Design, synthesis and biological evaluation of dihydroquinoxalinone derivatives as BRD4 inhibitors

Article abstract of DOI:10.1016/j.bioorg.2016.08.009

BRD4 plays a key role in transcriptional regulation. Recent biological and pharmacological studies have demonstrated that bromodomain-containing protein 4 (BRD4) is a viable drug target for cancer treatment. In this study, we synthesized a series of dihydroquinoxalinone derivatives and evaluated their BRD4 inhibitory activities, obtaining compound 5i with IC₅₀ value of 73?nM of binding activity in BRD4(1) and 258?nM of cellular activity in MV-4-11 cancer cell lines. Docking studies were performed to explain the structure-activity relationship. Based on its potent biochemical and anti-proliferative activity, the novel BRD4 inhibitor compound 5i, is a promising lead compound for further investigation.

Full text of DOI:10.1016/j.bioorg.2016.08.009

Bioorganic Chemistry 68 (2016) 236–244
Contents lists available at ScienceDirect
Bioorganic Chemistry
journal homepage: www.elsevier.com/locate/bioorg
Design, synthesis and biological evaluation of dihydroquinoxalinone
derivatives as BRD4 inhibitors
Yifei Yang a, Leilei Zhao b, Bin Xu b, LingYun Yang b, Jian Zhang b, Huibin Zhang b, Jinpei Zhou a,
⇑
a Department of Medicinal Chemistry, China Pharmaceutical University, 24 Tongjiaxiang, Nanjing 210009, PR China
b Center of Drug Discovery, State Key Laboratory of Natural Medicines, China Pharmaceutical University, 24 Tongjiaxiang, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
BRD4 plays a key role in transcriptional regulation. Recent biological and pharmacological studies have
demonstrated that bromodomain-containing protein 4 (BRD4) is a viable drug target for cancer treat-
ment. In this study, we synthesized a series of dihydroquinoxalinone derivatives and evaluated their
BRD4 inhibitory activities, obtaining compound 5i with IC50 value of 73 nM of binding activity in BRD4
(1) and 258 nM of cellular activity in MV-4-11 cancer cell lines. Docking studies were performed to
explain the structure-activity relationship. Based on its potent biochemical and anti-proliferative activity,
the novel BRD4 inhibitor compound 5i, is a promising lead compound for further investigation.
Ó 2016 Elsevier Inc. All rights reserved.
Received 17 June 2016
Revised 22 August 2016
Accepted 23 August 2016
Available online 24 August 2016
Keywords:
BRD4 inhibitors
Cancer
Molecular docking
Dihydroquinoxalinone derivatives
1. Introduction
of chromatin [17]. Interestingly, BRD4 interacts with P-TEFb
through a conserved P-TEFb-interacting domain and promotes
Bromodomain-containing protein 4 (BRD4) is a member of the
bromodomain and extra-terminal domain (BET) family of proteins
which can activate transcription through recognizing specific e-N-
phosphorylation of RNA Pol II to resume transcription elongation
[21,23,24]. In addition, BRD4 mediates transcriptional activation
of target genes such as the MYC oncogene [25–27]. Above all,
BRD4 bromodomain protein has developed to be an interesting
drug target for the treatment of cancer [28–30], obesity [31],
kidney disease [32], lung fibrosis [33] and other inflammatory
diseases [34].
acetyl modified lysine residues found within histone tails [1–3]. As
known, acetylation of histones lysine residues plays an essential
role in the epigenetic regulation of gene expression [4]. Lots of
studies have revealed the conserved roles of the BET protein in
gene expression regulation [5–8]. Regulating function of BET bro-
modomains to modulate the gene expression has become a hot
research field [9]. To date, there are plenty of crystal structures
of BRDs for BET family protein uncovered in the Protein Data Bank
(http://www.rcsb.org.pdb) [10–14]. The BET family includes BRD2,
BRD3, BRD4, and BRDT, each of which contains two bromodomain
modules [15,16]. There are obvious structural similarities among
However, JQ1, a potent BRD4 inhibitor, has serious preclinical
toxicity in vivo and was used as a tool compound [35]. Therefore,
novel BRD4 inhibitors with excellent ADMET properties
were needed. Recently several reports described 3,5-dimethyli-
soxazoles as potent inhibitors of BRD4 [36–47], including com-
pound 28a with IC50 value of 180 nM of binding activity [48]. In
this study, compound 28a was docked into BRD4 crystal complex
(PDB id: 3P5O) and the docking conformation was analyzed in
detail. Its 3,5-dimethylisoxazole as an acetylated lysine mimic
anchored into Kac pocket through a directed hydrogen bond inter-
action formed with the conserved Asn140 at the bottom of the
pocket (Fig. 1A). From the docking conformation, we also found
that the 4-cyano benzyl of compound 28a was able to interact with
WPF shelf composed of W81, P82 and F83. However, the com-
pound 28a fails to interact with the waters around ZA channel
region and the 1-bromobenzyl substitution was far from the WPF
region. In addition, Rooney et al. employ a dihydroquinoxalinone
scaffold as the KAc mimic in a series of CREBBP bromodomain
ligands. The amide of the dihydroquinoxalinone formed two
the BRDs, such as a left-handed four-helix bundle (
aA, aB, aC,
aZ) [17]. The BET proteins have emerged as promising drug targets
and play important roles in human disease, including viral infec-
tions and cancers [18–20].
Bromodomain protein 4 (BRD4) is considered as the best stud-
ied member of the BET family [21]. It is a nuclear protein that plays
a vital role in maintaining chromatin architecture [22]. Impor-
tantly, BRD4 remains related with chromatin throughout the cell
cycle. Therefore BRD4 could directly keep higher-order structure
⇑
Corresponding author.
E-mail address: jpzhou668@163.com (J. Zhou).
http://dx.doi.org/10.1016/j.bioorg.2016.08.009
0045-2068/Ó 2016 Elsevier Inc. All rights reserved.

Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
237
Fig. 1. (A) Docking conformation of compound 28a in BRD4 protein (PDB id: 3P5O). (B) Docking conformation of compound 5a in BRD4 protein (PDB id: 3P5O). (C)
Superimposition docking conformation of compound 28a (Green) and compound 5a (Blue) in BRD4 protein (PDB id: 3P5O). (D) Superimposition docking conformation of
compound 5a (Blue) and compound 5d (Yellow) in BRD4 protein (PDB id: 3P5O).
hydrogen bonds with N1168, and the methyl group resided at the
base of the Kac binding pocket [49]. On the basis of that analysis,
the 2,3-dihydro-1H-benzo[d]imidazole core was replaced by dihy-
droquinoxalinone skeleton to build interaction with ZA Channel
region (Fig. 1B). Meanwhile the interaction between the 1-
bromobenzyl substitution of dihydroquinoxalinone and WPF shelf
was strengthened. Most of these dihydroquinoxalinone analogues
were proved to be novel BRD4 inhibitors by biochemical activity
and cellular activity in vitro. Among all the synthesized com-
pounds, compound 5i with excellent BRD4 inhibition activity
(70 nM) and anti-proliferative activity (258 nM) was considered
to be a promising lead compound worthy of further investigation.
tion and Suzuki coupling reaction to give the target compound
5a–k.
The dihydroquinoxalinone derivative 8 was synthesized start-
ing from intermediate 3a as shown in Scheme 2. The intermediate
6 was obtained by methylation reaction using starting material 3a
and iodomethane. The intermediate 6 was further treated with
para-methoxybenzaldehyde in tetrahydrofuran at room tempera-
ture to prepare the key intermediate 7. Then compound 8 was
obtained
by
Suzuki
coupling
reaction
using
3,5-
dimethylisoxazole-4-boronic acid pinacol ester.
2.2. Biological activity and SAR study
2. Results and discussion
In order to improve the BRD4 inhibitory activity, the dihydro-
quinoxalinone skeleton was designed and synthesized to replace
the benzimidazolone core of compound 28a. The binding activity
was investigated by an AlphaScreen assay to prove the rational
structure-activity relationship (Table 1). Compound 28a was used
as positive control. The binding activities are presented in Table 1.
The initially benzimidazolone core of compound 28a was replaced
by dihydroquinoxalinone skeleton to afford compound 5a with
binding activity (IC50 = 75 nM). This demonstrated that dihydro-
quinoxalinone skeleton was feasible as a core of BRD4 inhibitors.
Then various bromobenzyl or naphthene substituents were
adopted to explore structure-activity relationship at WPF shelf of
BRD4. The result showed that benzyl group gives a relatively better
activity than aliphatic ring when interacting with BRD4 WPF shelf.
Meanwhile, benzyl group with electron-donating substituents (5b)
exhibited a better effect on inhibition than benzyl group with
electron-withdrawing substituents (5c). This series was docked
into BRD4(1) protein (PDB id: 3P5O) by Glide docking protocol in
2.1. Chemistry
The general schemes for synthesis of dihydroquinoxalinone
derivatives 5a–k and 8 are summarized in Scheme 1 and 2. The
structures of dihydroquinoxalinone derivatives were confirmed
by 1H NMR and 13C NMR spectrum, mass spectrometry, and IR
spectrum. Purification by column chromatography was carried
out over silica gel (200–300 mesh) and checked for purity using
HPLC before being tested in biological evaluation (purity was
>97%).
As shown in Scheme 1, the commercially starting material com-
pound 1 was treated with different amino acid to afford the desired
intermediates 2a–g by a substitution reaction. Then the intermedi-
ates 2a–g were further converted to intermediates 3a–g by reduc-
tion and cyclization with sodium hydrosulfite in water at room
temperature. The obtained 3a–g were treated by reductive amina-

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Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
Scheme 1. Reagents and conditions: (a) K2CO3, amino acid, H2O, EtOH, reflux; (b) K2CO3, H2O, RT; (c) Na2S2O4, H2O, RT; (d) dibutyltin dichloride, Phenylsilane,
benzaldehydes/cyclic ketones, THF, RT; (e) phenylboronic acids, Pd(PPh3)4, Na2CO3, N2, H2O/EtOH/Toluene, 80 °C
Scheme 2. Reagents and conditions: (f) NaH, DMF, CH3I, 0 °C; (d) dibutyltin dichloride, phenylsilane, p-methoxybenzaldehyde, THF, RT; (e) phenylboronic acids, Pd(P(Ph)3)4,
Na2CO3, N2, H2O/EtOH/Toluene, 80 °C.
Table 1
Structures and BRD4-BD1 inhibitory effects of compounds 5a–e and compound 8.
Compound
Conformation
R
R2
R3
H
BRD4(1) IC50 (l
mol/L)b
28ac
5a
0.190 0.013
0.075 0.021
5b
R
H
0.160 0.011
5c
5d
5e
R
R
R
H
H
H
0.440 0.024
0.370 0.017
NAa
8
R
0.220 0.015
a
b
c
Mean of the BRD4-BD1 inhibitory activity IC50 > 1 lm.
IC50 values for BRD4-BD1 activities presented are the mean SD values of three independent determinations.
Used as positive control.

Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
239
Table 2
of methyl group at R3 in compound 8 also weakens the binding
efficiency.
Structures and BRD4-BD1 inhibitory effects of compounds 5f–k.
Based on these results above, we explored various substituent
groups into the R2 position of the lead compound 5a to obtain com-
pound 5f–i (Table 2). Gratifyingly, compound 5i exhibited a slight
improvement on potency compared with compound 5a when a
hydrophobic ethyl group at R2 position was introduced. Compound
5h with a hydrophilic substituent (methylol) at R2 position exhib-
ited a markedly reduced inhibitory activity in the dihydroquinox-
alinone core. The superimposition docking conformation of 5h
and 5i is shown in Fig. 2B. Therefore, a hydrophobic group was nec-
essary to increase the lipophilic effects with the hydrophobic WPF
shelf, but the introduction of hydrophobic group (benzyl group)
exhibited unfavorable steric to decrease the binding efficiency.
For this reason, compound 5j has a weaker inhibitory activity with
IC50 value of 151 nM. Compounds 5a and 5j were docking into
BRD4(1) protein to explain the structure-activity relationship.
The superimposition of compound 5a and 5j in BRD4(1) protein
is shown in Fig. 2C. In addition, the ‘‘S” configuration of the stere-
ogenic center at R1 position decreased the binding efficiency.
According to the overlap docking conformation, a small hydropho-
bic group (ethyl group) at R2 position was necessary for the
improvement of binding efficiency.
Compound Conformation R1
R2
BRD4(1) IC50
mol/L)b
(l
5f
S
NAa
5g
5h
S
NAa
R
0.650 0.021
5i
R
R
R
0.073 0.012
0.151 0.015
0.500 0.026
2.3. In-vitro anticancer evaluation against cancer cell line MV-4-11
5j
BRD4 inhibitors showed potent effects in cellular systems,
including inhibition of proliferation. We further tested whether
the compounds 5a and 5i could inhibit cancer cell lines MV-4-11.
Then, we assessed the effect of compounds 5a and 5i on cell prolif-
eration in abroad panel of human cancer cell lines, MV-4-11 [50–
52]. As shown in Table 3, compound 5i significantly inhibited the
proliferation of MV-4-11 cell lines with IC50 value of 258 nM. These
results established that compound 5i could be a promising lead
compound for anti-cancer.
5k
H
a
Mean of the BRD4-BD1 inhibitory activity IC50 > 1
IC50 values for BRD4-BD1 activities presented is the mean SD value of three
independent determinations.
lm.
b
Table 3
Schrodinger 10.2. Compared to other substituent at R1 region, ben-
zyl group was considered as lead candidate with the best biological
activity. The docking conformations of compounds 5a and 5d are
shown in Fig. 1D. The docking conformation shows that the
cyclopentane group of compound 5d was 7.65 Å from the Pro82.
This distance from cyclopentane group to Pro82 indicates com-
pound 5d cannot stretch into WPF hydrophobic pocket. On the
contrary, the benzyl group of compound 5a could occupy WPF
region. This subtle difference may lead to the decrease of biology
value (IC50). In addition, compared to compound 5b, the introduction
Anti-proliferative activities of compound 5i and 5a on MV-4-11 cell lines.
Compound no.
mw
IC50 (MV-4-11 cell lines) (
l
mol/L)b
clogPc
28aa
5a
5i
330.39
347.42
361.45
456.99
0.632 0.013
0.378 0.021
0.258 0.014
4.136 0.028
2.809
3.540
4.069
4.820
(+)-JQ1
a
Used as positive control.
IC50 values for BRD4-BD1 activities presented is the mean SD value of three
b
independent determinations.
c
clogP values were estimated with ChemDraw Ultra, version 14.0.
Fig. 2. (A) Superimposition docking conformation of compound 5a (Blue) and compound 5i (Pink) in BRD4 protein (PDB id: 3P5O). (B) Superimposition docking conformation
of compound 5h (Gray) and compound 5i (Pink) in BRD4 protein (PDB id: 3P5O). (C) Superimposition docking conformation of compound 5j (Orange) and compound 5i (Pink)
in BRD4 protein (PDB id: 3P5O).

240
Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
13.23 (bs, 1H), 8.42 (s, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.15 (s, 1H),
3. Conclusion
6.88 (d, J = 7.0 Hz, 1H), 4.19 (s, 2H).
In summary, we designed and synthesized a series of novel
dihydroquinoxalinone derivatives as BRD4 inhibitors. Most of
these compounds were evaluated as excellent BRD4 suppressant
in vitro. Especially compound 5i had excellent BRD4 inhibition
activity (IC50 73 nM) better than the compound 28a (IC50
180 nM). Meanwhile, compound 5a and compound 5i exhibited
potency of anti-proliferative activity in MV-4-11 cancer cell lines.
In addition, docking studies were performed to claim the
structure-activity relationship. All of these results demonstrated
that compound 5i was a potent BRD4 inhibitor and worthy for fur-
ther investigation.
4.1.1.6. (5-Bromo-2-nitrophenyl)-d-phenylalanine (2f). Yellow solid,
96%;ESI-MS m/z 364.0 [M+H]+; 1H NMR (300 MHz, DMSO-d6) d:
13.41 (s, 1H), 8.24 (d, J = 7.9 Hz, 1H), 7.98 (d, J = 9.1 Hz, 1H),
7.33–7.11 (m, 6H), 6.92–6.82 (m, 1H), 4.91 (m, 1H), 3.26–3.11
(m, 2H).
4.1.2. General procedure for the preparation of 3a–f
To a solution of 2a–g (1 g, 3.46 mmol) and K2CO3 (0.96 g,
6.92 mmol) in water was added sodium hydrosulfite (3.01 g,
17.3 mmol) in portions. Then the mixture was stirred for overnight.
The white precipitate was collected by filtration to afford 3a–g
without further purification.
4. Experiment
4.1.2.1. (R)-6-bromo-3-methyl-3,4-dihydroquinoxalin-2(1H)-one (3a).
White solid, yield 45%; ESI-MS m/z 239.9 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.27 (s, 1H), 6.81 (d, J = 2.0 Hz, 1H), 6.73
(dd, J = 8.2, 2.0 Hz, 1H), 6.64 (d, J = 8.2 Hz, 1H), 6.26 (s, 1H), 3.81
(q, J = 6.3 Hz, 1H), 1.24 (d, J = 6.6 Hz, 3H).
4.1. Chemistry
All chemicals (reagent grade) used were purchased from Sino-
pharm Chemical Reagent Co., Ltd. (China) and treated with stan-
dard
methods
before
use.
Purifications
by
column
chromatography were carried out over silica gel (200–300 mesh).
Melting points were measured on capillary tube and were uncor-
rected. IR spectra (in KBr pellets) were taken using Shimadzu FT-
IR-8400S spectrophotometer. 1H and 13C NMR spectra (DMSO-d6)
were measured on Bruker AV-300 spectrometer at 25 °C and refer-
enced to TMS. Chemical shifts were reported in ppm (d) using the
residual solvent line as internal standard. Analytical thin-layer
chromatography (TLC) was performed on the glass-backed silica
gel sheets (silica gel 60 Å GF 254). High-resolution mass spectra
were recorded using an Agilent QTOF 6520 (Beijing, China).
4.1.2.2. (S)-6-bromo-3-methyl-3,4-dihydroquinoxalin-2(1H)-one (3b).
Off white solid, yield 40%; ESI-MS m/z 239.9 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.29 (s, 1H), 6.81 (d, J = 2.0 Hz, 1H), 6.73
(dd, J = 8.2, 2.0 Hz, 1H), 6.63 (d, J = 8.2 Hz, 1H), 6.27 (s, 1H), 3.80
(m, 1H), 1.23 (d, J = 6.5 Hz, 3H).
4.1.2.3. (R)-6-bromo-3-(hydroxymethyl)-3,4-dihydroquinoxalin-2(1H)-
one (3c). White solid, yield 40%; ESI-MS m/z 255.9 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.36 (s, 1H), 6.87 (s, 1H), 6.78–6.49 (m,
2H), 6.31 (s, 1H), 4.98 (s, 1H), 3.83 (s, 1H), 3.59 (s, 2H).
4.1.1. General procedure for the preparation of 2a–f
4.1.2.4. (R)-6-bromo-3-ethyl-3,4-dihydroquinoxalin-2(1H)-one (3d).
White solid, yield 40%; ESI-MS m/z 254.0 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.31 (s, 1H), 6.85 (d, J = 2.1 Hz, 1H), 6.70
(dd, J = 8.2, 2.1 Hz, 1H), 6.61 (d, J = 8.2 Hz, 1H), 6.29 (s, 1H), 3.76–
3.66 (m, 1H), 1.72–1.52 (m, 2H), 0.91 (t, J = 7.4 Hz, 3H).
To
a
solution of 4-bromo-2-fluoro-1-nitrobenzene (1 g,
4.55 mmol), K2CO3 (0.78 g, 5.68 mmol) and different amino acid
(5.45 mmol) in ethanol (5 ml) and water (4 ml) were refluxed at
100 °C for 10 h. After cooling to the room temperature, pH value
was adjusted to 2 with 1 M HCl solution. The mixture was filtered
and the precipitate was collected to afford 2a–g as golden yellow
solid.
4.1.2.5. 6-Bromo-3,4-dihydroquinoxalin-2(1H)-one (3e). Off white
solid, yield 43%; ESI-MS m/z 225.9 [M+H]+; 1H NMR (300 MHz,
DMSO-d6) d: 10.33 (s, 1H), 8.19 (s, 1H), 7.97 (d, J = 2.2 Hz, 1H),
7.70 (dd, J = 8.8, 2.2 Hz, 1H), 7.24 (d, J = 8.8 Hz, 1H), 3.78 (s, 2H).
4.1.1.1. (5-Bromo-2-nitrophenyl)-d-alanine (2a). Golden yellow
solid, yield 98.2%; ESI-MS m/z 287.9 [M+H]+; 1H NMR (300 MHz,
DMSO-d6) d: 8.39 (d, J = 6.8 Hz, 1H), 8.02 (d, J = 9.1 Hz, 1H), 7.23
(s, 1H), 6.90 (dd, J = 9.1, 1.7 Hz, 1H), 4.62–4.46 (m, 1H), 1.46 (d,
J = 6.9 Hz, 3H).
4.1.2.6. (R)-3-benzyl-6-bromo-3,4-dihydroquinoxalin-2(1H)-one (3f).
White solid, yield 45%; ESI-MS m/z 316.0 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.34 (s, 1H), 7.33–7.10 (m, 5H), 6.83 (s,
1H), 6.67 (dd, J = 8.2, 1.7 Hz, 1H), 6.57 (d, J = 8.2 Hz, 1H), 6.19 (s,
1H), 4.08 (t, J = 4.8 Hz, 1H), 2.99–2.78 (m, 2H).
4.1.1.2. (R)-2-((5-bromo-2-nitrophenyl)amino)butanoic acid (2b).
Sandy solid, yield 94%; ESI-MS m/z 301.9 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 8.44 (d, J = 7.3 Hz, 1H), 8.02 (d, J = 9.1 Hz,
1H), 7.25 (d, J = 1.9 Hz, 1H), 6.88 (dd, J = 9.1, 1.9 Hz, 1H), 4.62–
4.43 (m, 1H), 1.98–1.77 (m, 2H), 0.87 (t, J = 7.3 Hz, 3H).
4.1.3. General procedure for the preparation of 4a–b
A mixture of 3a–b (0.5 mmol),3,5-dimethyl-4-(4,4,5,5-tetrame
thyl-1,3,2-dioxaborolan-2-yl)isoxazole (0.5 mmol), sodium carbon-
ate (1.5 mmol), Tetrakis (triphenylphosphine) palladium
(0.05 mmol) in a mixed solvent of water (3 mL), ethanol (1 mL)
and toluene (3 mL) was refluxed under N2 for 12 h. After the reaction
was completed, the mixture was diluted with water (20 mL) and fil-
tered withdiatomite. Then the filtrate was extracted withethyl acet-
ate (10 mL Â 3). The combined organic layers were dried over
anhydrous Na2SO4 and concentrated under reduced pressure to give
brown oil. The resulting crude oil was purified by silica gel column
chromatography (petroleum ether/ethylacetate, 3:1) to afford 4a–b.
4.1.1.3. (5-Bromo-2-nitrophenyl)-d-serin (2c). Yellow solid, yield
91%; ESI-MS m/z 303.9 [M+H]+.
4.1.1.4. (5-Bromo-2-nitrophenyl)-l-alanine (2d). Golden yellow
solid, yield 94%; ESI-MS m/z 287.9 [M+H]+; 1H NMR (300 MHz,
DMSO-d6) d: 13.33 (bs, 1H), 8.39 (d, J = 7.1 Hz, 1H), 8.02 (d,
J = 9.1 Hz, 1H), 7.24 (s, 1H), 7.00–6.88 (m, 1H), 4.63–4.50 (m, 1H),
1.45 (d, J = 7.0 Hz, 3H).
4.1.3.1.
(R)-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-3,4-dihydro-
4.1.1.5. (5-Bromo-2-nitrophenyl)glycine (2e). Brown solid, yield
92%; ESI-MS m/z 273.9 [M+H]+; 1H NMR (300 MHz, DMSO-d6) d:
quinoxalin-2(1H)-one (4a). Light yellow solid, yield 65%; ESI-MS
m/z 257.1 [M+H]+; 1H NMR (300 MHz, DMSO-d6) d: 10.29 (s, 1H),

Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
241
6.80 (d, J = 7.9 Hz, 1H), 6.66 (s, 1H), 6.59 (d, J = 7.9 Hz, 1H), 6.13 (s,
1H), 3.83 (q, J = 6.1 Hz, 1H), 2.36 (s, 3H), 2.19 (s, 3H), 1.27 (d,
J = 6.1 Hz, 3H).
z): 370.1 [M+Na]+; Anal. calcd. for C21H21N3O2: C, 72.60; H, 6.09;
N, 12.10. Found: C, 72.56; H, 6.07; N, 12.06.
4.1.5.2. 2(R)-6-(3,5-dimethylisoxazol-4-yl)-4-(4-methoxybenzyl)-3-
methyl-3,4-dihydroquinoxalin-2(1H)-one (5b). Light yellow solid,
yield 78%; m.p.: 107–109 °C; IR (KBr, cmÀ1): 3451.33, 1684.65,
1512., 1245.82, 1033.52, 815.47; 1H NMR (300 MHz, DMSO-d6) d:
10.52 (s, 1H), 7.28 (d, J = 8.5 Hz, 2H), 6.95–6.80 (m, 3H), 6.67 (d,
J = 7.9 Hz, 1H), 6.50 (s, 1H), 4.50 (d, J = 15.1 Hz, 1H), 4.24 (d,
J = 15.1 Hz, 1H), 3.91 (d, J = 6.7 Hz, 1H), 3.72 (s, 3H), 2.17 (s, 3H),
2.00 (s, 3H), 1.09 (d, J = 6.7 Hz, 3H); 13C NMR (75 MHz, DMSO-d6)
d: 168.1, 164.4, 159.0, 158.4, 158.0, 133.2, 129.2, 128.5, 126.4,
124.1, 118.8, 116.0, 114.9, 114.0, 57.3, 55.0, 50.3, 40.0, 39.7, 39.4,
39.2, 38.9, 13.0, 11.0, 10.1. MS (ESI, m/z): 400.1 [M+Na]+; Anal.
calcd. for C22H23N3O3: C, 70.01; H, 6.14; N, 11.13. Found: C,
70.01; H, 6.12; N, 11.16.
4.1.3.2.
(S)-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-3,4-dihydro-
quinoxalin-2(1H)-one (4b). Light yellow solid, yield 74%; ESI-MS
m/z 257.1 [M+H]+; 1H NMR (300 MHz, DMSO-d6) d: 10.30 (s, 1H),
6.80 (d, J = 7.9 Hz, 1H), 6.65 (s, 1H), 6.62–6.54 (m, 1H), 6.13 (s,
1H), 3.83 (q, J = 7.0 Hz, 1H), 2.36 (s, 3H), 2.19 (s, 3H), 1.27 (d,
J = 7.0 Hz, 3H).
4.1.4. General procedure for the preparation of 4c–f
A mixture of 3c–f (2.07 mmol), benzaldehydes (6.22 mmol),
phenylsilane (0.67 g, 6.22 mmol) and dibutyltin dichloride
(0.69 g, 2.28 mmol) in THF (10 mL) was stirred for 8 h at room tem-
perature. Then the mixture was quenched with water and
extracted with EtOAc (3 Â 20 ml). The combined organic layers
were dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give yellow oil. The resulting crude oil was
purified by silica gel column chromatography (petroleum ether/
ethyl acetate, 3:1) to afford 4c–f.
4.1.5.3.
3(R)-4-(4-chlorobenzyl)-6-(3,5-dimethylisoxazol-4-yl)-3-
methyl-3,4-dihydroquinoxalin-2(1H)-one (5c). Light yellow solid,
yield 70%; m.p.: 104–106 °C; IR (KBr, cmÀ1): 3445.43, 1685.47,
1522.60, 1393.00, 815.47; 1H NMR (300 MHz, DMSO-d6) d: 10.55
(s, 1H), 7.39 (s, 4H), 6.89 (d, J = 7.9 Hz, 1H), 6.67 (d, J = 7.9 Hz,
1H), 6.39 (s, 1H), 4.56 (d, J = 15.9 Hz, 1H), 4.36 (d, J = 15.9 Hz,
1H), 4.00 (q, J = 6.8 Hz, 1H), 2.12 (s, 3H), 1.94 (s, 3H), 1.12 (d,
J = 6.8 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) d: 168.0, 164.3,
157.9, 136.8, 132.8, 131.4, 128.9, 128.4, 126.4, 124.1, 119.0,
115.9, 115.0, 114.0, 112.9, 58.0, 50.3, 40.0, 39.7, 39.4, 39.2, 38.9,
24.9, 13.4, 10.9, 10.0. MS (ESI, m/z): 404.0 [M+Na]+; Anal. calcd.
for C21H2°ClN3O2: C, 66.05; H, 5.28; N, 11.00. Found: C, 66.03; H,
5.25; N, 11.06.
4.1.4.1.
(R)-4-benzyl-3-(hydroxymethyl)-6-methyl-3,4-dihydro-
quinoxalin-2(1H)-one (4c). Yellow solid, yield 81%; ESI-MS m/z
347.2 [M+H]+.
4.1.4.2. (R)-4-benzyl-6-bromo-3-ethyl-3,4-dihydroquinoxalin-2(1H)-
one (4d). Light yellow solid, yield 80%; ESI-MS m/z 344.1 [M+H]+;
1H NMR (300 MHz, DMSO-d6) d: 10.56 (s, 1H), 7.35–7.25 (m, 5H),
6.84–6.66 (m, 3H), 4.69 (d, J = 15.6 Hz, 1H), 4.36 (d, J = 15.6 Hz,
1H), 3.84–3.73 (m, 1H), 1.70–1.43 (m, 2H), 0.81 (t, J = 7.5 Hz, 3H).
4.1.5.4. 4(R)-4-cyclopentyl-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-
3,4-dihydroquinoxalin-2(1H)-one (5d). Light yellow solid, yield
71%; m.p.: 111–113 °C; IR (KBr, cmÀ1): 3457.23, 2938.05,
1692.34, 1520.44, 1392.90, 815.47; 1H NMR (300 MHz, DMSO-d6)
d: 10.47 (s, 1H), 6.89 (d, J = 8.1 Hz, 1H), 6.79–6.64 (m, 2H), 3.95
(q, J = 6.7 Hz, 1H), 3.90–3.78 (m, 1H), 2.39 (s, 3H), 2.22 (s, 3H),
1.97 (s, 2H), 1.60 (s, 6H), 0.99 (d, J = 6.7 Hz, 3H). 13C NMR
(75 MHz, DMSO-d6) d: 168.5, 164.4, 158.1, 133.6, 127.2, 124.1,
119.2, 116.2, 115.5, 115.0, 58.6, 53.6, 40.0, 39.7, 39.4, 39.2, 38.9,
29.9, 23.7, 23.2, 14.3, 11.3, 10.5. MS (ESI, m/z): 348.1 [M+Na]+;
Anal. calcd. for C19H23N3O2: C, 70.13; H, 7.12; N, 12.91. Found: C,
70.11; H, 7.14; N, 12.87.
4.1.4.3. (R)-3,4-dibenzyl-6-bromo-3,4-dihydroquinoxalin-2(1H)-one
(4e). White solid, yield 83%; ESI-MS m/z 406.1 [M+H]+; 1H NMR
(300 MHz, DMSO-d6) d: 10.57 (s, 1H), 7.26 (m, 8H), 7.07 (d,
J = 6.1 Hz, 2H), 6.77 (d, J = 5.9 Hz, 2H), 6.63 (d, J = 8.8 Hz, 1H),
4.57 (d, J = 15.4 Hz, 1H), 4.16 (d, J = 15.4 Hz, 1H), 4.08 (t,
J = 6.2 Hz, 1H), 2.80 (m, 2H).
4.1.4.4.
6-Bromo-4-(4-chlorobenzyl)-3,4-dihydroquinoxalin-2(1H)-
one (4f). Light brown solid, yield 81%; ESI-MS m/z 350.0 [M+H]+;
1H NMR (300 MHz, DMSO-d6) d: 10.57 (s, 1H), 7.42 (d, J = 8.3 Hz,
2H), 7.33 (d, J = 8.3 Hz, 2H), 6.85–6.70 (m, 3H), 4.45 (s, 2H), 3.78
(s, 2H).
4.1.5.5.
5(R)-4-cyclohexyl-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-
4.1.5. General procedure for the preparation of 5a–g
3,4-dihydroquinoxalin-2(1H)-one (5e). Light yellow solid, yield
75%; m.p.: 116–118 °C; IR (KBr, cmÀ1): 3433.63, 2926.25,
1683.08, 1516.76, 1440.15, 1301.66, 1024.68; 1H NMR (300 MHz,
DMSO-d6) d: 10.42 (s, 1H), 6.88 (d, J = 7.9 Hz, 1H), 6.76 (s, 1H),
6.71 (dd, J = 7.9, 1.6 Hz, 1H), 4.02–3.92 (m, 1H), 3.50–3.38 (m,
1H), 2.39 (s, 3H), 2.22 (s, 3H), 2.01–1.92 (m, 1H), 1.81–1.69 (m,
2H), 1.67–1.24 (m, 7H), 1.02 (d, J = 6.7 Hz, 3H); 13C NMR
(75 MHz, DMSO-d6) d: 168.0, 164.4, 157.8, 133.3, 126.9, 124.3,
118.8, 116.1, 115.2, 114.9, 58.3, 51.4, 40.0, 39.7, 39.4, 39.2, 38.9,
31.2, 30.9, 25.6, 25.4, 25.2, 24.9, 17.0, 11.3, 10.5. MS (ESI, m/z):
362.2 [M+Na]+; Anal. calcd. for C20H25N3O2: C, 70.77; H, 7.42; N,
12.38. Found: C, 70.71; H, 7.44; N, 12.37.
A mixture of 4a–b (2.07 mmol), benzaldehydes/cyclic ketones
(6.22 mmol), phenylsilane (0.67 g, 6.22 mmol) and dibutyltin
dichloride (0.69 g, 2.28 mmol) in THF (10 mL) was stirred for 8 h
at room temperature. Then the mixture was quenched with water
and extracted with EtOAc (3 Â 20 ml). The combined organic layers
were dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give yellow oil. The resulting crude oil was
purified by silica gel column chromatography (petroleum ether/
ethyl acetate, 3:1) to afford target products 5a–g.
4.1.5.1.
1(R)-4-benzyl-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-3,4-
dihydroquinoxalin-2(1H)-one (5a). Light yellow solid, yield 70%;
m.p.: 112–114 °C; IR (KBr, cmÀ1): 3185.84, 2920.3, 1689.57,
1522.49, 1444.04, 1393.57, 739.46, 514.83; 1H NMR (300 MHz,
DMSO-d6) d: 10.54 (s, 1H), 7.40–7.20 (m, 5H), 6.84 (d, J = 8.2 Hz,
1H), 6.79–6.66 (m, 2H), 4.58 (d, J = 15.3 Hz, 1H), 4.27 (d,
J = 15.3 Hz, 1H), 3.84 (q, J = 6.7 Hz, 1H), 1.07 (d, J = 6.7 Hz, 3H);
13C NMR (75 MHz, DMSO-d6) d: 168.1, 164.3, 157.9, 137.7, 133.0,
128.5, 127.0, 126.4, 124.1, 118.8, 116.0, 114.9, 114.0, 57.8, 51.1,
40.3, 40.0, 39.7, 39.5, 39.2, 38.9, 38.6, 13.3, 10.9, 10.0. MS (ESI, m/
4.1.5.6. 6(S)-6-(3,5-dimethylisoxazol-4-yl)-4-(4-methoxybenzyl)-3-
methyl-3,4-dihydroquinoxalin-2(1H)-one (5f). Light yellow solid,
yield 57%; m.p.: 107–109 °C; IR (KBr, cmÀ1): 3445.43, 1685.40,
1512.58, 1246.17, 1033.52, 815.47; 1H NMR (300 MHz, DMSO-d6)
d: 10.55 (s, 1H), 7.28 (d, J = 8.1 Hz, 2H), 6.96–6.85 (m, 3H), 6.67
(d, J = 7.8 Hz, 1H), 6.50 (s, 1H), 4.49 (d, J = 15.2 Hz, 1H), 4.24 (d,
J = 15.2 Hz, 1H), 4.00–3.85 (m, 1H), 3.72 (s, 3H), 2.16 (s, 3H), 1.99
(s, 3H), 1.09 (d, J = 6.5 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) d:

242
Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
168.1, 164.4, 163.2, 158.4, 157.9, 133.2, 130.7, 129.2, 128.4, 126.4,
124.1, 118.8, 116.0, 114.9, 114.0, 57.3, 55.0, 50.3, 40.0, 39.7, 39.5,
39.2, 13.0, 11.0, 10.1. MS (ESI, m/z): 400.1 [M+Na]+; Anal. calcd.
for C22H23N3O3: C, 70.01; H, 6.14; N, 11.13. Found: C, 70.01; H,
6.17; N, 11.21.
(s, 1H), 7.37–6.98 (m, 10H), 6.76 (d, J = 8.0 Hz, 1H), 6.58 (d,
J = 7.0 Hz, 1H), 6.42 (s, 1H), 4.54 (d, J = 15.5 Hz, 1H), 4.20 (s, 2H),
2.83 (s, 2H), 2.15 (s, 3H), 1.99 (s, 3H); MS (ESI, m/z): 446.1 [M
+Na]+; Anal. calcd. for C27H25N3O2: C, 76.57; H, 5.95; N, 9.92.
Found: C, 76.55; H, 5.90; N, 9.92.
4.1.5.7. 7(S)-4-cyclopentyl-6-(3,5-dimethylisoxazol-4-yl)-3-methyl-
3,4-dihydroquinoxalin-2(1H)-one (5g). Light yellow solid, yield
75%; m.p.: 110–113 °C; IR (KBr, cmÀ1): 3457.23, 2943.95,
1692.60, 1520.70, 1393.06, 815.47; 1H NMR (300 MHz, DMSO-d6)
d: 10.47 (s, 1H), 6.89 (d, J = 8.3 Hz, 1H), 6.73 (d, J = 3.7 Hz, 2H),
4.04–3.90 (m, 1H), 3.90–3.76 (m, 1H), 2.38 (s, 3H), 2.21 (s, 3H),
1.97 (s, 2H), 1.59 (s, 6H), 0.99 (d, J = 6.7 Hz, 3H); 13C NMR
(75 MHz, DMSO-d6) d: 168.5, 164.3, 158.0, 133.6, 127.2, 124.1,
119.2, 116.1, 115.4, 115.0, 58.5, 53.6, 39.7, 39.4, 39.2, 38.9, 29.9,
23.7, 23.2, 14.3, 11.3, 10.5. MS (ESI, m/z): 348.1 [M+Na]+; Anal.
calcd. for C19H23N3O2: C, 70.13; H, 7.12; N, 12.91. Found: C,
70.11; H, 7.17; N, 12.85.
4.1.6.4. 4-(4-Chlorobenzyl)-6-(3,5-dimethylisoxazol-4-yl)-3,4-dihy-
droquinoxalin-2(1H)-one (5k). Light yellow solid, yield 62%; m.p.:
150–152 °C; IR (KBr, cmÀ1): 3427.73, 2922.92, 2849.56, 2353.98,
1680.99, 1403.61, 794.84; 1H NMR (300 MHz, DMSO-d6) d: 10.58
(s, 1H), 7.44–7.34 (m, 4H), 6.89 (d, J = 7.9 Hz, 1H), 6.70–6.62 (m,
1H), 6.47 (s, 1H), 4.46 (s, 2H), 3.86 (s, 2H), 2.16 (s, 3H), 1.99 (s,
3H); 13C NMR (75 MHz, DMSO-d6) d: 165.4, 164.3, 157.8, 136.3,
134.7, 131.3, 129.1, 128.5, 126.4, 123.9, 118.6, 115.9, 115.2,
114.2, 112.5, 70.2, 52.5, 52.0. MS (ESI, m/z): 390.0 [M+Na]+; Anal.
calcd. for C20H18ClN3O2: C, 65.31; H, 4.93; N, 11.42. Found: C,
65.30; H, 4.91; N, 11.44.
4.1.7. Preparation of (R)-6-bromo-1,3-dimethyl-3,4-
4.1.6. General procedure for the preparation of 5h–k
dihydroquinoxalin-2(1H)-one (6)
A mixture of 4c-f (0.5 mmol),3,5-dimethyl-4-(4,4,5,5-tetrame
thyl-1,3,2-dioxaborolan-2-yl) isoxazole (0.5 mmol), sodium car-
bonate (1.5 mmol), Tetrakis (triphenylphosphine) palladium
(0.05 mmol) in a mixed solvent of water (3 mL), ethanol (1 mL)
and toluene (3 mL) was refluxed under N2 for 12 h. After the reac-
tion was completed, the mixture was diluted with water (20 mL)
and filtered with diatomite. Then the filtrate was extracted with
ethyl acetate (10 mL Â 3). The combined organic layers were dried
over anhydrous Na2SO4 and concentrated under reduced pressure
to give brown oil. The resulting crude oil was purified by silica
gel column chromatography (petroleum ether/ethyl acetate, 3:1)
to afford target products.
To a solution of 3a (1.52 g, 6.30 mmol) in Dimethyl Formamide
was added 60% NaH (0.38 g, 9.64 mmol) in portions at ice bath.
Then the mixture was stirred for 20 min. Methyl iodide (0.59 mL,
9.46 mmol) was added dropwise and then was stirred for another
one hour at room temperature. The mixture was quenched with
saturated NaHCO3 solution and extracted with 20 ml  3. The com-
bined organic layers were washed with brine (30 mL), dried over
Na2SO4, and evaporated in vacuo to give brown oil. The resulting
crude oil was purified by silica gel column chromatography (petro-
leum ether/ethyl acetate, 3:1) to afford 6 (1.04 g, 64%) as a light
yellow solid. ESI-MS m/z 254.0 [M+H]+; 1H NMR (300 MHz,
DMSO-d6): d: 6.89–6.86 (m, 3H), 6.36 (s, 1H), 3.85 (q, J = 6.6 Hz,
1H), 3.21 (s, 3H), 1.25 (d, J = 6.6 Hz, 3H).
4.1.6.1.
8(R)-4-benzyl-6-(3,5-dimethylisoxazol-4-yl)-3-(hydrox-
ymethyl)-3,4-dihydroquinoxalin-2(1H)-one (5h). Light brown solid,
yield 66%; m.p.: 122–124 °C; IR (KBr, cmÀ1): 3410.03, 2926.25,
1681.77, 1524.00, 1409.74, 1065.93, 694.66. 1H NMR (300 MHz,
DMSO-d6) d: 10.62 (s, 1H), 7.48–7.09 (m, 5H), 6.81 (d, J = 7.5 Hz,
1H), 6.55 (d, J = 7.5 Hz, 1H), 6.32 (s, 1H), 5.03 (s, 1H), 4.70 (d,
J = 16.1 Hz, 1H), 4.53 (d, J = 16.1 Hz, 1H), 4.07 (s, 1H), 3.66 (s, 2H),
2.06 (s, 3H), 1.89 (s, 3H); 13C NMR (75 MHz, DMSO-d6) d: 166.0,
138.1, 133.8, 132.0, 131.5, 131.3, 128.8, 128.6, 128.5, 126.7,
125.9, 123.7, 117.6, 114.6, 112.8, 65.2, 60.9, 52.1, 40.0, 39.7, 39.4,
39.2, 38.9, 10.9, 10.0. MS (ESI, m/z): 386.1 [M+Na]+; Anal. calcd.
for C21H21N3O3: C, 69.41; H, 5.82; N, 11.56. Found: C, 69.40; H,
5.81; N, 11.58.
4.1.8. Preparation of (R)-6-bromo-4-(4-methoxybenzyl)-1,3-
dimethyl-3,4-dihydroquinoxalin-2(1H)-one (7)
A mixture of 6 (0.85 g, 3.30 mmol), p-methoxybenzaldehyde
(1.22 mL, 10.00 mL), phenylsilane (1.23 mL, 10.00 mmol) and dibu-
tyltin dichloride (1.11 g, 3.67 mmol) in THF (4 mL) was stirred for
8 h at room temperature. Then the mixture was quenched with
water and extracted with EtOAc (3 Â 20 ml). The combined organic
layers were dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give yellow oil. The resulting crude oil was
purified by silica gel column chromatography (petroleum ether/
ethyl acetate, 3:1) to afford 7 (1.02 g, 81%). ESI-MS m/z 344.0 [M
+H]+; 1H NMR (300 MHz, DMSO-d6) d: 7.27 (d, J = 8.5 Hz, 2H),
6.98 (s, 2H), 6.93 (s, 1H), 6.90 (s, 1H), 6.87 (s, 1H), 4.51 (d,
J = 14.6 Hz, 1H), 4.15 (d, J = 14.6 Hz, 1H), 3.90 (q, J = 6.7 Hz, 1H),
3.74 (s, 3H), 3.26 (s, 3H), 0.99 (d, J = 6.7 Hz, 3H).
4.1.6.2. (R)-4-benzyl-6-(3,5-dimethylisoxazol-4-yl)-3-ethyl-3,4-dihy-
droquinoxalin-2(1H)-one (5i). Light yellow solid, yield 52%; m.p.:
112–114 °C; IR (KBr, cmÀ1): 3439.53, 2967.55, 1688.83, 1381.22,
1230.94, 1151.38, 821.36, 732.97; 1H NMR (300 MHz, DMSO-d6)
d: 10.54 (s, 1H), 7.33 (d, J = 4.3 Hz, 4H), 7.27–7.16 (m, 1H), 6.85
(d, J = 7.9 Hz, 1H), 6.62 (d, J = 7.9 Hz, 1H), 6.43 (s, 1H), 4.67 (d,
J = 15.7 Hz, 1H), 4.44 (d, J = 15.7 Hz, 1H), 3.92–3.85 (m, 1H), 2.10
(s, 3H), 1.93 (s, 3H), 1.73–1.61 (m, 1H), 1.59–1.48 (m, 1H), 0.85
(t, J = 7.5 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) d: 157.8, 137.6,
135.5, 128.6, 128.5, 126.9, 126.5, 124.0, 118.4, 116.1, 114.7,
113.3, 73.2, 63.3, 51.4, 45.2, 40.0, 39.7, 39.4, 39.2, 38.9, 24.9,
22.5, 10.9, 10.1, 9.9. MS (ESI, m/z): 384.1 [M+Na]+; Anal. calcd.
for C22H23N3O2: C, 73.11; H, 6.41; N, 11.63. Found: C, 73.14; H,
6.41; N, 11.58.
4.1.9. Preparation of (R)-6-(3,5-dimethylisoxazol-4-yl)-4-(4-
methoxybenzyl)-1,3-dimethyl-3,4-dihydroquinoxalin-2(1H)-one (8)
A mixture of 7 (0.20 g, 0.53 mmol), 3,5-dimethyl-4-(4,4,5,5-tet
ramethyl-1,3,2-dioxaborolan-2-yl) isoxazole (0.12 g, 0.53 mmol),
sodium carbonate (0.17 g, 1.60 mmol), Tetrakis (triphenylphos-
phine) palladium (0.06 g, 0.053 mmol) in a mixed solvent of water
(3 mL), ethanol (1 mL) and toluene (3 mL) was refluxed under N2
for 12 h. After the reaction was completed, the mixture was diluted
with water (20 mL) and filtered with diatomite. Then the filtrate
was extracted with ethyl acetate (10 mL Â 3). The combined
organic layers were dried over anhydrous Na2SO4 and concen-
trated under reduced pressure to give brown oil. The resulting
crude oil was purified by silica gel column chromatography (petro-
leum ether/ethyl acetate, 3:1) to afford target product 8 (light yel-
low solid, yield 52%). m.p.: 118–120 °C; IR (KBr, cmÀ1): 3436.73,
2959.18, 1677.19, 1512.41, 1243.83, 816.33; 1H NMR (300 MHz,
4.1.6.3. (R)-3,4-dibenzyl-6-(3,5-dimethylisoxazol-4-yl)-3,4-dihydro-
quinoxalin-2(1H)-one (5j). Light brown solid, yield 52%; m.p.:
106–108 °C; IR (KBr, cmÀ1): 3427.73, 1681.93, 1522.49, 1394.39,
1230.94, 744.75, 691.71; 1H NMR (300 MHz, DMSO-d6) d: 10.57

Y. Yang et al. / Bioorganic Chemistry 68 (2016) 236–244
243
DMSO-d6) d: 7.29 (d, J = 8.3 Hz, 2H), 7.12 (d, J = 7.9 Hz, 1H), 6.90 (d,
J = 8.3 Hz, 2H), 6.81 (d, J = 7.9 Hz, 1H), 6.56 (s, 1H), 4.50 (d,
J = 14.7 Hz, 1H), 4.24 (d, J = 14.7 Hz, 1H), 4.06–4.01 (m, 1H), 3.72
(s, 3H), 3.33 (s, 3H), 2.18 (s, 3H), 2.01 (s, 3H), 1.04 (d, J = 7.0 Hz,
3H). 13C NMR (75 MHz, DMSO-d6) d: 167.5, 164.6, 158.4, 158.0,
134.7, 128.9, 128.7, 128.6, 124.6, 119.0, 114.8, 114.1, 114.0, 58.7,
57.3, 55.0, 50.4, 40.0, 39.7, 39.4, 39.2, 38.9, 28.5, 12.3, 11.0, 10.1.
MS (ESI, m/z):414.1 [M+Na]+; Anal. calcd. for C23H25N3O3: C,
70.57; H, 6.44; N, 10.73. Found: C, 70.55; H, 6.51; N, 10.68.
Hi-Tech Innovation Project for the R&D of Novel Drugs (No.
2013ZX09301303-002).
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