Stable bicyclic functionalized nitroxides: The synthesis of derivatives of aza-nortropinone–5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene 8-oxyls

Article abstract of DOI:10.3390/molecules26103050

In recent decades, bicyclic nitroxyl radicals have caught chemists’ attention as selective catalysts for the oxidation of alcohols and amines and as additives and mediators in directed C-H oxidative transformations. In this regard, the design and development of synthetic approaches to new functional bicyclic nitroxides is a relevant and important issue. It has been reported that imidazo[1,2-b]isoxazoles formed during the condensation of acetylacetone with 2-hydroxyaminooximes having a secondary hydroxyamino group are recyclized under mild basic catalyzed conditions to 8-hydroxy-5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octenes. The latter, containing a sterically hindered cyclic N-hydroxy group, upon oxidation with lead dioxide in acetone, virtually quantitatively form stable nitroxyl bicyclic radicals of a new class, which are derivatives of both 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (TEMPON) and 3-imidazolines.

Full text of DOI:10.3390/molecules26103050

molecules
Article
Stable Bicyclic Functionalized Nitroxides: The Synthesis of
Derivatives of Aza-nortropinone–5-Methyl-3-oxo-6,8-
diazabicyclo[3.2.1]-6-octene 8-oxyls
Larisa N. Grigor’eva, Alexsei Ya. Tikhonov *, Konstantin A. Lomanovich and Dmitrii G. Mazhukin *
N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Academician Lavrentiev Ave. 9,
630090 Novosibirsk, Russia; lng@nioch.nsc.ru (L.N.G.); lomanovich@nioch.nsc.ru (K.A.L.)
* Correspondence: alyatikh@nioch.nsc.ru (A.Y.T.); d-mazhukin@yandex.ru (D.G.M.);
Tel.: +7-383-330-8867 (A.Y.T.); +7-383-330-6852 (D.G.M.)
Abstract: In recent decades, bicyclic nitroxyl radicals have caught chemists’ attention as selective
catalysts for the oxidation of alcohols and amines and as additives and mediators in directed C-H
oxidative transformations. In this regard, the design and development of synthetic approaches to new
functional bicyclic nitroxides is a relevant and important issue. It has been reported that imidazo[1,2-
b]isoxazoles formed during the condensation of acetylacetone with 2-hydroxyaminooximes having a
secondary hydroxyamino group are recyclized under mild basic catalyzed conditions to 8-hydroxy-
5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octenes. The latter, containing a sterically hindered cyclic
N-hydroxy group, upon oxidation with lead dioxide in acetone, virtually quantitatively form sta-
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ble nitroxyl bicyclic radicals of a new class, which are derivatives of both 2,2,6,6-tetramethyl-4-
ꢀꢁꢂꢃꢄꢅꢆ
oxopiperidine-1-oxyl (TEMPON) and 3-imidazolines.
Citation: Grigor’eva, L.N.; Tikhonov,
A.Y.; Lomanovich, K.A.; Mazhukin,
Keywords: bicyclic nitroxide; condensation; acetylacetone; base-catalyzed recyclization; 3-imidazoline
nitroxide; TEMPON; 8-hydroxy-5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene
D.G. Stable Bicyclic Functionalized
Nitroxides: The Synthesis of
Derivatives of Aza-nortropinone–5-
Methyl-3-oxo-6,8-diazabicyclo[3.2.1]-
6-octene 8-oxyls. Molecules 2021, 26,
1. Introduction
3050. https://doi.org/10.3390/
molecules26103050
Among various types of organic radicals, nitroxyl radicals (NRs) are the most stable,
most robust, and best-studied class of paramagnetic species. In addition to numerous
publications on their synthesis and properties [1–9], more and more reports include numer-
Academic Editor: Mario Waser
ous examples of their use. Thus, NRs are actively employed as oxidants and as trapping
reagents in synthetic chemistry [10], as ligands in metal complex catalysis [11], as agents in
nitroxide-mediated radical polymerization [12], as molecular spin probes [13], and spin
labels [14] and in many other applications [15,16].
Received: 20 April 2021
Accepted: 18 May 2021
Published: 20 May 2021
In the last decade, new areas of the use of nitroxides have been actively developing.
For instance, NRs can participate in UV—or visible-light—excited photosynthesis owing
to their ability to participate in light-induced hydrogen atom transfer processes [17–19].
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
A popular NR, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), has been employed as a
mediator for obtaining N-centered radicals during electrosynthesis. For example, TEMPO-
mediated electrolysis of anilides or ketoximes generates amidyl or iminoxy radicals that
trigger subsequent cyclization via an interaction of the forming radical with an unsaturated
part of the molecule thereby leading to the closure of a pyrroline-type structure or cyclic
nitrone [20]. With a sterically more hindered dispirocyclic NR under the conditions of
electrochemical synthesis from sodium azide, it is possible to generate an azidyl radi-
cal, which at low temperatures can react twice with alkenes thus implementing alkene
diazidation [21]. Taking into account all these “old” and new abilities of stable NRs, it
seems worthwhile to direct our efforts toward the design and synthesis of new classes
of functional nitroxides that may find applications in various modern fields of materials
chemistry and biology.
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Molecules 2021, 26, 3050. https://doi.org/10.3390/molecules26103050
https://www.mdpi.com/journal/molecules

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Bicyclic NRs (BNRs) have been known for a long time, starting from the 1960s
(
Figure 1, structures
alcohols, in particular, those containing a sterically hindered hydroxyl group, began rel-
atively recently [22 26]. Other functional groups—primary amines [27], diols [28], and
A–G); however, their active use as catalysts for the oxidation of
–
cyclic carbamates [29]—as it turned out, are also prone to oxidative transformations in the
presence of BNR mediators, forming nitriles and lactone derivatives. Moreover, BNRs were
recently shown to be superior additives for enantioselective Ni-catalyzed diarylation of
vinylarenes [30] and as mediators for electrochemical
morpholine, and pyrrolidine [31].
α-cyanation of secondary piperidines,
Figure 1. Selected examples of known BNRs: (
quinuclidine N-oxyl [33]; ( ) nortropane N-oxyl [34]; (
azabicyclo[3.3.1]nonane N-oxyl (ABNO) [36]; ( ) 2-azaadamantane N-oxyl (AZADO) [37]; and
A
) Norpseudopelleterine N-oxyl [32]; (B) trimethyliso-
C
D
) dimethylnortropinone N-oxyl [35]; (
E) 9-
F
(G) 9-azanoradamantane N-oxyl (nor-AZADO) [38]. (H) The BNRs obtained in this work.
In the present work, we report the first synthesis of BNRs that are derivatives of
5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene 8-oxyl, combining 2,2,6,6-tetramethyl-4-
oxopiperidine-1-oxyl (TEMPON) and 3-imidazoline NR moieties (Figure 1, structure H).
This paper is a tribute to Professor Leonid B. Volodarsky who was the founder of the
school of stable NRs of the imidazoline series and has inspired our research by his brilliant
ideas for over 40 years.
2. Results
2.1. The Synthesis of Bicyclic Nitroxides
Previously, it has been demonstrated that the reaction of acetylacetone with 2-hydrox-
yamino oximes having a hydroxyamino group at the secondary carbon atom, 1a
ceeds through a number of intermediates in tautomeric equilibrium with each other and
finally leads to derivatives of tetrahydroimidazo[1,2-b]isoxazoles 2a 39]. Refluxing of
–d, pro-
–d [
the latter in aqueous alkali is accompanied by the loss of a water molecule followed by
recyclization, which entails the opening of the five-membered heterocycle and the forma-
tion of a six-membered tetrahydropyridine derivative (cyclic enaminoketone 3a
main product and 3-acetyl-1-hydroxypyrrole 4a as a minor product. We found that this
alkali-catalyzed reaction for compounds 2a under much milder conditions (cooling at
0–5 ◦C, 3–4 h) yields a completely different result: bicycle
becomes the main product. Its
structure was proved on the basis of spectral and analytical data. For instance, IR spectra
of compounds 5a
(in KBr) contain intense bands at 1714–1716 and 1629–1639 cm−1
corresponding to stretching vibrations of isolated C=O and C=N bonds, as well as a band at
3600 cm−1 (in CHCl3), corresponding to the presence of an OH group. Compounds 5a
,c,d
–d) as the
–d
,c,d
5
,
c,
d
,
do not have absorption maxima in the UV spectrum above 220 nm, indicating the absence
of a nitrone group or a conjugated system of unsaturated moieties in the molecule. Analysis
of their 1H and 13C NMR spectra points to the existence of bicycle
5 in the form of a mixture
of three isomers, one of which is cyclic hemiacetal 5C and the other two are the respective
hydroxyamino derivatives in which the N-OH bond is oriented either toward the keto

Molecules 2021, 26, 3050
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group (compound 5O
in the PMR spectra of compounds 5a
are observed (at 7.45–7.54, 7.88–7.92, and 8.29–8.33 ppm), corresponding to protons of
the C-OH group of the hemiacetal and N-OH protons for isomers 5O cis and 5O trans
The ratio of these species is shifted toward the prevailing, hypothetically, 5O trans con-
formation (47–65%), while the proportion of 5C is 22–44%, and the minor component is
always 5O
cis (1–13%). This structural assignment is confirmed by the 13C NMR spectra
of bicycles 5a , in which carbonyl C atoms for 5O cis and 5O trans isomers are clearly
distinguished at ~206 and 207 ppm, whereas the unique quaternary carbon atom of 5C
linked to two oxygen atoms, appears at 114 ppm (Supplementary Materials, Figures S1–S6).
-
cis) or away from it (compound 5O
-
trans
)
(Scheme 1). Accordingly,
,c,d in a weak field, three distinct broadened signals
-
-
.
-
-
,
c,d
-
-
,
Scheme 1. Interaction of 2-hydroxyamino oximes 1a
b]isoxazoles 2a–d.
–d with acetylacetone and base-mediated transformations of imidazo[l,2-
We found that bicycle
oxidized by lead dioxide to form a stable NR (6a
Acetone is a solvent of choice for the preparation of BNRs; in this case, the yields of
compounds 6a could reach 80–87% (Scheme 1). It should be noted that to achieve
the highest yield of the radical, pure recrystallized samples of bicycle should be used
because the presence of impurities in the substrate affects the stability of during its
5, encumbered with a sterically hindered N-OH group, can be
,
6c, or 6d) as the only reaction product.
,c,d
5
6

Molecules 2021, 26, 3050
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isolation, namely, during the concentration of the reaction solution. BNRs 6a
,
c,
d
are solid
purple fine-crystalline compounds stable for a long time in a solid state in ambient air at
room temperature in the dark. The radicals are readily soluble in polar organic solvents
and organochlorides and relatively well soluble in water; for example, the solubility of
BNR 6c at ambient temperature is ~2.5
×
10−2 M. Their IR spectra resemble those of their
diamagnetic precursors, except that the band at >3100 cm−1 is absent due to vibrations of
the N-OH group.
In the mass spectra of BNRs 6, along with the molecular ion peak at 181 (6a), 207 (6c),
or 221 (6d), M + 1 ions are present, and the most intense peak belongs to the M
corresponding to a loss of the N-O group (Supplementary Materials).
−
30 ion
To confirm the structure of the obtained BNRs 6, they were reduced to their diamagnetic
precursors 5 via a reaction with zinc powder in the presence of NH4Cl in aqueous acetone.
2.2. EPR Study of New Bicyclic Nitroxides 6
The EPR spectra of BNRs
6 recorded for degassed solutions in toluene represent
triplets with hyperfine coupling (hfc) constant aN of ~1.855 mT, for one of which (radical
6c), a fine structure is observed due to the presence of a hyperfine interaction with hydrogen
−
−
atoms in the CH2 and CH3 groups (Figure 2).
NR 6c
giso = 2.00664
aN = 1.855 mT
NR 6a
giso = 2.0054
aN = 1.852 mT
NR 6d
giso = 2.00667
aN = 1.841 mT
348.0
348.2
348.4
348.6
348.8
349.0
349.2
349.4
346
348
350
B, mT
352
354
B, mT
(B)
(A)
Figure 2.
(A) EPR spectra of new bicyclic nitroxides, from the top: radicals 6c, 6a, and 6d. (B) The center line in the spectrum
of NR 6c. The black line: experimental spectrum, red line: mathematical reconstruction.
A similar splitting of the triplet has been documented earlier by Rassat et al. for the
NR series of nortropinone (Figure 1, structure ), in which the hydrogen atoms located
on the - and
0-carbon atoms of the carbonyl group (C-2,4) interact with an unpaired
D
α
α
electron, with the aH constant of 0.25 mT (2H) or 0.12 mT (2H) depending on whether
they are in the axial or equatorial position of the six-membered ring [35]. In our case, due
to the asymmetry of all carbon atoms in 6c, the hfc constants for all the different types of
hydrogen atoms differ from each other and are in the range from 0.095 to 0.217 mT for
methylene protons in the carbonyl and decrease from 0.054 mT for protons of the CH2
group on the six-membered ring closest to the radical center down to ~0.001 mT for the
most distant protons of the aliphatic ring (Supplementary Materials, Figure S14).

Molecules 2021, 26, 3050
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To the best of our knowledge, such BNRs containing both structural fragments of
2,5-dihydroimidazole and 4-piperidone have not been described previously in the literature.
There is only one known example of the formation of bicyclic 1-hydroxy-3-imidazoline
3-oxide
with a low yield upon extra-long heating in a sealed ampoule, presumably through
tetrahydropyridine intermediate (Scheme 2) [40]. Although the putative structure of
compound has been confirmed by X-ray diffraction data [41], later, German researchers
found that most of the reactions of bicyclic proceed through the monocyclic tautomeric
form of methylnitrone 10 42]. They also showed that during the oxidation of solutions of
N-hydroxy derivative in various polar and nonpolar solvents by Ag2O, PbO2, hydrogen
7, which is carried out via the cyclodimerization of 3-methylbut-3-en-2-one oxime
8
9
7
7
[
7
peroxide, or tert-butoxy or benzoyloxy radicals, the corresponding BNR 11 is generated
with a typical aN of 1.9 mT and an isotropic g-factor (giso) of 2.0058–2.0066 [43]. Neverthe-
less, the publications just cited do not say how stable the nitroxide 11 prepared in an EPR
spectrometer ampoule is.
O
O
N
N
[O]
N
O
N
OH
N
OH
HO
OH
N
N
80o C
375 h
12%
7
11
+
N
OH
O
N
8
8
9
N
OH
10
Scheme 2. Formation of 8-hydroxy-1,4,5,7-tetramethyl-6,8-diazabicyclo[3.2.1]oct-6-ene 6-oxide
unsaturated oxime 8.
7 via cyclodimerization of
According to our observations and similar data in the literature, now we can propose a
mechanism of base-catalyzed recyclization of imidazoisoxazoles . The mechanism consists
of the transformation of into an intermediate (2H-imidazole N-oxide 2H Im Scheme 1),
2
2
-
;
whose subsequent intramolecular interaction with the enolate anion generated in the
reaction can occur both at the carbon atom of the nitrone group and at the carbon atom of the
imino group, thus leading either to the product of kinetic control (bicyclic hydroxylamine
5
in the former case) or to the product of thermodynamic control (4-pyridone-2-ketoxime 3
in the latter case). Indeed, in the control experiment, when we introduced a crystallized
sample of bicycle 5d into the reaction, with short heating (60–65 ◦C) of this aqueous
solution in 0.25N NaOH, 5d virtually quantitatively turned into a single product, pyridone
3d, which was isolated and characterized by NMR spectroscopy.
3. Materials and Methods
3.1. General Information
Fourier transform infrared spectra (FT-IR) were recorded in KBr pellets on a Bruker
Vector-22. UV-Vis spectra were obtained for EtOH solutions of bicyclic nitroxides 6a,c,d
using a Hewlett-Packard HP 8453 spectrophotometer. 1H NMR and 13C NMR spec-
tra were recorded on Bruker AV-300, AV-400, DRX-500, and AV-600 spectrometers at
300/400/500/600 and 75/100/125/150 MHz, respectively, for 3–10% solutions of com-
pounds 2, 3, and 5 in DMSO-d6; the positions of signals were determined relative to a
residual proton signal [DMSO-d6 (2.50 ppm) for 1H spectra] or a carbon signal [DMSO-d6
(39.4 ppm) for 13C spectra] of the deuterated solvent. The analyses were performed on
a Thermo Scientific DFS (Double Focusing System) high-resolution mass spectrometer

Molecules 2021, 26, 3050
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(Thermo Electron Corp., Waltham, MA, USA); the recording mode was electron ionization
with an ionizing electron energy of 70 eV, and exact mass was measured relative to the
lines of the standard perfluorokerosene.
Elemental analyses were carried out on a Euro EA 3000 automatic CHNS analyzer.
Melting points were determined by means of an FP 81 HT instrument (Mettler Toledo,
Columbus, OH, USA). Column chromatography and thin-layer chromatography (TLC)
were conducted using Acros silica gel 60A (0.035–0.070 mm) and Sorbfil PTLC-AF-UV 254
(Imid, Krasnodar, Russia), respectively, with eluents EtOAc and CHCl3-MeOH.
EPR spectra of nitroxides 6a,c,d were acquired by means of a Bruker Elexsys E540
X-band continuous-wave EPR spectrometer at 295 K for diluted (10−4 M) and oxygen-free
toluene solutions. Experimental settings for 6c were as follows: microwave power, 0.63 mW;
modulation frequency, 100 kHz; modulation amplitude, 0.002 mT; the number of points,
2048; sweep magnetic field, 6 mT; the number of scans, 160; and the time constant, 20 ms.
Experimental settings for 6a,d were as follows: microwave power, 2.0 mW; modulation
frequency, 100 kHz; modulation amplitude, 0.05 mT; the number of points, 1024; sweep
magnetic field, 10 mT; the number of scans, 8; and the time constant, 20 ms. The following
experimental settings for the center line of 6c were applied: microwave power, 0.63 mW;
modulation frequency, 100 kHz; modulation amplitude, 0.002 mT; the number of points,
1048; sweep magnetic field, 1.5 mT; the number of scans, 196; and the time constant,
20 ms. To determine giso values, X-band continuous-wave EPR spectra of a mixture of
the analyzed radical (6a, 6c, or 6d) with TEMPO were recorded. Then, the known giso
of TEMPO was used for a spectrum simulation, and the target giso value was excluded.
Simulations of solution EPR lines were carried out in the Easy Spin software, which is
available at www.easyspin.org (accessed on 18 May 2021).
3.2. Synthesis
3.2.1. Imidazo[1,2-b]isoxazoles 2a,c,d
Compounds 2a,c,d were synthesized via a reaction of 2-hydroxyamino oximes 1a,c,d [44]
with acetylacetone [39]. Their 1H NMR spectra and other data are consistent with those
described in the literature.
6-Hydroxy-2,3,6,7a-tetramethyl-3,6,7,7a-tetrahydroimidazo[1,2-b]isoxazole 1-oxide (2a) (a mix-
ture of three diastereomers). 13C NMR (150 MHz, DMSO-d6),
δ, ppm: 9.8, 10.17*, 10.2
(C(2)-CH3); 12.2, 16.8*, 18.5 (C(3)-CH3); 19.2, 23.7*, 24.7, 24.8, 25.3*, 25.9 (C(7a)-CH3 and
C(6)-CH3); 48.9, 49.6, 50.6* (C-7); 62.2, 66.3*, 70.5 (C-3); 98.3, 98.9*, 99.2 (C-7a); 100.4, 101.3*,
104.9 (C-6); 137.1, 138.3*, 139.3 (C-2). (The asterisk denotes a minor isomer).
2-Hydroxy-2,3a-dimethyl-2,3,3a,5,6,7,8,8a-octahydrobenzo[d]isoxazolo[2,3-a]imidazole 4-oxide (2c
)
(a mixture of two diastereomers). 13C NMR (75 MHz, DMSO-d6),
δ, ppm: 23.6, 23.7*, 23.8*,
24.7, 25.2* (C-6, C-7, C-8); 24.7*, 24.8, 25.1*, 25.6 (C(3a)-CH3 and C(2)-CH3); 32.5*, 34.6 (C-5);
48.8, 50.4* (C-3); 69.0*, 72.7 (C-8a); 99.6, 100.0* (C-3a); 102.2*, 106.0 (C-2); 140.9*, 141.7 (C-4a);
(The asterisk denotes a minor isomer).
2-Hydroxy-2,3a-dimethyl-3,3a,5,6,7,8,9,9a-octahydro-2H-cyclohepta[d]isoxazolo[2,3-a]imidazole 4-
oxide (2d) (a mixture of six diastereomers). 13C NMR (150 MHz, DMSO-d6),
δ, ppm: 12.8,
13.5, 18.6*, 18.7, 19.9, 23.6, 24.7, 24.9, 25.3, 25.4, 25.96, 26.03* (C(3a)-CH3 and C(2)-CH3);
24.0, 24.2, 24.3, 24.5*, 24.7, 25.5, 26.6*, 26.8, 26.9, 27.0, 27.7, 27.8, 27.9, 27.94, 28.7, 28.8, 28.9*,
29.2, 30.5*, 30.53, 30.58, 33.9, 34.1, 34.8, 36.4* (C-5, C-6, C-7, C-8, C-9); 48.7, 49.1, 49.8*, 50.6,
56.3 (C-3); 66.7, 68.1*, 69.6, 71.2, 75.4, 77.8 (C-9a); 98.3, 98.7, 99.0, 99.6*, 100.7*, 101.7, 102.8,
105.4, 105.9, 106.8 (C-3a and C-2); 141.2, 141.6, 142.8*, 143.7, 144.2, 145.3 (C-4a); (The asterisk
denotes the main isomer).

Molecules 2021, 26, 3050
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3.2.2. The General Synthetic Procedure for the Recyclization of Imidazo[1,2-b]isoxazoles
2a,c,d to 8-Hydroxy-5-methyl-3-oxo-6,8-diazabicyclo[3,2,1]-6-octenes 5a,c,d
◦
To a cold solution (0–5 C) of imidazoisoxazole
2 (10 mmol) in 10–15 mL of water,
a solution of 10 mmol of NaOH in 14 mL of water was added with stirring for 20 min
and was kept on ice for 3 h. The reaction mixture was neutralized with 10% hydrochloric
acid in an ice-water bath, and NaCl was added until saturation at room temperature. The
precipitate of bicycle
5 was filtered off, washed with a small amount of ice-cold water, and
dried until constant weight.
8-Hydroxy-1,5,7-trimethyl-6,8-diazabicyclo[3.2.1]oct-6-en-3-one (5a), colorless crystals, isolated
yield 0.72 g (40%), m.p. 175–177 ◦C (i-PrOH). Elemental analysis: found: C, 59.42; H, 7.90;
N, 15.45; calcd. for C9H14N2O2: C, 59.32; H, 7.74; N, 15.37%. IR spectrum,
3124, 2978, 2847, 1714 (C=O), 1637 (C=N), 1456, 1375, 1315, 1242. 1H NMR (400 MHz,
DMSO-d6), , ppm (J, Hz): A: Main isomer (keto form): 1.16 (s, 3H, CH3); 1.29 (s, 3H, CH3);
ν
, cm−1, (KBr):
δ
1.87 (s, 3H, CH3); 1.95 (d, J = 4.4, 1H, CH); 1.99 (d, J = 4.4, 1H, CH); 2.47 (d, J = 17.0, 1H,
CH); 2.57 (d, J = 17.0, 1H, CH); 8.33 (brs, 1H, N-OH). B: Cyclic hemiacetal: 1.21 (s, 3H,
CH3); 1.34 (s, 3H, CH3); 1.90 (s, 3H, CH3); 1.49 (dd, J = 2.8, 11.8, 1H, CH); 1.57 (d, J = 11.8,
1H, CH); 1.67 (dd, J = 2.8, 11.8, 1H, CH); 1.80 (d, J = 11.8, 1H, CH); 7.45 (brs, 1H, C-OH).
C: Minor isomer (keto form): 1.29 (s, 3H, CH3); 1.41 (s, 3H, CH3); 1.91 (s, 3H, CH3); 2.29
(dd, J = 2.1, 17.3, 1H, CH); 2.43 (dd, J = 2.1, 17.3, 1H, CH); 7.88 (brs, 1H, N-OH). 13C NMR
(100 MHz, DMSO-d6), δ, ppm: A: Main isomer (keto form): 16.4 (CH3); 18.8 (CH3); 23.1
(5-CH3); 40.7 (CH2); 42.0 (CH2); 69.8 (C-1); 85.8 (C-5); 177.8 (C=N); 206.9 (C=O). B: Cyclic
hemiacetal: 16.7 (CH3); 21.5 (CH3); 25.8 (5-CH3); 44.5 (CH2); 45.9 (CH2); 76.1 (C-1); 93.6
(C-5); 114.0 (NOCO); 177.3 (C=N). C: Minor isomer (keto form): 16.4 (CH3); 17.1 (CH3);
21.3 (CH3); 49.0 (CH2); 50.3 (CH2); 75.1 (C-1); 90.6 (C-5); 175.1 (C=N); 205.8 (C=O).
8-Hydroxy-5-methyl-1,7-tetramethylene-6,8-diazabicyclo[3.2.1]oct-6-en-3-one (5c), colorless rhom-
bic crystals, isolated yield 1.04 g (50%), m.p. 188–190 ◦C (abs EtOH). Elemental analysis:
found: C, 63.50; H, 7.90; N, 13.33; calcd. for C11H16N2O2: C, 63.44; H, 7.74; N, 13.45%. IR
spectrum,
ν
, cm−1, (KBr): 3402, 3099, 2941, 2862, 1715 (C=O), 1639 (C=N), 1452, 1313, 1238,
1
1009, 955, 812. H NMR (600 MHz, DMSO-d6),
δ
, ppm (J, Hz): A: Main isomer (keto form):
1.25–1.87 (m, 6H, (CH2)3); 1.30 (s, 3H, Me); 1.91 (d, J = 12.0, 1H, CH); 1.97 (d, J = 12.0, 1H,
CH); 2.14–2.70 (m, 2H, 2CH); 2.49 (d, J = 12.0, 1H, CH); 2.56 (d, J = 12.0, 1H, CH); 8.29 (brs,
1H, N-OH). B: Cyclic hemiacetal: 1.25–1.87 (m, 10H, (CH2)3 and CH2COCH2); 1.37 (s, 3H,
Me); 2.12–2.70 (m, 2H, 2CH); 7.54 (s, 1H, C-OH). C: Minor isomer (keto form): 1.25–1.87 (m,
6H, (CH2)3); 1.45 (s, 3H, CH3); 2.14–2.32 (m, 3H, 3CH); 2.43 (d, J = 18.0, 1H, CH); 2.57 (d,
J = 18.0, 1H, CH); 2.60–2.70 (m, 1H, CH); 7.89 (brs, 1H, N-OH). D: tetrahydropyridone 3c (as
an impurity): 1.25–2.25 (m, 8H, (CH2)4); 1.88 (s, 3H, CH3); 2.16 (d, JAB = 18.0, 1H, CH); 2.49
(d, JAB = 18.0, 1H, CH); 4.57 (s, 1H, CH=); 7.27 (s, 1H, NH); 10.75 (s, 1H, =NOH). 13C NMR
(150 MHz, DMSO-d6), δ, ppm: A: Main isomer (keto form): 23.0 (Me); 21.0, 24.3; 29.2, 34.5
((CH2)4); 42.5, 46.2 (CH2-C=O); 68.9 (N-C-C=N); 85.9 (N-C-N); 178.7 (C=N); 207.0 (C=O).
B: Cyclic hemiacetal: 25.7 (CH3); 21.6, 24.2, 29.7, 36.9 ((CH2)4); 44.0, 44.8 (CH2-C=O); 76.2
(N-C-C=N); 94.2 (N-C-N); 114.1 (NOCO); 178.2 (C=N). C: Minor isomer (keto form): 21.5
(Me); 21.2, 24.9, 28.2, 29.0 ((CH2)4); 48.2, 50.3 (CH2-C=O); 74.9 (N-C-C=N); 91.0 (N-C-N);
175.7 (C=N); 205.8 (C=O). D: tetrahydropyridone 3c (as an impurity): 20.6 (CH3); 20.1, 20.4,
25.4, 38.2 ((CH2)4); 44.8 (C-5); 58.6 (C-6); 96.9 (C-3); 156.9, 160.4 (C-2 and C-7); 190.0 (C-4).
8-Hydroxy-5-methyl-1,7-pentamethylene-6,8-diazabicyclo[3.2.1]oct-6-en-3-one (5d), colorless
◦
rhombic crystals, isolated yield 1.11 g (50%), m.p. 203–204 C (abs EtOH). Elemental
analysis: found: C, 65.10; H, 8.31; N, 12.62; calcd. for C12H18N2O2: C, 64.84; H, 8.16; N,
12.60%. IR spectrum, ν
, cm−1, (KBr): 3400, 3093, 2926, 2852, 1716 (C=O), 1630 (C=N), 1448,
1
1313, 1182, 991, 825. H NMR (400 MHz, DMSO-d6), δ, ppm (J, Hz): A: Main isomer (keto
form): 1.06–1.90 (m, 8H, (CH2)4); 1.30 (s, 3H, Me); 1.94 (d, J = 16.0, 1H, CH); 2.00 (d, J = 16.0,
1H, CH); 2.17–2.42 (m, 2H, 2CH); 2.46 (d, J = 16.0, 1H, CH); 2.61 (d, J = 16.0, 1H, CH); 8.28

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(brs, 1H, N-OH). B: Cyclic hemiacetal: 1.06–1.90 (m, 12H, (CH2)4 and CH2COCH2); 1.34 (s,
3H, Me); 2.17–2.42 (m, 2H, 2CH); 7.51 (brs, 1H, C-OH). 13C NMR (100 MHz, DMSO-d6),
δ,
ppm: A: Main isomer (keto form): 23.4 (CH3); 23.4, 28.4, 30.7, 32.0, 32.1 ((CH2)5); 41.2, 42.8
(CH2-C=O); 73.5 (N-C-C=N); 85.9 (N-C-N); 182.2 (C=N); 207.3 (C=O). B: Cyclic hemiacetal:
26.0 (CH3); 24.3, 27.8, 32.1, 32.5, 35.4 ((CH2)5); 44.5, 46.2 (CH2-C=O); 80.1 (N-C-C=N); 93.8
(N-C-N); 113.9 (NOCO); 181.9 (C=N).
3.2.3. The General Procedure for the Oxidation of 8-Hydroxy-6,8-diazabicyclo[3.2.1]oct-
6-en-3-ones 5a,c,d
An amount of 8-hydroxydiazabicyclooctene
5 (0.55 mmol) was stirred in 10 mL of
acetone for 5 min; after that, 2.0 mmol of PbO2 was added, and the suspension was stirred
for 3 h at room temperature and passed through a fine pore glass filter, and the precipitate
was washed thoroughly with Me2CO (3
the solid residue was diluted with 2 mL of EtOAc and flash chromatographed with an
ethyl acetate on a column with silica to obtain a colored fraction containing radical . The
×
5 mL). Combined filtrates were evaporated, and
6
solvent was evaporated, the residue was treated with cold hexane, and the precipitate of
NR 6 was filtered off.
1,5,7-Trimethyl-6,8-diazabicyclo[3.2.1]oct-6-en-3-one 8-oxyl (6a). Light purple fine crystals,
isolated yield 81 mg (81%), m.p. 66–68 ◦C (hexane), Rf 0.45 (EtOAc). Elemental analysis:
found: C, 59.42; H, 7.60; N, 15.41; calcd. for C9H13N2O2: C, 59.65; H, 7.23; N, 15.46%.
IR spectrum,
1236. UV (EtOH),
ν
, cm−1, (KBr): 2991, 2937, 2893, 1718 (C=O), 1626 (C=N), 1450, 1371, 1313,
λ
max
nm, (lg ε): 298 (2.65), 312 (2.65). High-resolution mass spectrometry
(HRMS), electrospray ionization (EI): Observed 181.0970; C9H13N2O2 ([M]+); calculated
181.0972. ESR (PhMe): br. triplet; aN1 = 1.852 mT; giso = 2.0054.
5-Methyl-1,7-tetramethylene-6,8-diazabicyclo[3.2.1]oct-6-en-3-one 8-oxyl (6c). Purple needle
crystals, isolated yield 92 mg (81%), m.p. 102–104 ◦C (hexane), Rf 0.45 (EtOAc). Elemental
analysis: found: C, 63.55; H, 7.31; N, 13.30; calcd. for C11H15N2O2: C, 63.75; H, 7.30; N,
13.52%. IR spectrum,
1317, 1238, 1167. UV (EtOH),
ν
, cm−1, (KBr): 2951, 2862, 1716 (C=O), 1628 (C=N), 1448, 1392, 1373,
λ
nm, (lg
ε
): 235 (3.30). HRMS (EI): Observed 207.1126;
C11H15N2O2 ([M]+); calculated 207.1126. ESR (PhMe): aN1 = 1.855 mT; aH1 = 0.217 mT;
aH2 = 0.1798 mT; aH3 = 0.119 mT; aH4 = 0.095 mT; aH5,H6,H7 = 0.041 mT; aH8 = 0.054 mT
max
;
aH9 = 0.025 mT; aH10 = 0.022 mT; aH11 = 0.019 mT; aH12 = 0.015 mT; aN2 = 0.009 mT;
aH13 = 0.004 mT; aH14 = 0.002 mT; aH15 = 0.001 mT; giso = 2.00664.
5-Methyl-1,7-pentamethylene-6,8-diazabicyclo[3.2.1]oct-6-en-3-one-8-oxyl (6d). Dark purple
needles, isolated yield 106 mg (87%), m.p. 98–100 ◦C (hexane), Rf 0.45 (EtOAc). Elemental
analysis: found: C, 65.22; H, 8.01; N, 12.57; calcd. for C12H17N2O2: C, 65.14; H, 7.74; N,
12.66%. IR spectrum, ν
, cm−1, (KBr): 2941, 2929, 2856, 1722 and 1711 (C=O), 1616 (C=N),
1446, 1383, 1313, 1317, 1227, 1182. UV (EtOH),
λ
nm, (lg ε): 238 (3.30). HRMS (EI):
max
Observed 221.1286; C12H17N2O2 ([M]+); calculated 221.1285. ESR (PhMe): br. triplet;
aN1 = 1.841 mT; giso = 2.00667.
3.2.4. The General Procedure for the Reduction of NRs 6a,c,d
To a stirred solution of nitroxide
6 (1.89 mmol) in 15 mL of acetone, zinc powder
(123 mg, 1.89 mmol) was added, followed by dropwise addition of a solution of NH4Cl
(158 mg, 2.27 mmol) in 1 mL of water during 15 min. The mixture was incubated for
25 min, the precipitate was filtered off, and the filtrate was evaporated. The residue
was treated with cold water, and the precipitate was filtered off, washed with a minimal
amount of water and diethyl ether, and dried until constant weight. The yield of bicyclic
hydroxylamine 5 depends on its solubility in water and varies from 48% (for compound
5a) up to 79% (for 5d). IR spectra of the obtained compounds are consistent with the IR
spectra of those synthesized through the recyclization of 2.

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3.2.5. Base-Promoted Recyclization of 8-Hydroxy-6,8-diazabicyclo[3.2.1]oct-6-en-3-one 5d
into Tetrahydropyridine-4-one 3d
A solution of 21 mg (0.525 mmol) of NaOH in 1 mL of water was added to a suspension
of 111 mg (0.5 mmol) of bicycle 5d in 1 mL of water, and the resulting solution was stirred
◦
at 65 C for 150 min. The reaction mixture was cooled to ambient temperature, neutralized
with 10% aq HCl and kept at 3 ◦C for 15 h. The precipitate of 3d was filtered off, washed
with 1 mL of ice water, and dried to constant weight. The aqueous filtrate was saturated
with NaCl and kept at room temperature for 2 d; the precipitate was filtered off and washed
with a minimal volume of ice water to isolate an additional amount of 3d.
7-(Hydroxyimino)-2-methyl-1-azaspiro[5.6]dodec-2-en-4-one (3d). Pale fine crystal powder,
1
total isolated yield 84 mg (76%). Its IR and H NMR spectra are consistent with those
described in the literature [39]. 13C NMR (75 MHz, DMSO-d6),
δ, ppm: 20.5 (CH3); 22.9
(C-11); 23.3 (C-8); 25.7 (C-9); 30.3 (C-10); 39.3 (C-12); 44.9 (C-5); 61.5 (C-6); 96.8 (C-3); 159.2,
159.8 (C-2 and C-7); 190.2 (C-4).
4. Conclusions
It was shown in this work that imidazo[1,2-b]isoxazoles formed by the reaction of
acetylacetone with 2-hydroxyaminooximes having a secondary hydroxyamino group are
key compounds for the synthesis of sterically hindered hydroxylamines, precursors of
the corresponding BNRs containing two functional groups in a paramagnetic molecule.
It should be pointed out that these nitroxides simultaneously contain TEMPON and 3-
imidazoline frames in the molecular core; this dualism may open up new possibilities
for the synthesis of functional DNP agents, radical polymerization mediators, catalysts
of one-electron oxidation, spin-labeled chelating ligands, and other constructs. Being
aza-analogs of NRs of the nortropinone series, radicals 6 can become convenient starting
compounds for the synthesis of polyfunctional hydrophilic nitroxides of the 3-imidazoline
series (cf. [45]).
Considering the increased interest of researchers in BNRs as promising agents for
the oxidation of various alcohols, amines, and related compounds (including sterically
hindered ones), further work is being planned regarding the synthesis, reactivity evaluation,
transformation of the keto function, and determination of electrochemical characteristics of
BNRs of the 6,8-diazabicyclo [3.2.1]-6-octene 8-oxyls series.
Supplementary Materials: The following are available online: IR, NMR, and mass spectra data for
bicycles 5 and for BRNs 6 as well as hfc constants for the EPR spectrum of 6c.
Author Contributions: Conceptualization, A.Y.T. and L.N.G.; methodology, A.Y.T., L.N.G. and
D.G.M.; validation, K.A.L. and L.N.G.; formal analysis, A.Y.T. and D.G.M.; investigation, L.N.G.,
A.Y.T., K.A.L. and D.G.M.; writing—original draft preparation, L.N.G., A.Y.T. and D.G.M.; writing—
review and editing, D.G.M. and A.Y.T.; supervision, D.G.M. All authors have read and agreed to the
published version of the manuscript.
Funding: This research was funded by the Ministry of Science and Higher Education of Russia
(project number 14.W03.31.0034).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: We thank the Multi-Access Chemical Research Center SB RAS for spectral and
analytical measurements.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of compounds 5a,c,d and 6a,c,d are available from the authors (A.Y.T.
or D.G.M.) upon request.

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