Facile synthesis of 3-formyl-2,2,5,5-tetramethyl-1-oxypyrroline

Article abstract of DOI:10.1055/s-1999-3538

The spin-label compound 3-formyl-2,2,5,5-tetramethyl-1-oxypyrroline was synthesized from inexpensive starting materials in a sequence of four experimentally undemanding, but high yield achieving transformations. The key step is reduction of an N-methoxy-N-methylcarboxamide to an aldehyde functionality.

Full text of DOI:10.1055/s-1999-3538

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)DFLOHꢄ6\QWKHVLVꢄRIꢄꢁꢅ)RUP\OꢅꢆꢇꢆꢇꢈꢇꢈꢅWHWUDPHWK\OꢅꢀꢅR[\S\UUROLQH
Sebastian W. Stork, Marvin W. Makinen*
Department of Biochemistry and Molecular Biology, The University of Chicago, 920 East 58th Street, Chicago, Illinois 60637, USA
Fax +1(773)7020439; Eꢀmail:ꢁmꢀmakinen@uchicago.edu
5HFHLYHGꢁꢂꢁ)HEUXDU\ꢁꢃꢄꢄꢄꢅꢁUHYLVHGꢁꢆꢄꢁ0DUFKꢁꢃꢄꢄꢄꢁ
This route allows high yields of the spin-label aldehyde ꢀ
from easily available, inexpensive starting materials by a
sequence of only four transformations.
$EVWUDFWꢉ The spin-label compound 3-formyl-2,2,5,5-tetramethyl-
1-oxypyrroline was synthesized from inexpensive starting materials
in a sequence of four experimentally undemanding, but high yield
achieving transformations. The key step is reduction of an 1ꢀmeth-
oxyꢀ1ꢀmethylcarboxamide to an aldehyde functionality.
The synthetic route to ꢀ is given in the Scheme. The key
step is reduction of the 1ꢀmethoxyꢀ1ꢀmethylcarboxamide
ꢊ to the aldehyde ꢀ. In addition to synthesis of ꢀ, we also
describe application of the synthesis method for site-spe-
cific enrichment of the spin-label aldehyde to yield ꢀD and
ꢀE using deuterated reagents en route in the synthetic
pathway (see experimental). Groesbeck and Lugtenburg7
have reported an analogous procedure starting with the
parent 3-carboxylic acid of ꢀ which is considerably more
expensive than the starting material 2,2,6,6-tetramethylpi-
peridin-4-one. In contrast to their method, we found the
route given in the Scheme to result consistently in high
yields of all intermediates and of the product ꢀ for both
natural abundance isotope composition and for site-spe-
cifically deuterated analogs. Site-specifically deuterium-
enriched analogs are of special advantage in electron nu-
clear double resonance studies.8–10
.H\ꢄZRUGVꢉ nitroxyl spin-label, 3-formyl-2,2,5,5-tetramethyl-1-ox-
ypyrroline, synthesis, site-specific deuteration, electron magnetic
resonance
Spin-label compounds are frequently used as structural
and spectroscopic probes in biochemical and biophysical
studies through application of electron magnetic reso-
nance techniques.1–3 A critical objective, therefore, is to
develop a suitable functionalization of the spin-label for
preparing analogs. A key compound in many syntheses is
the spin-label aldehyde ꢀ.4–6 Synthesis of a spin-label
compound with an aldehyde functionality is often prob-
lematic since its creation by reducing or oxidizing agents
easily proceeds to the alcohol or acid state, respectively,
or causes, on the other hand, reduction of the nitroxyl
group. In this article we describe a synthetic route to ꢀ that
utilizes the reduction of the corresponding 1ꢀmethoxyꢀ1ꢀ
methyl-substituted carboxamide by DIBAL or LiAlH4.
TLC: flexible sheets with silica gel 60 and fluorescence indicator,
IB2-F254 (0.2 mm, Merck). Melting points are not corrected. NMR
spectra: Bruker Instruments AMX-500 spectrometer. IR spectra:
Nicolet 20SXB FTIR-spectrometer. Chemicals of the highest purity
available were purchased from Aldrich, Inc. (Milwaukee, WI), and
were used as received. THF was dried by refluxing for 3 d over
LiAlH4 followed by distillation. Elemental analyses were per-
formed by Midwest Microlab (Indianapolis, IN) or Desert Analytics
Laboratory (Tucson, AZ). Mass spectrometric analyses were car-
ried out by the mass spectrometry facilities in the School of Chem-
ical Sciences at the University of Illinois (Urbana, IL).
Synthesis 1999, No. 8, 1309–1312 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢀꢁꢀꢂ
S. W. Stork, M. W. Makinen
3$3(5
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S\UUROLQHꢄꢌꢊꢍ
2,2,6,6-Tetramethylpiperidin-4-one (100 g, 0.644 mol) was dis-
solved in glacial AcOH (395 mL) under water bath cooling. A solu-
tion of Br2 (205.8 g, 1.288 mol) in AcOH (285 mL) was added
dropwise over the course of 6 h. After 1 d, the reaction mixture was
filtered. The isolated solid was washed with AcOH (200 mL), H2O
(200 mL), and finally with Et2O (2 x 200 mL). After air drying for
7–10 d the product was obtained as a light beige powder;
yield:229.55 g (90%); mp 201∞C (dec.).
IR (KBr): n = 3425(br), 2891, 2696, 1752, 640 cm–1.
MS: m/z (%) = 313 (29.60, M+– HBr), 298 (16.54, 313 – CH3), 234
(28.27, 313 – Br), 152 (16.65), 136 (13.93, 270+H+), 112 (38.64),
98 (87.24, 112 – CH2), 83 (26.19), 58.2 [100.00, HN=C(CH3)2].
The free radical intermediate ꢊ was generated by adapting the meth-
od of Rozantsev.11 Compound ꢁ (11.5 g, 54.1 mmol) was mixed
with disodium tungstate dihydrate (0.46 g, 1.35 mmol) and disodi-
um ethylendiaminetetraacetate dihydrate (0.46 g, 1.35 mmol) in
H2O (90 mL). H2O2 (30%, 9.1 mL) was added slowly to avoid gen-
eration of heat. Except for occasional shaking, the mixture was left
undisturbed in the dark at r.t. for 5 d. The pH was then adjusted to
5–6 with concd HCl and the mixture extracted with EtOAc (7 x 20
mL). Occasionally the product was observed as brown, not well de-
fined crystals. These were not isolated but were redissolved in
EtOAc. The combined organic phases were dried (MgSO4), suction
filtered over silica gel (1–2 cm), and the solvent was evaporated in
vacuo yielding 9.46 g (76.1%) of large, orange needles; mp 56.0–
56.5∞C.
1H NMR (CDCl3/MeOH-G4, 2:1, v/v): d = 1.45 (s, 6 H, 2 CH3), 1.88
(s, 6 H, 2 CH3), 5.63 (s, 2 H, 2 CHBr).
13C NMR (CDCl3/MeOH-G4, 2:1, v/v): d = 22.91 (CH3), 27.96
(CH3), 59.08 (CHBr), 65.85 (Cq), 189.07 (C=O).
IR (KBr): n = 2975, 2934, 1652, 1635, 1616, 1465, 1429, 1366,
1161 cm–1.
MS: m/z = 227 (24.18, M+), 197 (16.04, M+– NO), 167 [27.37, 227
– N(OCH3)CH3], 152 (27.04, 167 – CH3), 137 [76.80, 197 –
N(OCH3)CH3], 122 (9.27, 152 – NO), 109 (100.00, 137 – CO), 81
(16.02, ), 67 (21.98, 109 – C3H6).
C9H16Br3NO calcd
(393.1) found
C
27.44
27.88
H
4.09
4.20
N
3.56
3.47
ꢁꢅꢌ1ꢀPHWKR[\ꢀ1ꢀPHWK\OFDUEDPR\OꢍꢅꢆꢇꢆꢇꢈꢇꢈꢅWHWUDPHWK\OS\UURꢅ
OLQHꢄꢌꢁꢍ
C11H19N2O3 calcd
(227.3) found
C
58.13
57.88
H
8.43
8.33
N
12.33
12.16
To a solution of 1,2ꢀdimethylhydroxylamine hydrochloride (135.8
g, 1.392 mol) in H2O (680 mL) cooled in a water bath was added
Et3N (680 mL, 4.872 mol). The water bath was removed and com-
pound ꢆ (457.0 g, 1.160 mol) was added as a solid in small portions
over a period of 6 h under vigorous stirring. The flask was closed
with a rubber stopper and the mixture was stirred for 6 h more at r.t.
and then heated to 50∞C (oil bath temperature) for another 6 h. After
cooling to r.t., the pH was adjusted to 9.3 with 30% aq NaOH, and
the mixture was extracted with EtOAc (5 x 400 mL). The combined
organic phases were dried (MgSO4) and the solvent evaporated in
vacuo. The remaining crude product was fractionated over a 2 x 30
cm Vigreux-column. At 75–80∞C/20 mbar ca 10 mL of a yellow oil
were isolated which by NMR was identified as mainly unreacted
1,2ꢀdimethylhydroxylamine. A second fraction of ca 5 mL of a yel-
low oil was obtained at 100–105∞C/20 mbar which was comprised
of a mixture of unidentified compounds and ꢁ. The Vigreux-column
was removed, and a fraction boiling at 77–80∞C/0.5 mbar was col-
lected. The product consisted of 208.1 g (85%) of a yellow oil that
darkened upon standing. For this reason the product was used for
the next step of the synthesis immediately upon fractionation. The
crude product containing ꢁ could be purified also by dissolving in
EtOAc (4 mL per g crude product) to which an equal volume of 2
M HCl was added. The aqueous phase was then separated and under
ice cooling brought to pH 9.3 with aq 30% NaOH. This phase was
then extracted thrice with EtOAc (3 times one-third the volume of
the aqueous phase). The organic phases were combined, dried with
MgSO4, and the solvent removed in vacuo. Analyses of ꢁ purified
by distillation or by extraction were indistinguishable.
ꢁꢅ)RUP\OꢅꢆꢇꢆꢇꢈꢇꢈꢅWHWUDPHWK\OꢅꢀꢅR[\S\UUROLQHꢄꢌꢀꢍ
A. Reduction of ꢊ with DIBAL: Compound ꢊ (18.94 g, 83.33 mmol)
was dissolved in anhyd Et2O (100 mL), and the solution was cooled
to –78∞C. A 1.0 M solution of DIBAL (100mL, 100 mmol) in hex-
anes was added dropwise over a period of 30 min. The reaction was
stirred for 5 min and slowly poured into 2 M HCl (60 mL) which
had been previously cooled and maintained at 0∞C. During addition
the temperature has to be rigorously controlled since a rise in tem-
perature during hydrolysis causes a drastic reduction in yield and
quality of the product material. The mixture was then slowly
brought to r.t. The organic phase was separated, and the aqueous
phase extracted with EtOAc (5 x 80 mL). The combined organic
phases were dried (MgSO4) and the solvent was evaporated in vac-
uo. The crude product was purified by silica gel chromatography.
For 15 g of product 300 g of silica gel (70–230 mesh, 60 Å) in a
6 x 45 cm column were used. The eluent was Et2O/pentane (2:1,
v/v). After solvent removal the final product was isolated as light
yellow crystals (12.32 g, 88%); mp 79.5–80.0∞C.
IR (KBr): n = 2964, 2931, 1688, 1623, 1462, 1416, 1369, 1360,
1182 cm–1.
MS: m/z (%) = 168 (60.91, M+), 153 (8.30, M+– CH3), 138 (80.76,
M+– NO), 123 (100.00, 153 – NO, 138 – CH3), 120 (42.59, 138 –
H2O), 105 (36.43, 120 – CH3), 95 (60.38, 123 – CO).
C9H14NO2
(168.2)
calcd
C
64.26
64.06
H
8.39
8.49
N
8.33
8.32
found
IR (film): n = 3325 (w), 2966, 1658, 1635, 1616, 1461, 1416, 1360,
1194, 1181 cm–1.
MS: m/z (%) = 213 (9.21, M++H), 197 (100.00, M+– CH3), 137
[64.83, 197 – N(OCH3)CH3], 109 (35.36, 137 – CO).
1H NMR (CDCl3): d = 1.35 (s, 6 H, 2 CH3), 1.42 (s, 6 H, 2 CH3),
3.25 (s, 3 H, NCH3), 3.64 (s, 3 H, OCH3), 6.13 (s, 1 H, =CH).
Reduction with DIBAL was also tried in THF as solvent. Under the
same conditions only small amounts of aldehyde could be found in
the reaction mixture. This procedure was, therefore, not pursued
further.
B. Reduction of ꢊ with LiAlH4: Compound ꢊ (29.55 g, 13.00 mmol)
was dissolved in anhyd THF (300 mL) and the solution cooled to –
78∞ C. LiAlH4 (5 g, 13.18 mmol) was added in small portions over
5 min. Thereafter the solution was immediately poured into 2 M
HCl (300 mL) which had been previously cooled to 0∞C as de-
scribed in part A above. After slow warming to r.t., the organic
phase was separated and the aqueous phase extracted with EtOAc
(5 x 80 mL). The combined organic phases were dried (MgSO4)
13C NMR (CDCl3): d = 30.47 (CH3), 30.74 (CH3), 33.79 (NCH3),
61.41 (OCH3), 64.84 (Cq), 68.97 (Cq), 139.49 (=C), 141.28 (=CH),
166.93 (C=O).
C11H20N2O2 calcd
(212.3) found
C
62.23
62.00
H
9.50
9.34
N
13.20
13.67
Synthesis 1999, No. 8, 1309–1312 ISSN 0039-7881 © Thieme Stuttgart · New York

3$3(5
Facile Synthesis of 3-Formyl-2,2,5,5-tetramethyl-1-oxypyrroline
ꢀꢁꢀꢀ
and the solvent was evaporated in vacuo. The crude product (16.75
g) was purified by silica gel chromatography as described in part A
above. After solvent removal 11.21 g (50.1%) of yellow crystals
were obtained; mp 79.5–80.0∞C.
in vacuo, yielding 12.52 g (67.9%) of a dark yellow oil as crude
product. This was used without further purification.
IR (film): n = 3345, 2965, 2932, 1652, 1622, 1605, 1462, 1443,
1413, 1369, 1200, 1180, 974 cm–1.
IR (KBr): n = 2964, 2931, 1688, 1623, 1462, 1416, 1369, 1360,
1H NMR (CDCl3): d = 1.35 (s, 6 H, 2 CH3), 1.42 (s, 6 H, 2 CH3),
1182 cm–1.
3.25 (s, 3 H, NCH3), 3.64 (s, 3 H, OCH3), 6.13 (s, 1 H, =CH).
MS: m/z (%) = 168 (73.89, M+), 153 (9.86, M+– CH3), 138 (81.47,
M+– NO), 123 (100.00, 153 – NO, 138 – CH3), 120 (43.10, 138 –
H2O), 105 (39.60, 120 – CH3), 95 (63.60, 123 – CO).
13C NMR (CDCl3): d = 30.47 (CH3), 30.74 (CH3), 33.79, 61.41,
64.84 (Cq), 68.97 (Cq), 139.49 ( =C), 141.28 ( =CHD), 166.93
(C=O).
C9H14NO2
(168.2)
calcd
C
64.26
64.09
H
8.39
8.29
N
8.33
8.23
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>ꢊꢅ +@S\UUROLQHꢄꢌꢊDꢍ
found
The amine ꢁD was oxidized to the spin-label-compound ꢊD as de-
scribed for ꢊ above. From ꢁD (12.52 g, 54.8 mmol), 4.93 g (32%) of
the crude product ꢊD was obtained as large orange needles; mp
56.5∞C.
C. Reduction of ꢊ with LiAl2H4: For this reaction the same proce-
dure described under part B above was employed, but as reducing
agent LiAl2H4 (5.0 g, 11.91 mmol) was used, yielding 8.05 g
(36.6%) of ꢀD as yellow crystals; mp 80∞C.
IR (KBr): n = 2975, 2927, 1653, 1646, 1602, 1466, 1440, 1429,
1419, 1383, 1358, 973 cm–1.
IR (KBr): n = 2978, 2931, 2869, 2106, 1673, 1661, 1477, 1456,
1443, 1380, 1355, 1348, 1284, 1161 cm–1.
MS: m/z (%) = 228 (19.59, M+), 198 (17.01, M+– NO), 168 [30.96,
M+– N(OCH3)CH3], 153 (33.30, 168 – CH3), 138 (84.15, 168 –
NO), 123 (7.31, 153 – NO), 110 (100.00, 138 – CO), 82 (14.37, 123
– C3H6), 68 (21.56, 110 – C3H6), 56 (14.06, NC(CH3)2).
MS: m/z (%) = 169 (62.55, M+), 154 (9.51, M+– CH3), 139 (84.81,
M+– NO), 124 (100.00, 154 – NO, 139 – CH3), 121 (46.50, 139 –
H2O), 106 (40.98, 121 – CH3), 96 (58.12, 124 – CO).
C11H182HN2O3 calcd
C
57.87
57.67
H
8.39
8.46
N
12.27
12.23
C9H132HNO2 calcd
C
63.88
63.46
H
8.34
8.30
N
8.28
8.54
(228.3) found
(169.2) found
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For the synthesis of 3-formyl-2,2,5,5-tetramethyl-1-oxy[4-2H]pyr-
roline (ꢀE), 2,2,6,6-tetramethylpiperidin-4-one (19.95 g, 128.5
mmol) was first brominated in AcOH/AcOD (87 mL, Aldrich, 2H-
content >98%) by dropwise addition of Br2 (44.15 g, 276.3 mmol)
dissolved in AcOD (54 mL) to form product ꢆD. From this reaction
mixture 43.43 g of ꢆD (84.9%) were isolated as described above for
ꢆ. According to NMR spectra, the 3,5-positions were substituted
with 2H to ca 75%. To enhance the extent of deuteration in these po-
sitions, all of the reaction product was suspended again in a mixture
of AcOD (697.5 mL), D2O (Aldrich, 2H-content 99.9%, 618.3 mL),
Compound ꢊD (2.13g, 9.33 mmol) was finely dispersed in Et2O (50
mL), cooled to –78∞C, and a 1.0 M solution of DIBAL (12.0 mL,
12.0 mmol) in hexanes was added dropwise within 10 min. After
completion of addition the mixture was stirred for 5 min at –78∞C
and then poured into 2 M HCl (50 mL), stirred, and maintained at
0∞C. The mixture was brought slowly to r.t. After hydrolysis (ca 3
h) the Et2O phase was separated, the aqueous phase extracted with
EtOAc (5 times one half the volume of the aqueous phase), and the
combined organic phases dried (MgSO4) and evaporated in vacuo.
The aldehyde ꢀE was isolated as yellow crystals (1.38 g, 87.4%);
mp 80.0 °C.
2
and 47% DBr in D2O (Aldrich, H-content >99%, 100 mL) under
vigorous stirring for 5 weeks.12 From this mixture 34.40 g (79.2%)
of ꢆD were recovered by filtration and air drying. NMR analysis
showed that the extent of deuteration at the 3,5-position to be at
least 90%; mp 202∞C (dec.).
IR (KBr): n = 2978, 1936, 1869, 2834, 2736, 1679, 1605, 1478,
1457, 1450, 1430, 1374, 1370, 1354, 1341, 1275, 1163, 643, 509
cm–1.
MS: m/z (%) = 169 (46.22, M+), 139 (69.65, M+– NO), 124 (100.00,
139 – CH3), 121 (37.38, 139 – H2O), 110 (21.28, 96 (67.54, 124 –
CO), 82 (21.46, 124 – C3H6), 68 (38.77, 110 – C3H6), 56 [19.97,
NC(CH3)2].
IR (KBr): n = 3425(w), 2891, 2696, 1750, 640 cm–1.
MS: m/z (%) = 315 (24.04, M+– DBr), 300 (11.23, 315 – CH3), 236
(18.76, 315 – Br), 113 (26.51), 98 (82.39, 113 – CHD), 84 (19.39),
71 (16.52), 58 [100.00, HN = C(CH3)2].
1H NMR (CDCl3/MeOH-G4, 2:1, v/v): d = 1.45 (s, 6 H, 2 CH3), 1.88
(s, 6 H, 2 CH3), 5.63 (weak s, remaining CHBr).
C9H132HNO2 calcd
C
63.88
62.63
H
8.34
8.34
N
8.28
8.20
(169.2) found
13C NMR (CDCl3/MeOH-G4, 2:1, v/v): d = 22.91 (CH3), 27.96
(CH3), 59.08 (CHBr), 65.85 (Cq), 189.07 (C=O).
$FNQRZOHGJHPHQW
C9H142H2Br3NO calcd
C
27.16
27.11
H
4.05
3.95
N
3.52
3.48
This work was supported by grants of the Petroleum Research Fund
of the American Chemical Society (32541 – AC4) and the National
Institutes of Health (GM21900).
(398.0) found
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+@S\UUROLQHꢄꢌꢁDꢍ
5HIHUHQFHV
This substance was obtained by adding ꢆD (34.40 g, 86.4 mmol) in
small portions to a vigorously stirred mixture of 1,2ꢀdimethylhy-
droxylamine (10.12 g, 103.7 mmol) in H2O (74 mL) and Et3N (50.6
mL, 362.9 mmol). After addition, the mixture was stirred for 3 h, fil-
tered, brought to pH 9.3 with aq 30% NaOH and extracted 5 times
with EtOAc (half the volume of the aqueous phase). The organic
phases were combined, dried (MgSO4), and the solvent evaporated
(1) 6SLQꢁ/DEHOLQJꢇꢁ7KHRU\ꢁDQGꢁ$SSOLFDWLRQVꢅ Berliner, L. J., Ed.;
Academic Press: New York, 1976, p 592.
(2) %LRORJLFDOꢁ0DJQHWLFꢁ5HVRQDQFHꢇꢁ9ROꢇꢁꢈꢉꢁ6SLQꢁ/DEHOLQJꢇꢁ
7KHRU\ꢁDQGꢁ$SSOLFDWLRQVꢉ Berliner, L. J.; Reuben, J., Eds.;
Plenum Press: New York, 1989, p 450.
Synthesis 1999, No. 8, 1309–1312 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢀꢁꢀꢆ
S. W. Stork, M. W. Makinen
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(9) Mustafi, D.; Makinen, M. W. $SSOꢇꢁ0DJQꢇꢁ5HVRQꢇ ꢀꢃꢃꢆ, ꢋ,
(3) %LRORJLFDOꢁ0DJQHWLFꢁ5HVRQDQFHꢇꢁ9ROꢇꢁꢃꢊꢉꢁ6SLQꢁ/DEHOLQJꢇꢁ7KHꢁ
1H[Wꢁ0LOOHQLXPꢉ Berliner, L. J. Ed.; Plenum Press: New York,
1998, p 423.
(4) Koch, T. R.; Kuo, L. C.; Douglas, E. G.; Jaffer, S.; Makinen,
M. W. -ꢇꢁ%LROꢇꢁ&KHPꢇ ꢀꢃꢎꢃ, ꢆꢂꢊ, 12310.
(5) Hideg, K.; Hankovszky, H. O.; Lex, L.; Kulcsar, Gy. 6\QWKHVLV
ꢀꢃꢏꢂ, 911.
321.
(10) Stork, S. W., Ozhogina, O. A.; Cunningham, M. A.; Bash, P.
A.; Lee, R.; Makinen, M. W., manuscript in preparation.
(11) Rozantsev, E. G. )UHHꢁ1LWUR[\Oꢁ5DGLFDOV; Plenum Press: New
York, 1970, p 205.
(12) Lin, Y. W.; Teicher, B. A. -ꢇꢁ/DEHOOHGꢁ&RPSGꢇꢁ5DGLRSKDUP.
ꢀꢃꢃꢂ, ꢆꢈ, 62.
(6) Mustafi, D.; Boisvert, W. E.; Makinen, M. W. -ꢇꢁ$Pꢇꢁ&KHPꢇꢁ
6RFꢇ ꢀꢃꢃꢁ, ꢃꢃꢂ, 3674.
(7) Groesbeek, M.; Lugtenburg, J. 5HFOꢇꢁ7UDYꢇꢁ&KLPꢇꢁ3D\Vꢀ%DV
ꢀꢃꢃꢈ, ꢃꢃꢊ, 403.
(8) Mustafi, D.; Boisvert, W. E.; Makinen, M. W. -ꢇꢁ$Pꢇꢁ&KHPꢇꢁ
6RFꢇ ꢀꢃꢃꢁ, ꢃꢃꢂ, 3683.
Article Identifier:
1437-210X,E;1999,0,08,1309,1312,ftx,en;C00699SS.pdf
Synthesis 1999, No. 8, 1309–1312 ISSN 0039-7881 © Thieme Stuttgart · New York