Effective 2,6-substitution of piperidine nitroxyl radical by carbonyl compound

Article abstract of DOI:10.1016/j.tet.2010.02.004

Nitroxyl radicals (nitroxides) with unpaired electron are widely used as antioxidants, contrast agents, and spin probes. Although piperidine nitroxyl radicals have many applications, these are mainly tetramethylpiperidine compounds, and only a few reports consider the substitution of N-O surround as a reaction site, such as 2,2,6,6-tetrasubstituted piperidine nitroxyl radicals. Our results revealed that the 2,6-position of the 2,2,6,6-tetramethylpiperidin-4-one compound was substituted by cyclohexyl groups to produce 2,2,6,6-tetrasubstituted piperidin-4-one derivatives under mild reaction conditions. An interesting result was obtained by using ¹⁵N-labeled NH₄Cl instead of ¹⁴NH₄Cl: it gave ¹⁵N-labeled 2,2,6,6-tetrasubstituted piperidin-4-one-1-oxyls with a high ¹⁵N content. In conclusion, the new method for the synthesis of nitroxyl radicals readily yields 2,2,6,6-tetrasubstituted piperidin-4-one under mild conditions.

Full text of DOI:10.1016/j.tet.2010.02.004

Tetrahedron 66 (2010) 2311–2315
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Effective 2,6-substitution of piperidine nitroxyl radical by carbonyl compound
Kiyoshi Sakai a, Ken-ichi Yamada a, Toshihide Yamasaki a, Yuichi Kinoshita a, Fumiya Mito a,
,b,
Hideo Utsumi a
*
a Department of Bio-function Science, Faculty of Pharmaceutical Sciences, Kyushu University, 3-1-1 Higashi-ku, Maidashi, Fukuoka 812-8582, Japan
b Innovation Center for Medical Redox Navigation, Kyushu University, 3-1-1 Higashi-ku, Maidashi, Fukuoka 812-8582, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Nitroxyl radicals (nitroxides) with unpaired electron are widely used as antioxidants, contrast agents, and
spin probes. Although piperidine nitroxyl radicals have many applications, these are mainly tetrame-
thylpiperidine compounds, and only a few reports consider the substitution of N–O surround as a reaction
site, such as 2,2,6,6-tetrasubstituted piperidine nitroxyl radicals. Our results revealed that the 2,6-position
of the 2,2,6,6-tetramethylpiperidin-4-one compound was substituted by cyclohexyl groups to produce
2,2,6,6-tetrasubstituted piperidin-4-one derivatives under mild reaction conditions. An interesting result
was obtained by using 15N-labeled NH4Cl instead of 14NH4Cl: it gave 15N-labeled 2,2,6,6-tetrasubstituted
piperidin-4-one-1-oxyls with a high 15N content. In conclusion, the new method for the synthesis of
nitroxyl radicals readily yields 2,2,6,6-tetrasubstituted piperidin-4-one under mild conditions.
Ó 2009 Published by Elsevier Ltd.
Received 28 October 2009
Received in revised form 30 January 2010
Accepted 1 February 2010
Available online 4 February 2010
Keywords:
Nitroxide
Radical
Redox
ESR
1. Introduction
type nitroxyl radicals. These results suggest that the development of
a 2,6-substituted compound would potentially have a new scope and
Nitroxyl radicals (nitroxides) with unpaired electron are widely
used as antioxidants,1–3 contrast agents,4,5 spin probes,6–8 spin la-
bels,9,10 hindered amine light stabilizers,11 nitroxide-mediated radi-
cal polymerization agents,12,13 and radical batteries.14,15 In particular,
the most commonly used nitroxyl radicals, tempol, are starting to be
used in human applications. For example, the results of a phase I
study have shown that radiation-induced alopecia can be prevented
by the topical application of tempol, which is a typical piperidine
nitroxyl radical, to the scalp.16 Such piperidine nitroxyl radicals are
usually formed by the reaction of 4-oxo-2,2,6,6-tetramethylpiper-
idine, which is known as triacetonamine, because it is synthesized
from three molecules of acetone and ammonium chloride.17
Although piperidine nitroxyl radicals have wide applications, there
are mainly tetramethylpiperidine compounds and only a few reports
considering the substitution of N–O surround as reaction site, such as
2,2,6,6-tetrasubstituted piperidine nitroxyl radicals. Miura et al.
successfully synthesized 7-aza-15-oxodispiro[5.1.5.3]hexadec-7-yl-
oxyl using 2,2,4,6,6-pentamethyltetrahydropyrimidine (acetonine)
as the intermediate.12 Wetter et al. synthesized 2,2,6,6-tetraethylpi-
peridin-4-one-1-oxyl with bisphosphonate as the starting material.13
Recently, we also reported that 2,2,6,6-tetraethylpiperidin-4-one-1-
oxyl was synthesized from acetonine and tetrahydro-4H-thiopyran-
4-one. This compound has the characteristic of ascorbic acid
resistance,18 and therefore, differs from commonly used piperidine
application. Therefore, we attempted to develop an effective method
of synthesis for a series of 2,6-substituted piperidine nitroxyl radicals.
2. Results and discussion
First, we focused on the steric properties of the existing com-
pound 2,2,6,6-tetramethylpiperidin-4-one-1-oxyl. It has a twist-
boat conformation19 and a bulky tetramethyl group substituted at
the 2,6-position. Hence, we expected that the carbonyl group of the
2,2,6,6-tetramethylpiperidin-4-one compound (1b), as a precursor
of 2,2,6,6-tetramethylpiperidin-4-one-1-oxyl, would be enolized
easily. Our results revealed that the four methyl groups at the
2,6-position in the commercially available compound 1a were
substituted by cyclohexyl groups to produce 2,6-substituted
piperidin-4-one derivative 2 under mild reaction conditions (yield
34%, Scheme 1). Subsequent oxidation with hydrogen peroxide in
Scheme 1. Synthetic route for 7-aza-15-oxodispiro[5.1.5.3]hexadec-7-yl-7-oxyl.
Reagents and conditions: (a) cyclohexanone, NH4Cl, DMSO, 60 ꢀC, 5 h, 34% from 1a, 16%
from 1b; (b) H2O2, Na2WO4$2H2O, EtOH, rt, 24 h, 83%.
* Corresponding author. Tel.: þ81 92 642 6621; fax: þ81 92 642 6626.
E-mail address: utsumi@pch.phar.kyushu-u.ac.jp (H. Utsumi).
0040-4020/$ – see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tet.2010.02.004

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K. Sakai et al. / Tetrahedron 66 (2010) 2311–2315
the presence of sodium tungstate gave the nitroxyl radical 3. As an
alternative method, we added the strong base, Triton B, to the re-
action mixture of 1a, and the yield was increased to 48%. We as-
sumed that the carbonyl group was more easily enolized by the
Triton B and the aldol reaction was more advanced. However, we
did not try other base reagents, so further investigation was needed
to clarify the effect of the base compound on the reaction yield. The
four methyl groups in 2,2,6,6-tetramethylpiperidin-4-one (1b) was
also substituted by carbonyl compounds to afford 2,2,6,6-tetra-
substituted piperidin-4-one derivatives. This reaction was induced
using the same method as that used to produce 1a and cyclohex-
anone; however, the reaction time was 15–20 h, and the yield was
low (16%). This reaction also required the four methyl substituents
of piperidine, because when piperidin-4-one was used as a sub-
strate, it was unable to react with cyclohexanone under the same
conditions (data not shown). Instead of cyclohexanone, a substrate
containing ketone was used to produce the corresponding 2,2,6,6-
tetrasubstituted piperidin-4-one (Scheme 2; compounds 8 and 14
were synthesized from compounds 6 and 12, respectively). Tetra-
hydro-4H-pyran-4-one, tetrahydro-4H-thiopyran-4-one, and 1-
acetyl-piperidin-4-one gave compounds 4, 6, and 10, respectively.
However, there was not much difference in the yield according to
the type of heteroatom on the ring. Compound 6 was oxidized with
H2O2 and the sulfide was also oxidized to sulfone to afford
compound 7. Moreover, the desulfurization of compound 6 with
Raney-Ni afforded compound 8. Spiro compounds such as 4-(eth-
ylenedioxy) cyclohexanone also afforded a moderate yield of
2,2,6,6-tetrasubstituted piperidin-4-one derivatives 13. In addition,
the ketal hydrolysis of compound 12 afforded compound 14. Fur-
ther, the use of steroidal ketone produced compound 16, and this
structure was confirmed by X-ray crystallographic analysis (Fig. 1).
Figure 1. Molecular structure of compound 16 with thermal ellipsoids at the 50%
probability level.
An interesting result was obtained by using 15N-labeled NH4Cl
instead of 14NH4Cl; this gave 15N-labeled 2,2,6,6-tetrasubstituted
piperidin-4-one-1-oxyls 17, 18, and 19 with a high 15N content
(>98%, Scheme 3). These results suggested that the external NH4X
compound was the source of nitrogen during the reaction. The ba-
sicity of the starting compound 1a or 1b was greater than that of
ammonia, so the HX of NH4X was quenched by the compound 1a or
1b and the ammonia was produced. From the results, we inferred
that the reaction mechanism consists of the following six steps in
one reaction mixture (Scheme 4); 2,2,6,6-tetramethylpiperidine re-
acts with carbonyl compound via crossed aldol condensation. The
steric hindrance between the carbonyl compound and the N-methyl-
substituted piperidine leads to Grob-type fragmentation when an
ammonium substrate is added to the reaction mixture. After
a crossed aldol condensation reaction, acetone and ammonia species
such as methylamine are eliminated. Finally, the ring is restructured.
The yield of each six reaction steps would be more than 80%, based
on the calculation from the final product yield. However, this is
a tentative mechanism, and an in-depth study should be conducted
to obtain detailed information on the actual reaction mechanism.
Scheme 3. Synthetic route for 15N-labeled nitroxyl radicals. Reagents and conditions:
(a) cyclohexanone, 15NH4Cl, DMSO, 60 ꢀC, 5 h; (b) H2O2, Na2WO4$2H2O, EtOH, rt, 24 h;
Scheme 2. Synthetic route for 2,2,6,6-tetrasubstituted piperidine nitroxyl radicals,
reagents and conditions: (a) carbonyl compound, NH4Cl, DMSO, 60 ꢀC, 5 h; (b) H2O2,
Na2WO4$2H2O, EtOH, rt, 24 h; (c) Raney-Ni, EtOH, 60 ꢀC, 2 h; (d) HCl, AcOH, 70 ꢀC, 5 h.
15
(c) tetrahydro-4H-pyran-4-one, NH4Cl, DMSO, 60 ꢀC, 5 h; (d) 1,4-cyclohexandione
monoethylene acetal, 15NH4Cl, DMSO, 60 ꢀC, 5 h; (e) HCl, AcOH, 70ꢀC, 5 h.

K. Sakai et al. / Tetrahedron 66 (2010) 2311–2315
2313
10 mmol) or 2,2,6,6-tetramethylpiperidin-4-one (1b) (1.55 g,
10 mmol) and cyclohexanone (2.94 g, 30 mmol) in dimethyl sulf-
oxide (15 mL) at room temperature. The mixture was then heated
for 5 h at 60 ꢀC. It was diluted with H2O (40 mL), acidified with 7%
aq HCl (10 mL), and extracted with ether (ꢁ3) to remove a neutral
fraction. The reaction mixture was adjusted to pH 9 using 10% aq
K2CO3 and then extracted with AcOEt (ꢁ4). The AcOEt extract was
washed with brine, dried over anhydrous sodium sulfate, and
concentrated in vacuo. The residue was separated by column
chromatography with hexane/AcOEt (85:15) to afford 7-azadispir-
o[5.1.5.3]hexadecan-15-one (2) (799 mg, 34%; 376 mg,16% when 1b
was used as the starting compound) as a white solid after re-
crystallization. As an alternative, Triton B (2 ml) was added to
a stirred solution of 1a (1.69 g, 10 mmol), cyclohexanone (2.94 g,
30 mmol), and NH4Cl (2.68 g, 50 mmol) in dimethyl sulfoxide
(15 mL), and stirred for 2.5 h at 50 ꢀC. The reaction mixture was
worked up in the same manner and gave compound 2 (48% yield);
mp: 103 ꢀC (hexane/AcOEt, lit.12 mp: 101–103 ꢀC,); MS (FABþ):
236.3 (Mþþ1); nmax/cmꢂ1: 1689 (C]O); 1H NMR (400 MHz; CDCl3)
Scheme 4. Tentative mechanism for 2,6-substitution of piperidin-4-one.
d
(ppm): 1.36–1.46 (8H, m), 1.47–1.56 (8H, m), 1.68–1.71 (4H, m),
2.31 (4H, s); 13C NMR (75 MHz; CDCl3)
d
(ppm): 22.53 (CH2), 25.87
(CH2), 40.96 (CH2), 52.47 (C), 57.02 (CH2), 211.57 (C]O). Found: C,
76.51; H, 10.68; N, 5.93%. Calcd for C15H25NO: C, 76.55; H, 10.71; N,
5.95%.
3. Conclusion
In conclusion, the new method for the synthesis of nitroxyl
radicals readily yields 2,2,6,6-tetrasubstituted piperidin-4-one
under mild conditions. 15N-labeled nitroxyl radicals can be syn-
thesized by using 15N-labeled NH4Cl as an external source of ni-
trogen. The electron spin resonance spectral characteristics of the
nitrogen nuclei of nitroxyl radicals containing 15N nuclei (I¼1/2) are
different from those of the 14N-substituted molecule (I¼1). A re-
duction in the spectral multiplicity of the 15N analogue leads to an
increase in its sensitivity when it is subjected to a magnetic
field.20,21 Different types of 2,6-substituted nitroxyl radicals can be
synthesized by the new method. The new nitroxyl radicals could be
used as antioxidants, contrast agents, and nitroxide-mediated
radical polymerization agents.
4.3. 7-Aza-15-oxodispiro[5.1.5.3]hexadec-7-yl-7-oxyl (3)
(general procedure for oxidation of 2,2,6,6-tetrasubstituted
piperidin-4-one)
Compound 2 (235 mg, 0.99 mmol) and Na2WO4$2H2O (50 mg,
0.15 mmol) were added in ethanol (10 mL); H2O2 (30%, 2 mL) was
slowly added to the solution, which was stirred for 24 h at room
temperature. After stirring, the solution was saturated with K2CO3
and extracted with chloroform. The chloroform extracts were dried
over anhydrous sodium sulfate and evaporated. The residue was
recrystallized from hexane to give a pale yellow scale-like crysþtal
(83% yield, 208 mg); mp: 114.2 ꢀC (lit.12 mp 114–116 ꢀC); MS (FAB ):
250.3 (Mþ); nmax/cmꢂ1: 1717 (C]O). Found: C, 71.86; H, 9.62; N,
5.57%. Calcd for C15H24NO2: C, 71.96; H, 9.66; N, 5.59%;
AN¼1.407 mT.
4. Experimental section
4.1. General
All commercially available materials were used as received. TLC:
4.4. 7-Aza-3,11-dioxadispiro[5.1.5.3]hexadecan-15-one (4)
silica gel 60 plates (Merck) detected with iodine vapor. Column
chromatography: WakogelÒ C-200 (75–150
mm, Wako Pure Chem-
Tetrahydro-4H-pyran-4-one (3.00 g, 30 mmol) was used in
place of cyclohexanone according to the method for compound 2.
The product was separated by column chromatography with CHCl3
to afford a white prismatic crystal; yield: 32%; mp: 167 ꢀC (AcOEt);
MS (FABþ): 240.1 (Mþþ1); nmax/cmꢂ1: 1692 (C]O); 1H NMR
ical Industries, Ltd., Osaka, Japan). Melting points were measured
with the Yanaco micro melting point apparatus MP-500P (Yanaco,
Kyoto, Japan). Mass spectra were measured on a JMS-600H (JEOL,
Akishima, Japan). IR spectra were recorded by using the attenuated
total reflection (ATR) method on an FT/IR-4200 (JASCO, Japan). 1H
NMR spectra were recorded on a Unity INOVA 400 (400 MHz,
Varian) or INOVA 500 plus (500 MHz, Varian) using tetramethylsi-
(400 MHz; CDCl3)
d
(ppm): 1.64 (8H, t, J¼5.6 Hz), 2.40 (4H, s), 3.54–
3.60 (4H, m), 3.81–3.87 (4H, m); 13C NMR (75 MHz; CDCl3)
(ppm):40.61 (CH2), 52.21 (C), 54.96 (CH2), 63.85 (CH2), 209.34
d
lane (TMS) as an internal standard. Chemical shifts (d) are given
(C]O). Found: C, 65.17; H, 8.85; N, 5.86%. Calcd for C13H21NO3: C,
65.25; H, 8.84; N, 5.85%.
from TMS (0 ppm) and coupling constants are expressed in hertz
(Hz). 13C NMR spectra were recorded on an ECA300 (75 MHz, JEOL,
Akishima, Japan). Elemental analyses were performed using the
center of elemental analysis with a CHN CORDER MT-6 (Yanaco,
Kyoto, Japan). ESR spectra were recorded on a JES-RE1X (JEOL,
Akishima, Japan), and nitroxides were dissolved in 10 mM of
phosphate buffer (pH 7.4).
4.5. 7-Aza-3,11-dioxa-15-oxodispiro[5.1.5.3]hexadec-7-yl-7-
oxyl (5)
Compound 4 (130 mg, 0.54 mmol) was oxidized according to the
method for compound 3. The crude product was separated by
column chromatography with CHCl3 and recrystallized from AcOEt
to afford compound 5 (110þmg, 80%) as an orange crystal; mp:
149.5 ꢀC (AcOEt); MS (FAB ): 255.2 (Mþþ1); nmax/cmꢂ1: 1717
(C]O). Found: C, 61.23; H, 7.97; N, 5.40%. Calcd for C13H20NO4: C,
61.40; H, 7.93; N, 5.51%; AN¼1.364 mT.
4.2. 7-Azadispiro[5.1.5.3]hexadecan-15-one (2) (general
procedure for 2,2,6,6-tetrasubstituted piperidin-4-one)
NH4Cl (3.21 g, 60 mmol) was added portionwise to a stirred
solution of 1,2,2,6,6-pentamethylpiperidin-4-one (1a) (1.69 g,

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K. Sakai et al. / Tetrahedron 66 (2010) 2311–2315
4.6. 7-Aza-3,11-dithiadispiro[5.1.5.3]hexadecan-15-one (6)
d
(ppm): 1.58–1.66 (8H, m), 2.08 (6H, s), 2.38 (4H, s), 3.37–3.71(8H,
m); 13C NMR (75 MHz; CDCl3)
d
(ppm): 21.48 (CH3), 37.59 (CH2),
Tetrahydro-4H-thiopyran-4-one (3.49 g, 30 mmol) was used in
place of cyclohexanone according to the method for compound 2.
The product was separated by column chromatography with hex-
ane/AcOEt (1:1) and recrystallized from AcOEt to afford compound
6 as a dark scale-like crystal; yield: 30%; mp: 155–157 ꢀC (AcOEt);
MS (FABþ): 272.2 (Mþþ1); nmax/cmꢂ1: 1697 (C]O); 1H NMR
39.96 (CH2), 42.62 (CH2), 52.41 (C), 55.94 (CH2), 168.93 (C]O),
208.71 (C]O); HRMS (FABþ): found: 322.2104. Calcd for
C17H28N3O3 ([MþH]þ): 322.2131.
4.11. 3,11-Diacetyl-15-oxo-3,7,11-
triazadispiro[5.1.5.3]hexadec-7-yl-7-oxyl (11)
(400 MHz; CDCl3)
(4H, s), 2.43–2.50 (4H, m), 2.88–2.95 (4H, m); 13C NMR (75 MHz;
CDCl3) (ppm): 24.26 (CH2), 41.66 (CH2), 53.22 (C), 55.94 (CH2),
d (ppm): 0.8 (1H, br s), 1.75–1.90 (8H, m), 2.27
Compound 10 was oxidized according to the method for com-
pound 3. The product was separated by column chromatography
with CHCl3/MeOH (95:5) and recrystallized from hexane/AcOEt to
afford compound 11 as a yellow solid; mp: 162–163 ꢀC (hexane/
AcOEt); MS (FABþ): 337.3 (Mþþ1); nmax/cmꢂ1: 1720 (C]O), 1637
(–N–CO–CH3); HRMS (FABþ): found: 337.1996. Calcd for C17H27N3O4
([MþH]þ): 337.2002.; AN¼1.359 mT.
d
209.24 (C]O). Found: C, 57.54; H, 7.78; N, 5.11%. Calcd for
C13H21NOS2: C, 57.52; H, 7.80; N, 5.16%.
4.7. 7-Aza-3,11-dithia-3,3,11,11,15-
pentaoxodispiro[5.1.5.3]hexadec-7-yl-7-oxyl (7)
Compound 6 (210 mg, 0.78 mmol) and Na2WO4$2H2O (118 mg,
0.36 mmol) were added in methanol (3 mL); H2O2 (30%, 1 mL) was
slowly added to the solution, which was stirred for 48 h at room
temperature. After stirring, the solution was saturated with K2CO3
and extracted with CHCl3/MeOH (3:1). The extracts were dried over
anhydrous sodium sulfate and evaporated. The residue was
recrystallized from H2O at a 29% yield (80 mg) to afford compound
7 as a pale yellow crystal; mp: 212.5–214.2 ꢀC; MS (FABþ): 351.0
(Mþþ1); nmax/cmꢂ1: 1712 (C]O), 1287 (SO2), 1122 (SO2). Found: C,
44.38; H, 5.79; N, 3.93%. Calcd for C13H21NO6S2: C, 44.56; H, 5.75; N,
4.00%; AN¼1.295 mT.
4.12. 1,4,14,17-Tetraoxa-9-
azatetraspiro[4.2.1.2.4.2.3.2]tetracosan-21-one (12)
1,4-Cyclohexanedione monoethylene acetal was used in place of
cyclohexanone according to the method for compound 2. The
product was separated by column chromatography with CHCl3 and
recrystallized from hexane/AcOEt to afford compound 12 as a whþite
powder; yield: 32%; mp: 190.6 ꢀC (hexane/AcOEt); MS (FAB ):
352.4 (Mþþ1). nmax/cmꢂ1: 1699 (C]O), 1083 (–O–CH2–CH2–O–);
1H NMR (400 MHz; CDCl3)
d (ppm): 1.56–1.74 (12H, m), 1.85–1.92
(4H, m), 2.32 (4H, s), 3.93(8H, br s); 13C NMR (75 MHz; CDCl3)
(ppm):31.14 (CH2), 37.86 (CH2), 52.84 (C), 56.38 (CH2), 64.41 (CH2),
d
4.8. 2,2,6,6-Tetraethylpiperidin-4-one (8)
64.43 (CH2), 108.42 (C), 210.59 (C–O). Found: C, 64.95; H, 8.30; N,
4.05%. Calcd for C19H29NO5: C, 64.93; H, 8.32; N, 3.99%.
A suspension of freshly prepared Raney-Ni in ethanol (8 mL)
was added to a well-stirred solution of compound 6 (489 mg,
1.8 mmol) in ethanol (15 mL), and the mixture was stirred at 60 ꢀC.
After 2 h, the mixture was filtered through a pad of CeliteÒ and the
filtrates were concentrated in vacuo. The residue was separated by
column chromatography with CHCl3 to give an orange oil 8 at a 15%
yield (57 mg); MS (FABþ): 212.2 (Mþþ1); 1H NMR (400 MHz;
4.13. 1,4,14,17-Tetraoxa-9-aza-21-
oxotetraspiro[4.2.1.2.4.2.3.2]tetracos-9-yl-9-oxyl (13)
Compound 12 was oxidized according to the method for com-
pound 3. The product was separated by column chromatography
with CHCl3 and recrystallized from hexane/AcOEt to afford com-
pound 13 as an orange needle; yield: 50%; mp: 183.2–184.2 ꢀC
CDCl3)
d (ppm): 0.8–0.9 (12H, m), 1.35–1.55 (8H, m), 2.27 (4H, s).
Found: C, 73.81; H, 11.99; N, 6.25%. Calcd for C13H25NO: C, 73.88; H,
11.92; N, 6.63%. The analytical data are in agreement with literature
values.13
(hexane/AcOEt); MS (FABþ): 367.3 (Mþþ1); nmax/cmꢂ1
: 1713
(C]O). Found: C, 62.18; H, 7.38; N, 3.88%. Calcd for C19H28NO6: C,
62.28; H, 7.70; N, 3.82%; AN¼1.374 mT.
4.9. 2,2,6,6-Tetraethylpiperidin-4-one-1-oxyl (9)
4.14. 7-Aza-3,11,15-trioxodispiro[5.1.5.3]hexadecane (14)
Compound 8 was oxidized according to the method for com-
pound 3. The analytical data are in agreement with literature
values.13
Tenpercent aq HCl (0.2 mL) was dropped portionwise into awell-
stirred solution of compound 12 (536 mg, 1.53 mmol) in H2O (2 mL)
and acetic acid (5 mL) and the mixture was heated at 70 ꢀC. The
reaction mixture was diluted with H2O and adjusted to pH 7, then
extracted with AcOEt. The extract was dried over anhydrous sodium
sulfate and concentrated in vacuo. The residue was separated by
column chromatography (hexane/AcOEt, 1:4) to afford compound
14 after recrystallization using hexane and AcOEt at an 82%þyield
(330 mg) as awhite powder; mp: 99.7 ꢀC; MS (FABþ): 264.2 (M þ1);
4.10. 3,11-Diacetyl-3,7,11-triazadispiro[5.1.5.3]hexadecan-15-
one (10)
NH4Cl (3.21 g, 6 equiv) was added portionwise to a stirred so-
lution of 1,2,2,6,6-pentamethylpiperidin-4-one (1a) (0.845 g,
5 mmol) and 1-acetyl-piperidin-4-one (2.12 g, 15 mmol) in di-
methyl sulfoxide (12 mL) at room temperature. The mixture was
then heated for 8 h at 60 ꢀC. It was diluted with H2O (40 mL),
acidified with 7% aq HCl (10 mL), and extracted with ether (ꢁ3) to
remove a neutral fraction. The reaction mixture was adjusted to pH
9 using 10% aq K2CO3 and then extracted with CHCl3 (ꢁ4). The
CHCl3 extract was washed with brine, dried over anhydrous sodium
sulfate, and concentrated in vacuo. The residue was separated by
column chromatography with CHCl3/MeOH (95:5) and recrystal-
lized from hexane/AcOEt to afford compound 10 (328 mg, 21%) as
nmax/cmꢂ1: 1705 (C]O); 1H NMR (400 MHz; CDCl3)
d
(ppm): 1.24
(1H, br s),1.87–2.05 (8H, m), 2.29–2.67 (8H, m), 2.49 (4H, s); 13C NMR
(75 MHz; CDCl3) (ppm): 37.25 (CH2), 39.15 (CH2), 52.56 (C), 56.72
d
(CH2), 208.61 (C]O), 209.93 (C]O). Found: C, 68.39; H, 8.08; N,
5.34%. Calcd for C15H21NO3: C, 68.42; H, 8.04; N, 5.32%.
4.15. 7-Aza-3,11,15-trioxodispiro[5.1.5.3]hexadec-7-yl-7-oxyl (15)
Compound 14 (215 mg, 0.82 mmol) was oxidized according to
the method for compound 3. The product was separated by column
chromatography with CHCl3 and recrystallized from hexane/AcOEt
to afford compound 15 (170 mg, 75%) as a yellow solid; mp: 161.5 ꢀC
a pale yellow solid; mp: 164.2 ꢀC; MS (FABþ): 322.3 (Mþþ1); nmax
/
cmꢂ1: 1687 (C]O), 1634 (N–Ac); 1H NMR (400 MHz; CDCl3)

K. Sakai et al. / Tetrahedron 66 (2010) 2311–2315
2315
(hexane/AcOEt); MS (FABþ): 279.3 (Mþþ1); nmax/cmꢂ1
:
1708
yield). The product was oxidized according to the method for com-
(C]O). Found: C, 64.71; H, 7.25; N, 5.06%. Calcd for C15H20NO4: C,
64.73; H, 7.24; N, 5.03%; AN¼1.351 mT.
pound 3 to afford the title compound asþa yellow solid þ(38% yield);
mp: 165.8 ꢀC (hexane/AcOEt); MS (FAB ): 280.22 (M þ1); nmax
/
cmꢂ1: 1707 (C]O). Found: C, 64.47; H, 7.22; N, 5.01%. Calcd for
C15H2015NO4: C, 64.50; H, 7.22; N, 5.01%; A15N¼1.888 mT.
4.16. Bis(170
b a a
-hydroxy-170 -methyl-50 -androstan-30-spiro)-
2,6-piperidin-4-one (16)
Supplementary data
170 -Hydroxy-170
b a-androstan-3-one (2.25 g, 7.39 mmol) was
used in place of cyclohexanone according to the method for com-
pound 2 and compound 16 (210 mg, 0.32 mmol, 9%) was obtained
as a white solid. This compound was identified with X-ray crys-
tallographic analysis; mp: 292.7 ꢀC (toluene); MS (FABþ): 648.0
(Mþ); nmax/cmꢂ1: 1705 (C]O), 2920 (OH, br); 1H NMR (500 MHz;
Crystallographic data for compound 16 have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication No. CCDC 761583. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK, (fax: þ44 (0)1223 336 033 or e-mail:
deposit@ccdc.cam.ac.uk).
CDCl3)
d (ppm): 0.60–0.64 (2H, m), 0.81 (6H, s), 0.83 (6H, s), 0.86–
0.88 (2H, m), 0.99–1.05 (2H, m), 1.15–1.28 (14H, m), 1.21 (6H, s),
1.32–1.34 (2H, m), 1.40–1.48 (7H, m), 1.54–1.60 (7H, m), 1.65–1.81
Acknowledgements
(7H, m), 2.41 (4H, s); 13C NMR (75 MHz; CDCl3)
d (ppm): 12.07,
14.07, 20.76, 23.32, 25.92, 28.65, 31.74, 35.25, 35.85, 36.46, 37.13,
39.08, 42.54, 44.01, 45.65, 50.26, 50.75, 54.53, 57.81, 81.80, 211.71.
CCDC No. 761583.
We thank Dr. Ryoichi Ando for X-ray crystallographic analysis.
This study was partially supported by the Development of Systems
and Technology for Advanced Measurement and Analysis of the
Japan Science and Technology Agency and a Grant-in-Aid for Young
Scientists from the Japan Society for the Promotion of Science.
4.17. 7-[15N]-Aza-15-oxodispiro[5.1.5.3]hexadec-
7-yl-7-oxyl (17)
References and notes
15NH4Cl was used in place of NH4Cl according to the general
procedure for compound
2
to afford 7-[15N]-azadispir-
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115.4–117.4 ꢀC (hexane); MS (FABþ): 251.3 (Mþ); nmax/cmꢂ1: 1717
(C]O). Found: C, 71.67; H, 9.79; N, 5.67%. Calcd for C15H2415NO2: C,
71.68; H, 9.62; N, 5.57%; A15N¼1.974 mT.
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15NH4Cl and tetrahydro-4H-pyran-4-one were used in place of
NH4Cl and cyclohexanone, respectively, according to the general
procedure for compound 2, to afford 7-[15N]-aza-3,11-dioxadispir-
o[5.1.5.3]hexadecane-15-one as a white prismatic crystal (32%
yield). The product was oxidized according to the method for
compound 3 to afford the title compound as an orange crystal (45%
yield); mp: 171.7–172.0 ꢀC (AcOEt); MS (FABþ): 256.23 (Mþþ1);
nmax/cmꢂ1: 1719 (C]O). Found: C, 61.18; H, 7.90; N, 5.52%. Calcd for
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4.19. 7-[15N]-Aza-3,11,15-trioxodispiro[5.1.5.3]hexadec-7-yl-7-
oxyl (19)
15NH4Cl and 1,4-cyclohexanedione monoethylene acetal were
used in place of NH4Cl and cyclohexanone, respectively, according to
the general procedure forcompound2 to afford 9-[15N]-aza-1,4,14,17-
tetraoxatetraspiro[4.2.1.2.4.2.3.2]tetracosan-21-one as a white pow-
der (33% yield). The ketal group of the product was hydrolyzed
according to the method for compound 14 to afford 7-[15N]-aza-
3,11,15-trioxodispiro[5.1.5.3]hexadecane as a white powder (74%
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