The Reactions of 6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonanes with Methanesulfonyl Chloride or PPh3-CBr4

Abstract

Activation of a hydroxyl group towards nucleophilic substitution via reaction with methanesulfonyl chloride or PPh₃-CBr₄ system is a commonly used pathway to various functional derivatives. The reactions of (5R(S),6R(S))-1-X-6-(hydroxymethyl)-2,2-dimethyl- 1-azaspiro[4.4]nonanes 1a–d (X = O·; H; OBn, OBz) with MsCl/NR₃ or PPh₃-CBr₄ were studied. Depending on substituent X, the reaction afforded hexahydro-1H,6H-cyclopenta[c]pyrrolo[1,2-b]isoxazole (2) (for X = O), a mixture of 2 and octahydrocyclopenta[c]azepines (4–6) (for X = OBn, OBz), or perhydro-cyclopenta[2,3]azeto[1,2-a]pyrrol (3) (for X = H) derivatives. Alkylation of the latter with MeI with subsequent Hofmann elimination afforded 2,3,3-trimethyl-1,2,3,4,5,7,8,8a-octahydrocyclopenta[c]azepine with 56% yield.

1. Introduction

Activation of a hydroxyl group towards nucleophilic substitution via reaction with methanesulfonyl chloride or PPh₃-CBr₄ system is a commonly used pathway to various functional derivatives [1,2,3]. Recently, we reported on simple synthesis of (5R(S),6R(S))-2,2-dimethyl-6-(hydroxymethyl)-1-azaspiro[4.4]nonane-1-oxyl 1a from commercially available 5,5-dimethyl-1-pyrroline N-oxide (DMPO) [4]. The above nitroxide and its analogs were prepared as single enantiomeric pair with hydroxymethyl group close to the nitroxide one. Close proximity of hydroxy and nitroxide groups make these compounds potential precursors of rigid spin labels, which might allow precise distance measurements via site-directed spin labeling—PELDOR technique [5,6]. Synthesis of these spin labels would require replacement of the hydroxyl group to methanethiosulfonate or maleimido moiety, and Appel reaction or treatment with methanesulfonyl chloride seems to be a proper step towards this direction.

Here, we describe the reactions of 1a and its diamagnetic analogs (5R(S),6R(S))-1-X-6-(hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonanes 1b–d (X = OBn; OBz; H) with MsCl/NR₃ or PPh₃-CBr₄. Surprisingly, none of these reactions allowed us to isolate expected bromomethyl- or mesyloxymethyl-substituted compounds. The product structure was dependent on substituent X in the position 1 of the pyrrolidine ring. For nitroxide (X = O·) hexahydro-1H,6H-cyclopenta[c]pyrrolo[1,2-b]isoxazole (2) was isolated, unsubstituted amine (X = H) afforded unknown perhydro-cyclopenta[2,3]azeto[1,2-a]pyrrol (3), while alkoxy- or acyloxyamines (X = OBn, OBz) afforded unknown octahydrocyclopenta[c]azepine derivatives (4c–d, 5c–d and 6c–d) (Scheme 1). To the best of our knowledge, the ring junction similar to perhydrocyclopenta[2,3]azeto[1,2-a]pyrrol was only observed in cis,cis,cis,cis-[5.5.5.4]-1-azafenestrane system [7]. Azepanes are of continuous interest for preparation of bioactive compounds. This scaffold is present in numerous pharmaceuticals [8,9,10,11,12,13,14,15,16].

2. Results and Discussion

Both mesylation and the Appel reaction were successfully used in nitroxide chemistry, however, the Appel reaction often gives low to moderate yields of brominated nitroxides [17], while the yields of OMs derivatives are high [18,19,20]. The reactions of (5R(S),6R(S))-6-(hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-oxyl (1a) with MsCl/NEt₃ or with PPh₃-CBr₄ afforded no expected nitroxides 7. The reaction mixtures turned brown with significant tarring and the earlier described 3,3-dimethyloctahydrocyclopenta[c]pyrrolo[1,2-b]isoxazole 2 [4] was isolated from both reaction mixtures with a moderate yield (Scheme 2).

Unusual behavior of 1a can only be explained by close proximity of hydroxymethyl and nitroxide groups in this molecule (ca. 3 Å distance). The possible mechanism of the formation of 2 is shown in Scheme 3. The reaction must anyway include one-electron reduction and cyclization via nucleophilic substitution steps. It is not clear whether activation of nitroxide oxygen to nucleophilic attack with one-electron transfer is required for a cyclization step or cyclic radical-cation can be formed spontaneously first and then undergoes one-electron reduction. Tertiary amines and phosphines are known to act as single-electron donors in various reactions, see for example [21,22].

Conversion into alkoxy groups is a convenient way of the protection of nitroxide groups from reactions onto the oxygen atom [23]. We have previously reported on the synthesis of alkoxyamine 9 via treatment of corresponding nitroxide 8 with benzyl bromide in the presence of in situ formed Cu (I) using Matyjaszewski’s procedure [4]. Alkaline hydrolysis of 9 afforded 1c with nearly quantitative yield, and subsequent treatment with MsCl in the presence of a base gave a mixture of products (Scheme 4).

Column chromatography allowed us to isolate two individual compounds, and in addition, an inseparable mixture of two compounds was collected. HPLC showed that the mixture was composed of two compounds (Figure S62). The main component was isolated from the mixture using preparative HPLC. Non-polar character of the products allowed us to discard quaternary alkoxyammonium salt structure, which could be a result of intramolecular alkylation on alkoxyamine nitrogen in analogy to literature [24,25,26]. IR and NMR spectra allowed us to identify one of the isolated products as 2 (preparative yield about 13%). The formation of product 2 in this process may occur via intramolecular alkylation onto O-alkoxyamine atom of intermediate 10 to give cation 11 with subsequent nucleophilic substitution (Scheme 5).

HRMS spectra of the other two isolated compounds showed the same molecular ions ([M]⁺ ca. 271, see Experimental part) with a formula C₁₈H₂₅NO, correspondent to products of the formal elimination of water from 1c. The NMR spectra confirmed that both isomers retained benzyloxy moieties. The multiplets at 5.2–5.4 ppm in ¹H NMR spectra and signals at 137–138 and 148–150 ppm in ¹³C NMR indicated the formation of a trisubstituted ethylene moiety. Assignment of the structure of the isomers was performed on the basis of ¹H-¹H COSY, ¹H-¹³C HMBC and ¹H-¹³C HSQC NMR spectra (see Figures S42–S47 and Appendix A). At room temperature, some signals in the spectra of 5c were broadened, presumably due to slow conformational changes, therefore, the spectra acquired at 333 K with more narrow lines were used for assignments. Analysis of interactions in the 2D spectra allowed us to conclude that compounds 4c and 5c have the same octahydrocyclopenta[c]azepine backbone, differing only in the position of the double >C=CH bond, which is located in 5- or 7-membered ring.

The NMR analysis of the fraction containing 4c (major isomer) obtained via column chromatography showed the extra signals. These signals may belong to a third isomer, 6c, in which a fully substituted double bond is located between the rings. Indeed, in the ¹H NMR spectrum of this fraction recorded at 333 K (Figure S20) a singlet at 1.18 (s, 6H), multiplet signals of five methylene groups in the region of 1.64–2.40 ppm partially overlapped with the signals of the major isomer 4c, the singlet signal of the methylene group at the heteroatom (3.51 ppm) and the signal of the benzyl methylene group at 4.63 ppm were observed. In the ¹³C NMR spectrum (Figure S22), two signals in the low-field region at 138.17 and 138.27 ppm were observed, which correspond to the carbons of tetrasubstituted C=C bond. The ratio of the products was determined using the NMR spectrum of the reaction mixture: 4c/5c/6c/2 = 12.63/3.28/1/9.42.

The possible mechanism of 4c, 5c, and 6c formation is likely to imply intramolecular alkylation to give N-benzyloxyammonium salt 12 with subsequent Hofmann-type elimination, however, one cannot exclude a contribution of Cope-type elimination via 5-membered transition state with coordination of alkoxyamine oxygen in 1c with the most closely located hydrogen atom (methylene hydrogen of cyclopentane ring, ca. 2.5 Å distance) (Scheme 6) [27]. This may account for predominant formation of isomer 4c. Some examples of the conversion of 1-methyl-1-azoniabicyclo[3.2.0]heptanes into azepane derivatives are known [9,28], but in these examples ring opening occurred via nucleophilic substitution, not via elimination.

Under the Appel reaction conditions, 1c was converted into the same products 4c, 5c, 6c and 2 in the ratio of 3.89/1.35/1/4.61, respectively, according to NMR. Total preparative yield of the isomers 4c, 5c, and 6c was 34%.

With the replacement of benzyl group in 1c with electron-withdrawing benzoyl, one could make the N-O-R moiety less prone to intramolecular alkylation. To prepare N-benzoyloxy derivative 1d, the nitroxide 1a was treated with benzhydrazide in the presence of excess MnO₂ [29]. This method allowed us to avoid the acylation of hydroxymethyl group and afforded 1d with the yield of 81% as colorless oil not susceptible to oxidation to nitroxide (Scheme 7). The presence of free hydroxy group in 1d was confirmed with IR absorption band at 3452 cm⁻¹ and a broad singlet (1H) at 5.29 ppm in the ¹H NMR spectrum.

Reaction of 1d with PPh₃-CBr₄ afforded a mixture, from which three compounds were isolated (Scheme 8).

One of the isolated compounds, a yellow crystalline solid, showed intense triplet in EPR spectrum with a_N = 1.49 mT (Figure S57). HRMS with a molecular ion, [M⁺] = 302.1752, corresponding to the formula C₁₈H₂₄NO₃, IR spectrum with strong absorption band at 1716 cm⁻¹ and no absorption above 3100 cm⁻¹ favored the structure 13, which could result from transesterification and oxidation. The structure was confirmed with ¹H NMR spectrum recorded after reduction of the nitroxide 13 to corresponding ammonium salt 14 with Zn in the presence of trifluoroacetic acid (Scheme 9).

The remaining two products were colorless diamagnetic oils. The ¹H and ¹³C NMR spectra of both compounds revealed broadening of some signals similarly to those of 5c. A detailed analysis of the ¹H-¹³C HMBC, ¹H-¹³C HSQC, and ¹H-¹H COSY spectra of isolated compounds (Figures S48–S53, Appendix A) allowed us to conclude that major and minor products are isomeric octahydrocyclopenta[c]azepines 4d and 5d. The presence of impurity signals (singlet signal at 1.24 ppm, multiplet signals of five methylene groups in the region of 1.70–2.40 ppm partially overlapped with the signals of the major isomer 5d, the singlet signal of the methylene group at the heteroatom at 3.78 ppm in ¹H NMR spectrum and two signals in the low-field region at 130.84 and 138.45 ppm in ¹³C NMR spectrum) of some 5d samples may indicate the formation of the third isomer 6d; however, this isomer was not isolated and no other confirmation for this was obtained.

Formation of both 4 and 5 presumably occurs via corresponding benzyl- or benzoyloxyammonium salts 12. However, we did not find references on perhydro-cyclopenta[2,3]azeto[1,2-a]pyrrolium salts in literature. To the best of our knowledge, similar ring junction was never observed before, except for cis,cis,cis,cis-[5.5.5.4]-1-azafenestrane derivatives [7]. These strained structures may be unstable themselves or in the presence of bases, which are present in the reaction mixtures during mesylation or Appel reaction. So easy cleavage of C-N bond in perhydro-cyclopenta[2,3]azeto[1,2-a]pyrrolium salts inspired us to study the behavior of ((5R(S),6R(S))-6-(hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonane 1b under Appel reaction conditions and mesylation. Both reactions afforded the same amine 3 with the yield of 65–77%. The amine 3 was characterized as a hydrobromide, a colorless crystalline compound (Scheme 10).

Analysis of the ¹H NMR spectrum fine structure of the obtained substance, in addition to signals of pair methyl groups, indicates the presence of two isolated spin systems (Figures S60 and S61, Table S2). To confirm the structure of isolated substance, ¹H-¹H COSY, ¹H-¹³C HMBC, and ¹H-¹³C HSQC NMR spectra were recorded (Figures S39–S41, Appendix A). In the ¹H-¹³C HMBC spectrum, the long-range interactions are observed between two geminal hydrogen atoms in the weakest field (i.e., adjacent to heteroatom) and carbons of geminal methyl groups and node carbon atom, confirming C(5)-N bond formation. The single crystal X-ray analysis of the hydrobromide 3×HBr provided unambiguous confirmation of the structure (Figure 1, see Figure S59).

The reaction of amine 3 with an excess of methyl iodide gave a crystalline substance, the NMR spectra of which are very similar to the spectra of 3×HBr, however, in ¹H NMR, a singlet with the intensity of 3H at 2.94 ppm is observed, and in ¹³C NMR, a signal at 38.72 ppm is observed, which confirms methylation at the nitrogen atom with the formation of 15, while maintaining the strained tricyclic structure (Scheme 11).

Salt 15 was then treated with wet silver (I) oxide and heating of the resulting quaternary ammonium hydroxide gave a mixture of several compounds. According to GC-MS data, two compounds with M = 179 g/mol (that corresponds to elimination of HI) were observed in the reaction mixture, 83 and 1%, and a product with M = 197 g/mol (14% according to GC) (Figure S58, Table S1). The latter compound may correspond to the amino alcohol, a typical byproduct in Hofmann elimination conditions, a result of the substitution reaction [27]. The preparative yield of the main product was 70%, the minor products were not isolated. Analysis of the ¹H and ¹³C NMR data allows us to conclude that the isolated product contains a double bond of the >C=CH type; therefore, the assumption of the possible formation of a spirocyclic structure with an exomethylene fragment should be rejected. 2D-NMR spectra (Figures S54–S56, Appendix A) confirmed the octahydrocyclopenta[c]azepine structure of compound 16 and showed that the double carbon-carbon bond is located in the 5-membered ring.

3. Materials and Methods

3.1. General Information

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) (see Figures S1–S11). UV spectra were acquired on a HP Agilent 8453 spectrometer (Agilent Technologies, Santa Clara, CA, USA) in ethanol solutions (concentration ~10⁻⁴ M) (see Figure S12). ¹H NMR spectra were recorded on a Bruker AV 300 (300.132 MHz), AV 400 (400.134 MHz), AV III 500 (500.030 MHz), AV 600 (600.300 MHz), and DRX 500 (500.130 MHz) spectrometers (Bruker, Billerica, MA, USA). ¹³C NMR spectra were recorded on a Bruker AV 300 (75.467 MHz), AV 400 (100.614 MHz), AV III 500 (125.730 MHz), AV 600 (150. 945 MHz), and DRX 500 (125.758 MHz) spectrometers (see Figures S13–S56). ¹⁵N NMR spectrum was obtained on a Bruker Avance III 500 FT-spectrometer (Bruker, Billerica, MA, USA) (50.67 MHz) as projection of 2D ¹H-¹⁵N-correlation. The ¹⁵N NMR chemical shifts are referred to external standard of formamide (δ(¹⁵N) = 112.5 ppm). All the NMR spectra were acquired for 5–10% solutions in CDCl₃, CD₃OD, or CDCl₃-CD₃OD mixtures at 300 K or in DMSO-d₆ at 333 K using the signal of the solvent as a standard. HRMS analyses were performed with High Resolution Mass Spectrometer DFS (Thermo Electron, Waltham, MA, USA). GC-MS analyses were performed with Chromato-mass spectrometer Agilent 6890 MSD Agilent 5973. HPLC analyses were carried out using an HPLC-UV (Agilent 1100, Agilent Technologies Inc., USA) with an Zorbax C8 column (250 mm × 4.6 mm with 5 µm particle size; Agilent Technologies Inc., USA), and the mobile phase were delivered at a flow rate of 1.0 mL/min. Sample was dissolved in acetonitrile (25 mg/mL). The injection volume was 20 µL, and the column temperature was 35 °C. Mobile phase used was a mixture acetonitrile/water (8:2 v/v).

The structure of compound 3×HBr was determined by single-crystal X-ray analysis (see SI). The X-ray diffraction experiment was carried out on a Bruker KAPPA APEX II diffractometer (graphite-monochromated Mo Kα radiation). Reflection intensities were corrected for absorption by SADABS program [30]. The structure of salt 3×HBr was solved by direct methods using the SHELXS-97 program [31] and refined as a 2-component inversion twin by anisotropic (isotropic for all H atoms) full-matrix least-squares method against F2 of all reflections by SHELXL-2014 [32]. The positions of the hydrogen atoms were calculated geometrically and refined in riding model. Crystallographic data for 3×HBr have been deposited at the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1940904. Copy of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK; fax: [phone] or e-mail: [email protected]; internet: www.ccdc.cam.ac.uk (accessed on 13 August 2021).

The reactions were monitored by thin layer chromatography (TLC) on Merck TLC Silica gel 60 F₂₅₄ plates or Merck TLC Aluminium oxide 60 F₂₅₄, neutral, plates. Kieselgel 60 (Macherey-Nagel GmbH & Co. KG, Düren, Germany) and alumina were utilized as sorbent for the column chromatography.

The EPR spectrum of nitroxide 13 in methanol solution was recorded with an Elexsys E540 X-band spectrometer (Bruker, Billerica, MA, USA) in a 100 µL quartz capillary for 0.1 mM solution, 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, 21.39 ms. The EasySpin software (Version 5.2.28, easyspin.org, [33]) was employed for simulation of spectra.

3.2. Synthesis

3.2.1. Reaction of 6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-oxyl 1a with Methanesulfonyl Chloride

Triethylamine (0.174 g, 1.71 mmol) was added to a solution of nitroxide 1a (0.20 g, 1.01 mmol) in dry CHCl₃ (5 mL) at 0 °C. Then MsCl (0.139 g, 1.21 mmol) was added dropwise to mixture under cooling on an ice bath. The mixture was stirred at room temperature for 6 h. The reaction was monitored by TLC (SiO₂, hexane-diethyl ether 1:1 mixture; stained with Dragendorff’s reagent, R_f(2) = 0.45). The reaction mixture was washed with a saturated solution of NaCl (2×10 mL), the organic layer was dried with anhydrous Na₂SO₄. After evaporation of the solvent under reduced pressure the crude residue was purified by column chromatography (SiO₂, hexane-diethyl ether 1:1 mixture as an eluent) to give 3,3-dimethyloctahydrocyclopenta[c]pyrrolo[1,2-b]isoxazole (2): 0.119 g, yield 65%. Physical properties and spectral characteristics coincide to the literature data [4].

3.2.2. Reaction of 6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-oxyl 1a with PPh₃-CBr₄

Carbon tetrabromide (0.84 g, 2.66 mmol) and PPh₃ (0.70 g, 2.66 mmol) were added to a solution of nitroxide 1a (0.25 g, 1.26 mmol) in dry CH₂Cl₂ (6 mL) and the reaction mixture was stirred at room temperature for 12 h to complete the reaction. The progress of reaction was monitored by TLC (SiO₂, hexane-diethyl ether 1:1 mixture; stained with Dragendorff’s reagent). After evaporation of the solvent under reduced pressure, the crude residue was purified by column chromatography (SiO₂, hexane-diethyl ether 1:1 mixture as an eluent) to give 2, 0.091 g, yield 40%. Physical properties and spectral characteristics coincide to the literature data [4].

3.2.3. Reaction of (2,2-Dimethyl-1-azaspiro[4.4]nonan-6-yl)methanol 1b with Methanesulfonyl Chloride

Approximately three-fold excess of cold liquid NMe₃ (1.53 mL, 17.34 mmol), prepared from aqueous solution of NMe₃ and excess of solid NaOH, was added to the solution of 1b (1.058 g, 5.78 mmol) in dry CHCl₃ at 0 °C. Then MsCl (1.06 g, 9.25 mmol) was added dropwise to the mixture under cooling on an ice bath and the reaction mass was stirred at room temperature for 12 h. The progress of the reaction was monitored by TLC (SiO₂, methanol-ethyl acetate 1:4 mixture as an eluent; stained with iodine vapor, R_f(3salt) = 0.15). The reaction mixture was evaporated under reduced pressure. The residue was dissolved in excess of aqueous solution of NaOH and extracted with diethyl ether (3 × 10 mL). The combined organic layers were dried with anhydrous Na₂CO₃. After removal of the solvent under atmosphere pressure, the crude residue was purified by vacuum sublimation (P = 14 mm Hg, T = 110–120 °C) to give 3.

(5aS(R),8aR(S))-3,3-Dimethyloctahydrocyclopenta[2,3]azeto[1,2-a]pyrrole (3): 0.73 g, yield 77%. Colorless liquid. IR (neat) ν_max: 2943, 2860, 2829, 1460, 1444, 1429, 1379, 1363, 1333, 1302, 1290, 1280, 1257, 1246, 1232, 1223, 1207, 1196, 1184, 1147, 1134, 1101, 1084, 1070, 1037, 980, 970, 951, 926, 904, 897, 872, 769, 717, 627 cm⁻¹; ¹H NMR (600 MHz; CDCl₃, δ): 0.91 (s, 3H, CH₃), 1.08 (s, 3H, CH₃), 1.37 (ddd, J_d1 = 6.5 Hz, J_d2 = 12.9 Hz, J_d3 = 13.0 Hz, 1H, C(8)H₂), 1.51 (dddd, J_d1 = 7.2 Hz, J_d2 = 8.1 Hz, J_d3 = 12.5 Hz, J_d4 = 12.08 Hz, 1H, C(6)H₂), 1.59 (dd, J_d1 = 6.5 Hz, J_d2 = 11.5 Hz, 1H, C(2)H₂), 1.61 (dd, J_d1 = 7.0 Hz, J_d2 = 12.8 Hz, 1H, C(6)H₂), 1.65 (dd, J_d1 = 6.4 Hz, J_d2 = 13.0 Hz, 1H, C(8)H₂), 1.74 (dd, J_d1 = 7.5 Hz, J_d2 = 12.9 Hz, 1H, C(1)H₂), 1.81 (ddd, J_d1 = 6.5 Hz, J_d2 = 7.2 Hz, J_d3 = 12.5 Hz, 1H, C(7)H₂), 1.88 (ddd, J_d1 = 6.5 Hz, J_d2 = 12.7 Hz, J_d3 = 12.9 Hz, 1H, C(1)H₂), 1.96 (ddd, J_d1 = 7.5 Hz, J_d2 = 11.5 Hz, J_d3 = 12.7 Hz, 1H, C(2)H₂), 2.15 (dddt, J_d1 = 6.4 Hz, J_d2 = 7.0 Hz, J_d3 = 12.7 Hz, J_t = 12.5 Hz, 1H, C(7)H₂), 2.29 (ddd, J_d1 = 4.4 Hz, J_d2 = 8.1 Hz, J_d3 = 9.3 Hz, 1H, C(5a)H), 2.76 (dd, J_d1 = 4.4 Hz, J_d2 = 9.8 Hz, 1H, C(5)H₂), 3.30 (dd, J_d1 = 9.3 Hz, J_d2 = 9.8 Hz, 1H, C(5)H₂); ¹³C{¹H} NMR (150 MHz; CDCl₃, δ): 22.16 (CH₃), 25.89 C(7), 27.85 (CH₃), 31.14 C(6), 33.83 C(1), 36.95 C(2), 38.94 C(5a), 39.37 C(8), 48.96 C(5), 61.95 C(3), 82.21 C(8a).

3.2.4. Reaction of (5aS,8aR)-3,3-Dimethyloctahydrocyclopenta[2,3]azeto[1,2-a]pyrrole 3 with HBr

An aqueous solution of HBr was added dropwise to a solution of amine 3 (0.27 g, 1.64 mmol) in diethyl ether (3 mL) with stirring to pH = 2–3. Then the organic and aqueous phases were separated, the aqueous one was evaporated to dryness under reduced pressure. The solid residue was crystallized from iso-propanol to give 3×HBr.

(5aS(R),8aR(S))-3,3-Dimethyloctahydro-1H-cyclopenta[2,3]azeto[1,2-a]pyrrol-4-ium bromide (3×HBr): 0.35 g, yield 87%. Colorless crystals, m.p. 194.5 °C with decomposition (iso-propanol). Elemental analysis: found: C, 53.66; H, 8.11; N, 5.70; Br, 32.49; calcd. for C₁₁H₂₀BrN: C, 53.67; H, 8.19; N, 5.69; Br, 32.46%; IR (KBr) ν_max: 3014, 2960, 2928, 2868, 2800, 2754, 2733, 2673, 2950, 2632, 2596, 2571, 2546, 2521, 2496, 2482, 2442, 2415, 2363, 2334, 1470, 1446, 1425, 1406, 1387, 1381, 1350, 1331, 1317, 1304, 1273, 1242, 1213, 1192, 1171, 1146, 1117, 1090, 1065, 1034, 997, 985, 964, 947, 927, 893, 862, 845, 804, 787, 650, 627, 588, 486 cm⁻¹; ¹H NMR (600 MHz; CDCl₃, δ): 1.39 (s, 3H, CH₃), 1.46 (s, 3H, CH₃), 1.82 (dddd, J_d1 = 6.5 Hz, J_d2 = 7.1 Hz, J_d3 = 13.0 Hz, J_d4 = 13.7 Hz, 1H, C(6)H₂), 1.89 (dd, J_d1 = 6.6 Hz, J_d2 = 13.7 Hz, 1H, C(6)H₂), 1.90 (ddd, J_d1 = 6.9 Hz, J_d2 = 13.1 Hz, J_d3 = 14.8 Hz, 1H, C(8)H₂), 2.11 (dd, J_d1 = 6.4 Hz, J_d2 = 13.4 Hz, 1H, C(2)H₂), 2.13 (ddd, J_d1 = 6.5 Hz, J_d2 = 6.9 Hz, J_d3 = 13.3 Hz, 1H, C(7)H₂), 2.19 (dd, J_d1 = 7.1 Hz, J_d2 = 13.7 Hz, 1H, C(1)H₂), 2.22 (dd, J_d1 = 6.1 Hz, J_d2 = 14.8 Hz, 1H, C(8)H₂), 2.30 (ddddd, J_d1 = 6.1 Hz, J_d2 = 6.5 Hz, J_d3 = 13.0 Hz, J_d4 = 13.1 Hz, J_d5 = 13.3 Hz, 1H, C(7)H₂), 2.36 (ddd, J_d1 = 6.4 Hz, J_d2 = 13.3 Hz, J_d3 = 13.7 Hz, 1H, C(1)H₂), 2.53 (ddd, J_d1 = 7.1 Hz, J_d2 = 13.3 Hz, J_d3 = 13.3 Hz, 1H, C(2)H₂), 2.80 (ddd, J_d1 = 5.6 Hz, J_d2 = 7.1 Hz, J_d3 = 9.4 Hz, 1H, C(5a)H), 3.51 (dd, J_d1 = 5.6 Hz, J_d2 = 12.3 Hz, 1H, C(5)H₂), 4.19 (dd, J_d1 = 9.4 Hz, J_d2 = 12.3 Hz, 1H, C(5)H₂); ¹³C{¹H} NMR (150 MHz; CDCl₃, δ): 21.31 (CH₃), 24.80 (CH₃), 25.79 C(7), 31.34 C(6), 32.53 C(1), 37.38 C(8), 37.42 C(2), 39.48 C(5a), 49.85 C(5), 67.74 C(3), 91.79 C(8a).

3.2.5. Reaction of ((5R(S),6R(S))-2,2-Dimethyl-1-azaspiro[4.4]nonan-6-yl)methanol 1b with PPh₃-CBr₄

Carbon tetrabromide (1.80 g, 5.53 mmol) and PPh₃ (1.45 g, 5.53 mmol) were added to a solution of 1b (0.40 g, 2.21 mmol) in dry CH₂Cl₂ (8 mL) and the reaction mixture was stirred at room temperature for 12 h to complete the reaction. The progress of reaction was monitored by TLC (SiO₂, methanol-ethyl acetate 1:6 mixture; stained with iodine vapor, R_f(3×HBr) = 0.2). After evaporation of the solvent under reduced pressure, the crude residue was purified by column chromatography (SiO₂, methanol-ethyl acetate 1:5 mixture) and recrystallized from iso-propanol to give 3×HBr, 0.35 g, yield 65%.

3.2.6. (1-(Benzyloxy)-2,2-dimethyl-1-azaspiro[4.4]nonan-6-yl)-methanol (1c)

An excess of KOH suspension in methanol-iso-propanol 1:3 mixture (8 mL) was added to a solution of alkoxyamine 9 [4] (0.342 g, 1.03 mmol) in iso-propanol (25 mL). The reaction mixture was left at room temperature for 12 h to complete the reaction. The progress of reaction was monitored by TLC (SiO₂, hexane-diethyl ether 1:1 mixture; stained with Dragendorff’s reagent, R_f(1c) = 0.45). Methanol and iso-propanol were evaporated under reduced pressure. A saturated solution of NaCl (15 mL) was added to the residue, and the resulting mixture was extracted with CHCl₃ (3 × 15 mL). The combined organic extracts were dried with anhydrous Na₂SO₄. After evaporation of the solvent under reduced pressure, the crude residue was purified by column chromatography (SiO₂, hexane-diethyl ether 1:1 mixture) to give 1c.

(1-(Benzyloxy)-2,2-dimethyl-1-azaspiro[4.4]nonan-6-yl)-methanol (1c): 0.28 g, yield 94%. Colorless oil. Elemental analysis: found: C, 74.85; H, 9.46; N, 4.91; calcd. for C₁₈H₂₇NO₂: C, 74.70; H, 9.40; N, 4.73%; IR (neat) ν_max: 3427, 3089, 3064, 3032, 2958, 2870, 1608, 1497, 1454, 1363, 1317, 1257, 1155, 1082, 1028, 908, 845, 752, 735, 696, 613 cm⁻¹; ¹H NMR (500 MHz; CDCl₃, δ): 1.21 (s, 3H), 1.31 (s, 3H), 1.40–1.57 (m, 3H), 1.60–1.80 (m, 5H), 1.80–1.90 (m, 2H), 2.30–2.38 (m, 1H), 3.78–3.82 (m, 2H), 4.76 (d, J_d = 10 Hz, 1H), 4.94 (d, J_d = 10 Hz, 1H), 5.56 (br.s, 1H), 7.25–7.36 (m, 5H); ¹³C{¹H} NMR (125 MHz; CDCl₃, δ): 22.55, 22.81, 27.15, 29.96, 34.44, 35.99, 36.09, 50.41, 64.47, 66.57, 76.46, 78.81, 127.83, 128.17, 128.39, 137.12.

3.2.7. Reaction of (1-(Benzyloxy)-2,2-dimethyl-1-azaspiro[4.4]nonan-6-yl)-methanol 1c with Methanesulfonyl Chloride

Triethylamine (0.62 g, 6.16 mmol) was added to a solution of 1c (1.018 g, 3.52 mmol) in dry CHCl₃ (15 mL) at 0 °C. Then MsCl (0.605 g, 5.29 mmol) was added dropwise to the mixture upon stirring and cooling on an ice bath, and the reaction mass was left at room temperature for 72 h. The reaction was monitored by TLC (SiO₂, hexane-diethyl ether 1:2 mixture, stained with Dragendorff’s reagent; R_f(1c) = 0.55). The mixture was concentrated under reduced pressure, the crude residue was subjected to column chromatography (SiO₂, hexane-diethyl ether 20:1 mixture, stained with Dragendorff’s reagent; R_f(4c) = 0.4, R_f(5c) = 0.3, and then: hexane-diethyl ether 1:2 mixture, R_f(2) = 0.45) to give 2, 5c, and a mixture of 4c and 6c 10:1 with preparative yields 13, 12, and 48%, respectively. Pure 4c was isolated using preparative HPLC (Zorbax C8 (250 mm × 4.6 mm, i.d., 5 µm); mobile phase: acetonitrile/water (8:2 v/v).

2-(Benzyloxy)-3,3-dimethyl-1,2,3,4,5,7,8,8a-octahydrocyclopenta[c]azepine (4c): Colorless oil. Elemental analysis: C, 79.86; H, 9.21; N, 5.17; calcd. for C₁₈H₂₅NO: C, 79.66; H, 9.28; N, 5.16%; HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₅NO)⁺: 271.1931; found: 271.1932. IR (neat) ν_max: 3109, 3090, 3066, 3034, 2931, 2850, 1944, 1647, 1606, 1497, 1454, 1433, 1379, 1362, 1307, 1288, 1265, 1242, 1209, 1176, 1155, 1140, 1126, 1084, 1041, 1030, 997, 958, 931, 910, 870, 847, 733, 696, 665, 648, 617, 600 cm⁻¹; ¹H NMR (400 MHz; CDCl₃, δ): 1.07 (s, 3H, CH₃), 1.25 (s, 3H, CH₃), 1.32 (ddd, J_d1 = 3.1 Hz, J_d2 = 7.1 Hz, J_d3 = 14.6 Hz, 1H, C(4)H₂), 1.34 (tdd, J_t = 7.5 Hz, J_d1 = 9.0 Hz, J_d2 = 12.6 Hz, 1H, C(8)H₂), 1.85–1.95 (m, 1H, C(4)H₂), 2.01 (dtd, J_d1 = 4.1 Hz, J_t = 8.0 Hz, J_d2 = 12.6 Hz, 1H, C(8)H₂), 2.17–2.26 (m, 2H, C(7)H₂), 2.26–2.38 (m, 2H, C(5)H₂), 2.82 (dd, J_d1 = 10.7 Hz, J_d2 = 13.2 Hz, 1H, C(1)H₂), 2.90–2.98 (m, 1H, C(8a)H), 3.07 (dd, J_d1 = 3.4 Hz, J_d2 = 13.2 Hz, 1H, C(1)H₂), 4.64 (d, J_d = 10.8 Hz, 1H, CH₂Ph), 4.67 (d, J_d = 10.8 Hz, 1H, CH₂Ph), 5.29–5.32 (m, 1H, C(6)H), 7.25–7.29 (m, 1H, Ph), 7.31–7.37 (m, 4H, Ph); ¹³C{¹H} NMR (100 MHz; CDCl₃, δ): 19.81 (CH₃), 24.27 C(5), 28.87 (CH₃), 29.53 C(8), 30.87 C(7), 36.75 C(4), 45.41 C(8a), 57.66 C(1), 61.35 C(3), 75.44 (CH₂Ph), 122.95 C(6), 127.48 (Ph), 128.10 (Ph), 128.22 (Ph), 137.73 (Ph), 147.92 C(5a).

2-(Benzyloxy)-3,3-dimethyl-1,2,3,4,6,7,8,8a-octahydrocyclopenta[c]azepine (5c): 0.11 g, yield 12%. Colorless oil. HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₅NO)⁺: 271.1931; found: 271.1927. IR (neat) ν_max: 3088, 3063, 3030, 2947, 2897, 2864, 1606, 1587, 1497, 1452, 1433, 1377, 1360, 1331, 1306, 1275, 1221, 1194, 1176, 1155, 1132, 1105, 1082, 1040, 1005, 968, 935, 912, 876, 852, 825, 798, 748, 733, 696, 677, 654, 634, 609, 600, 565, 542, 463; ¹H NMR (500 MHz; DMSO-d₆, 333 K, δ): 0.98 (s, 3H, CH₃), 1.20 (s, 3H, CH₃), 1.20–1.28 (m, 1H, C(8)H₂), 1.36–1.46 (m, 1H, C(7)H₂), 1.60–1.68 (m, 1H, C(7)H₂), 1.72–1.82 (m, 1H, C(4)H₂), 1.83–1.93 (m, 1H, C(8)H₂), 2.23 (br.s, 2H, C(6)H₂), 2.44 (br.s, 1H, C(4)H₂), 2.68 (m, 1H, C(1)H₂), 2.78 (br.s, 1H, C(8a)H), 3.04 (dd, J_d1 = 12.4 Hz, J_d2 = 2.4 Hz, 1H, C(1)H₂), 4.58–4.66 (m, 2H, CH₂Ph), 5.36–5.42 (m, 1H, C(5)H), 7.24–7.36 (m, 5H, Ph); ¹³C{¹H} NMR (125 MHz; DMSO-d₆, 333 K, δ): 18.75 (CH₃), 25.12 C(7), 29.57 (CH₃), 32.04 C(8), 33.31 C(6), 36.91 (C4), 39.00 C(8a), 53.87 C(1), 59.20 C(3), 74.40 (CH₂Ph), 116.72 C(5), 127.34 (Ph), 127.96 (Ph), 128.08 (Ph), 137.57 (Ph), 149.34 C(5a); ¹⁵N NMR (51 MHz, HCONH₂, DMSO-d₆, δ): 185.7.

2-(Benzyloxy)-3,3-dimethyl-1,2,3,4,5,6,7,8-octahydrocyclopenta[c]azepine (6c): (the data from the NMR spectrum of the mixture): ¹H NMR (500 MHz; DMSO-d₆, 333 K, δ): 1.18 (s, 6H), 1.65–1.69 (m, 2H), 1.75–1.83 (m, 2H), 2.05–2.10 (m, 2H), 2.15–2.40 (m, 4H), 3.50 (s, 2H), 4.63 (br.s., 2H), 7.30–7.42 (m. 5H); ¹³C{¹H} NMR (125 MHz; DMSO-d₆, 333 K, δ): 22.45, 24.33, 24.90, 37.66, 37.80, 38.89, 51.87, 61.38, 75.71, 128.09, 128.65, 129.04, 133.04, 138.17, 138.27.

3.2.8. Reaction of (1-(Benzyloxy)-2,2-dimethyl-1-azaspiro[4.4]nonan-6-yl)-methanol 1c with PPh₃-CBr₄

Carbon tetrabromide (1.21 g, 3.63 mmol) and PPh₃ (0.98 g, 3.75 mmol) were added to a solution of 1c (0.35 g, 1.21 mmol) in dry CHCl₃ (7 mL) and the reaction mixture was stirred at room temperature for 48 h. The reaction was monitored by TLC (SiO₂, hexane-diethyl ether 1:2 mixture). After evaporation of the solvent under reduced pressure, the crude residue was subjected to column chromatography (SiO₂, hexane-diethyl ether 20:1 mixture as an eluent) to give a mixture of 4c, 5c, and 6c (0.111 g, yield 34%).

3.2.9. 6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-yl benzoate (1d)

Manganese dioxide (7.47 g, 85 mmol) was added to a solution of nitroxide 1а (0.85 g, 4.29 mmol) in diethyl ether (20 mL). A solution of benzhydrazide (1.16 g, 8.58 mmol) in methanol was added dropwise to the resulting mixture, and the reaction mass was stirred for 30 min. Then manganese dioxide was filtered off through celite, the filtrate was evaporated, and the residue was purified by column chromatography (SiO₂, hexane-diethyl ether 1:1 mixture as an eluent, detected under UV lamp and stained with Dragendorff’s reagent, R_f(1d) = 0.45).

6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-yl benzoate (1d): 1.06 g, yield 81%. Yellowish oil. Elemental analysis: C, 70.98; H, 8.41; N, 4.86; calcd. for C₁₈H₂₅NO₃: C, 71.26; H, 8.31; N, 4.62%; HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₅NO₃)⁺: 303.1829; found: 303.1826. IR (neat) ν_max: 3452, 3088, 3064, 3032, 2958, 2875, 1740, 1601, 1583, 1491, 1452, 1410, 1385, 1367, 1315, 1261, 1244, 1176, 1113, 1082, 1063, 1026, 1001, 976, 947, 924, 897, 885, 854, 802, 710, 687, 667, 646, 617 cm⁻¹; NMR (400 MHz; CDCl₃, δ): 1.19 (s, 3H), 1.25 (s, 3H), 1.38–1.56 (m, 2H), 1.56–1.69 (m, 3H), 1.70–1.92 (m, 4H), 2.04 (dt, J_d = 12.4 Hz, J_t = 8.5 Hz, 1H), 2.32–2.41 (m, 1H), 3.61 (dd, J_d1 = 6.8 Hz, J_d2 = 12.1 Hz, 1H), 3.80 (dd, J_d1 = 2.5 Hz, J_d2 = 12.1 Hz, 1H), 5.29 (br.s, 1H), 7.39–7.46 (m, 2H), 7.52–7.59 (m, 1H), 7.97–8.03 (m, 2H); ¹³C{¹H} NMR (100 MHz; CDCl₃, δ): 21.88, 22.35, 26.05, 27.93, 35.08, 35.41, 35.46, 51.50, 62.32, 67.59, 77.79, 128.51, 128.40, 129.31, 133.17, 166.01.

3.2.10. Reaction of (1-(Benzoyloxy)-2,2-dimethyl-1-azaspiro[4.4]nonan-6-yl)-methanol 1d with PPh₃-CBr₄

Carbon tetrabromide (0.638 g, 1.92 mmol) and PPh₃ (0.50 g, 1.92 mmol) were added to a solution of 1d (0.194 g, 0.64 mmol) in dry CHCl₃ (6 mL) and the reaction mixture was stirred at room temperature for 12 h. The reaction was monitored by TLC (SiO₂, hexane-diethyl ether 2:1 mixture; detected under UV lamp and stained with Dragendorff’s reagent). After evaporation of the solvent under reduced pressure, the crude residue was subjected to column chromatography (SiO₂, from hexane-ethyl acetate 50:1 to hexane-ethyl acetate 25:1 mixture as an eluent) to give 4d, 5d, and 13.

3,3-Dimethyl-1,4,5,7,8,8a-hexahydrocyclopenta[c]azepin-2(3H)-yl benzoate (4d): 0.055 g, yield 30%. Colorless oil. Elemental analysis: C, 75.88; H, 8.25; N, 4.80; calcd. for C₁₈H₂₃NO₂: C, 75.76; H, 8.12; N, 4.62%; HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₃NO₂)⁺: 285.1723; found: 285.1720. IR (neat) ν_max: 3088, 3061, 3037, 2976, 2931, 2850, 1741, 1645, 1601, 1583, 1491, 1450, 1383, 1363, 1313, 1259, 1238, 1176, 1157, 1126, 1084, 1063, 1024, 999, 960, 935, 906, 889, 874, 862, 802, 708, 687, 671, 646 cm⁻¹; ¹H NMR (600 MHz; CDCl₃, δ): 1.19 (s, 3H, CH₃), 1.22 (s, 3H, CH₃), 1.34 (dddd, J_d1 = 6.6 Hz, J_d2 = 6.9 Hz, J_d3 = 8.7 Hz, J_d4 = 13.0 Hz, 1H, C(8)H₂), 1.46 (ddd, J_d1 = 4.1 Hz, J_d2 = 5.2 Hz, J_d3 = 15.0 Hz, 1H, C(4)H₂), 2.04 (dddd, J_d1 = 4.7 Hz, J_d2 = 8.4 Hz, J_d3 = 8.7 Hz, J_d4 = 12.7 Hz, 1H, C(8)H₂), 2.07–2.18 (m, 1H, C(4)H₂), 2.17–2.29 (m, 2H, C(7)H₂), 2.37–2.48 (m, 2H, C(5)H₂), 3.06 (dd, J_d1 = 11.0 Hz, J_d2 = 13.0 Hz, 1H, C(1)H₂), 3.15–3.29 (m, 2H, C(8a)H + C(1)H₂), 5.30–5.32 (m, 1H, C(6)H), 7.39–7.43 (m, 2H, Ph), 7.51–7.55 (m, 1H, Ph), 7.98–8.02 (m, 2H, Ph); ¹³C{¹H} NMR (150 MHz; CDCl₃, δ): 20.30 (CH₃), 24.23 C(5), 29.29 (CH₃), 29.41 C(8), 30.77 C(7), 35.55 C(4), 44.02 C(8a), 59.15 C(1), 62.27 C(3), 123.80 C(6), 128.27 (Ph), 129.23 (Ph), 129.58 (Ph), 132.74 (Ph), 146.74 C(5a), 164.97 (C = O).

3,3-Dimethyl-3,4,6,7,8,8a-hexahydrocyclopenta[c]azepin-2(1H)-yl benzoate (5d): 0.027 g, yield 15%. Colorless oil. HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₃NO₂)⁺: 285.1723; found: 285.1725. IR (neat) ν_max: 3088, 3061, 3032, 2949, 2931, 2899, 2866, 2854, 1743, 1601, 1583, 1491, 1450, 1435, 1381, 1362, 1331, 1313, 1257, 1248, 1192, 1176, 1130, 1084, 1063, 1024, 1007, 993, 970, 939, 893, 870, 851, 831, 802, 754, 710, 688, 671 cm⁻¹; ¹H NMR (500 MHz; DMSO-d₆, δ): 1.10 (s, 3H, CH₃), 1.16 (s, 3H, CH₃), 1.21–1.28 (m, 1H, C(8)H₂), 1.38–1.49 (m, 1H, C(7)H₂), 1.61–1.71 (m, 1H, C(7)H₂), 1.84–1.96 (m, 2H, C(4)H, C(8)H), 2.28 (br.s, 2H, C(6)H₂), 2.62 (br.s, 1H, C(4)H), 2.91 (br.s, 1H, C(8a)H), 2.99 (t, J_t = 12.4 Hz, 1H, C(1)H₂), 3.12 (dd, J_d1 = 12.4 Hz, J_d2 = 2.0 Hz, 1H, C(1)H₂), 5.44–5.51 (m, 1H, C(5)H), 7.52 (t, J_t = 7.7 Hz, 2H, m-Ph), 7.64 (tt, J_t1 = 7.5 Hz, J_t2 = 1.1 Hz, 1H, p-Ph), 7.94 (dd, J_d1 = 8.2 Hz, J_d2 = 1.1 Hz, 2H, o-Ph); ¹³C{¹H} NMR (125 MHz; DMSO-d₆, δ): 19.25 (CH₃), 25.05 C(7), 29.34 (CH₃), 31.83 (C8), 33.36 C(6), 36.62 C(4), 38.61 C(8a), 55.36 C(1), 59.89 C(3), 116.70 C(5), 128.69 (m-Ph), 128.73 (o-Ph), 129.22 (i-Ph), 133.03 (p-Ph), 149.51(C(5a), 163.84 (C=O); ¹⁵N NMR (51 MHz, HCONH₂, DMSO-d₆, δ): 194.4.

6-((Benzoyloxy)methyl)-2,2-dimethyl-1-oxo-1-azaspiro[4.4]nonan-1-oxyl (13): 0.027 g, yield 14%. Yellow crystalline solid, m.p. 68.2 °C with decomposition (hexane). Elemental analysis: C, 71.77; H, 7.97; N, 4.68; calcd. for C₁₈H₂₄NO₃: C, 71.50; H, 8.00; N, 4.63%; HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₈H₂₄NO₃)⁺: 302.1751; found: 302.1752. IR (KBr) ν_max: 3070, 3063, 2968, 2928, 2873, 2854, 1716, 1601, 1583, 1491, 1454, 1406, 1371, 1360, 1350, 1313, 1282, 1273, 1250, 1203, 1180, 1163, 1130, 1115, 1084, 1072, 1022, 991, 968, 951, 928, 893, 849, 717, 688, 665, 590, 569, 447 cm⁻¹; UV (EtOH) λmax (log ε): 229 (4.06).

3.2.11. Reduction of Nitroxide 13 with Zn in CF₃COOH for NMR

A suspension of zinc dust (100 mg) in a solution of nitroxide 13 (0.015 g, 0.050 mmol) in CD₃OD (0.4 mL) in a small glass vial was heated to reflux upon vigorous stirring and trifluoroacetic acid (0.1 mL) was added dropwise. The mixture was stirred for 10–15 min, and the solution was transferred into an NMR tube through a pipette tip with a tightly inserted paper filter. The vial was rinsed with a small portion of CDCl₃–CD₃OD mixture and this solution was filtered into the same NMR tube until the normal NMR sample volume was reached.

(5R(S),6R(S))-6-((Benzoyloxy)methyl)-2,2-dimethyl-1-azaspiro[4.4]nonan-1-ium 2,2,2-trifluoroacetate (14): ¹H NMR (400 MHz; CDCl₃–CD₃OD mixture, δ): 1.47 (s, 3H), 1.50 (s, 3H), 1.71–1.87 (m, 2H), 1.88–2.07 (m, 3H), 2.08–2.24 (m, 2H), 2.44–2.56 (m, 1H), 4.36 (dd, J_d1 = 7.1 Hz, J_d2 = 11.8 Hz, 1H), 4.48 (dd, J_d1 = 6.3 Hz, J_d2 = 11.8 Hz, 1H), 7.40–7.50 (m, 2H), 7.55–7.63 (m, 1H), 7.93–8.03 (m, 2H).

3.2.12. 3,3,4-Trimethyloctahydro-1H-cyclopenta[2,3]azeto[1,2-a]pyrrol-4-ium iodide (15)

Iodomethane (0.23g, 1.64 mmol) was added to a solution of 3 (0.09 g, 0.545 mmol) in dry diethyl ether (2 mL) and the reaction mass was left over 12 h. Then the precipitate formed was filtered off and washed with diethyl ether.

3,3,4-Trimethyloctahydro-1H-cyclopenta[2,3]azeto[1,2-a]pyrrol-4-ium iodide (15): 0.133 g, yield 80%. White crystalline solid, m.p. 187.2–187.4 °C. Elemental analysis: C, 47.19; H, 7.23; N, 4.56; calcd. for C₁₂H₂₂IN: C, 46.91; H, 7.22; N, 4.59%. IR (KBr) ν_max: 3007, 2956, 2874, 2833, 2816, 2756, 2600, 2580, 1470, 1443, 1433, 1396, 1383, 1358, 1335, 1315, 1304, 1294, 1275, 1240, 1217, 1196, 1178, 1149, 1138, 1099, 1086, 1068, 1034, 986, 953, 931, 916, 895, 881, 852, 839, 796, 779, 735, 642, 606, 571, 499, 411; ¹H NMR (300 MHz; CDCl₃, δ): 1.34 (s, 3H), 1.53 (s, 3H), 1.80–2.39 (m, 8H), 2.50 (dd, J_d1 = 5.9 Hz, J_d2 = 16.0 Hz, 1H), 2.60 (ddd, J_d1 = 6.2 Hz, J_d2 = 13.0 Hz, J_d3 = 13.1 Hz, 1H), 2.77–2.89 (m, 1H), 2.94 (c, 3H), 3.52 (dd, J_d1 = 6.2 Hz, J_d2 = 12.6 Hz, 1H), 4.65 (dd, J_d1 = 9.9 Hz, J_d2 = 12.6 Hz, 1H); ¹³C{¹H} NMR (75 MHz; CDCl₃, δ): 20.72, 24.74, 26.90, 30.09, 33.16, 33.44, 37.80, 38.72, 40.20, 62.65, 74.37, 99.18.

3.2.13. 2,3,3-Trimethyl-1,2,3,4,5,7,8,8a-octahydrocyclopenta[c]azepine (16)

Wet silver (I) oxide (5.94 mmol) and water (12 mL) were added to salt 15 (0.729 g, 2.37 mmol) and stirred for 12 h. Then the solid residue was filtered off, the filtrate was concentrated in vacuum. The residue was heated on a water bath under reflux until the disappearance of a solid insoluble in diethyl ether, and then was extracted with diethyl ether. After evaporation of the solvent under reduced pressure, the crude residue was purified by column chromatography (alumina, pentane-diethyl ether 6:1 as an eluent) to give 16.

2,3,3-Trimethyl-1,2,3,4,5,7,8,8a-octahydrocyclopenta[c]azepine (16): 0.296 g, yield 70%. Colorless oil. HRMS (EI/DFS) m/z [M]⁺ calcd. for (C₁₂H₂₁N)⁺: 179.1669; found: 179.1668. Elemental analysis: C, 80.36; H, 11.82; N, 7.60; calcd. for C₁₂H₂₁N: C, 80.38; H, 11.81; N, 7.81%. IR (neat) ν_max: 3466, 3039, 2964, 2945, 2926, 2847, 2798, 2789, 2777, 1649, 1618, 1464, 1454, 1429, 1377, 1362, 1344, 1315, 1288, 1267, 1236, 1205, 1174, 1128, 1109, 1088, 1049, 1024, 1005, 974, 952.7 941, 918, 908, 852, 800, 694, 638, 577, 530, 488. ¹H NMR (600 MHz; CDCl₃, δ): 0.94 (s, 3H, CH₃), 1.09 (dd, J_d1 = 0.4 Hz, J_d2 = 0.7 Hz, 3H, CH₃), 1.22 (dddd, J_d1 = 8.8 Hz, J_d2 = 8.9 Hz, J_d3 = 9.2 Hz, J_d4 = 12.5 H, 1H, C(8)H₂), 1.32–1.39 (m, 1H, C(4)H₂), 1.65–1.71 (m, 1H, C(4)H₂), 1.94 (dddd, J_d1 = 5.1 Hz, J_d2 = 5.5 Hz, J_d3 = 7.8 Hz, J_d4 = 12.6 Hz, 1H, C(8)H₂), 2.15–2.21 (m, 2H, C(7)H₂), 2.23–2.29 (m, 2H, C(5)H₂), 2.32 (s, 3H, CH₃), 2.35 (dd-quartet, J_d1 = 4.2 Hz, J_d2 = 13.9 Hz, J_quartet = 0.4 Hz, 1H, C(1)H₂), 2.55 (dd, J_d1 = 10.7 Hz, J_d2 = 13.9 Hz, 1H, C(1)H₂), 2.72–2.79 (m, 1H, C(8a)H), 5.20–5.22 (m, 1H, C(6)H); ¹³C{¹H} NMR (150 MHz; CDCl₃, δ): 20.27 (CH₃), 24.70 C(5), 28.19 (CH₃), 29.35 C(8), 30.83 C(7), 38.72 (NCH₃), 39.95 C(4), 48.35 C(8a), 55.96 C(3), 57.34 C(1), 122.48 C(6), 148.90 C(5a).

4. Conclusions

Thus, conversion of hydroxyl group in (5R(S),6R(S))-6-(hydroxymethyl)- 2,2-dimethyl-1-X-1-azaspiro[4.4]nonanes (X = H, O·, OBn, OBz) into good leaving group always leads to intramolecular alkylation. The structure of the cyclic products depends on the substituent X at N-1 atom. If X = OBn or OBz, the initially formed 1-OBn- or 1-OBz-1-azoniabicyclo[3.2.0]heptanes undergo spontaneous azetidine ring opening with formation of mixture of isomeric octahydrocyclopenta[c]azepine derivatives. Alternatively, thermal degradation of 3,3,4-trimethyloctahydro-1H-cyclopen-ta[2,3]azeto[1,2-a]pyrrol-4-ium hydroxide gives 2,3,3-trimethyl-1,2,3,4,5,7,8,8a-octa-hydrocyclopenta[c]azepine with high selectivity.

The heterocycles with spiro-(2-hydroxymethyl)cyclopentane moieties can be easily prepared from cyclic nitrones via addition of pent-4-enylmagnesium bromide-oxidation-intramolecular cycloaddition-isoxazolidine ring opening sequence [4,34,35]. Here, we demonstrated an example of conversion of these spirocyclic structures into azepane derivatives. Similar transformations may provide a promising pathway to valuable scaffolds for the synthesis of biologically active compounds.

Author Contributions

Synthesis, Y.V.K. and A.I.T.; NMR experiments I.V.E.; X-ray analyses, T.V.R.; EPR spectra, N.B.A.; spectral data analysis I.V.E., Y.V.K., D.A.M., S.A.D.; HPLC compound isolation and analysis, Y.S.S. and D.N.P.; writing—original draft preparation, Y.V.K. and D.A.M.; writing—review and editing, I.A.K., D.A.M., S.A.D.; supervision, project administration, I.A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by RFBR 18-53-76003-ERA-A (ERA.Net RUS+ project ST2017-382: NanoHyperRadicals).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article and supplementary material.

Acknowledgments

Authors 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 the compounds 3, 15, 16 are available from the authors.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figure and Schemes

Scheme 1. The reactions of 6-(hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonanes 1a–d with MsCl/NR3 and PPh3-CBr4.

Scheme 2. The reactions of 1a with MsCl/NEt3 and PPh3-CBr4.

Scheme 3. The possible mechanism of the formation of 2 in the reactions of 1a with MsCl/NEt3 and PPh3-CBr4.

Scheme 4. Synthesis of alkoxyamine 1c and its reaction with MsCl/NEt3. The yield of 4c (pure fraction isolated using preparative HPLC) was 25%. The ratio in the reaction mixture (NMR): 4c/5c/6c/2 = 12.63/3.28/1/9.42.

Scheme 5. The possible mechanism of product 2 formation.

Scheme 6. The possible mechanisms of the formation of products 4c–d, 5c–d, and 6c–d.

Scheme 7. Synthesis of N-benzoyloxy derivative 1d.

Scheme 8. Reaction of 1d with PPh3-CBr4.

Scheme 9. Reduction of nitroxide 13 with Zn in CF3COOH.

Scheme 10. Reaction of 1b with MsCl/NMe3 or PPh3-CBr4.

Figure 1. The structure of 3×HBr according to X-ray analysis.

Scheme 11. Methylation of 3 with subsequent Hofmann cleavage of quaternary ammonium hydroxide.

Supplementary Materials

Figures S1–S11: IR spectra of 1c–d, 3, 3×HBr, 4c–d, 5c–d, 13, 15, 16; Figure S12: UV spectrum of 13; Figures S13–S56: NMR spectra of 1c–d, 3, 3×HBr, 4c–d, 5c–d, 13, 14, 15, 16; Figure S57: EPR spectrum of 13; Figure S58 and Table S1: gas chromatography data of Hofmann elimination of 15; Figure S59: X-ray analysis data of 3×HBr; Figures S60, S61 and Table S2: NMR spectrum fine structure analysis of 3×HBr; Figure S62: HPLC analysis data of 4c and 6c mixture and 4c are available online

References

1. Nelson, J.D. Aliphatic Nucleophilic Substitution. Practical Synthetic Organic Chemistry: Reactions, Principles, and Techniques; 2nd ed. Caron, S. John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2020; Chapter 1 pp. 1-71.

2. Spener, F. Reactions of aliphatic methanesulfonates. Chem. Phys. Lipids; 1973; 11, pp. 229-243. [DOI: https://dx.doi.org/10.1016/0009-3084(73)90063-7]

3. Appel, R. Tertiary Phosphane/Tetrachloromethane, a Versatile Reagent for Chlorination, Dehydration, and P–N Linkage. Angew. Chem. Int. Ed.; 1975; 14, pp. 801-811. [DOI: https://dx.doi.org/10.1002/anie.197508011]

4. Khoroshunova, Y.V.; Morozov, D.A.; Taratayko, A.I.; Gladkikh, P.D.; Glazachev, Y.I.; Kirilyuk, I.A.; Beilstein, J. Synthesis of 1-azaspiro[4.4] nonan-1-oxyls via intramolecular 1,3-dipolar cycloaddition. Org. Chem.; 2019; 15, pp. 2036-2042. [DOI: https://dx.doi.org/10.3762/bjoc.15.200]

5. Roser, P.; Schmidt, M.J.; Drescher, M.; Summerer, D. Site-directed spin labeling of proteins for distance measurements in vitro and in cells. Org. Biomol. Chem.; 2016; 14, pp. 5468-5476. [DOI: https://dx.doi.org/10.1039/C6OB00473C] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27181459]

6. Fielding, J.; Consilio, M.G.; Heaven, G.; Hollas, M.A. New developments in spin labels for pulsed dipolar EPR. Molecules; 2014; 19, pp. 16998-17025. [DOI: https://dx.doi.org/10.3390/molecules191016998] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25342554]

7. Denmark, S.E.; Montgomery, J.I. Synthesis of cis,cis,cis,cis-[5.5.5.4]-1-Azafenestrane with Discovery of an Unexpected Dyotropic Rearrangement. Angew. Chem.; 2005; 44, pp. 3732-3736. [DOI: https://dx.doi.org/10.1002/anie.200500367]

8. Masson, G.; Rioton, S.; Pardo, D.G.; Cossy, J. Synthesis of 2-Fluoroalkyl 4-Substituted Azepanes. Eur. J. Org. Chem.; 2019; 2019, pp. 5497-5507. [DOI: https://dx.doi.org/10.1002/ejoc.201900613]

9. Sivaprakasam, M.; Couty, F.; David, O.; Marrot, J.; Sridhar, R.; Srinivas, B.; Rao, K.R. A Straightforward Synthesis of Enantiopure Bicyclic Azetidines. Eur. J. Org. Chem.; 2007; pp. 5734-5739. [DOI: https://dx.doi.org/10.1002/ejoc.200700616]

10. Holmes, B.; Ward, A. Meptazinol. A Review of its Pharmacodynamic and Pharmacokinetic Properties and Therapeutic Efficacy. Drugs; 1985; 30, pp. 285-312. [DOI: https://dx.doi.org/10.2165/00003495-198530040-00001] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/2998723]

11. Soderquist, C.J.; Bowers, J.B.; Crosby, D.G. Dissipation of molinate in a rice field. J. Agric. Food Chem.; 1977; 25, pp. 940-945. [DOI: https://dx.doi.org/10.1021/jf60212a022]

12. Zaimoku, H.; Taniguchi, T. Redox Divergent Synthesis of Fawcettimine-TypeLycopodiumAlkaloids. Chem. A Eur. J.; 2014; 20, pp. 9613-9619. [DOI: https://dx.doi.org/10.1002/chem.201403163] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25042023]

13. Williams, D.R.; Fromhold, M.G.; Earley, J.D. Total Synthesis of (−)-Stemospironine. Org. Lett.; 2001; 3, pp. 2721-2724. [DOI: https://dx.doi.org/10.1021/ol016336a] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/11506618]

14. Eguchi, S. Recent progress in the synthesis of heterocyclic natural productsby the Staudinger/intramolecular aza-Wittig reaction. ARKIVOC; 2005; 2, pp. 98-119. [DOI: https://dx.doi.org/10.3998/ark.5550190.0006.208]

15. Albertini, E.; Barco, A.; Benetti, S.; De Risi, C.; Pollini, G.P.; Zanirato, V. Efficient Synthesis of Chiral N-Tosyl-3,4-Disubstituted Hexahydroazepins from D-(-)-Quinic Acid. Synlett; 1996; 1996, pp. 29-30. [DOI: https://dx.doi.org/10.1055/s-1996-5304]

16. Ducandas, C.; Lasne, M.C.; Moreau, B. Syntheses of piperidine and perhydroazepine derivatives, precursors of two selective antagonists of muscarinic M2 receptors: AF-DX 384 and its perhydroazepine isomer. J. Chem. Soc. Perkin Trans. 1; 1996; pp. 2925-2932. [DOI: https://dx.doi.org/10.1039/p19960002925]

17. Úr, G.; Kálai, T.; Balog, M.; Bognár, B.; Gulyás-Fekete, G.; Hideg, K. Synthesis of New Pyrroline Nitroxides with Ethynyl Functional Group. Synth. Commun.; 2015; 45, pp. 2122-2129. [DOI: https://dx.doi.org/10.1080/00397911.2015.1066391]

18. Bushmakina, N.G.; Misharin, A.Y. A Simple Synthesis of 4-Amino-2,2,6,6-tetramethyl-1-piperidinyloxy Radical. Synthesis; 1986; 1986, 966. [DOI: https://dx.doi.org/10.1055/s-1986-31841]

19. Dobrynin, S.; Kutseikin, S.; Morozov, D.; Krumkacheva, O.; Spitsyna, A.; Gatilov, Y.; Silnikov, V.; Angelovski, G.; Bowman, M.K.; Kirilyuk, I. et al. Human Serum Albumin Labelled with Sterically-Hindered Nitroxides as Potential MRI Contrast Agents. Molecules; 2020; 25, 1709. [DOI: https://dx.doi.org/10.3390/molecules25071709]

20. Kálai, T.; Balog, M.; Jekö, J.; Hideg, K. Synthesis and Reactions of a Symmetric Paramagnetic Pyrrolidine Diene. Synthesis; 1999; pp. 973-980. [DOI: https://dx.doi.org/10.1055/s-1999-3502]

21. Yasui, S.; Shioji, K.; Tsujimoto, M.; Ohno, A. Reactivity of a trivalent phosphorus radical cation as an electrophile toward pyridine derivatives. Heteroat. Chem.; 2000; 11, pp. 152-157. [DOI: https://dx.doi.org/10.1002/(SICI)1098-1071(2000)11:2<152::AID-HC11>3.0.CO;2-0]

22. Hu, J.; Wang, J.; Nguyen, T.H.; Zheng, N. The chemistry of amine radical cations produced by visible light photoredox catalysis. Beilstein J. Org. Chem.; 2013; 9, pp. 1977-2001. [DOI: https://dx.doi.org/10.3762/bjoc.9.234]

23. Chalmers, B.; Morris, J.C.; Fairfull-Smith, K.; Grainger, R.; Bottle, S.E. A novel protecting group methodology for syntheses using nitroxides. Chem. Commun.; 2013; 49, pp. 10382-10384. [DOI: https://dx.doi.org/10.1039/C3CC46146G]

24. Cardona, F.; Goti, A.; Picasso, S.; Vogel, P.; Brandi, A. Polyhydroxypyrrolidine Glycosidase Inhibitors Related to (+)-Lentiginosine. J. Carbohydr. Chem.; 2000; 19, pp. 585-601. [DOI: https://dx.doi.org/10.1080/07328300008544101]

25. Tufariello, J.J.; Tegeler, J.J. Nitrone cycloaddition. A route to the lupin class of alkaloids. Tetrahedron Lett.; 1976; 17, pp. 4037-4040. [DOI: https://dx.doi.org/10.1016/S0040-4039(00)92569-3]

26. McCaig, A.E.; Meldrum, K.P.; Wightman, R.H. Synthesis of Trihydroxylated Pyrrolizidines and Indolizidines using Cycloaddition Reactions of Functionalized Cyclic Nitrones, and the Synthesis of (+)- and (-)-Lentiginosine. Tetrahedron.; 1998; 54, pp. 9429-9446. [DOI: https://dx.doi.org/10.1016/S0040-4020(98)00572-9]

27. Cope, A.C.; Trumbull, E.R. Olefins from Amines: The Hofmann Elimination Reaction and Amine Oxide Pyrolysis. Org. React.; 1960; 11, pp. 317-493. [DOI: https://dx.doi.org/10.1002/0471264180.or011.05]

28. Ebnőter, A.; Jucker, E. Ringerweiterungen bei Reaktionen von 2-[N-Methyl-pyrrolidinyl-(2)]-1chlor-äthan und 2-[N-Methyl-piperidyI-(2)]-l-chlor-athan mit nucleophilen Reagenzien. Untersuchungen über synthetische Arzneimittel, 13. Mitteilung. Helv. Chim. Acta.; 1964; 47, pp. 745-756. [DOI: https://dx.doi.org/10.1002/hlca.19640470308]

29. Braslau, R.; Anderson, M.O.; Rivera, F.; Jimenez, A.; Haddad, T.; Axon, J.R. Acyl hydrazines as precursors to acyl radicals. Tetrahedron; 2002; 58, pp. 5513-5523. [DOI: https://dx.doi.org/10.1016/S0040-4020(02)00490-8]

30. Shelrick, G.M. SADABS, v. 2008/1; Bruker AXS Inc.: Madison, WI, USA, 2008.

31. Sheldrick, G.M. SHELX-97 Programs for Crystal Structure Analysis (Release 97-2). University of Göttingen, Germany, 1997. Sheldrick, G.M. Acta Crystallogr. Sect. A; 2008; 64, pp. 112-122. [DOI: https://dx.doi.org/10.1107/S0108767307043930] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/18156677]

32. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Crystallogr. Sect. C.; 2015; 71, pp. 3-8. [DOI: https://dx.doi.org/10.1107/S2053229614024218] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25567568]

33. Stoll, S.; Schweiger, A. EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. J. Magn. Reson.; 2006; 178, pp. 42-55. [DOI: https://dx.doi.org/10.1016/j.jmr.2005.08.013] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/16188474]

34. Morozov, D.A.; Kirilyuk, I.A.; Komarov, D.A.; Goti, A.; Bagryanskaya, I.Y.; Kuratieva, N.V.; Grigorev, I.A.J. Synthesis of a Chiral C₂-Symmetric Sterically Hindered Pyrrolidine Nitroxide Radical via Combined Iterative Nucleophilic Additions and Intramolecular 1,3-Dipolar Cycloadditions to Cyclic Nitrones. J. Org. Chem.; 2012; 77, pp. 10688-10698. [DOI: https://dx.doi.org/10.1021/jo3019158] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23130653]

35. Edeleva, M.V.; Parkhomenko, D.A.; Morozov, D.A.; Dobrynin, S.A.; Trofimov, D.G.; Kanagatov, B.; Kirilyuk, I.A.; Bagryanskaya, E.G. Controlled/living polymerization of methyl methacrylate using new sterically hindered imidazoline nitroxides prepared via intramolecular 1,3-dipolar cycloaddition reaction. J. Polym. Sci. Part A Polym. Chem.; 2014; 52, pp. 929-943. [DOI: https://dx.doi.org/10.1002/pola.27071]