Synthesis and evaluation of cyclic nitrone derivatives as potential anti-cancer agents

Article abstract of DOI:10.1007/s00044-021-02729-2

Nitrones have been found to exhibit attractive biological values as immuno spin trapping agents. However, successful clinical cases of nitrone therapeutics are still lacking. Herein we report the synthesis and antiproliferative activity of a series of structurally diverse nitrone derivatives against a panel of 5 cancer cell lines, based on which indole- and pyrrole-fused were further evaluated by analogue preparation and in-vitro screening. Analogues with moderate to good potency were identified. This study shows the promise for further pursuit of nitrone-type small molecules in chemotherapy. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00044-021-02729-2

MEDICINAL
CHEMISTRY
RESEARCH
Medicinal Chemistry Research (2021) 30:1309–1316
https://doi.org/10.1007/s00044-021-02729-2
ORIGINAL RESEARCH
Synthesis and evaluation of cyclic nitrone derivatives as potential
anti-cancer agents
1
Wei Zhou1 Dongyan Ju1 Yuhui Ao1 Yu Liu1 Jinbo Zhao
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Received: 8 January 2021 / Accepted: 5 April 2021 / Published online: 18 May 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Nitrones have been found to exhibit attractive biological values as immuno spin trapping agents. However, successful
clinical cases of nitrone therapeutics are still lacking. Herein we report the synthesis and antiproliferative activity of a series
of structurally diverse nitrone derivatives against a panel of 5 cancer cell lines, based on which indole- and pyrrole-fused
were further evaluated by analogue preparation and in-vitro screening. Analogues with moderate to good potency were
identified. This study shows the promise for further pursuit of nitrone-type small molecules in chemotherapy.
Graphical Abstract
●
●
●
Keywords Cyclic nitrone Metal catalysis Anti-proliferative activity In-vitro screening
Introduction
radicals [3] and the biology system (Fig. 1) [4]. In the
biomedical field, their fascinating radical trapping cap-
ability has endowed nitrone derivatives promising ther-
apeutic agents in the eradication of reactive oxygen species
(ROS), which are related to many pathological conditions,
including neurodegenerative diseases, cancer, stroke, dia-
betes, et al. [5–8]. However, despite this attractive property,
commonly applied nitrone-based therapeutic agents or
probes are far from being diverse. Novelli et al. first
demonstrated the therapeutic application of nitrone by
showing that phenyl tert-butyl nitrone (PBN) can protect
rats from lethal trauma and circulatory shock [9, 10]. Since
then, PBN analogues have been widely explored, and the
2,4-disulfonylphenyl PBN derivative, NXY-059, was
found to be safe in humans and efficacious in the treatment
of acute ischemic stroke [11]. Although it did not pass
clinical phase III, its highly attractive safety profile incites
intensive interest in further development. NXY-059 was
found to be very effective in the treatment of several pre-
clinical glioma models [12, 13], and shows an effect in the
treatment of neurogenerative diseases in combination with
Nitrone derivatives are not only highly versatile synthetic
intermediate, most notably for their [3 + 2] dipolar
cycloaddition reactions [1, 2], but also useful radical trap in
electron paramagnetic resonance (EPR) studies of organic
Supplementary information The online version contains
supplementary material available at https://doi.org/10.1007/s00044-
021-02729-2.
* Yuhui Ao
aoyuhui@ccut.edu.cn
* Yu Liu
yuliu@ccut.edu.cn
* Jinbo Zhao
zhaojinbo@ccut.edu.cn
1
Department of Chemistry and Biology, Jilin Provincial Key
Laboratory of Carbon Fiber Development and Application,
Changchun University of Technology, Changchun, China

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Medicinal Chemistry Research (2021) 30:1309–1316
Results and discussion
Initially, a few representative nitrone compounds of dif-
ferent structural subtypes were synthesized according to a
mild Cu- and Pd-catalyzed intramolecular Cope-type ami-
nation protocol from alkenyl oxime was first evaluated
[16, 17]. The antiproliferative effects of the compounds
were initially performed at 40 μM concentration against a
panel of 5 cancer cell lines, including HL-60 (leukemia),
A549 (lung cancer), SMMC-7721 (liver cancer), MCF-7
(breast cancer), as well as SW480 (colon cancer), by MTS
assay, as summarized in Fig. 2.
As can be seen from Fig. 2, among the five type of
nitrone-type compounds, the pyrrole- and indole-fused
nitrone derivatives 4a and 5a showed promising anti-
proliferative activities, with 4a being selective against
A549 cell line, and 5a showing selectivity against HL-60
cell line, respectively. While compounds 1 and 2 also
showed some activities against specific cell lines (MCF-7
for 1a and SW-480 for 1b, 1c, and 2), these compounds
were not considered further as their activities against other
cell lines were much less pronounced. Based on this
observation, a panel of pyrrole- and indole-fused nitrone,
namely, 3,4-dihydropyrrolo[1,2-a]pyrazine 2-oxide deri-
vatives and 3,4-dihydropyrazino[1,2-a]indole 2-oxide
derivatives were synthesized and evaluated.
The pyrrole-fused nitrone derivatives were prepared by a
three-step sequence from the corresponding pyrrole-2-
aldehyde or 2-acetylpyrrole, involving N-allylation, oxime
formation, and Pd-catalyzed intramolecular Cope-type
hydroamination (Scheme 1) [The structure and naming of
the oxime starting materials is provided in Fig. S1 of the
supplementary material]. The last step constituents a highly
effective neutral annulation protocol for the assembly of these
6-membered cyclic nitrones, tolerating a variety of sub-
stituents at 3-, 4-, and 5-position, including even Bromo and
iodine atoms, which are typically labile under Pd catalysis.
Initial screening at 40 μM drug concentration was exe-
cuted and the results are summarized in Fig. 3. From the
data some interesting points can be inferred: (1) like 5a, a
majority of compounds displayed selective inhibition
against HL-60 cell line. (2) 5d unexpectedly showed
selectivity towards A549 cell line, suggesting that 4-
position on the pyrrole ring may be further exploited for
selective purposes. The relatively good efficacy against
A549 displayed by 5b, 5d, and 5f may be attributed to the
electron-withdrawing property of the substituents at 4- and
5-positions. (3) These compounds also generally demon-
strate activity against SMMC-7721 cell line, but their
activities against MCF-7 and SW480 remain low. Among
these analogues, 5-iodo analogue 5l displayed the highest
activity against SW480 cell line among these series of
compounds (31.5%) [The structure and naming of the
Fig. 1 Nitrone-type small molecules as functional entities due to their
superb radical trapping properties
neurotropic compounds [14]. Floyd et al. found that PBN
was promising in the well-known dietary choline-
deficiency rat liver cancer model [15], and was highly
effective against hepatocarcinoma and glioma models [16]
by oral delivery. The effects were attributed to suppression
of oxidative damage by ROS while causing very little
toxicity. In addition, PBN was also shown to decrease
iNOS activity [17], making it potentially useful in brain
tumors, where increasing levels of iNOS are found. In the
past two decades, there is increasingly well recognized that
ROS plays a key role in the sustainment of cancer and as a
potential target for cancer chemotherapy [18–22]. As ROS
and oxidative stress can induce cancer, and transformed
cells seem to generate more ROS compared to their normal
counterparts, applying ROS modulators such as nitrones as
chemotherapy agents may be a highly promising strategy.
Despite these facts, however, the identification of novel
nitrone-based therapeutic agents remains highly under-
developed owing to the lack of synthetically available
nitrone subtypes. Especially, in view of the low toxicity
profile of these compounds, development of novel nitrone-
type agents would not only be highly desirable for the
capability to bring up novel therapeutic subtypes, but also
provides new avenues for the elucidation of the mechanism
of action of these compounds.
We recently engaged in the development of novel
methodologies for the preparation of nitrones, and docu-
mented several metal-catalyzed routes for their efficient
preparation [23–25]. Given the high promise of nitrone and
the available compound library produced from these stu-
dies, we initiated a program aimed at evaluating in-vitro
anti-proliferative screening processes against cancer cell
lines. Herein we would like to report our preliminary results
in the identification of two types of N-heterocycle fused
nitrones as potential anti-cancer agents.

Medicinal Chemistry Research (2021) 30:1309–1316
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Fig. 2 Antiproliferative
activities of nitrones 1–5
incubated at 40 μM against a
panel of 5 cancer cell lines
Scheme 1 Synthesis of 3,4-
dihydropyrrolo[1,2-a]pyrazine
2-oxide derivatives
Fig. 3 In vitro antiproliferative
activities of 5a–5p incubated at
40 μM against a panel of 5
cancer cell lines
oxime starting materials is provided in Fig. S1 of the sup-
plementary material].
Furthermore, several indole-fused nitrones 4b–e were
also synthesized and evaluated. These compounds were
obtained from the corresponding carboxylic acids, which
were converted to Weinreb amides, followed by methyl
lithium addition to afford the corresponding methyl ketones.
Then, following a similar sequence as above, the corre-
sponding nitrones were obtained in high yields (Scheme 2).
The preliminary antiproliferative effects of these com-
pounds at 40 μM against the five cancer cell lines are shown in
Fig. 4. It can be inferred from Fig. 4 that compared to the

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Medicinal Chemistry Research (2021) 30:1309–1316
Scheme 2 Synthetic scheme for the indole-fused nitrones 4
Fig. 4 In vitro anti-proliferation
of indole-based nitrone
analogues 4a–4g at 40 μM
against 5 cancer cell lines
activity of 4a, introduction of 6- and 7-substituents most sig-
nificantly improved the activities against HL-60 cells.
Appreciable improvement in the activities against SMMC-
7721 was observed for 4b–4e, but not for 4f and 4g, which is
in line with the results against HL-60 cell line. Similar patterns
were also found for the activities against MCF-7. Compared to
4a, substituted analogues 4b–4g showed even worse activity
against A549 and SW-480 cell lines. These observations on
the cell line specificity suggest a particular mechanism of
action that targets more effectively against HL-60, SMMC-
7721, and MCF-7 but not for A549 and SW480 cell lines, on
which further studies are required. In contrast, the compara-
tively inferior overall activities displayed by 4f and 4g suggest
that functionalization at the 3-position of indole may be det-
rimental, and that the nitrone-substitution R’ is beneficial to
the anti-proliferative effects.
After obtaining preliminary in vitro single-dose data, we
subsequently selected several compounds that displayed the
highest activities in the initial screening for the determination
of IC50 values. The results were summarized in Table 1. As
can be seen, while unfortunately, 5d and 5l were found to be
ineffective in the dose-dependent activity test, showing that
their activities may vary depending on stability reasons,
compound 4b was found to be moderately active against
SMMC-7721 cell line; and 4d was found to be a good
candidate against leukemia model HL-60 cell line, demon-
strating an IC50 of 36.16 μM, showing that these compounds
hold promise for further applications in anti-cancer phar-
maceutics, and selectivity against different cell lines may be
realized by substitution modulation. [There seem to be some
discrepancy of activity between single dose and dose-
dependent activities of the tested compounds; we suspect
that this may arise from the mediocre stability of these
compounds in solution under aerobic conditions].
Conclusion
In summary, we have prepared two types of novel nitrone-
type compounds in moderate to good yields. Some of the
new compounds exhibited anticancer activities against the
human tumor cell lines HL-60, A549, SMMC-7721, MCF-7,
and SW-480 in the 40 μM initial screening, based on which
an indole-fused nitrone analogue, 4d, with good anti-
proliferative activities against HL-60 cell line, is identified.
The data support the conclusion that 3,4-dihydropyrazino

Medicinal Chemistry Research (2021) 30:1309–1316
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Table 1 IC50 values determined
Cmpd.
IC50/μM
of selected compounds
HL-60
A549
SMMC-7721
MCF-7
SW-480
5d
>200
>200
>200
>200
>200
5l
>200
>200
>200
>200
>200
4b
62.35 1.11
36.16 0.89
4.96 0.28
<0.008
170.02 3.50
>200
105.66 15.54
>200
>200
>200
4d
>200
>200
DPP
Taxol
25.33 0.73
<0.008
11.13 1.22
0.134 0.011
18.30 1.33
<0.008
21.74 0.30
<0.008
[1,2-a]indole 2-oxide derivatives are promising novel scaf-
fold for anti-tumor drug development. We anticipate that
these promising results may incite more extensive interest in
the development of nitrone-based therapeutic agents. Further
improvement upon these discoveries is undergoing in our
laboratory.
2–4 h before being tested by absorption by a microplate
reader (MULTISKAN FC) at 492 nm. Cells incubated in 1%
DMSO were used as 100% proliferation (i.e., DMSO = 100%
growth) and the relative growth for each compound was
compared to 1% DMSO. DPP and Taxol were used as
positive control. IC50 values of DPP and Taxol were calcu-
lated by Reed and Muench method from two separate
experiments performed in triplicate.
Experimental
IC50 value determination: cells were grown and seeded as
above. Compounds of five different concentrations (200,
100, 50, 25, and 12.5 μM), along with positive controls
(DPP and Taxol) were added to the cell plates, and the Cells
were returned to the incubator for an additional 48 h. After
48 h, for attaching cells media was removed, and MTS
(20 μL/cell) and media (100 μL/cell) were added. For non-
attaching cells, 100 μL of supernatant was removed and
20 μL of MTS is added. Cells are then cultivated for a
further 2–4 h before being tested by absorption by a
microplate reader (MULTISKAN FC) at 492 nm. Cells
incubated in 1% DMSO were used as 100% proliferation
(i.e., DMSO = 100% growth) and the relative growth for
each compound was compared to 1% DMSO. IC50 values
were calculated by Reed and Muench method from two
separate experiments performed in triplicate. DPP and
Taxol were used as positive control.
Synthesis of nitrones: to a solution of oximes (0.2 mmol,
1.0 equiv.) in 2 ml anhydrous DCM was added PdCl2
(CH3CN)2 (0.02 mmol, 10 mol%). The resulting solution
was heated to 100 oC for 20 h. The solvent was removed in
vacuo and the residue was purified by flash chromatograph
to afford nitrones. Compounds 1a [24], 1b [24], 1c [24],
2 [24], 3 [24], 4a [25], 4g [25], 5a [25], 5j [25], 5k [25], 5l
[25], 5m [24], 5n [25], 5o [24] were synthesized following
previously reported procedures.
Unless otherwise noted, all reactions were carried out with
distilled and dried solvents a nitrogen atmosphere. All
commercial chemicals were used without further purifica-
tion. Tetrahydrofuran and toluene was distilled from cal-
cium hydride. All Pd(CH3CN)2Cl2, NaOt-Bu, NH2OH·HCl,
ketones and aldehydes were purchased from commercial
sources. 1H NMR and 13C NMR spectra were recorded on a
Bruker 400 spectrometer at 22 oC. Chemical shifts (δ) were
expresses in part per million (ppm) relative to internal
standards (0 ppm (TMS) for 1H NMR and 77.0 ppm
(CDCl3) for 13C NMR). High resolution mass spectra were
recorded on a Bruker microTOF spectrometer. Flash chro-
matography was performed on silica gel 60 particle size
300–400 mesh ASTM, purchase from Taizhou, China. Cells
were and obtained from Shanghai National Collection of
Authenticated Cell Cultures (HL-60, A549, MCF-7, and
SW480) or Beijing Bei Na Biotechnology Institute
(SMMC-7721) and maintained in DMEM or RPMI1640
(Gibco) supplemented with 10% FBS.
MTS cell anti-proliferation assay
40 μM screening: cells were grown to confluence, seeded
(3000–15,000 cells/well, 100 μL total media) in clear, flat-
bottom 96-well plates and allowed to attach overnight.
Compounds at 40 μM in DMSO (final volume: 200 μL) was
added. Three blank cells were reserved for blank control (1%
DMSO). Cells were returned to the incubator for an additional
48 h. After 48 h, for attaching cells media was removed, and
MTS (20 μL/cell) and media (100 μL/cell) were added. For
non-attaching cells, 100 μL of supernatant was removed and
20 μL of MTS is added. Cells are then cultivated for a further
8-Chloro-1,3-dimethyl-3,4-dihydropyrazino[1,2-a]indole
2-oxide (4b): following the general produce, the reaction of
oxime S18 (0.2 mmol, 49.7 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product (4b) (46.8 mg, 94 %
yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.56
(d, J = 1.6 Hz, 1H), 7.21 (dd, J = 8.7, 1.8 Hz, 1H), 7.15 (d,
J = 8.7 Hz, 1H), 6.58 (s, 1H), 4.37 (dd, J = 15.9, 4.9 Hz,
2H), 4.11 (dd, J = 12.1, 3.2 Hz, 1H), 2.41 (s, 3H), 1.57 (d,

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Medicinal Chemistry Research (2021) 30:1309–1316
J = 6.7 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 135.76 (s),
135.55 (s), 131.78 (s), 129.74 (s), 126.34 (s), 124.13 (s),
120.74 (s), 109.74 (s), 101.30 (s), 63.57 (s), 45.45 (s), 16.94
(s), 13.45 (s). HRMS(ESI) calcd for C13H14ClN2O+ ([M+
H]+): 249.0789, found 249.0786.
90.90 (s), 63.81 (s), 55.71 (s), 45.28 (s), 16.48 (s), 13.65 (s).
HRMS(ESI) calcd for C14H16BrN2O2
323.0390, found 323.0393.
+
([M+H]+):
6,7-Dibromo-1,3-dimethyl-3,4-dihydropyrrolo[1,2-a]
pyrazine 2-oxide (5b): following the general produce, the
reaction of oxime S2 (0.2 mmol, 64.4 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product 5b (33.0 mg, 51%
yield)as a colorless oil; 1H NMR (400 MHz, CDCl3) δ 6.40
(s, 1H), 4.44–4.13 (m, 2H), 3.98 (d, J = 13.0 Hz, 1H), 2.27
(s, 3H), 1.53 (d, J = 6.7 Hz, 3H); 13C NMR (101 MHz,
CDCl3) δ 134.42 (s), 126.89 (s), 111.22 (s), 107.17 (s),
100.93 (s), 62.93 (s), 48.38 (s), 16.24 (s), 12.98 (s). HRMS
(ESI) calcd for C9H11Br2N2O+ ([M+H]+): 320.9233, found
320.9225.
1,3,8-Trimethyl-3,4-dihydropyrazino[1,2-a]indole
2-
oxide (4c): following the general produce, the reaction
of oxime S19 (0.2 mmol, 45.7 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product (4c) (37.1 mg, 81%
yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.39
(s, 1H), 7.12 (d, J = 2.7 Hz, 2H), 6.57 (s, 1H), 4.33 (dd, J =
19.0, 6.4 Hz, 2H), 4.19–3.98 (m, 1H), 2.42 (d, J = 5.4 Hz,
6H), 1.56 (d, J = 6.5 Hz, 3H). 13C NMR (101 MHz, CDCl3)
δ 135.97 (s), 135.88 (s), 130.56 (s), 129.99 (s), 129.02 (s),
125.60 (s), 121.00 (s), 108.39 (s), 101.66 (s), 63.48 (s),
45.30 (s), 21.26 (s), 16.90 (s), 13.43 (s). HRMS(ESI) calcd
for C14H17N2O+ ([M+H]+): 229.1335, found 229.1344.
8-Methoxy-1,3-dimethyl-3,4-dihydropyrazino[1,2-a]
1,3-Dimethyl-6-nitro-3,4-dihydropyrrolo[1,2-a] pyrazine
2-oxide (5c): following the general produce, the reaction of
oxime S3 (0.2 mmol, 42.0 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product 5c (39.3 mg, 93%
yield) as a colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.27
(d, J = 4.0 Hz, 1H), 6.37 (d, J = 4.5 Hz, 1H), 4.83 (dd, J =
14.4, 5.3 Hz, 1H), 4.74 (dd, J = 14.4, 4.9 Hz, 1H), 4.34 (dd,
J = 11.4, 6.0 Hz, 1H), 2.35 (s, 3H), 1.58 (t, J = 5.2 Hz, 3H);
13C NMR (101 MHz, CDCl3) δ 137.95 (s), 134.00 (s),
130.79 (s), 115.35 (s), 108.17 (s), 63.01 (s), 48.03 (s),
indole 2-oxide (4d): following the general produce, the
reaction of oxime S20 (0.2 mmol, 48.9 mg), PdCl2(CH3CN)
2 (0.02 mmol, 5.2 mg) afforded product (4d) (44.4 mg, 91%
yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.13
(d, J = 8.9 Hz, 1H), 7.04 (d, J = 2.1 Hz, 1H), 6.93 (dd, J =
8.9, 2.2 Hz, 1H), 6.57 (s, 1H), 4.33 (dd, J = 19.0, 6.5 Hz,
2H), 4.07 (dd, J = 12.2, 3.0 Hz, 1H), 3.83 (s, 3H), 2.41 (s,
3H), 1.56 (d, J = 6.5 Hz, 3H). 13C NMR (101 MHz, CDCl3)
δ 154.73 (s), 135.90 (s), 132.82 (s), 131.04 (s), 129.23 (s),
114.52 (s), 109.54 (s), 102.70 (s), 101.72 (s), 63.46 (s),
55.73 (s), 45.48 (s), 16.90 (s), 13.42 (s). HRMS(ESI) calcd
+
16.04 (s), 13.03 (s). HRMS(ESI) calcd for C9H12N3O3
([M+H]+): 210.0873, found 210.0879.
1,3-Dimethyl-7-nitro-3,4-dihydropyrrolo[1,2-a] pyrazine
2-oxide (5d): following the general produce, the reaction of
oxime S4 (0.2 mmol, 42.0 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product 5d (34.3 mg, 81%
yield) as a colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.58
(s, 1H), 6.83 (s, 1H), 4.45 (dd, J = 13.4, 4.5 Hz, 1H), 4.35
(s, 1H), 4.02 (dd, J = 13.3, 3.7 Hz, 1H), 2.33 (s, 3H), 1.55
(d, J = 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 138.17
(s), 134.05 (s), 125.83 (s), 122.65 (s), 103.24 (s), 63.15 (s),
49.02 (s), 29.54 (s), 16.19 (s), 13.04 (s). HRMS(ESI) calcd
+
for C14H17N2O2 ([M+H]+): 245.1285, found 245.1279.
7-Methoxy-1,3-dimethyl-3,4-dihydropyrazino[1,2-a]
indole 2-oxide (4e): following the general produce, the
reaction of oxime S21 (0.2 mmol, 48.9 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product (4e) (42.7 mg, 87 %
yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.47
(d, J = 8.7 Hz, 1H), 6.78 (dd, J = 8.7, 2.0 Hz, 1H), 6.67
(s, 1H), 6.59 (s, 1H), 4.45–4.23 (m, 2H), 4.07 (dd, J = 12.4,
3.3 Hz, 1H), 3.86 (s, 3H), 2.39 (s, 3H), 1.56 (d, J = 6.7 Hz,
3H). 13C NMR (101 MHz, CDCl3) δ 157.96 (s), 138.51 (s),
136.08 (s), 129.61 (s), 123.02 (s), 122.23 (s), 110.88 (s),
102.42 (s), 92.01 (s), 63.24 (s), 55.55 (s), 45.29 (s),
+
for C9H12N3O3 ([M+H]+): 210.0873, found 210.0879.
7-Bromo-3-methyl-3,4-dihydropyrrolo[1,2-a]pyrazine 2-
oxide (5e): following the general produce, the reaction of
oxime S5 (0.2 mmol, 46.0 mg), PdCl2(CH3CN)2
(0.02 mmol, 5.2 mg) afforded product 5e (22.7 mg, 49%
yield) as a colorless oil; 1H NMR (400 MHz, CDCl3) δ 7.52
(s, 1H), 6.72 (s, 1H), 6.27 (s, 1H), 4.31 (dd, J = 13.1,
4.5 Hz, 1H), 4.19 (d, J = 6.0 Hz, 1H), 3.90 (dd, J = 13.1,
4.0 Hz, 1H), 1.52 (d, J = 6.7 Hz, 3H); 13C NMR (101 MHz,
CDCl3) δ 125.12 (s), 124.33 (s), 122.62 (s), 111.36 (s),
98.47 (s), 63.49 (s), 48.95 (s), 16.12 (s). HRMS(ESI) calcd
for C8H10BrN2O+ ([M+H]+): 228.9971, found 228.9966.
6,7-Diiodo-1,3-dimethyl-3,4-dihydropyrrolo[1,2-a] pyr-
azine 2-oxide (5f): following the general produce, the
reaction of oxime S6 (0.1 mmol, 41.6 mg), PdCl2(CH3CN)2
(0.01 mmol, 2.6 mg) afforded product 5f (36.0 mg, 87%
yield) as a colorless oil; 1H NMR (400 MHz, CDCl3) δ 6.37
+
16.93 (s), 13.41 (s). HRMS(ESI) calcd for C14H17N2O2
([M+H]+): 245.1285, found 245.1279.
10-Bromo-8-methoxy-1,3-dimethyl-3,4-dihydropyrazino
[1,2-a]indole 2-oxide (4f): following the general produce,
the reaction of oxime S22 (0.2 mmol, 64.6 mg), PdCl2
(CH3CN)2 (0.02 mmol, 5.2 mg) afforded product (4f)
(42.9 mg, 65% yield) as a colorless oil. 1H NMR (400 MHz,
CDCl3) δ 7.10 (d, J = 9.4 Hz, 1H), 6.96 (d, J = 7.4 Hz, 2H),
4.32 (dd, J = 21.4, 8.7 Hz, 2H), 4.14–3.97 (m, 1H), 3.86 (s,
3H), 2.69 (s, 3H), 1.51 (d, J = 6.6 Hz, 3H). 13C NMR
(101 MHz, CDCl3) δ 155.26 (s), 135.82 (s), 130.81 (s),
129.13 (s), 126.48 (s), 116.20 (s), 109.67 (s), 100.80 (s),

Medicinal Chemistry Research (2021) 30:1309–1316
1315
(s, 1H), 4.32–4.13 (m, 2H), 3.96 (dd, J = 13.0, 3.7 Hz, 1H),
2.24 (s, 3H), 1.50 (d, J = 6.6 Hz, 3H); 13CNMR (101 MHz,
CDCl3) δ 134.08 (s), 126.91 (s), 111.00 (s), 106.95 (s),
100.82 (s), 62.86 (s), 48.31 (s), 16.14 (s), 12.84 (s). HRMS
(ESI) calcd for C9H11I2N2O+ ([M+H]+): 416.8955, found
416.8954.
Acknowledgements We are grateful to National Natural Science
Foundation of China (21871045), the natural science foundation of
Jilin Province (20190201070JC) and Changchun University of Tech-
nology for generous financial support.
Compliance with ethical standards
3,3-Dimethyl-3,4-dihydropyrrolo[1,2-a] pyrazine 2-oxide
(5g): following the general produce, the reaction of oxime S7
(0.2 mmol, 32.8 mg), PdCl2(CH3CN)2 (0.02 mmol, 5.2 mg)
afforded product 5g (8.4 mg, 26% yield) as a colorless oil; 1H
NMR (400 MHz, CDCl3) δ 7.60 (s, 1H), 6.71 (s, 1H),
6.31–6.09 (m, 2H), 4.01 (s, 2H), 1.50 (s, 7H); 13C NMR
(101 MHz, CDCl3) δ 126.04 (s), 124.24 (s), 123.00 (s),
110.83 (s), 109.46 (s), 66.47 (s), 54.62 (s), 23.18 (s). HRMS
(ESI) calcd for C9H13N2O+ ([M+H]+): 165.1022, found
165.1030.
Conflict of interest The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
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6.70 (s, 1H), 6.40 (d, J = 2.6 Hz, 1H), 4.50 (dd, J = 13.2,
3.9 Hz, 1H), 4.28 (s, 1H), 4.19 (dd, J = 13.2, 3.8 Hz, 1H),
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