Synthesis of 3-amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane

Article abstract of DOI:10.1080/00397910903289255

The synthesis of 3-amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane is described. Key steps include the addition of prenyl magnesium bromide to a 4-methoxybenzylimine without reversal of stereochemistry and the iodine-initiated aminocyclization to form the azaspirocycle. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903289255

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Synthesis of 3-Amino-2,2-dimethyl-8-
thia-1-azaspiro[4.5]decane
Wenpeng Mai a , Sarah A. Green a , Dallas K. Bates a & Shiyue Fang a
a Department of Chemistry , Michigan Technological University ,
Houghton, Michigan, USA
Published online: 05 Aug 2010.
To cite this article: Wenpeng Mai , Sarah A. Green , Dallas K. Bates & Shiyue Fang (2010)
Synthesis of 3-Amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:17, 2571-2577,
DOI: 10.1080/00397910903289255
To link to this article: http://dx.doi.org/10.1080/00397910903289255
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Synthetic Communications1, 40: 2571–2577, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903289255
SYNTHESIS OF 3-AMINO-2,2-DIMETHYL-8-THIA-1-
AZASPIRO[4.5]DECANE
Wenpeng Mai, Sarah A. Green, Dallas K. Bates,
and Shiyue Fang
Department of Chemistry, Michigan Technological University, Houghton,
Michigan, USA
The synthesis of 3-amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane is described. Key
steps include the addition of prenyl magnesium bromide to a 4-methoxybenzylimine
without reversal of stereochemistry and the iodine-initiated aminocyclization to form the
azaspirocycle.
Keywords: Grignard reactions; heterocycle; iodoaminocyclization; prenylation; spiro compound
INTRODUCTION
The 1-azaspirocycle is an important structural motif in many alkaloid natural
products.[1] For example, cephalotaxine contains a 1-azaspiro[4.4]nonane skeleton,[2]
cocculolidine contains a 1-azaspiro[4.5]decane skeleton,[3] and fasicularin contains a
1-azaspiro[5.5]undecane skeleton.[4] The major challenges for the construction of
such skeletons come from the formation of the tetra-substituted tertiary carbon
center bearing the nitrogen atom that will become the spirocyclic center in the final
product and the formation of the two rings of the system. To meet these challenges,
several synthetic approaches have been developed, which include the construction of
the tertiary nitrogen-bearing carbon center followed by closing the heterocycle,[5] the
construction of the tertiary nitrogen-bearing carbon center followed by closing the
carbocycle,[6] and the construction of the tertiary nitrogen-bearing carbon center
and formation of the spirocycle in a single step.[7] Among the 1-azaspirocycles, some
contain other heteroatoms such as sulfur in the ring system and have biological
activities. For example, several compounds that contain an 8-thia-1-azaspiro[4.5]
decane skeleton have been found to be effective inhibitors of undecaprenyl pyropho-
sphate synthase,[8] and several others have been used as pesticides.[9] Many of these
compounds can potentially be prepared in a way similar to those that do not contain
heteroatoms other than nitrogen in the spirocyclic system.
Received March 27, 2009.
Address correspondence to Shiyue Fang, Department of Chemistry, Michigan Technological
University, 1400 Townsend Drive, Houghton, MI 49931, USA. E-mail: shifang@mtu.edu
2571

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W. MAI ET AL.
RESULTS AND DISCUSSION
Recently, we have been interested in the preparation of the azaspirocycle
3-amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane (1). Besides the two common
challenges for the preparation of this class of compounds, the hindered tetra-
substituted carbon-2 was expected to cause additional difficulties for the synthesis.
This tetra-substituted carbon along with the tertiary spirocyclic center makes the
secondary amine at position-1 highly hindered. On the other hand, we envisioned
that the two methyl groups at carbon-2 may facilitate the synthesis if the spirocyclic
system is formed through the formation of the bond between nitrogen-1 and
carbon-2 using a cationic approach, that is, using nitrogen-1 to attack a tertiary
cation on carbon-2 (Scheme 1). One method for the formation of a cation on
carbon-2 is halogenation of the double bond between carbon-2 and carbon-3 of
the enamine 3. Using this approach, the halogen atom on carbon-3 in the spirocycli-
zation product 2 is well suited for conversion to the primary amino group at
carbon-3 in 1. Gratifyingly, a literature search revealed that Diaba et al. used such
an iodoaminocyclization method for converting compound 4 to 5 (Scheme 1).[10]
With this information in mind, the remaining potential challenges are stereoselective
formation of 3 and the conversion of the secondary iodo function of 2 to the amino
function of 1, which are adjacent to a tetra-substituted carbon.
The synthesis was initiated with the preparation of imine 7 from tetrahydro-
4H-thiopyran-4-one (6)[11] and benzyl amine. Compound 7 could be identified using
gas chromatography–mass spectrometry (GC-MS) and purified by distillation under
vacuum, but it could not be purified by column chromatography. Analysis by NMR
using CDCl3 as solvent also failed because of hydrolysis. After vacuum distillation, 7
was treated with excess prenyl magnesium bromide (8)[12] to give the spirocyclization
precursor 9, which was fully characterized (Scheme 2). It is remarkable that addition
of 8 to imine 7 proceeded with complete stereocontrol; no product arising from the
addition of a reversed prenyl group to 7 was observed.[13] Unfortunately, all attempts
to convert 9 to the 1-azaspirocycle 10 under various idodoaminocyclization
conditions (9=I2=5% NaHCO3=CH2Cl2=rt; 9=NIS=K2CO3=CH2Cl2=rt) failed. Because
Scheme 1. Synthesis plan.

3-AMINO-2,2-DIMETHYL-8-THIA-1-AZASPIRO[4.5]DECANE
2573
Scheme 2. Reagents and conditions: (a) BnNH2, PhH, reflux; (b) prenyl magnesium, Grignard (8), Et2O,
rt, 71% from 6; and (c) I2, NaHCO3, CH2Cl2, 0%.
Diaba et al. could convert 4 to 5 smoothly in excellent yield,[10] we believe that our
failure to form 10 may be attributed to the high steric hindrance around the nitrogen
atom in the product.[10,14] Accordingly, to facilitate the spirocyclization, we turned to
making the protecting-group-free amine 13, which would be easier to cyclize because
of the reduced steric demand for the carbon–nitrogen bond-formation reaction.
To synthesize 13, the 4-methoxybenzylimine 11 was prepared from ketone 6
and 4-methoxybenzylamine (Scheme 3). After purification by vacuum distillation,
11 was treated with excess 8, and the desired product 12 was obtained in 60% yield;
no reversed prenylation product was detected.[13] The 4-methoxybenzyl group in 12
was next removed by treatment with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
to give the spirocyclization precursor 13 in 70% yield.[15] Iodoaminospirocyclization
of 13 was performed at room temperature using I2 in CH2Cl2 under weakly basic
conditions[10]; the desired 1-azaspirocycle 14 was obtained in 50% yield. To install
the amino group at carbon-3, the iodo function was replaced with an azido group
to give 15,[16] which was reduced under hydrogenation conditions to give the target
1-azaspirocycle 1 in quantitative yield.[17] Despite the steric hindrance from the
adjacent tetra-substituted carbon-2, these functional group conversion reactions
proceeded smoothly with excellent yields.
In conclusion, we have successfully synthesized 3-amino-2,2-dimethyl-8-thia-1-
azaspiro[4.5]decane (1) in six steps with a total yield of 15% starting from the
commercially available tetrahydro-4H-thiopyran-4-one (6). It is remarkable that
Scheme 3. Reagents and conditions: (a) PhH, reflux; (b) 8, Et2O, rt, 60% from 6; (c) DDQ, CH2Cl2, H2O,
rt, 15 h, 70%; (d) I2, NaHCO3 (aq), CH2Cl2, rt, 12 h, 50%; (e) NaN3, MeOH, H2O, reflux, 12 h, 77%; and
(f) 10% Pd=carbon, H2 (1 atm), EtOH, rt, 24 h, 95%.

2574
W. MAI ET AL.
azaspirocycle 10 was not accessible from 9 using an iodoaminocyclization approach.
This was probably due to the high steric hindrance of 9, while the less sterically
hindered 13 could be cyclized to give 14 in good yield. Prenylation of benzylimines
such as 7 and 11 gave products with retention of stereochemistry of the prenyl group.
EXPERIMENTAL
N-[Tetrahydro-4-prenyl-2H-thiopyran-4-yl]-N-benzylamine (9)
Tetrahydro-4H-thiopyran-4-one (6, 4.64 g, 40 mmol; commercially available
but prepared according to the reported procedure[11]) and benzylamine (4.4 mL,
40 mmol) were dissolved in benzene (100 mL), and the solution was heated to reflux
for 5 h with azeotropic removal of water using a Dean–Stark apparatus. After
cooling to rt, the solvent was removed under reduced pressure, and the residue
was distilled under vacuum, giving imine 7 as a colorless liquid (7.8 g, 95%). Analysis
by NMR using CDCl3 as solvent failed because of hydrolysis, but GC-MS showed
that the product was pure and had the correct molecular weight. Prenyl magnesium
bromide (8), which was prepared from prenyl bromide (1.7 mL, 15 mmol) and mag-
nesium turnings (1.8 g, 75 mmol) in dry Et2O (40 mL) at rt according to a reported
procedure, was added dropwise to the solution of imine 7 (1.4 g, 7.0 mmol) in dry
Et2O (20 mL).[12] The mixture was stirred at rt overnight, poured into saturated
NH4Cl, and extracted with Et2O. The organic extracts were washed with brine, dried
over anhydrous Na2SO4, and concentrated under reduced pressure. Purification by
flash-column chromatography (SiO2, hexanes–EtOAc, 5:1) gave 9 as a light yellow
1
thick oil (1.4 g, 71%): Rf ¼ 0.4 (SiO2, hexanes–EtOAc, 5:1); H NMR (400 MHz,
CDCl3): d 7.37 (d, J ¼ 6.8 Hz, 2H), 7.31 (t, J ¼ 7.2 Hz, 2H), 7.24 (t, J ¼ 7.2 Hz,
1H), 5.15 (tt, J ¼ 7.2, 1.2 Hz, 1H), 3.59 (s, 2H), 3.09 (dt, J ¼ 13.6, 11.2 Hz, 2H),
2.33 (dt, J ¼ 13.2, 4.4 Hz, 2H), 2.15 (d, J ¼ 7.6 Hz, 2H), 1.92 (dt, J ¼ 14.4, 3.2 Hz,
2H), 1.72 (s, 3H), 1.68 (dt, J ¼ 14.4, 2.4 Hz, 2H), 1.64 (s, 3H); 13C NMR
(100 MHz, CDCl3): d 141.30, 134.59, 128.34, 128.21, 126.85, 118.55, 53.43, 45.12,
37.52, 36.52, 26.18, 23.56, 18.17. HRMS-ESI m=z [M þ H]þ calcd. for C17H26NS:
276.1786; found 276.1793.
N-[Tetrahydro-4-prenyl-2H-thiopyran-4-yl]-N-
(4-methoxy-benzyl)amine (12)
Imine 11 was prepared from 6 (4.64 g, 40 mmol) and 4-methyoxybenzylamine
(5.2 mL, 40 mmol) as a colorless liquid (8.9 g, 95%) after distillation under vacuum
using the procedure for the preparation of 7. Like 7, analysis of 11 by NMR using
CDCl3 as solvent failed because of hydrolysis. Prenyl magnesium bromide (8), which
was prepared from prenyl bromide (9.24 mL, 80 mmol) and magnesium turnings
(9.6 g, 400 mmol),[12] was added dropwise to the solution of 11 (8.9 g, 38 mmol) in
dry Et2O (70 mL). After stirring at rt overnight, the mixture was poured into
saturated NH4Cl, and the product was extracted into Et2O. The extracts were
washed with brine, dried over anhydrous Na2SO4, and concentrated. Purification
by flash-column chromatography (SiO2, hexanes=EtOAc, 5:1) gave 12 as a light yel-
1
low liquid (7.0 g, 60%): Rf ¼ 0.4 (SiO2, hexanes=EtOAc, 5:1); H NMR (400 MHz,

3-AMINO-2,2-DIMETHYL-8-THIA-1-AZASPIRO[4.5]DECANE
2575
CDCl3): d 7.27 (d, J ¼ 8.4 Hz, 2H), 6.85 (d, J ¼ 8.4 Hz, 2H), 5.14 (t, J ¼ 7.2 Hz, 1H),
3.78 (s, 3H), 3.52 (s, 2H), 3.07 (dt, J ¼ 13.6, 2.4 Hz, 2H), 2.33 (d, J ¼ 13.6, 2H), 2.14
(d, J ¼ 7.2 Hz, 2H), 1.91 (d, J ¼ 14.4 Hz, 2H), 1.71 (s, 3H), 1.64–1.66 (m, 5H); 13C
NMR (100 MHz, CDCl3): d 158.6, 134.5, 133.4, 129.3, 118.6, 113.7, 55.3, 53.4,
44.5, 37.5, 36.5, 26.2, 23.6, 18.2. HRMS-ESI m=z [M þ H]þ calcd. for C18H28NOS:
306.1892; found 306.1887.
Tetrahydro-4-prenyl-2H-thiopyran-4-amine (13)
DDQ (227 mg, 1.0 mmol) was added to a solution of 12 (280 mg, 0.92 mmol) in
CH2Cl2=H2O (5:1; 18 mL). After stirring at rt overnight, the mixture was poured into
saturated NaHCO3 (40 mL). The organic components were extracted with CH2Cl2,
and the extracts were dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. Purification by flash-column chromatography (SiO2, hexanes=
EtOAc=Et3N, 10:10:1) gave 13 as a colorless liquid (119 mg, 70%): Rf ¼ 0.25 (SiO2,
1
hexanes=EtOAc=Et3N, 10:10:1); H NMR (400 MHz, CDCl3): d 5.17 (tt, J ¼ 7.6,
1.6 Hz, 1H), 2.84 (dt, J ¼ 10.8, 3.6 Hz, 2H), 2.45 (dt, J ¼ 13.6, 4.0 Hz, 2H), 2.02 (d,
J ¼ 7.6 Hz, 2H), 1.61–1.71 (m, 10H); 13C NMR (100 MHz, CDCl3): d 135.2, 118.5,
50.4, 42.2, 39.0, 26.1, 24.2, 18.1. HRMS-ESI m=z [M þ H]þ calcd. for C10H20NS:
186.1316; found 186.1320.
2,2-Dimethyl-3-iodo-8-thia-1-azaspiro[4.5]decane (14)
The solution of I2 (650 mg, 2.6 mmol) in CH2Cl2 (20 mL) was added dropwise to
a solution of 13 (400 mg, 2.2 mmol) in CH2Cl2 (40 mL) and 15% NaHCO3 (20 mL).
After stirring at rt overnight, the reaction was quenched with sat. Na2S2O3 (20 mL),
and the mixture was extracted with CH2Cl2. The organic extracts were dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure. Purification
by flash-column chromatography (SiO2, hexanes=EtOAc, 1:1) gave 14 as colorless
1
thick oil (336 mg, 50%): Rf ¼ 0.5 (SiO2, hexanes=EtOAc, 1:1); H NMR (400 MHz,
CDCl3): d 3.85 (dd, J ¼ 7.2, 4.4 Hz, 1H), 2.47–2.53 (m, 1H), 2.56–2.63 (m,
2H), 2.67–2.75 (m, 1H), 2.34 (dd, J ¼ 13.2, 6.8 Hz, 1H), 2.11 (t, J ¼ 11.6 Hz, 1H),
1.63–1.72 (m, 1H), 1.73–1.81 (m, 2H), 1.89–1.95 (m, 1H), 1.25 (s, 3H), 1.21 (s, 3H);
13C NMR (100 MHz, CDCl3): d 62.1, 60.1, 48.7, 42.5, 40.8, 34.6, 28.5, 28.0, 25.9,
25.8. HRMS-ESI m=z [M þ H]þ calcd. for C10H19INS: 312.0283; found 312.0282.
3-Azido-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane (15)
NaN3 (100 mg, 1.5 mmol) was added to the solution of 14 (320 mg, 1.0 mmol)
in MeOH=H2O (4:1, 30 mL), and the mixture was heated to reflux for 12 h. After
cooling to rt, MeOH was removed under reduced pressure. The remaining mixture
was extracted with CH2Cl2 and dried over anhydrous Na2SO4. Purification by
flash-column chromatography (SiO2, hexanes=EtOAc, 1:1) gave 15 as a colorless
thick oil (179 mg, 77%): Rf ¼ 0.35 (SiO2, hexanes=EtOAc, 1:1); 1H NMR
(400 MHz, CDCl3): d 3.62 (t, J ¼ 6.4 Hz, 1H), 2.66–2.72 (m, 2H), 2.51–2.58 (m,
2H), 2.07 (dd, J ¼ 13.2, 6.4 Hz, 1H), 1.71–1.82 (m, 4H), 1.86–1.89 (m, 1H), 1.16 (s,
3H), 1.11 (s, 3H); 13C NMR (100 MHz, CDCl3): d 70.0, 62.4, 58.6, 42.24, 42.20,

2576
W. MAI ET AL.
42.0, 29.7, 26.15, 26.10, 25.2. HRMS-ESI m=z [M þ H]þ calcd. for C10H19N4S:
227.1330; found 227.1329.
3-Amino-2,2-dimethyl-8-thia-1-azaspiro[4.5]decane (1)
The solution of 15 (30 mg, 0.13 mmol) in anhydrous ethanol (10 mL) was stir-
red over 10% palladium on charcoal (0.01 g) under 1 atm of hydrogen gas overnight.
The mixture was filtered through a celite pad, and the filtrate was concentrated under
reduced pressure. The residue was purified by flash-column chromatography (SiO2,
CH2Cl2=MeOHEt3N, 20:1:0.25) giving 1 as a thick colorless oil (24.7 mg, 95%):
1
Rf ¼ 0.2 (SiO2, CH2Cl2=MeOH=Et3N, 20:1:0.25); H NMR (400 MHz, CDCl3): d
3.0 (dd, J ¼ 6.4, 4.0 Hz, 1H), 2.48–2.70 (m, 4H), 2.02 (dd, J ¼ 12.8, 6.8 Hz, 1H),
1.62–1.90 (m, 4H), 1.37 (dd, J ¼ 12.8, 10 Hz, 1H), 1.10 (s, 3H), 0.98 (s, 3H); 13C
NMR (100 MHz, CDCl3): d 60.6, 60.5, 50.4, 45.3, 42.9, 41.6, 29.1, 26.2, 26.1, 23.9.
HRMS-ESI m=z [M þ H]þ calcd. for C10H21N2S: 201.1425; found 201.1428.
ACKNOWLEDGMENTS
Financial support from the National Institutes of Health (R01 DA017201) and
the Michigan Technological University Chemistry Department; the assistance from
Mr. Jerry L. Lutz (NMR), Mr. Shane Crist (computation), and Mr. Dean W.
Seppala (electronics); and an NSF equipment grant (CHE-9512445) are all gratefully
acknowledged.
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