.
Figure 2. Compound 6l as a starting point in the optimization of the aliphatic series.
Table 3. Biological Activity of 2(3)-(4-Fluorophenyl)-3(2)-(pyridin-
4-yl)quinoxalines 6h-k, 6m, 6n, 6r, 6x, and 6y (effect of quinoxaline
core substitution)
Figure 3. SAR of pyridinylquinoxaline derivatives 6.
Figure 4. After geometric optimization, inhibitor 6f was docked in
the p38R active center using an induced fit docking tool.21 As the
protein model, the X-ray structure of Protein Data Bank entry
1YWR22 was used. Possible hydrogen bonds and π-π stacking
interactions are shown as dashed lines.
Met109 in the hinge region. Another possible hydrogen bond
interaction could be formed between N1 of the quinoxaline
core and the side chain of Lys53. The 4-fluorophenyl ring
binds to hydrophobic region I (selectivity pocket), which is
mediated by the presence of the gatekeeper residue Thr106.
The isopropyl moiety interacts with hydrophobic region II,
leading to a gain in inhibitory activity. Another possible
ligand-protein interaction could be a π-π stacking between
the aromatic side chain of Tyr35 and the phenyl system of the
quinoxaline core.
a Mean values ( SEM of three experiments.
(ii) Pyridinylpyridopyrazines. The biological activities of
pyridinylpyridopyrazines 9a-d are listed in Table 4. Com-
pound 9a is the only isomer of this series showing an IC50
value in the low micromolar range comparable to that of 6a.
Introducing a nitrogen atom into the carbocyclic part of
the quinoxaline ring can influence the atomic electrostatic
potential of the nitrogen which forms a hydrogen bond (HB)
to Lys53. An increase in the electrostatic potential should
result in a stronger HB and in an increase in inhibitory
potency.
groups or chlorine atoms at positions 6 and 7 of the quinoxa-
line core results in a 3- or 5-fold decrease in activity,
respectively (compare 6f, 6h, and 6j).
The best compound of this series, 6f, was docked into the
ATP binding site of p38R MAP kinase. Possible interactions
between ATP competitive inhibitor 6f and the ATP binding
site, which is located in the cleft between the N- and
C-terminal domains of p38R MAP kinase, are depicted in
Figure 4. The nitrogen of the pyridin-4-yl moiety and the
amino function at the pyridine C2 position form crucial
hydrogen bonds to the backbone NH and CO groups of
With the Jaguar batch script hydrogen_bond.py (which is
available in the Jaguar package),20 we calculated the HB

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3 1133
binding energies from Lys53 to the quinoxaline and pyrido-
pyrazine derivatives 6a, 9a, and 9b. The three compounds
were first docked with an induced fit docking tool21 into
the X-ray structure of Protein Data Bank entry 1YWR22
(Figure 5). The best docking poses were used for further
calculations. For the calculation of the binding energies of
the hydrogen-bonded complexes, we selected the fast mode,
which uses the DFT energies instead of the LMP2 energies.
Additionally, all torsions were frozen during optimizations.
The HB binding energies of compounds 6a, 9a, and 9b are
-19.15, -24.73, and -13.30 kcal/mol, respectively (Table 5).
To compare the inhibitory activity of these three com-
pounds, we calculated the quotient (Q). Q is the ratio
between the IC50 of the reference compound of the kinase
assay (SB203580) and the IC50 of the tested compound.
3-(4-Fluorophenyl)-2-(pyridin-4-yl)pyrido[2,3-b]pyrazine
(9a), having the highest quotient (Q = 0.014), also exhibited
the highest HB binding energy between N4 and the ε-amino
group of the Lys53 side chain of p38R MAP kinase. Con-
versely, 9b with the lowest HB binding energy shows the
lowest inhibitory activity (Q = 0.004).
The biological activity of pyridinylpyridopyrazines 9e-h
bearing an isopropylamino moiety at the pyridine C2 posi-
tion is listed in Table 6. The SAR of compounds 9a-d with
respect to the position of the nitrogen atom of the pyrido-
pyrazine core can be transferred to the compounds of series
9e-h. Thus, the best compound of this series was 9e,
exhibiting an IC50 value in the low double-digit nanomolar
range.
A comparison to quinoxaline derivative 6f and pyrido-
pyrazine compounds 9e and 9f (Figure 6) emphasizes the
influence of the nitrogen atom in the carbocyclic part of the
quinoxaline ring controlling the inhibitory activity. These
data support the proposed hypothesis that increasing the
strength of the HB to Lys53 improves the potency of the
inhibitor.
Table 4. Biological Activity of Pyridinylpyridopyrazines 9a-d
Inhibition of p38r MAP Kinase versus JNK3. A compar-
ison of inhibition of p38R MAP kinase and JNK3 for selected
compounds as well as details of molecular geometry for com-
pounds with the 4-fluorophenyl/pyridin-4-yl pharmacophore
connected to five-membered (SB203580) or six-membered
Table 5. Hydrogen Bond (HB) Energies between the Heterocyclic Core
and the Amino Function of the Side Chain of Lys53
6a
-19.15 -24.73 -13.30
Q [IC50(SB203580)/IC50(test compound)] 0.010 0.014 0.004
9a
9b
HB energy (kcal/mol)
a Mean values ( SEM of three experiments.
Figure 5. After geometric optimization, compounds 6a, 9a, and 9b were docked in the p38R active center using an induced fit docking tool.21
For the protein model, we used the X-ray structure of Protein Data Bank entry 1YWR.22 Possible hydrogen bonds are shown as dashed lines,
and their lengths are given.

1134 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3
Koch et al.
heterocyclic core structures (quinoxaline derivatives 6a and 6f)
is given in Table 7.
4-fluorophenyl/pyridin-4-yl pharmacophore of the six-mem-
bered ring inhibitors (quinoxalines) and the larger gatekeeper
residue in JNK3 (Met146) could prevent appropriate binding
access of quinoxalines to JNK3 (Figure 7). Pyridinylquinoxa-
line 6f and pyridinylimidazole SB203580 have IC50 values for
inhibition of p38R MAP kinase in the double-digit nanomolar
range (Table 7). Perhaps because of these geometrical differ-
ences in the 4-fluorophenyl/pyridin-4-yl system, quinoxaline
derivative 6f shows a significant decrease in the level of
inhibition of JNK3 (IC50 = 4 μM) compared to that of
SB203580 (IC50 = 0.3 μM).
The six-membered heterocyclic quinoxaline core is fixing
the 4-fluorophenyl/pyridin-4-yl pharmacophore with exo-
cyclic bond angles between 123.3° and 125.3°. The distance
between the 4-fluorophenyl and pyridin-4-yl ring (tip to tip)
˚
ranges from 7.24 to 7.36 A. The five-membered imidazole
derivative SB203580 shows larger exocyclic angles (>131.4°)
and therefore a significantly longer distance (8.28 A) between
the two aromatic rings.
˚
The gatekeeper residue in p38R MAP kinase, Thr106, is
controlling the access of the 4-fluorophenyl ring to hydro-
phobic region I. The more closed conformation of the
Conclusion
In summary, we report short and straightforward syntheses
for producing differently substituted quinoxalines 6a-y and
pyridopyrazines 9a-h as novel p38R MAP kinase inhibitors.
The various substituted 3-(2-alkyl/phenylalkylaminopyridin-
4-yl)-2-(4-fluorophenyl)quinoxalines were prepared in three
steps starting from ethanone 2e (route B) via oxidation
to diketone 3e, click chemistry to quinoxalines 7b-d, and
nucleophilic aromatic displacement of the fluoro atom with
amines.
Table 6. Biological Activities of Isopropylamino-Substituted Pyridi-
nylpyridopyrazines 9e-h
Introduction of an amino function bearing a small aliphatic
moiety at the pyridine C2 position led to another hydrogen
bond to the hinge region as well as possible interaction with
hydrophobic region II.
In contrast to the pyridinylimidazole SB203580 which in-
hibits both p38R MAP kinase and JNK3, the pyridinylquinox-
alines show a clear decline in the level of inhibition of JNK3.
Introducing a nitrogen atom in the heterocyclic-free ring of
the quinoxaline core influences the strength of the HB binding
Table 7. Inhibition of p38R MAP Kinase/JNK3 for Quinoxaline Deri-
vatives 6a and 6f and Details of Molecular Geometry for Compounds
with the 4-Fluorophenyl/pyridin-4-yl Pharmacophore Connected to
Five- or Six-Membered Heterocyclic Core Structures
IC50a (μM)
˚
˚
compound
p38R
JNK3
31.8c
3.95 ( 0.2
a (A) b (A) R (deg) β (deg)
6a
6f
3.15 ( 0.33
0.081 ( 0.005
7.24d 1.39 125.2 125.3
7.36e 1.43 123.3 124.3
SB203580 0.043 ( 0.001b 0.290 ( 0.05 8.28f 1.49 131.5 131.5
a Mean values ( SEM of three experiments. b Mean values ( SEM of
81 experiments. c Percent inhibition at 10 μM. d Calculated on the basis
of minimization by force field. e See ref 17. f See ref 7.
a Mean values ( SEM of three experiments.
Figure 6. Comparison of compounds 6f, 9e, and 9f (influence of the nitrogen atom).

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3 1135
Figure 7. Comparison of possible interactions between the five-membered heterocyclic inhibitor SB203580 (left) and the six-membered
heterocyclic inhibitors (right) with the binding site of p38R MAP kinase (top) and JNK3 (bottom).
energy between the heterocyclic core and the amino function of
the side chain of Lys53 in the p38R MAP kinase. To this end,
3-(4-fluorophenyl)-2-(2-isopropylaminopyridin-4-yl)pyrido-
[2,3-b]pyrazine (9e) was identified as the most promising
kinase inhibitor.
In future attempts to prepare potent, specific kinase inhi-
bitors, insight from our proposed binding mode may aid the
design of other inhibitor structures, and the described synth-
eses will allow a rapid access to more pyridinylquinoxalines
and -pyridopyrazines for further optimization.
compressed air cooled the reaction vessel to rt. The solvent was
removed under reduced pressure, and the residue was purified
by flash chromatography.
(i) 2-(4-Fluorophenyl)-3-(pyridin-4-yl)quinoxaline (6a). Com-
pound 6a was prepared according to general procedure A from
compound 3a (137 mg, 0.6 mmol), o-phenylenediamine (64 mg,
0.6 mmol), and 6 mL of a methanol/glacial acetic acid mixture
(9:1). Purification: flash chromatography (SiO2, petroleum
ether/ethyl acetate, 2:1 to 1:2). Yield: 163 mg (90%), colorless
solid. 1HNMR(DMSO-d6):δ7.19-7.28 (m, 2H, C3/C5-H 4-F-Phe),
7.46 (dd, J1 =4.5Hz, J2 = 1.6 Hz, 2H, C3/C5-H Pyr), 7.51-7.58 (m,
2H, C2/C6-H 4-F-Phe), 7.91-7.98 (m, 2H, Quinox), 8.17-8.22 (m,
2H, Quinox), 8.60 (dd, J1 = 4.4 Hz, J2 = 1.6 Hz, 2H, C2/C6-H Pyr).
EI-HRMS: calcd for C19H12FN3 m/z 301.1015, observed m/z
301.1026.
Experimental Section
General. All commercially available reagents and solvents
were used without further purification. The microwave reaction
was performed on a CEM Discover system. NMR data were
obtained on a Bruker Spectrospin AC 200 instrument at ambi-
ent temperature. High-resolution spectral mass data were
obtained on a Thermo Finnigan TSQ70 instrument. The purity
of the final compounds was determined by HPLC on a Hewlett-
Packard HP 1090 Series II liquid chromatograph using a Betasil
C8 column [150 mm ꢀ 4.6 mm (inside diameter), dp = 5 μm,
(Thermo Fisher Scientific, Waltham, MA)] at 230 and 254 nm
employing a gradient of 0.01 M KH2PO4 (pH 2.3) and methanol
as the solvent system with a flow rate of 1.5 mL/min; all final
compounds have a purity of >96% (see the Supporting
Information for details).
General Procedure for the Preparation of Quinoxalines 6a-k
via Click Chemistry (general procedure A). The R-diketone
(1 equiv), o-phenylenediamine (1 equiv), and a methanol/glacial
acetic acid mixture (9:1, v:v) were combined in a reaction vial.
The reaction vessel was heated in a microwave reactor for 5 min
at 160 °C (initial power of 250 W), after which a stream of
(ii) 3-(4-Fluorophenyl)-6-methoxy-2-(pyridin-4-yl)quinoxaline
(6d) and 2-(4-Fluorophenyl)-6-methoxy-3-(pyridin-4-yl)quinoxa-
line (6e). Compounds 6d and 6e were prepared according to
general procedure A from compound 3a (137 mg, 0.6 mmol),
4-methoxy-o-phenylenediamine (82 mg, 0.6 mmol), and 6 mL of
a methanol/glacial acetic acid mixture. Purification: flash chro-
matography (SiO2, petroleum ether/ethyl acetate, 2:1 to 1:2).
Yield of 6d: 82 mg (41%), colorless solid. Yield of 6e: 73 mg
(37%), colorless solid.
1
6d. H NMR (DMSO-d6): δ 3.98 (s, 3H, OCH3), 7.18-7.27
(m, 2H, C3/C5-H 4-F-Phe), 7.40-7.58 (m, 6H, C2/C6-H
4-F-Phe, C5/C7-H Quinox, C3/C5-H Pyr), 8.04-8.09 (m, 1H,
C8-H Quinox), 8.56-8.59 (m, 2H, C2/C6-H Pyr). EI-HRMS:
calcd for C20H14FN3O m/z 331.1120, observed m/z 331.1110.
The crystal structure of 6d was determined by X-ray analysis:
Enraf-Nonius CAD-4, Cu KR, SIR92, SHELXL97. Further
details of the crystal structure analysis are available in ref 13.
1
6e. H NMR (DMSO-d6): δ 3.98 (s, 3H, OCH3), 7.18-7.23
(m, 2H, C3/C5-H 4-F-Phe), 7.41-7.58 (m, 6H, C2/C6-H

1136 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3
Koch et al.
4-F-Phe, C5/C7-H Quinox, C3/C5-H Pyr), 8.05-8.10 (m, 1H,
C8-H Quinox), 8.55-8.58 (m, 2H, C2/C6-H Pyr). EI-HRMS:
calcd for C20H14FN3O m/z 331.1120, observed m/z 331.1103.
(iii) 4-[3-(4-Fluorophenyl)quinoxalin-2-yl]-N-isopropylpyri-
din-2-amine (6f). Compound 6f was prepared according to
general procedure A from 4b (44 mg, 0.15 mmol), o-phenylene-
diamine (17 mg, 0.15 mmol), and 2 mL of a methanol/glacial
acetic acid mixture. Purification: flash chromatography (SiO2,
petroleum ether/ethyl acetate, 3:1 to 1:1). Yield: 55 mg (99%),
colorless solid. 1H NMR (DMSO-d6): δ 1.09 (d, J = 6.4 Hz, 6H,
2ꢀ CH3), 3.83-4.00 (m, 1H, CH), 6.35 (dd, J1 = 5.2 Hz, J2 =
1.2 Hz, 1H, C5-H Pyr), 6.47 (d, J = 7.7 Hz, 1H, NH), 6.64 (s, 1H,
C3-H Pyr), 7.19-7.29 (m, 2H, C3/C5-H 4-F-Phe), 7.55-7.63
(m, 2H, C2/C6-H 4-F-Phe), 7.87-7.91 (m, 3H, C6/C7-H Quinox,
C6-H Pyr), 8.11-8.17 (m, 2H, C5/C8-H Quinox). EI-HRMS:
calcd for C22H19FN4 m/z 358.1594, observed m/z 358.1592.
The crystal structure of 6f was determined by X-ray analysis:
Enraf-Nonius CAD-4, Cu KR, SIR92, SHELXL97. Further
details of the crystal structure analysis are available in ref 17.
General Procedure for the Preparation of Quinoxalines 6l-v
via Nucleophilic Aromatic Displacement (general procedure B).
Compound 7b, 7c, or 7d and amine compound (excess) were
heated in a sealed glass tube at 160 °C. After the mixture had
been cooled to rt, the amine was evaporated and the residue was
purified by flash chromatography.
1:1). Yield: 109 mg (94%). 1H NMR (DMSO-d6): δ 0.82 (t, J =
6.7 Hz, 3H, C4H3 sec-butyl), 1.04 (d, J = 5.3 Hz, 3H, C1H3-
butylamine), 1.34-1.50 (m, 2H, C3H2 sec-butyl), 3.69-3.76 (m,
1H, C2H sec-butyl), 6.35-6.40 (m, 2H, NH, C5-H Pyr), 6.62 (s,
1H, C3-H Pyr), 7.19-7.27 (m, 2H, C3/C5-H 4-F-Phe), 7.55-7.61
(m, 2H, C2/C3-H 4-F-Phe), 7.88-7.90 (m, 3H, C6-H Pyr, C6/C7-
H Quinox), 8.12-8.15 (m, 2H, C4/C8-H Quinox). EI-HRMS:
calcd for C23H21FN4 m/z 372.1750, observed m/z 372.1732.
(v) N-sec-Butyl-4-[3-(4-fluorophenyl)-6,7-dimethylquinoxalin-
2-yl]pyridin-2-amine (6r). Compound 6r was prepared according
to general procedure B from 7c (100 mg, 0.29 mmol) and sec-
butylamine (0.47 mL, 4.6 mmol). Reaction time: 15 h. Purifica-
tion: flash chromatography (SiO2, petroleum ether/ethyl acet-
ate, 3:1 to 1:1). Yield: 101 mg (87%). 1H NMR (acetone-d6): δ
0.85-0.93 (m, 3H, C4H3 sec-butyl), 1.10-1.13 (m, 3H, C1H3
sec-butyl), 1.41-1.55 (m, 2H, C3H2 sec-butyl), 2.49 (s, 6H, 2ꢀ
CH3, Quinox), 3.76-3.83 (m, 1H, C2H sec-butyl), 5.58-5.62
(m, 1H, NH), 6.52-6.54 (m, 1H, C5-H Pyr), 6.60 (s, 1H, C3-H
Pyr), 7.10-7.18 (m, 2H, C3/C5-H 4-F-Phe), 7.58-7.64 (m, 2H,
C2/C3-H 4-F-Phe), 7.82 (s, 2H, C5/C8-H Quinox), 7.95-7.97 (m,
1H, C6-H Pyr). EI-HRMS: calcd for C25H25FN4 m/z 400.2063,
observed m/z 400.2057.
(vi) 4-[3-(4-Fluorophenyl)quinoxalin-2-yl]-N-isobutylpyridin-
2-amine (6s). Compound 6s was prepared according to general
procedure B from 7b (100 mg, 0.31 mmol) and isobutylamine
(0.52 mL, 5.0 mmol). Reaction time: 15 h. Purification: flash
chromatography (SiO2, petroleum ether/ethyl acetate, 3:1 to
1:1). Yield: 101 mg (87%). 1H NMR (DMSO-d6): δ 0.84 (d, J =
6.6 Hz, 6H, 2ꢀ CH3 isobutyl), 1.66-1.72 (m, 1H, C2H isobutyl),
2.84-3.00 (m, 2H, C1H2 isobutyl), 6.40-6.43 (m, 1H, NH), 6.59
(s, 1H, C3-H Pyr), 6.64-6.70 (m, 1H, C5-H Pyr), 7.19-7.28 (m,
2H, C3/C5-H 4-F-Phe), 7.54-7.61 (m, 2H, C2/C6-H Phe),
7.87-7.92 (m, 3H, 2ꢀ C-H Quinox, C6-H Pyr), 8.12-8.17 (m,
2H, 2ꢀ C-H Quinox). EI-HRMS: calcd for C23H21FN4 m/z
372.1750, observed m/z 372.1725.
General Procedure for the Preparation of Quinoxalines 6w-y
via Nucleophilic Aromatic Displacement (general procedure C).
Compound7b, 7c, or7dand 4-trans-aminocyclohexanol (8 equiv)
were combined in a reaction vial. The reaction vessel was heated
in a microwave reactor for 1 h at 135 °C (initial power of 250 W),
after which a stream of compressed air cooled the reaction vessel
tort. The reactionmixture was purified byflash chromatography.
(i) 4-{4-[3-(4-Fluorophenyl)quinoxalin-2-yl]pyridin-2-ylamin}-
cyclohexanol (6w). Compound 6w was prepared according to
general procedure C from 7b (167 mg, 0.52 mmol) and 4-trans-
aminocyclohexanol (0.48 g, 4.18 mmol). Purification: flash
chromatography (SiO2, petroleum ether/ethyl acetate, 1:1 to
0:1). Yield: 180 mg (83%). 1H NMR (DMSO-d6): δ 1.11-1.23
(m, 4H, 2ꢀ CH2 hydroxycyclohexyl), 1.79-1.83 (m, 4H, 2ꢀ
CH2 hydroxycyclohexyl), 3.40-3.58 (m, 2H, 2ꢀ CH hydroxy-
cyclohexyl, H2O in DMSO), 6.37-6.46 (m, 2H, NH, C5-H Pyr),
6.59 (s, 1H, C3-H Pyr), 7.23 (m, 2H, C3/C5-H 4-F-Phe),
7.54-7.61 (m, 2H, C2/C6-H 4-F-Phe), 7.85-7.91 (m, 3H, C6-
H Pyr, Quinox), 8.10-8.16 (m, 2H, Quinox). EI-HRMS: calcd
for C25H23FN4O m/z 414.1856, observed m/z 414.1866.
General Procedure for the Preparation of Pyridopyrazines
9a-h via Click Chemistry (general procedure D). The R-diketone
(1 equiv), the R-diaminopyridine (1 equiv), and a methanol/
glacial acetic acid mixture (9:1, v:v) were combined in a reaction
vial. The reaction vessel was heated in a microwave reactor for
5 min at 160 °C (initial power of 250 W), after which a stream of
compressed air cooled the reaction vessel to rt. The solvent was
removed under reduced pressure, and the residue was purified
by flash chromatography.
(i) 4-[3-(4-Fluorophenyl)quinoxalin-2-yl]-N-(3-methylbutan-2-
yl)pyridin-2-amine (6l). Compound 6l was prepared according
to general procedure B from 7b (76.5 mg, 0.24 mmol) and
3-methylbutan-2-amine (0.44 mL, 3.8 mmol). Reaction time:
15 h. Purification: flash chromatography (SiO2, petroleum
1
ether/ethyl acetate, 3:1 to 1:1). Yield: 90 mg (97%). H NMR
(CDCl3): δ 0.86-0.92 (m, 6H, 2ꢀ CH3), 1.05 (d, J = 6.6 Hz, 3H,
CH3), 1.59-1.72 (m, 1H, CH), 3.38-3.48 (m, 1H, CH), 4.59
(d, J = 8.6 Hz, 1H, NH), 6.43 (s, 1H, C3-H Pyr), 6.65-6.69 (m,
1H, C5-H Pyr), 7.04-7.13 (m, 2H, C3/C5-H 4-F-Phe), 7.55-7.62
(m, 2H, C2/C6-H 4-F-Phe), 7.78-7.84 (m, 2H, C6/C7-H
Quinox), 8.07 (d, J = 5.2 Hz, 1H, C6-H Pyr), 8.15-8.20 (m,
2H, C5/C8-H Quinox). EI-HRMS: calcd for C24H23FN4 m/z
386.1907, observed m/z 386.1890.
(ii) 4-[3-(4-Fluorophenyl)quinoxalin-2-yl]-N-[(R)-3-methylbutan-
2-yl]pyridin-2-amine (6o). Compound 6o was prepared according
to general procedure B from 7b (100 mg, 0.31 mmol) and (R)-(-)-
3-methylbutan-2-amine (1.2 mL, 10 mmol). Reaction time: 16 h.
Purification: flash chromatography (SiO2, petroleum ether/ethyl
acetate, 3:1 to 1:1). Yield: 117 mg (99%). 1H NMR (DMSO-d6): δ
0.79-0.85 (m, 6H, 2ꢀ CH3), 0.96-0.99 (m, 3H, CH3), 1.64-1.73
(m, 1H, CH), 3.67-3.74 (m, 1H, CH), 6.34-6.45 (m, 2H, NH,
C5-H Pyr), 6.64 (s, 1H, C3-H Pyr), 7.17-7.26 (m, 2H, C3/C5-H
4-F-Phe), 7.53-7.60 (m, 2H, C2/C6-H 4-F-Phe), 7.84-7.89 (m,
3H, C6-H Pyr, C6/C7-H Quinox), 8.10-8.15 (m, 2H, C5/C8-H
Quinox). EI-HRMS: calcd for C24H23FN4 m/z 386.1907, observed
m/z 386.1885.
(iii) 4-[3-(4-Fluorophenyl)quinoxalin-2-yl]-N-[(S)-3-methylbutan-
2-yl]pyridin-2-amine (6p). Compound 6p was prepared according
to general procedure B from 7b (100 mg, 0.31 mmol) and (S)-(þ)-
3-methylbutan-2-amine (0.56 mL, 5 mmol). Reaction time: 15 h.
Purification: flash chromatography (SiO2, petroleum ether/ethyl
acetate, 3:1 to 1:1). Yield: 73 mg (60%). 1H NMR (DMSO-d6): δ
0.79-0.85 (m, 6H, 2ꢀ CH3), 0.96-0.99 (m, 3H, CH3), 1.64-1.73
(m, 1H, CH), 3.67-3.74 (m, 1H, CH), 6.34-6.45 (m, 2H, NH, C5-
H Pyr), 6.64 (s, 1H, C3-H Pyr), 7.17-7.26 (m, 2H, C3/C5-H 4-F-
Phe), 7.53-7.60 (m, 2H, C2/C6-H 4-F-Phe), 7.84-7.89 (m, 3H, C6-
HPyr, C6/C7-H Quinox), 8.10-8.15(m, 2H, C5/C8-HQuinox). EI-
HRMS: calcd for C24H23FN4 m/z 386.1907, observed m/z
386.1892.
(i) 3-(4-Fluorophenyl)-2-(pyridin-4-yl)pyrido[2,3-b]pyrazine (9a)
and 2-(4-Fluorophenyl)-3-(pyridin-4-yl)pyrido[2,3-b]pyrazine (9b).
Compounds 9a and 9b were prepared according to general
procedure D from compound 3a (113 mg, 0.5 mmol), 2,3-diami-
nopyridine (54 mg, 0.5 mmol), and 2 mL of a methanol/glacial
acetic acid mixture (9:1). Isomers 9a and 9b were separated by
(iv) N-sec-Butyl-4-[3-(4-fluorophenyl)quinoxalin-2-yl]pyridin-
2-amine (6q). Compound 6q was prepared according to general
procedure B from 7b (100 mg, 0.31 mmol) and sec-butylamine
(0.51 mL, 5 mmol). Reaction time: 13 h. Purification: flash
chromatography (SiO2, petroleum ether/ethyl acetate, 3:1 to

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 3 1137
flash chromatography (SiO2, petroleum ether/ethyl acetate, 1:4 to
0:1). Yield of 9a: 67 mg (44%), colorless solid. Yield of 9b: 65 mg
(43%), colorless solid.
(2) Kumar, S.; Boehm, J.; Lee, J. C. p38 MAP kinases: Key signalling
molecules as therapeutic targets for inflammatory diseases. Nat.
Rev. Drug Discovery 2003, 2, 717–726.
(3) Kaminska, B. MAPK signalling pathways as molecular targets
for anti-inflammatory therapy: From molecular mechanisms
to therapeutic benefits. Biochim. Biophys. Acta 2005, 1754, 253–
262.
(4) Koch, P.; Baeuerlein, C.; Jank, H.; Laufer, S. Targeting the ribose
and phosphate binding site of p38 mitogen-activated protein
(MAP) kinase: Synthesis and biological testing of 2-alkylsulfanyl-,
4(5)-aryl-, 5(4)-heteroaryl-substituted imidazoles. J. Med. Chem.
2008, 51, 5630–5640.
9a. 1H NMR (DMSO-d6): δ 7.00-7.08 (m, 2H, C3/C5-H
4-F-Phe), 7.43-7.46 (m, 2H, C3/C5-H Pyr), 7.57-7.63 (m, 2H,
C2/C6-H 4-F-Phe), 7.72-7.78 (m, 1H, C7-H pyridopyrazine),
8.48-8.53 (m, 1H, C8-H pyridopyrazine), 8.64 (d, J = 5.7 Hz,
2H, C2/C6-H Pyr), 9.19-9.22 (m, 1H, C6-H pyridopyrazine).
EI-HRMS: calcd for C18H11FN4 m/z 302.0968, observed m/z
302.0930.
The crystal structure of 9a was determined by X-ray analysis:
Enraf-Nonius CAD-4, Cu KR, SIR92, SHELXL97. Further
details of the crystal structure analysis are available in ref 15.
9b. 1H NMR (DMSO-d6): δ 7.03-7.12 (m, 2H, C3/C5-H
4-F-Phe), 7.49-7.56 (m, 4H, C3/C5-H Pyr, C2/C6-H 4-F-Phe),
7.73-7.80 (m, 1H, C7-H pyridopyrazine), 8.49-8.54 (m, 1H,
C8-H pyridopyrazine), 8.61-8.64 (m, 2H, C2/C6-H Pyr),
9.19-9.21 (m, 1H, C6-H pyridopyrazine). EI-HRMS: calcd
for C18H11FN4 m/z 302.0968, observed m/z 302.0984.
The crystal structure of 9b was determined by X-ray analysis:
Enraf-Nonius CAD-4, Cu KR, SIR92, SHELXL97. Further
details of the crystal structure analysis are available in ref 16.
(ii) 3-(4-Fluorophenyl)-2-(2-isopropylaminopyridin-4-yl)pyrido-
[2,3-b]pyrazine (9e) and 2-(4-Fluorophenyl)-3-(2-isopropylamino-
pyridin-4-yl)pyrido[2,3-b]pyrazine (9f). Compounds 9e and 9f
were prepared according to general procedure D from compound
4b (115 mg, 0.4 mmol), 2,3-diaminopyridine (44 mg, 0.4 mmol),
and 3 mL of a methanol/glacial acetic acid mixture (9:1). Isomers
9e and 9f were separated by flash chromatography (SiO2, petro-
leum ether/ethyl acetate, 1:1 to 1:4). Yield of 9e: 46 mg (33%),
yellow solid. Yield of 9f: 62 mg (43%), yellow solid.
(5) Laufer, S.; Koch, P. Towards the improvement of the synthesis of
novel 4(5)-aryl-5(4)-heteroaryl-2-thio-substituted imidazoles and
their p38 MAP kinase inhibitory activity. Org. Biomol. Chem. 2008,
6, 437–439.
(6) Wagner, G.; Laufer, S. Small molecular anti-cykotine agents. Med.
Res. Rev. 2006, 26, 1–62.
(7) Peifer, C.; Kinkel, K.; Abadleh, M.; Schollmeyer, D.; Laufer, S.
From five- to six-membered rings: 3,4-Diarylquinolinone as lead
for novel p38MAP kinase inhibitors. J. Med. Chem. 2007, 50, 1213–
1221.
(8) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click chemistry: Diverse
chemical function from a few good reactions. Angew. Chem., Int.
Ed. 2001, 40, 2004–2021.
(9) Li, Z.; Li, W.; Sun, Y.; Huang, H.; Ouyang, P. Room temperature
facile synthesis of quinoxalines catalyzed by amidosulfonic acid.
J. Heterocycl. Chem. 2008, 45, 285–288.
(10) More, S. V.; Sastry, M. N. V.; Wang, C. C.; Yao, C. F. Molecular
iodine. A powerful catalyst for the easy and efficient synthesis of
quinoxalines. Tetrahedron Lett. 2005, 46, 6345–6348.
(11) More, S. V.; Sastry, M. N. V.; Yao, C. F. Cerium(IV) ammonium
nitrate (CAN) as a catalyst in tap water: A simple, efficient and
green approach to the synthesis of quinoxalines. Green Chem. 2006,
8, 91–95.
(12) Zhao, Z.; Wisnoski, D. D.; Wolkenberg, S. E.; Leister, W. H.;
Wang, Y.; Lindsley, C. W. General microwave-assisted protocols
for the expedient synthesis of quinoxalines and heterocyclic pyr-
azines. Tetrahedron Lett. 2004, 45, 4873–4876.
(13) Jahns, H.; Koch, P.; Schollmeyer, D.; Laufer, S. 3-(4-
Fluorophenyl)-6-methoxy-2-(4-pyridyl)quinoxaline. Acta Crystal-
logr. 2009, E65, o1626.
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development of practical catalysts for aromatic carbon-nitrogen
bond formation. Acc. Chem. Res. 1998, 31, 805–818.
(15) Koch, P.; Schollmeyer, D.; Laufer, S. 3-(4-Fluorophenyl)-
2-(4-pyridyl)pyrido[2,3-b]pyrazine. Acta Crystallogr. 2009, E65,
o2546.
(16) Koch, P.; Schollmeyer, D.; Laufer, S. 2-(4-Fluorophenyl)-
3-(4-pyridyl)pyrido[2,3-b]pyrazine. Acta Crystallogr. 2009, E65,
o2512.
9e. 1H NMR (DMSO-d6): δ 1.16 (d, J = 6.3 Hz, 6H, 2ꢀ CH3),
3.69-3.85 (m, 1H, CH), 4.55 (d, J = 7.8 Hz, 1H, NH), 6.51 (s,
1H, C3 Pyr), 6.63 (d, J = 5.2 Hz, 1H, C5 Pyr), 7.02-7.10 (m, 2H,
C3/C5-H 4-F-Phe), 7.66-7.77 (m, 3H, C7-H pyridopyrazine, C2/
C6-H 4-F-Phe), 8.08 (d, J = 5.2 Hz, 1H, C6-H Pyr), 8.49-8.54
(m, 1H, C8-H pyridopyrazine), 9.18-9.21 (m, 1H, C6-H
pyridopyrazine). EI-HRMS: calcd for C21H18FN5 m/z 359.1546,
observed m/z 359.1529.
The crystal structure of 9e was determined by X-ray analysis:
Enraf-Nonius CAD-4, Cu KR, SIR92, SHELXL97. Further
details of the crystal structure analysis are available in ref 17.
9f. 1H NMR (DMSO-d6): δ 1.17 (d, J = 6.2 Hz, 6H, 2ꢀ CH3),
3.70-3.86 (m, 1H, CH), 4.60 (d, J = 7.6 Hz, 1H, NH), 6.59 (d,
J = 5.2 Hz, 1H, C5 Pyr), 6.69 (s, 1H, C3 Pyr), 7.05-7.13 (m, 2H,
C3/C5-H 4-F-Phe), 7.58-7.64 (m, 2H, C2/C6-H 4-F-Phe),
7.72-7.78 (m, 1H, C7-H pyridopyrazine), 8.00 (d, J = 5.0 Hz,
1H, C6-H Pyr), 8.49-8.53 (m, 1H, C8-H pyridopyrazine),
9.18-9.20 (m, 1H, C6-H pyridopyrazine). EI-HRMS: calcd
for C21H18FN5 m/z 359.1546, observed m/z 359.1523.
(17) Koch, P.; Schollmeyer, D.; Laufer, S. 4-[3-(4-Fluorophenyl)quino-
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