Synthesis of a Nitroxide Spin-labeled Varenicline (Chantix) Derivative

Full text of DOI:10.1080/00304948.2021.1877997

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/uopp20
Synthesis of a Nitroxide Spin-labeled Varenicline
(Chantix) Derivative
Balázs Bognár, Mostafa Isbera & Tamás Kálai
To cite this article: Balázs Bognár, Mostafa Isbera & Tamás Kálai (2021) Synthesis of a Nitroxide
Spin-labeled Varenicline (Chantix) Derivative, Organic Preparations and Procedures International,
53:3, 311-315, DOI: 10.1080/00304948.2021.1877997
To link to this article: https://doi.org/10.1080/00304948.2021.1877997
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ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
2021, VOL. 53, NO. 3, 311–315
https://doi.org/10.1080/00304948.2021.1877997
OPPI BRIEF
Synthesis of a Nitroxide Spin-labeled Varenicline
(Chantix) Derivative
a
a,b
, Mostafa Isberaa , and Tamas Kalai
ꢀ
ꢀ
ꢀ ꢀ
Balazs Bognar
a
ꢀ
ꢀ
Institute of Organic and Medicinal Chemistry, Medical School, University of Pecs, Pecs, Hungary;
Szentagothai Research Centre, Pecs, Hungary
b
ꢀ
ꢀ
ARTICLE HISTORY Received 4 June 2020; Accepted 30 September 2020
The progress made during the past three decades on the synthesis and application of
stable nitroxide free radicals has facilitated understanding of the structure and function
of macromolecular systems.1 Spin-labeled drugs and biomolecules have been con-
structed for target-specific applications such as the study of biological systems,2 utiliza-
tion in therapy, and studying drug formulation problems.3 Our laboratory has a long-
standing interest in the synthesis and application of carbo- and heterocycle-fused pyrro-
line nitroxides;4 we recently reported the efficient synthesis of pyrroline nitroxide-fused
pyrazine and quinoxaline derivatives.5 Pyrazine derivatives exhibit numerous advanta-
geous pharmaceutical effects including antibacterial, antifungal, antiviral, anticancer,
antiproliferative, antidiabetic, diuretic, hypnotic, and analgesic properties.6 A notable
pyrazine derivative is the nicotinic receptor agonist varenicline 1 (Chantix, Scheme 1).
Clinically, it is effective as a smoking cessation drug, which attenuates the effect of nico-
tine by selectively binding to neuronal a4b2 nicotine acetylcholine receptors.7
Building on our previous work, we now report on an analog of varenicline, 7
(Scheme 2), which incorporates a rigid 1-oxyl-2,2,5,5-tetramethylpyrrolidine nitroxide
spin-label fused to the pyrazine ring. Commercially available dinitro compound 2 was
R
V
hydrogenated using a H-Cube Mini Plus flow reactor equipped with a 20% Pd(OH)2/
C cartridge at a pressure of 6 ꢀ 105 Pa H2 in a 1:1 (v/v) mixture of THF/MeOH to fur-
nish an intermediate diamine, which was condensed immediately with 1,2-diketone5 3
to yield pyrazine 4. The N-OMe function was deprotected with 2.0 equivalents of 3-
chloroperbenzoic acid8 (m-CPBA) in CH2Cl2 (DCM) to yield a mixture of nitroxide 5
and the pyrazine-N-oxide nitroxide 6 side product. Compound 6 was obtained as a
non-separable mixture of diastereomers. After chromatographic separation of 5 from 6,
the trifluoroacetyl group was removed from nitroxide 5 by treatment with aqueous
Na2CO3/MeOH9 to furnish the spin-labeled varenicline analog 7. In conclusion, a facile
method was developed for synthesis of a spin labeled derivative of varenicline without
affecting the key functional groups of the original molecule, providing an access for a
possible new theranostic10 agent.
ꢀ
ꢀ
ꢀ
CONTACT Tamas Kalai
tamas.kalai@aok.pte.hu
Institute of Organic and Medicinal Chemistry, Medical School,
ꢀ
University of Pecs, Pecs, Hungary
Supplemental data for this article is available online at https://doi.org/10.1080/00304948.2021.1877997
ß 2021 Taylor & Francis Group, LLC

312
B. BOGNÁR ET AL.
Scheme 1. Structure of varenicline.
Scheme 2. Synthesis of spin-labeled varenicline analog 7.
Experimental section
Mass spectra were recorded using a GCMS-2020 (Shimadzu Corporation) gas chromato-
graph mass spectrometer operated in EI mode (70 eV). Elemental analyses were meas-
ured on a Fisons EA 1110 CHNS elemental analyzer and melting points were
1
determined on a Boetius micro-melting point apparatus and are uncorrected. H NMR
and 13C NMR spectra were recorded in DMSO-d6 using a Bruker Avance III Ascend
500 spectrometer operating at 500 MHz and 125 MHz respectively; chemical shifts are
referenced to TMS. The in situ reduction of nitroxide radicals was achieved by the add-
ition of five equivalents of hydrazobenzene (DPPH)/radical. EPR spectra were recorded
on a MiniScope MS 200 spectrometer. IR spectra were obtained using a Bruker Alpha
FT-IR instrument with an ATR accessory on a diamond plate. Hydrogenations were
R
performed with a ThalesNano H-CubeV Mini Plus reactor with 20% Pd(OH)2/C cart-
ridge (Pearlman’s catalyst, Thalesnano, cat. no. THS-01115). Compound 2 was pur-
chased from Toronto Research Chemicals (cat. no. T293705). 1H NMR, 13C NMR,
spectra of all compounds and EPR spectrum of compound 7 can be found via the
“Supplementary Content” section of this article’s webpage.
2,2,2-Trifluoro-1-(2-methoxy-1,1,3,3-tetramethyl-2,3,6,7,9,10-hexahydro-6,10-
methanoazepino[4,5-g]pyrrolo[3,4-b]quinoxalin-8(1H)-yl)ethanone (4)
A solution of compound 2 (345 mg, 1.0 mmol) in a mixture of anhydrous THF/MeOH
R
(1:1 v/v, 80 mL) was reduced by hydrogenation using a H-CubeV Mini Plus flow reactor
equipped with a 20% Pd(OH)2/C cartridge at a pressure of 6 ꢀ 105 Pa H2 and a flow

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
313
rate of 0.7 mL minꢁ1. After consumption of the starting material (monitored by TLC,
Merck Silica gel 60 F254, hexane/EtOAc, 2:1), the solvents were evaporated. The residue
was dissolved in anhydrous EtOH (10 mL), and a solution of compound 3 (185 mg,
1.0 mmol) in anhydrous EtOH (10 mL) was added. The mixture was refluxed for 3 h
and allowed to stand in air overnight. The solvent was evaporated, and the residue was
purified by flash column chromatography (Merck Silica gel 60, 0.040-0.063 mm,
hexane–Et2O, 2:1) to give an off-white powder (278 mg, 64%); mp 200-202 ꢂC; Rf ¼
1
0.32 (Merck Silica gel 60 F254, hexane/EtOAc, 2:1); IR: (neat) 2980, 2932, 1685 cmꢁ1. H
NMR (DMSO-d6, 500 MHz): d (ppm) 1.49 (s, 12H), 2.18 (d, J ¼ 11 Hz, 1H), 2.28 (m,
1H), 3.35 (s, 2H), 3.56 (s, 2H), 3.78 (s, 3H), 3.89 (d, J ¼ 12 Hz, 1H), 4.37 (d, 1H,
J ¼ 12 Hz), 7.94 (s, 1H), 7.96 (s, 1H). 13C NMR (DMSO-d6, 125 MHz): d (ppm) 23.2
(2C), 28.9 (2C), 41.3 (2C), 48.6 (2C), 50.7 (1C), 65.7 (1C), 65.8 (2C), 122.4 (1C), 122.6
(1C), 142.8 (2C), 147.1 (2C), 147.6 (2C), 158.9 (d, J ¼ 9 Hz, 1C) (CF3 signal is missing).
MS (EI): m/z (%) ¼ 434 (3, [M]þ). 419 (100), 373 (22), 69 (7), 57 (11), 43 (13).
Anal. Calc. for C22H25F3N4O2: C, 60.82; H, 5.80; N, 12.90. Found: C, 60.78; H, 5.82; N, 12.72.
2,2,2-Trifluoro-1-(2-oxyl-1,1,3,3-tetramethyl-2,3,6,7,9,10-hexahydro-6,10-
methanoazepino[4,5-g]pyrrolo[3,4-b]quinoxalin-8(1H)-yl))ethanone radical (5) and
(6R(S),10R(S))-2-oxyl-1,1,3,3-tetramethyl-8-(2,2,2-trifluoroacetyl)-1,2,3,6,7,8,9,10-
octahydro-6,10-methanoazepino[4,5-g]pyrrolo[3,4-b]quinoxaline 4-oxide
radical (6)
To a stirred solution of compound 4 (220 mg, 0.506 mmol) in anhydrous DCM (20 mL),
3-chloroperbenzoic acid (ꢃ60%, 290 mg, 1.01 mmol, 2.0 eq) was added in 2-3 portions
at 0 ꢂC over a period of 10 min and stirring was continued at ambient temperature with
continuous monitoring by TLC (Merck Silica gel 60 F254, hexane/EtOAc, 2:1).
Consumption of the starting material (after ꢃ30 min) resulted in the formation of
deprotected nitroxide 5 together with pyrazine-N-oxide nitroxide 6 as a side product.
The solution was washed with 10% aq. Na2CO3 solution (20 mL ꢀ 2), and the organic
phase was separated, dried (MgSO4), filtered, and evaporated. The residue was purified
by flash column chromatography (Merck Silica gel 60, 0.040-0.063 mm, hexane–EtOAc,
2:1) to give compound 5 as a yellow powder (108 mg, 51%); mp 212–215 ꢂC; Rf ¼ 0.44
1
(Merck Silica gel 60 F254, CHCl3/Et2O, 2:1); IR: (neat) 2985, 2879, 1685 cmꢁ1. H-NMR
(DMSO-d6 þ (PhNH)2, 500 MHz): d (ppm) 1.43 (s, 6H), 1.44 (s, 6H), 2.13 (d,
J ¼ 11 Hz, 1H), 2.26 (m, 1H), 3.25 (d, J ¼ 12 Hz, 1H), 3.72 (d, J ¼ 12 Hz, 1H), 3.87 (d,
J ¼ 12 Hz, 1H), 4.25 (d, J ¼ 12 Hz, 1H), 7.64(s, 1H), 8.09 (s, 1H). 13C-NMR (DMSO-d6,
125 MHz): d (ppm) 25.1 (1C), 25.2 (1C), 25.4 (1C), 25.5 (1C), 41.4 (1C), 48.6 (2C), 50.7
(2C), 65.2 (2C), 116.5 (q, J ¼ 287 Hz, 1C), 122.4 (1C), 122.6 (1C), 142.8 (2C), 146.7
(2C), 147.2 (2C), 159.7 (d, J ¼ 8 Hz, 1C). MS (EI): m/z (%) ¼ 419 (70, [M]þ), 389 (100),
374 (36), 262 (58), 139 (26).
Anal. Calc. for C21H22F3N4O2: C, 60.14; H, 5.29; N, 13.36. Found: C, 60.02; H, 5.31; N, 13.25.
Compound 6 was obtained as a yellow powder (60 mg, 27%); mp 228–230 ꢂC; Rf ¼
0.37 (Merck Silica gel 60 F254, CHCl3/Et2O, 2:1); IR: (neat) 3012, 2871, 1688,
1
1583 cmꢁ1. H NMR (DMSO-d6 þ (PhNH)2, 500 MHz): d (ppm) 1.40 (s, 3H), 1.42 (s,
3H), 1.45 (s, 3H), 1.54 (s, 3H), 2.16 (d, J ¼ 11 Hz, 1H), 2.27 (m, 1H), 3.34 (d, J ¼ 12 Hz,

314
B. BOGNÁR ET AL.
1H), 3.53-3.58 (m, 2H), 3.75 (d, J ¼ 12Hz, 1H), 3.89 (d, J ¼ 12 Hz, 1H), 4.24 (d,
J ¼ 12 Hz, 1H), 7.98(d, J ¼ 13 Hz, 1H), 8.38 (d, J ¼ 15Hz, 1H). 13C NMR (DMSO-d6,
125 MHz): d (ppm) 21.9-22.3 (2C), 25.0-25.2 (2C), 44.3 (1C), 48.4 (2C), 50.6 (2C), 65.1
(1C), 66.4 (1C), 112.0 (1C), 122.7 (1C), 137.0 (1C), 141.0 (1C), 145.7 (1C), 147.1 (1C),
147.6 (1C), 148.7 (1C), 149.2 (1C), 162.1 (d, J ¼ 8 Hz, 1C). MS (EI): m/z (%) ¼ 435 (4,
[M]þ). 405 (39), 388 (100), 98 (52).
Anal. Calcd. for C21H22F3N4O3: C, 57.93; H, 5.09; N, 12.87. Found C, 58.02; H, 5.13;
N, 12.70.
1,1,3,3-Tetramethyl-3,6,7,8,9,10-hexahydro-6,10-methanoazepino[4,5-
g]pyrrolo[3,4-b]quinoxalin-2(1H)-yloxyl radical (7)
To a solution of compound 5 (100 mg, 0.23 mmol) MeOH (2.0 mL) was added in a
solution of Na2CO3 (48.7 mg, 0.46 mmol, 2.0 eq) in distilled water (2.0 mL). The mixture
was warmed to 70 ꢂC for 2 h, then the solvents were evaporated. The residue was treated
with distilled water (20 mL) and extracted with DCM (10 mL ꢀ 3). The organic phase
was separated, dried (MgSO4), filtered, evaporated, and the crude reaction mixture was
purified by flash column chromatography (Merck Silica gel 60, 0.040-0.063 mm,
hexane–EtOAc 2:1, then CHCl3/MeOH, 9:1) to give an orange powder (56 mg, 75%);
mp 205–207 ꢂC; Rf ¼ 0.33 (Merck Silica gel 60 F254, CHCl3/MeOH, 5:1); IR: (neat)
1
3321, 3076, 2974, 1574 cmꢁ1. H-NMR (DMSO-d6, 500 MHz): d (ppm) 1.46 (s, 12H),
1.89 (s, 1H), 1.95 (d, J ¼ 10 Hz, 1H), 2.31 (bs, 1H), 2.74 (d, J ¼ 11 Hz, 2H), 2.95 (d,
J ¼ 12Hz, 2H) 3.11 (s, 2H), 7.80 (s, 2H), 13C-NMR (DMSO-d6, 125 MHz): d (ppm) 25.3
(2C), 25.4 (2C), 42.1 (1C), 43.4 (2C), 50.2 (2C), 65.2 (2C), 122.6 (2C), 143.0 (2C), 149.2
(2C), 158.9 (2C). MS (EI): m/z (%) ¼ 323 (45, [M]þ), 293 (11), 280 (13), 250 (100),
98(49), 57(61). EPR triplet line, aN ¼ 15.3 G, radical content > 98% (in 0.15 M glycine
buffer, pH ¼ 3.1).
Anal. Calc. for C19H23N4O: C, 70.56; H, 7.17; N, 17.32. Found: C, 70.48; H, 7.09;
N, 17.18.
Acknowledgment
This work was supported by the Hungarian National Research, Development and Innovation
Office (FK124331) and GINOP-2.3.2-15-2016-00049. MI thanks the Hungarian Stipendium
Scholarship for support.
ORCID
ꢀ
ꢀ
Balazs Bognar
http://orcid.org/0000-0001-6695-6839
http://orcid.org/0000-0001-8539-3023
Mostafa Isbera
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