3.1. General Information
2,2,5,5-Tetraethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (
2) was prepared according to a published protocol [
20]. 3-Carboxy-2,2,5,5-tetraethylpyrrolidin-1-oxyl (
1) [
18] was prepared as described in the patent [
39], and the IR spectrum of the nitroxide is given in
Figure S18 (cf. [
18]). A
1H NMR spectrum of the corresponding hydroxylamine trifluoroacetate is depicted in
Figure S60. Commercially available reagents, such as
n-butyllithium and lithium diisopropylamide solutions, dimethyl fumarate, 2-amino-3,3-dimethylbutanoic acid and 2-pentanone were used as received from Acros Organics B.V.B.A, Geel, Belgium. Solvents were dried by standard procedures (described in the literature) prior to use. The IR spectra were recorded on a Bruker Vector 22 FT-IR spectrometer (Bruker, Billerica, MA, USA) in KBr pellets (1:150 ratio) or in neat samples (for oily compounds). UV spectra were acquired on a HP Agilent 8453 spectrometer (Agilent Technologies, Santa Clara, CA, USA) in ethanol solutions (concentration ~10
−4 M).
1H NMR spectra were recorded on a Bruker AV 300 (300.132 MHz), AM 400 (400.134 MHz), and AV 600 (600.300 MHz) spectrometers.
13C NMR spectra were recorded on a Bruker AV 300 (75.467 MHz), AM 400 (100.614 MHz), and AV 600 (151 MHz) spectrometers. All the NMR spectra were acquired for 5–10% solutions in CDCl
3 or a CDCl
3–CD
3OD 1:4 mixture at 300 K using the signal of the solvent as a standard. To confirm the structure of stable nitroxides,
1H NMR spectra were recorded of the solutions of corresponding hydroxylamines prepared via reduction of the nitroxide samples (10–20 mg) with Zn powder in a CD
3OD–CF
3COOH 10:1 mixture, see subsection 3.2.12.
The structures of compounds
4,
5, and
12 were determined by single-crystal X-ray analysis (
Table S2). X-ray diffraction data were obtained on a Bruker P4 for
12 and on a Bruker Kappa Apex II CCD diffractometer for
4 and
5 with Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator. Absorption corrections were applied empirically using
SADABS programs [
40] for
4 and
5 and by the integration method for
12. The structures were solved by direct methods and refined by the full-matrix least-squares method against all
F2 in anisotropic approximation (besides the H atoms) using the
SHELXL2014/7 software suite (Dept. of Structural Chemistry, University of Göttingen, Göttingen, Germany [
41]). The H atoms’ positions were processed via the riding model, except for the positions of hydroxyl groups in
5 (refined independently). There are two independent molecules in the asymmetric unit of the cell for
4. Datasets CCDC 1972133–1972135 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
[email protected].
HPLC analyses were performed with an Agilent 1100 liquid chromatography system (Agilent Technologies, Santa Clara, CA, USA) equipped with a quaternary pump, online degasser, autosampler, and diode array detector. Chromatographic separations were carried out on a ZORBAX SB-C18 column (150 mm × 4.6 mm, 5.0 μm). The flow rate was 0.6 mL/min. Detection was performed at 238 nm. Acetonitrile, methanol and water were used to prepare the mobile phase.
The reactions were monitored by thin layer chromatography (TLC) on Sorbfil UV-254 (Imid ltd, Krasnodar, Russia) and DC-Alufolien (Merck KGaA, Darmstadt, Germany) plates with chloroform, chloroform–hexane, diethyl ether–hexane, and ethyl acetate–hexane mixtures as eluents. Kieselgel 60 (Macherey-Nagel GmbH & Co. KG, Düren, Germany), and alumina were utilized as sorbents for the column chromatography.
The EPR spectra in water were recorded on a Bruker ER-200D-SRC spectrometer in 100 µL quartz capillary for 0.1 mM solutions degassed via bubbling with argon. Spectrometer settings: frequency, 9.87 GHz; microwave power, 5.0 mW; modulation amplitude, 0.05 mT; time constant, 50 ms; and conversion time, 5.12 ms. The EPR spectra in water–methanol solutions and in toluene were recorded with an Elexsys E540 X-band spectrometer (Bruker, Billerica, MA, USA) in a 100 µL quartz capillary for 0.1–0.3 mM solutions, with the following spectrometer settings: field center, 351.600 mT; sweep range, 10 mT; modulation amplitude, 0.15 mT; microwave power, 2 mW; time constant, 10.24 ms; and scan time, 41 ms. The EasySpin software (Version 5.2.28, easyspin.org, [
42]) was employed for simulation of spectra. For kinetic measurements, the EPR spectra were acquired at the same instrument settings but with a greater modulation amplitude (0.2 mT) to optimize the signal-to-noise ratio. Thermal stability of nitroxides
4 and
5 was studied in 1 mM solutions in toluene within a sealed 100 µL glass capillary using a double sample resonator (Bruker, ER 4105DR). One of the two samples with the nitroxide was placed into a water bath (95–100 °C) for 80 min incubation, and nitroxide decay was followed via comparison of intensities. Partition coefficients in a water–octanol mixture were estimated from the amplitudes of the EPR spectra of a nitroxide in water after extensive shaking with different portions of added octanol and separation.
For kinetic measurements in water, stock solutions of the nitroxide, ascorbic acid, and of glutathione in phosphate buffer (1 mM, pH 7.4) were prepared, and pH was adjusted to 7.4 with NaHCO3. All the solutions were deoxygenated with argon, carefully and quickly mixed in a small tube to attain final concentrations (nitroxide, 0.1–0.3 mM; GSH, 2 mM; and ascorbate, 166.7 mM) and were placed into an EPR capillary (50 μL). The capillary was sealed on both sides and placed into the EPR resonator. Alternatively, all the reagents were dissolved in a MeOH–H2O mixture (1:1), pH of the stock solutions was adjusted to 7.4 with NaHCO3, and the solutions were mixed in a similar manner to obtain final concentrations: nitroxide, 0.1 mM; GSH, 2 mM; and ascorbate, 500 mM. The decay of amplitude of the low-field component of the EPR spectrum was followed to obtain the kinetics. Initial part of the decay curves (up to 200 min) was used for fitting. Kinetics of the decay was fitted to a monoexponential function to calculate the first-order rate constants. Then, these constants were divided by the concentration of ascorbic acid to calculate the second-order reaction constants.
3.2. Synthesis
3.2.1. 5,5-Diethyl-1-hydroxy-2,2,4-trimethyl-2,5-dihydro-1H-imidazole (7)
It was prepared from 3-hydroxyamino-3-ethylpentan-2-one hydrochloride [
16] and acetone as described previously [
27].
3.2.2. (Z)-4-(3,3-Dimethyl-2-oxobutylidene)-5,5-diethyl-2,2-dimethylimidazolidin-1-oxyl (10)
A solution of 4.3 g (23.4 mmol) of imidazoline 6 in 30 mL of dry diethyl ether was added dropwise to 35 mL of a 2 M solution of lithium diisopropylamide in an argon atmosphere. After 50 min of stirring, ethyl pivalate (12 mL, 78.5 mmol) was added, and the reaction mixture was heated to reflux for 48 h. Then, the mixture was cooled to 5 °C, and 100 mL of water was added carefully. The organic layer was separated, and the aqueous layer was extracted with diethyl ether (30 mL × 3 times). The combined extract was agitated with 10 g of manganese dioxide for 1 h and dried over magnesium sulfate. The precipitate was filtered off, and the filtrate was evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel with gradient elution (from hexane to a diethyl ether–hexane 1:4 mixture) to give 10 (5.5 g, 88% yield) as a yellow crystalline solid, m.p. 84–86 °C (hexane). Elemental analysis: found: C, 67.66; H, 10.18; N, 10.46; calcd. for C15H27N2O2: C, 67.38; H, 10.18; N, 10.48%; IR (KBr) νmax: 3274, 2978, 2952, 2869, 1636, 1561, 1494, 1461, 1444, 1377, 1362, 1328, 1217, 1181, 1163, 1121, 1018, 968, 947, 874, 815, 793, 748, cm−1; UV (EtOH) λmax (log ε): 298 (4.27).
3.2.3. (Z)-1-(5,5-Diethyl-2,2-dimethyl-1-hydroxyimidazolidin-4-ylidene)-3,3-dimethylbutan-2-one (9)
Hydroxyamine 9 was prepared via hydrogenation of 10 on a Pd/C catalyst in methanol under atmospheric pressure and ambient temperature. Yield 97%, colorless crystalline solid, m.p. 108–111 °C (hexane). Elemental analysis: found: C, 68.03; H, 10.58; N, 9.79; calcd. for C15H28N2O2: C, 67.13; H, 10.52; N, 10.44%; IR (KBr) νmax: 3281 (br.), 2968, 2871, 1617, 1526, 1461, 1392, 1378, 1361, 1341, 1327, 1216, 1198, 1170, 1126, 1043, 1020, 988, 942, 910, 875, 812, 784, 752, 701, cm−1; UV (EtOH) λmax (log ε): 304 (4.28); 1H NMR(300 MHz; CDCl3, δ): 0.92 (t, J = 7.3 Hz, 6 H), 1.15 (s, 9H), 1.42 (s, 6H,), 1.69, 1.82 (ABq, Jq = 7.3 Hz, JAB = 14.5 Hz, both 2H), 5.01 (s, 1H), 5.35 (br. s, 1H), 10.12 (br. s, 1H); 13C{1H} NMR (75 MHz; CDCl3, δ): 8.94, 27.57, 27.80, 27.83, 41.41, 74.03, 79.97, 83.91, 165.45, 204.83.
3.2.4. Enaminoketone 9 Acid Hydrolysis and Recyclization
Method A: Similarly to a previously described procedure, [
25] a solution of 2.33 g (8.7 mmol) of enaminoketone
9 in methanol (10 mL) was diluted with 35% hydrochloric acid (10 mL), and the reaction mixture was kept for 84 h at ambient temperature. Then, methanol was removed under reduced pressure, and the mixture was neutralized with a saturated aqueous solution of sodium carbonate under argon and extracted with degassed chloroform (10 mL × 3 times). The combined extract was dried over magnesium sulfate and evaporated under reduced pressure. The residue was purified by recrystallization from hexane to isolate
6 in a 1.1 g (60%) yield.
Method B: Concentrated hydrochloric acid (40 mL) was added to a solution of 7.6 g (28.7 mmol) of 9 in 40 mL of methanol, and the reaction mixture was kept for 40 h at room temperature. Neutralization of the reaction mixture resulted in precipitation of colorless oil, which was separated and dissolved in chloroform, washed with brine and sodium carbonate, and dried over magnesium sulfate. Next, chloroform was removed under reduced pressure, and the residue was triturated with hexane. The formed precipitate was filtered off and recrystallized from hexane to obtain 13 (0.9 g, 15%). The aqueous solution after neutralization was processed as described in method A to obtain 6 (2.9 g, 47%). Compound 13 totally decomposed in a few days under aerobic conditions at room temperature. The mixture of decomposition products was separated by column chromatography on silica gel using a chloroform–hexane 1:4 mixture as an eluent to isolate 16 and 17.
5-(tert-Butyl)-2,2-diethyl-3-oxo-3,4-dihydro-2H-pyrrole 1-oxide (6): A colorless crystalline solid, m.p. 138–143 °C (hexane). Elemental analysis: found: C, 67.83; H, 10.36; N, 6.32; calcd. for C12H21NO2: C, 68.21; H, 10.02; N, 6.63%; IR (KBr) νmax: 2964, 2924, 2877, 2580 (br.), 1594, 1534 (br.), 1495, 1427, 1392, 1361, 1323, 1273, 1253, 1217, 1135, 1122, 1066, 1028, 951, 868, 792, 764, cm−1; UV (EtOH) λmax (log ε): 332 (3.97); 1H NMR (300 MHz; CDCl3, δ): Form A: 0.67 (br. m, 6H), 1.35 (s, 9H), 1.67, 1.92 (br. ABq, Jq = 6.8 Hz, JAB = 13.6 Hz, both 2H), 3.17 (br. s, 2H); Form B: 0.66 (br. m, 6H,), 1.35 (s, 9H), 1.67, 1.81 (br. ABq, Jq = 6.8 Hz, JAB = 13.6 Hz, both 2H), 4.96 (br. s, 1H), 10.22 (br. s, 1H); 13C{1H} NMR (75 MHz; CDCl3, δ): Form A: 7.64, 25.64, 28.14, 33.89, 42.87, 85.06, 151.31, 209.84; Form B: 7.64, 27.92, 34.38, 79.50, 94.50, 185.96, 196.55.
3-Amino-5-(tert-butyl)-2,2-diethyl-2H-pyrrole 1-oxide (13): A yellow crystalline solid. IR (KBr) νmax: 3320, 3150 (br.), 2972, 2877, 2737, 1661, 1588, 1484, 1459, 1407, 1378, 1359, 1337, 1299, 1255, 1179, 1149, 1090, 1055, 951, 870, 788, 763, 706, 674, 621, cm−1; 1H NMR (300 MHz; CDCl3, δ): 0.55 (t, J = 7.2 Hz, 6H), 1.26 (s, 9H), 1.40, 1.92 (ABq, Jq = 7.2 Hz, JAB = 13.8 Hz, both 2H), 4.63 (br. s, 2H), 5.06 (s, 1H); 13C{1H} NMR (75 MHz; CDCl3, δ): 7.21, 26.54, 28.70, 33.90, 77.01, 91.70, 153.59, 159.09.
5-(tert-Butyl)-2,2-diethyl-3-imino-4-oxo-3,4-dihydro-2H-pyrrole 1-oxide (16): Yellow oil. IR (neat) νmax: 3369, 3207, 2972, 2935, 2882, 2774, 1771, 1703, 1678, 1624, 1588, 1570, 1510, 1481, 1456, 1410, 1382, 1359, 1322, 1271, 1230, 1202, 1147, 1121, 1068, 1040, 1006, 964, 947, 864, 821, 791, 770, 753, 705, cm−1; M/z, found: M 224, (M-CH3) 209; calcd. for C12H20N2O2: 224; 1H NMR (400 MHz; CDCl3, δ): 0.63 (t, J = 7.4 Hz, 6H), 1.39 (s, 9H), 1.93, 2.08 (ABq, Jq = 7.4 Hz, JAB = 14.4 Hz, both 2H); 13C{1H} NMR (100 MHz; CDCl3, δ): 7.02, 25.71, 28.71, 33.94, 81.65, 153.93, 171.42, 178.29.
5-(tert-Butyl)-2,2-diethyl-3,4-dioxo-3,4-dihydro-2H-pyrrole 1-oxide (17): yellow crystalline solid, m.p. 31–31 °C (hexane). Elemental analysis: found: C, 64.29; H, 8.49; N, 6.49; calcd. for C12H19NO3: C, 63.98; H, 8.50; N, 6.22%; m/z, Found: M 225, (M-CH3) 210; calcd. for C12H19NO3: M 225; IR (KBr) νmax: 2927, 2855, 2769, 1769, 1696 (br.), 1503, 1454, 1408, 1381, 1361, 1320, 1226, 1192, 1120, 1085, 1039, 931, 870, 805, 772, 722, 687, cm−1; UV (EtOH) λmax (log ε): 239 (3.83), 328 (3.96); 1H NMR (400 MHz; CDCl3, δ): 0.69 (t, J = 7.4 Hz, 6H), 1.43 (s, 9H), 1.88, 2.00 (ABq, Jq = 7.4 Hz, JAB = 14.2 Hz, both 2H); 13C{1H} NMR (100 MHz; CDCl3, δ): 7.28, 25.71, 27.25, 34.41, 81.70, 159.63, 178.56, 197.35.
3.2.5. 3,3’-Bis(2-tert-butyl-5,5-diethyl-4-oxopyrrolinylidene) 1,1’-dioxide (12)
Manganese dioxide 0.75 g (8.63 mmol) was added to a solution of 0.15 g (0.71 mmol) of 6 in chloroform (3 mL), and the reaction mixture was stirred vigorously for 7 days. The precipitate was filtered off, and the filtrate was evaporated under reduced pressure. The residue was recrystallized from ethyl acetate to give 0.145 g (95%) of dimer 12 as a dark violet crystalline solid, m.p. 157–160 °C (ethyl acetate). Elemental analysis: found: C, 68.91; H, 9.27; N, 6.71; calcd. for C24H38N2O4: C, 68.87; H, 9.15; N, 6.69%; IR (KBr) νmax: 2972, 2928, 2850, 1701, 1505, 1489, 1459, 1422, 1379, 1359, 1298, 1219, 1118, 1050, 953, 879, 859, 778, 754, 717, 668, cm−1; UV (EtOH) λmax (log ε): 234 (3.83), 307 (3.93), 375 (3.70), 579 (4.03); 1H NMR (400 MHz; CDCl3, δ): 0.44, 0.63 (t, J = 7.35 Hz, both 6H), 1.39 (s, 18H), 1.66, 1.75 (ABq, Jq = 7.3 Hz, JAB = 14.0 Hz both 2H), 1.74, 1.89 (ABq, Jq = 7.4 Hz, JAB = 14.6 Hz, both 2H). 13C{1H} NMR (100 MHz; CDCl3, δ): 7.07, 7.39, 26.01, 27.72, 29.39, 36.03, 82.02, 198.35.
3.2.6. 5-(tert-Butyl)-5-butyl-2,2-diethyl-3-oxopyrrolidin-1-oxyl (18)
A warm solution of 0.5 g (2.37 mmol) of pyrroline 6 in dry benzene (30 mL) was added dropwise to a 2.6 M toluene solution of BuLi (14 mL) in an argon atmosphere. The reaction mixture was heated at 60 °C for 1 h and then kept at room temperature for 24 h. After that, the mixture was cooled to 0–5 °C and quenched with 30 mL of water. The organic layer was separated, and the aqueous layer was extracted with benzene. The combined extract was dried over magnesium sulfate and evaporated under reduced pressure. The residue was purified by column chromatography on silica gel with gradient elution (from tetrachloromethane to a chloroform–tetrachloromethane 1:5 mixture) to isolate 18 (0.39 g, 61%) as a yellow crystalline solid, m.p. 42–43 °C (hexane). Elemental analysis: found: C, 71.90; H, 11.49; N, 4.96; calcd. for C16H30NO2: C, 71.59; H, 11.27; N, 5.22%; HRMS (EI/DFS) m/z [M]+ calcd for (C16H30NO2)+: 268.2271; found: 268.2276. IR (KBr) νmax: 2969, 2874, 1747, 1483, 1470, 1407, 1396, 1379, 1366, 1337, 1297, 1277, 1213, 1161, 1148, 1109, 927, 888, 839, 785, 733, cm−1. UV (EtOH) λmax (log ε): 217 (3.22), 243 (3.19). HPLC UV 99.0355%.
3.2.7. Thermal Decomposition of Nitroxide 18 in the Presence of TEMPO
A mixture of 0.3 g (1.12 mmol) of nitroxide 18 and 1.6 g (10.3 mmol) of TEMPO was placed into a Schlenk flask and heated at 70–80 °C in vacuum for 7 days. The mixture was separated by column chromatography on silica gel with gradient elution (from hexane to a diethyl ether–hexane 1:2 mixture) to give 20 (0.09 g, 38%) and 21 (0.2 g, 85%).
1-(tert-Butoxy)-2,2,6,6-tetramethylpiperidine (20): Colorless oil. NMR
1H and
13C spectra match the literature data [
43,
44].
3,3’-Bis(2-butyl-5,5-diethyl-4-oxopyrrolinylidene) 1,1’-dioxide (21): A dark crimson crystalline solid, m.p. 95–100 °C (hexane). Elemental analysis: found: C, 69.06; H, 8.98; N, 6.55; calcd. for C24H38N2O4: C, 68.87; H, 9.15; N, 6.69%; IR (KBr) νmax: 2954, 2929, 2872, 1708, 1461, 1409, 1345, 1315, 1232, 1118, 1051, 935, 712, cm−1. UV (EtOH) λmax (log ε): 228 (3.89), 284 (3.71), 341 (3.81), 508 (4.19); 1H NMR (300 MHz; CDCl3, δ): 0.65 (t, J = 7.4 Hz, 3H), 0.86 (t, J = 7.4 Hz, 3H), 0.91 (t, J = 7.4 Hz, 3H), 1.39 (sex, J = 7.4 Hz, 2H), 1.60 (m, 2H,), 1.83, 1.94 (ABq, JAB = 14.3 Hz, Jq = 7.4 Hz, 4H), 2.95 (m, 2H); 13C{1H} NMR (75 MHz; CDCl3, δ): 7.02, 13.65, 22.24, 25.97, 27.14, 27.65, 80.57, 122.45, 159.55, 199.05.
3.2.8. 5-(tert-Butyl)-5-butyl-2,2-diethyl-3-hydroxypyrrolidin-1-oxyl (4)
A 1.3-fold excess of sodium borohydride (0.037 g, 0.97 mmol) was added in portions to a solution of nitroxide 18 (0.2 g, 0.75 mmol) in ethanol with stirring for 30 min. After that, ethanol was removed under reduced pressure, and the residue was diluted with an equal volume of water and extracted with diethyl ether. The extract was dried over magnesium sulfate and then evaporated under reduced pressure. The residue was recrystallized from hexane. Nitroxide 4 was isolated in a 0.197 g (98%) yield as a pale yellow crystalline solid, m.p. 88–90 °C (hexane). Elemental analysis: found: C 70.76; H 11.80; N 5.09%; calcd. for C16H32NO2, C 71.06; H 11.93, N 5.18%. HRMS (EI/DFS) m/z [M]+ calcd for (C16H32NO2)+: 270.2428; found: 270.2426. IR (KBr) νmax: 3392 (br.), 2963, 2936, 2877, 1461, 1405, 1383, 1367, 1311, 1282, 1221, 1144, 1122, 1095, 1056, 933, 829, 746, 671, cm−1. UV (EtOH) λmax (log ε): 236 (3.32). HPLC UV 96.2762%.
3.2.9. 5-(tert-Butyl)-3,4-bis(methoxycarbonyl)-2,2-diethylpyrrolidine (23a,b)
A mixture of 2-amino-3,3-dimethylbutanoic acid (1.33 g, 10 mmol), dimethyl fumarate (1.5 g, 10 mmol), diethylketone (10 mL, 100 mmol), DMF (10 mL), and toluene (10 mL) was placed into a Dean–Stark apparatus and stirred under reflux for 3 days. The solvent was distilled off in vacuum, and the residue was dissolved in ethyl acetate (30 mL). The solution was washed with aqueous sodium bicarbonate (50 mL × 3 times) and extracted with 5% sulfuric acid (20 mL × 3 times). The acidic extracts were basified with Na2CO3 to pH 7–8 and extracted with ethyl acetate (20 mL × 3 times). The extract was dried with Na2CO3, and the solvent was removed under reduced pressure to give 2.2 g (73%) of a crude diastereomeric mixture as yellow oil. The mixture was used in the next step without further purification. To confirm the structure, the isomers were separated by column chromatography on silica gel (eluent: hexane–ethyl acetate 50:1).
Isomer 23a: Yield 0.6 g (20%), colorless crystalline solid, m.p. 30.3–32.0 °C (hexane). Elemental analysis: found: C, 64.00; H, 9.76; N, 4.60; calcd. for C16H29NO4: C, 64.18; H, 9.76; N, 4.68%; HRMS (EI/ Agilent 7200 Accurate Mass Q-TOF GC/MS) m/z [M-CH3O]+ calcd. for C15H26NO3 268.1907, found 268.1913.IR (KBr) νmax: 3444, 2953, 2879, 1724, 1620, 1460, 1435, 1385, 1369, 1323, 1277, 1259, 1194, 1169, 1128, 1065, 1039, 1016, 974, 935, 893, 858, 804, 779, cm−1. 1H NMR (500 MHz; CDCl3, δ): 0.77 (t, J = 7.3 Hz, 3H), 0.88 (s, 9H), 0.93 (t, J = 7.5 Hz, 3H), 1.12 (dq, Jd = 14.0 Hz, Jq = 7.3 Hz, 1H), 1.38 (dq, Jd = 14.0 Hz, Jq = 7.3 Hz, 1H), 1.64 (br, 1H), 1.66 (dq, Jd = 13.6 Hz, Jq = 7.5 Hz, 1H), 1.70 (dq, Jd = 13.6 Hz, Jq = 7.5 Hz, 1H), 2.98 (d, J = 6.3 Hz, 1H), 3.02 (d, J = 3.0 Hz, 1H), 3.29 (dd, J1 = 6.3 Hz, J2 = 3.0 Hz, 1H), 3.57 (s, 3H), 3.58 (s, 3H). 13C{1H} NMR (125 MHz, CDCl3, δ): 8.3, 8.4, 24.8, 27.2, 29.8, 32.4, 50.1, 51.29, 51.33, 57.3, 67.9, 70.8, 173.4, 175.7.
Isomer 23b: Yield 1.36 g (46%), colorless oil. Elemental analysis: found: C, 64.17; H, 9.73; N, 4.68; calcd. for C16H29NO4: C, 64.18; H, 9.76; N, 4.68%. HRMS (EI/ Agilent 7200 Accurate Mass Q-TOF GC/MS) m/z [M-CH3O]+ calcd. for C15H26NO3 268.1907, found 268.1906. IR (neat) νmax: 2955, 2879, 1738, 1462, 1437, 1375, 1338, 1254, 1194, 1167, 1107, 1066, 1020, 982, 966, 820, 785, 744, cm−1. 1H NMR (500 MHz; CDCl3, δ): 0.79 (t, J = 7.4 Hz, 3H), 0.85 (s, 9H), 0.87 (t, J = 7.3 Hz, 3H), 1.25 (dq, Jd = 13.9 Hz, Jq = 7.4 Hz, 1H), 1.31 (dq, Jd = 13.9 Hz, Jq = 7.4, 1H), 1.36 (br, 1H), 1.56 (dq, Jd = 14.3 Hz, Jq = 7.3 Hz, 1H), 1.60 (dq, Jd = 14.3 Hz, Jq = 7.3 Hz, 1H), 3.04 (d, J = 8.8 Hz, 1H), 3.07 (dd, J1 = 8.8 Hz, J2 = 8.4 Hz, 1H), 3.19 (d, J = 8.4 Hz, 1H), 3.61 (s, 3H), 3.61 (s, 3H). 13C{1H} NMR (125 MHz, CDCl3, δ): 7.8, 7.9, 26.3, 28.6, 29.3, 33.2, 48.9, 51.3, 51.6, 57.3, 66.3, 70.1, 173.0, 175.3.
3.2.10. 5-(tert-Butyl)-2,2-diethyl-3,4-bis(hydroxymethyl)-3,4-dihydro-2H-pyrrole 1-oxide (24)
A solution of a crude mixture of amines 23a,b (6.38 g, 22 mmol) in dry diethyl ether (20 mL) was added dropwise to a stirred solution of LiAlH4 (1.56 g, 41 mmol) in dry diethyl ether (100 mL). The mixture was stirred at reflux for 1 h, then the flask was cooled in an ice bath and carefully quenched with 5 mL of 5% aqueous sodium hydroxide and 15 mL of water. The organic layer was separated via decantation, the wet precipitate was washed with diethyl ether (20 mL × 3 times), and the combined extract was evaporated under reduced pressure. The residue was dissolved in methanol (50 mL), mixed with a solution of sodium tungstate (0.7 g, 2.12 mmol) and EDTA disodium salt (0.71 g, 2.12 mmol) in water (30 mL), and hydrogen peroxide 30% (7 mL) was added. The solution was kept at ambient temperature for 3 days, then a catalytic amount of manganese dioxide (0.1 g, 1.2 mmol) was carefully added for quenching of remaining H2O2. After oxygen evolution ceased, the solution was evaporated in vacuum. The residue was triturated with chloroform, and the extract was dried with sodium carbonate. The solution was filtered, and the solvent was distilled off in vacuum. The residue was triturated with diethyl ether to give 4.16 g of crude 24 as a pinkish crystalline solid. The stock solution was evaporated under reduced pressure and purified by column chromatography on silica gel (eluent: methanol–ethyl acetate 1:100) to afford another portion of 24 (0.49 g).
24: Yield 4.65 g (85%), colorless crystals, m.p. 121.6–122.4 °C (ethyl acetate). Elemental analysis: found: C, 65.61; H, 10.83; N, 5.46; calcd. for C14H27NO3: C, 65.33; H, 10.57; N, 5.44%. HRMS (EI/DFS) m/z [M]+ calcd for (C14H27NO3)+ 257.1986, found 257.1985. IR (KBr) νmax: 3404, 3194, 2972, 2949, 2941, 2922, 2881, 2739, 1711, 1569,1487, 1454, 1396, 1373, 1331, 1292, 1242, 1157,1126, 1095, 1086, 1057, 1043, 1030, 1005, 943, 928, 901, 862, 796, 779, 739, 692, 669, 602, 575, 555, 486, 471, 444, 436, cm−1. UV (EtOH) λmax (log ε): 239 (3.97). 1H NMR (400 MHz; CDCl3, δ): 0.70 (t, J = 7.3 Hz, 3H), 0.83 (t, J = 7.4 Hz, 3H), 1.29 (s, 9H), 1.48 (dq, Jd = 14.5 Hz, Jq = 7.3 Hz, 1H), 0.59 (dq, Jd = 14.5 Hz, Jq = 7.3 Hz, 1H), 1.67 (dq, Jd = 14.5 Hz, Jq = 7.4 Hz, 1H), 1.75 (dq, Jd = 14.5 Hz, Jq = 7.3 Hz, 1H), 2.29 (ddd, J1 = 5.9 Hz, J2 = 10.0 Hz, J3 = 4.7 Hz, 1H), 2.83 (ddd, J1 = 5.9 Hz, J2 = 8.9 Hz, J3 = 3.0, 1H), 3.33 (t, J = 9.1 Hz, 1H), 3.61 (t, J = 9.3 Hz, 1H), 3.69 (br, 1H), 4.10 (br, 1H), 5.14 (br, 1H), 5.24 (br, 1H). 13C{1H} NMR (100 MHz, CDCl3, δ): 7.4, 9.1, 26.2, 27.3, 31.4, 34.4, 44.5, 50.6, 61.3, 64.8, 80.8, 152.9.
3.2.11. 2-(tert-Butyl)-2-butyl-5,5-diethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (5)
A solution of trimethylsilyl chloride (1.54 g, 14.1 mmol) in dry THF (10 mL) was added dropwise to a solution of nitrone 24 (1.65 g, 6.42 mmol) and triethylamine (1.3 g, 28.2 mmol) in dry THF (35 mL) upon stirring in ice bath. Then, the solvent was evaporated under reduced pressure, the residue was triturated with diethyl ether (10 mL × 5 times), and the precipitate was filtered off. The combined filtrate was concentrated under reduced pressure and dissolved in dry benzene (20 mL). Next, a solution of n-BuLi (1 M in hexane, 30 mL) was added in an argon atmosphere upon stirring in an ice bath. The reaction mixture was kept at ambient temperature for 3 days. The reaction was controlled with TLC on SiO2 (eluent: ethyl acetate–hexane 1:4), using H3[P(Mo3O10)4]⋅nH2O for staining. The mixture was quenched carefully with water until formation of two phases. The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (15 mL × 2 times). The combined extract was evaporated under reduced pressure, the residue was dissolved in methanol (20 mL), and an aqueous solution of PPTS (0.05 g in 3 mL) was added to remove the protective groups. The mixture was kept in ambient air for 3 days, and then methanol was removed under reduced pressure, and the residue was diluted with water (10 mL) and extracted with ethyl acetate (10 mL × 4 times). The extract was evaporated under reduced pressure, and the residue was separated by column chromatography on silica gel (eluent: ethyl acetate–hexane 1:4) to give 5 (1.31 g, 65%) and 26 (0.36 g, 20%). For purification, nitroxide 5 was recrystallized from a hexane–ethyl acetate (6:1) solution and then from an ethanol–water (2:1) solution.
2-(tert-Butyl)-2-butyl-5,5-diethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (5): A yellow crystalline solid, m.p. 109.4–113.8 °C (hexane–ethyl acetate 6:1). Elemental analysis: found: C, 68.90; H, 11.55; N, 4.58; calcd. for C18H36NO3: C, 68.75; H, 11.54; N, 4.45%. HRMS (EI/DFS) m/z [M]+ calcd. for (C18H36NO3)+ 314.2690, found 314.2688. IR (KBr) νmax: 3333, 2962, 2875, 1485, 1460, 1425, 1392, 1379, 1367, 1346, 1298, 1288, 1201, 1163, 1138, 1063, 1051, 1012, 995, 949, 922, 839, 795, 754, 739, 710, 604, 586, 478 cm−1. UV (EtOH), λmax (log ε): 239 (3.23). HPLC UV 96.8533%.
5-(tert-Butyl)-2,2-diethyl-3-(hydroxymethyl)-4-pentyl-3,4-dihydro-2H-pyrrole (26): colorless oil. Elemental analysis: found: C, 76.38; H, 12.08; N, 4.43; calcd. for C
18H
35NO: C, 76.81; H, 12.53; N, 4.98%. HRMS (EI/DFS) m/z [M]
+ calcd for (C
18H
35NO)
+ 281.2713, found 281.2710.IR (neat)
νmax: 3377, 2960, 2933, 2874, 1626, 1462, 1392, 1363, 1219, 1201, 1167, 1092, 1059, 1018, 955, 924, 837, 802, 727, 582 cm
−1. UV (EtOH) λ
max (log ε): 204 (3.01), 232 (2.52).
1H NMR (600 MHz; CDCl
3, δ): 0.81 (t,
Jt = 7.4 Hz, 3H), 0.84 (t,
Jt = 7.4 Hz, 3H), 0.85 (t,
Jt = 7.0 Hz, 3H), 1.16 (s, 9H), 1.17–1.41 (m, 7H), 1.41 (dq,
Jd = 14.2 Hz,
Jq = 7.2 Hz, 1H), 1.44 (dq,
Jd = 14.0 Hz,
Jq = 7.4 Hz, 1H), 1.53 (dq,
Jd = 14.0 Hz,
Jq = 7.4 Hz, 1H), 1.61 (dq,
Jd = 14.2 Hz,
Jq = 7.2 Hz, 1H), 1.84 (m, 1H), 1.96 (ddd,
Jd1 = 7.0 Hz,
Jd2 = 7.0 Hz,
Jd3 = 6.3 Hz, 1H), 2.64 (ddd,
Jd1 = 5.9 Hz,
Jd2 = 10.0 Hz,
Jd3 = 4.7 Hz, 1H), 3.47 (dd,
Jd1 = 10.6 Hz,
Jd2 = 6.8 Hz, 1H), 3.67 (dd,
Jd1 = 10.6 Hz,
Jd2 = 6.8 Hz, 1H).
13C{
1H} NMR (150 MHz, CDCl
3, δ): 8.33, 8.96, 13.89, 22.44, 27.30, 27.36, 29.35, 31.86, 32.62, 33.68, 35.98, 50.21, 53.64, 63.32, 76.46, 181.82. For 2D NMR correlation spectra
1H−
1H (COSY, mixing time 0.8 s) and
1H−
13C (HSQC, HMBC) see
Figures S64–S66.
3.2.12. The General Method for Nitroxides 4 and 5, Reduction with Zn in CF3COOH for NMR
Trifluoroacetic acid (20–50 μL) was added dropwise to a mixture of of the nitroxide (0.04–0.08 mmol), Zn dust (50–100 mg, 0.8–1.5 mmol) and CD3OD (0.3–0.4 mL). The reaction mass was incubated at ambient temperature for 15 min and then diluted with 0.3 mL of CDCl3; the precipitate was filtered off. The filtrate was placed into an NMR tube, and 1H spectra were recorded.
5-(tert-Butyl)-5-butyl-2,2-diethylpyrrolidine-1,3-diol 2,2,2-trifluoroacetate (4H): 1H NMR (400 MHz; CDCl3, CD3OD, CF3COOH, δ): 0.94 (t, J = 6.8 Hz, 3H), 1.03, 1.05 (t, J = 7.4 Hz, both 3H), 1.08 (s, 9H), 1.23 (m, 1H), 1.36 (m, 4H), 1.66 (m, 2H), 1.78-2.06 (m, 4H), 2.12 (m, 1H), 2.23 (m, 1H), 4.31 (m, 1H).
(2-(tert-Butyl)-2-butyl-5,5-diethyl-1-hydroxypyrrolidine-3,4-diyl)dimethanol 2,2,2-trifluoroacetate (5H): 1H NMR (400 MHz; CDCl3, CD3OD, CF3COOH, δ): 0.97 (t, J = 6.8 Hz, 3H), 1.03, 1.07 (t, J = 7.4 Hz, both 3H), 1.15 (s, 9H), 1.26–1.51 (m, 4H), 1.68–2.01 (m, 6H), 2.35 (m, 1H), 2.48 (m, 1H), 3.66 (m, 1H), 3.76 (m, 1H), 3.87 (m, 2H).