New Synthetic Methodology Toward Azaspiro-γ-Lactones by Oxidative C-H Spirocyclization

Article abstract of DOI:10.1055/s-0037-1611941

A new synthetic methodology for azaspiro-γ-lactones is reported. The key C-H spirolactonization was accomplished by employing iodobenzene diacetate and potassium bromide to afford a variety of azaspiro-γ-lactones in high yields. The reaction was also appl

Full text of DOI:10.1055/s-0037-1611941

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–D
letter
A
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T. Sengoku et al.
Letter
Synlett
New Synthetic Methodology Toward Azaspiro-γ-Lactones by
Oxidative C–H Spirocyclization
Tetsuya Sengokua
Yuichiro Nagaia
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Toshiyasu Inuzukab
Hidemi Yoda*a
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a Department of Applied Chemistry, Faculty of Engineering,
Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu
432-8561, Japan
yoda.hidemi@shizuoka.ac.jp
b Division of Instrumental Analysis, Life Science Research
Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
Received: 06.11.2018
Accepted after revision: 03.12.2018
Published online: 17.12.2018
(a) N,O-Spirocyclic natural products
OH
O
HO
DOI: 10.1055/s-0037-1611941; Art ID: st-2018-v0719-l
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Abstract A new synthetic methodology for azaspiro-γ-lactones is re-
ported. The key C–H spirolactonization was accomplished by employing
iodobenzene diacetate and potassium bromide to afford a variety of
azaspiro-γ-lactones in high yields. The reaction was also applicable to
the preparation of a bislactone derivative.
N
MeO
O
H
HO
lycoplanine A
elmenol G
penicitrinine A
(b) Spirocyclization of bifunctionalized ketones (our previous work)
COR
CONHPh
Key words C–H spirolactonization, hypervalent iodine, azaspiro-γ-lac-
tone, spiroheterocyclic compound, N,O-spirocycle
ROC
OH
N
PhHN
O
metal additives
O
O
N
O
+
X
X = SnBu3
O
N,O-Spirocycles are often found in the core structure of
natural products1 which exhibit unique bioactivities such
as calcium channel inhibitory activity (lycoplanine A),2 tu-
mor necrosis factor-α-related apoptosis-inducing ligand
(TRAIL) resistance-overcoming activity (elmenol G),3 and
antiproliferative activity on multiple tumor types (penici-
trinine A)4 (Scheme 1, a). Several types of N,O-spirocycles
have been constructed by cyclization approach using bi-
functionalized ketones,5 represented by N,O-acetalization
of amino hydroxy ketones.5f,i Recently, we reported the
analogous synthetic method starting from imide deriva-
tives, in which a γ-hydroxylactam intermediate underwent
in situ a ring opening–reclosure reaction (Scheme 1, b).6
The product, which we call azaspiro-γ-lactone, includes an
uncommon lactam spiro-fused to a lactone structure and is
expected to be a precursor of thermoplastics,7 but the sub-
strate scope of the reaction was limited to N-carbonyl-sub-
stituted derivatives prepared from phthalimide and ma-
leimide.
or
B
O
O
PhHN
NH O
ROC
O
O
O
O
N
Ph
ring opening
reclosure
azaspiro-γ-lactone
(c) Oxidative C–H spirocyclization (this work)
R
R
N
H
N
CO2H
O
O
O
O
oxidant
1
2
Scheme 1 Structures and synthetic methods of N,O-spirocyclic com-
pounds
and step-economic routes toward spiroheterocyclic mole-
cules.10 Indeed, Corey and his co-workers demonstrated the
oxidative formation of a lactone ring on the pentacyclic as-
pidosperma skeleton in their total synthesis of aspidophy-
tine.11 The White research group recently provided a syn-
thetic example of a lactone spiro-fused to a pyrane ring
through their metal (oxo)-promoted C–H hydroxylation.12
On the other hand, oxidative functionalization of C–H
bonds has gained attention in recent years,8 and elegant
studies on the functionalization of C–H bonds adjacent to a
heteroatom in heterocyclic molecules have been reported.9
This type of transformation undoubtedly constitutes direct
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

B
T. Sengoku et al.
Letter
Synlett
However, to our knowledge, no examples of C–H spirocy-
clization have been reported using a lactam derivative as a
substrate to date. In this communication, we describe our
development of a new synthetic method for azaspiro-γ-lac-
tones by oxidative C–H spirolactonization at the position
adjacent to a nitrogen atom of a lactam ring (Scheme 1, c).
Initially, we explored a promising oxidant for C–H spiro-
lactonization with lactam carboxylic acid 1a13 bearing a rel-
atively reactive benzylic proton. When 1a was treated with
inorganic oxidants such as cerium ammonium nitrate
(CAN), iodine, potassium persulfate, and Oxone in chloro-
benzene at 80 or 100 °C, only a small amount of the desired
spirolactone 2a was obtained (Table 1, entries 1–4). In con-
trast, iodobenzene diacectate (PIDA) that is the representa-
tive example of hypervalent iodine oxidants facilitated the
spirocyclization at 100 °C to give 2a in 71% yield along with
recovery of a trace amount of starting material (entry 5).
Because a subsequent attempt with an iodine(V) reagent,
Dess–Martin periodinane (DMP), resulted in poor yield (en-
try 6), we examined the reaction using PIDA in combination
with potassium bromide on the basis of the previous work
relating to benzylic C–H acetoxylation reported by Kita and
Dohi.14,15 In this case, the starting material was completely
consumed within 24 hours even at room temperature, giv-
ing rise to 2a in 84% yield (entry 7).16 Further screening of
the PIDA–KBr conditions revealed that the spirocyclization
is very sensitive to the reaction medium and worked well in
halogenated solvents. As a result, the reaction in dichloro-
methane or chlorobenzene afforded 2a in the highest yield
of 84% (entries 7–12). The use of a catalytic amount of po-
tassium bromide resulted in a reduced yield (entry 13). We
then evaluated the effect of additives. When the reaction
was performed with potassium fluoride, potassium chlo-
ride, or potassium iodide as an alternative to potassium
bromide, the product yield was significantly decreased to
3–29% (entries 14–16). The use of the bromide salt was es-
sential to obtain the product in good yields (entries 17–20),
indicating that the C–H spirocyclization would proceed
through a radical-induced hydrogen abstraction pathway.14,17
Thus, the optimal conditions for this transformation
were determined: 1.2 equivalents of PIDA in the presence of
potassium bromide (1.0 equivalents) in dichlorometh-
ane.18,19
Table 1 Screening of Conditions for Oxidative C–H Spirolactonization of 1a
Me
N
Me
N
CO2H
O
O
oxidant
additive
O
O
solvent (0.1 M), r.t.
1a
2a
Entry Oxidant (equiv) Additive
(equiv)
Solvent
Time (h) Yield (%)
1a
2a
3b
4a
5a
6b
7
CAN (3.0)
I2 (3.0)
–
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
24
6
trace
trace
trace
7
–
K2S2O8 (3.0)
Oxone (3.0)
PIDA (3.0)
DMP (3.0)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
–
12
6
–
–
6
71
24
84
13
33
79
49
84
17
3
–
12
24
24
24
24
24
24
24
24
24
24
24
24
24
24
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (0.2)
KF (1.0)
KCl (1.0)
KI (1.0)
NaBr (1.0)
ZnBr2 (1.0)
CuBr (1.0)
Et4NBr (1.0)
8
toluene
MeCN
TFEc
9
10
11
12
13
14
15
16
17
18
19
20
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
14
29
46
64
65
23
a Reactions were performed at 100 °C.
b Reactions were performed at 80 °C.
c TFE: 2,2,2-trifluoroethanol.
conducted in 2,2,2-trifluoroethanol (TFE) gave 2b in 66%
yield. The attempt to construct δ-lactone fused derivative
2h resulted in complex mixtures of undesired products as
1
judged by the H NMR spectrum of the reaction mixture.
We subsequently applied our method to the preparation of
nonaromatic derivatives 2i,j. This seemed to be challenging
due to the fact that Kita’s original protocol14 was reported
for the selective carboxylation of reactive benzylic C–H
bonds. Contrary to our expectation, oxidative C–H spirolac-
tonization successfully proceeded even at the nonactivated
position by employing modified conditions (2.4 equivalents
of PIDA and 2.0 equivalents of KBr at 40 °C), affording 2i and
2j in 87 and 64% yield, respectively. Finally, we investigated
the potential for the construction of O,O-spirocycles. Reac-
tion of lactone carboxylic acid readily proceeded under the
above-mentioned modified reaction conditions, furnishing
bislactone derivative 2k in 73% yield. These results suggest
Having the optimal conditions in hand, we next evaluat-
ed the scope of the spirolactonization (Scheme 2). Although
the reaction of N-unsubstituted lactam proceeded sluggish-
ly with poor conversions (2b, 13%), in the cases of N-alkyl,
N-aryl, and N-carbonyl substrates, the corresponding
azaspirolactones 2c–g were obtained in high yields (77–
82%). Notably, the cyclization of the N-benzyl derivative,
which has two benzylic carbons14 in its structure, occurred
predominantly at the desired position, providing 2d in 77%
yield.20 The poor yield of 2b can be attributed to the low
solubility of the substrate in CH2Cl2. In fact, the reaction
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

C
T. Sengoku et al.
Letter
Synlett
that the C–H spirocyclization reaction will have broad po-
tential utility in the syntheses of spiroheterocyclic com-
pounds.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611941.
S
u
p
p
orit
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gInformati
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S
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p
p
orit
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gInformati
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R
N
R
N
O
O
PIDA (1.2 equiv)
KBr (1.0 equiv)
CO2H
O
References and Notes
O
CH2Cl2 (0.1 M)
r.t., 24 h
1a–j
2a–j
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Ph
N
H
Bn
N
N
N
O
O
O
O
O
O
O
O
O
O
O
O
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2e
(78%)
2b
2c
(82%)
2d
(77%)
(13%)
(66%)a
Ac
N
Boc
N
Me
N
O
O
O
O
O
O
O
O
O
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2f
(79%)
2g
(82%)
2h
(0%)
Me
Me
N
N
O
O
O
O
O
O
2i
2j
(87%)b
(64%)b
O
O
O
PIDA (2.4 equiv)
KBr (2.0 equiv)
CO2H
O
O
O
CH2Cl2 (0.1 M)
40 °C, 12 h
2k
(73%)
1k
Scheme 2 Synthesis of spirolactone derivatives. a The reaction was
performed in TFE. b Reactions were performed with 2.4 equiv of PIDA
and 2.0 equiv of KBr for 12 h at 40 °C.
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Chang, F.-R. Org. Lett. 2014, 16, 1912. (i) Priyadarshini, S.; Amal
In conclusion, we have developed the new synthetic
methodology for azaspiro-γ-lactones. A key outcome of this
work is the PIDA/KBr-promoted C–H spirolactonization of
easily accessible lactam carboxylic acids. The protocol is ap-
plicable to the preparation of the bislactone compounds as
well as alicyclic azaspiro-γ-lactones. The simplicity and
broad applicability of this methodology will be valuable in
materials and medicinal chemistry. Further investigations
into the formation of other types of spirocycles such as lact-
am–lactam or lactam–ether structures are underway in our
laboratory, and the details of those reactions will be sub-
mitted as a full paper.
Funding Information
This work was supported financially in part by JSPS Kakenhi Grant
number JP15K21046 from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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Acknowledgment
We thank Mr. Shono Ichihara (Shizuoka University) for partial sup-
port of this research.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D

D
T. Sengoku et al.
Letter
Synlett
Joseph, P. J.; Lakshmi Kantam, M. Tetrahedron 2014, 70, 6068.
(j) Guo, S.; Yuan, Y.; Xiang, J. Org. Lett. 2013, 15, 4654. (k) Wang,
M.-K.; Zhou, Z.; Tang, R.-Y.; Zhang, X.-G.; Deng, C. Synlett 2013,
24, 737. (l) Kumar, G. S.; Pieber, B.; Reddy, K. R.; Kappe, C. O.
Chem. Eur. J. 2012, 18, 6124. (m) Mao, X.; Wu, Y.; Jiang, X.; Liu,
X.; Cheng, Y.; Zhu, C. RSC Adv. 2012, 2, 6733. (n) Lao, Z.-Q.;
Zhong, W.-H.; Lou, Q.-H.; Li, Z.-J.; Meng, X.-B. Org. Biomol. Chem.
2012, 10, 7869.
(15) When the reaction was performed under the classical C–H oxi-
dation conditions (PIDA, I2, hν) developed by Suárez, only 13% of
product was obtained along with 43% yield of recovered starting
material and a substantial amount of an unidentified side prod-
uct. For Suárez’s conditions, see: (a) Hernández, R.; Rivera, A.;
Salazar, J. A.; Suárez, E. J. Chem. Soc., Chem. Commun. 1980, 958.
(b) Calvert, M. B.; Sperry, J. Tetrahedron Lett. 2012, 53, 5426.
(16) In the absence of KBr, the use of 1.2 equivalents of PIDA at room
temperature gave only a trace amount of the desired product.
(17) Li, T.; Xiang, C.; Zhang, B.; Yan, J. Helv. Chim. Acta 2014, 97, 854.
(18) General procedure for spirolactonization: To a solution of 1
(21.9 mg, 0.100 mmol) in CH2Cl2 (1.0 mL) was added iodoben-
zene diacetate (PIDA, 38.7 mg, 0.120 mmol) and potassium
bromide (11.9 mg, 0.100 mmol) at room temperature under an
argon atmosphere. After stirring the suspension for 24 h, the
reaction was quenched by addition of saturated aqueous
NaHCO3 (5 mL). The resulting mixture was extracted with EtOAc
(5 mL × 3), washed with brine (5 mL), dried with Na2SO4, fil-
tered, and concentrated in vacuo to give a crude material. This
material was purified by column chromatography on silica gel
by using hexane/EtOAc as the eluent.
(19) Compound 2a: White solid; mp 230–232 °C; IR (KBr): 1774
(C=O), 1702 (C=O) cm–1; 1H NMR (300 MHz, CDCl3): δ 7.83 (m, 1
H, ArH), 7.66–7.49 (m, 3 H, ArH), 3.05 (s, 3 H, CH3), 3.04–2.98
(m, 2 H, CH2), 2.77 (ddd, J = 7.5, 9.9, 14.1 Hz, 1 H, CH2), 2.60 (ddd,
J = 7.5, 8.7, 14.1 Hz, 1 H, CH2); 13C NMR (75 MHz, CDCl3): δ 174.3
(C), 166.6 (C), 143.9 (C), 132.9 (CH), 130.7 (CH), 123.7 (CH),
121.4 (CH), 97.4 (C), 29.3 (CH2), 29.2 (CH2), 23.7 (CH3). Anal.
Calcd for C12H11NO3: C, 66.35; H, 5.10; N, 6.45. Found: C, 66.12;
H, 4.88; N, 6.24.
(10) For selected reviews on spirocyclization, see: (a) Singh, F. V.;
Kole, P. B.; Mangaonkar, S. R.; Shetgaonkar, S. E. Beilstein J. Org.
Chem. 2018, 14, 1778. (b) Subba Reddy, B. V.; Nair, P. N.; Antony,
A.; Srivastava, N. Eur. J. Org. Chem. 2017, 5484. (c) Reddy, C. R.;
Prajapti, S. K.; Warudikar, K.; Ranjan, R.; Rao, B. B. Org. Biomol.
Chem. 2017, 15, 3130. (d) Quach, R.; Furkert, D. P.; Brimble, M.
A. Org. Biomol. Chem. 2017, 15, 3098. (e) Wagner, B.; Belger, K.;
Minkler, S.; Belting, V.; Krause, N. Pure Appl. Chem. 2016, 88,
391. (f) Favre, S.; Vogel, P.; Gerber-Lemaire, S. Molecules 2008,
13, 2570. (g) Sinibaldi, M.; Canet, I. Eur. J. Org. Chem. 2008, 4391.
(11) He, F.; Bo, Y.; Alton, J. D.; Corey, E. J. J. Am. Chem. Soc. 1999, 121,
6771.
(12) Bigi, M. A.; Reed, S. A.; White, M. C. J. Am. Chem. Soc. 2012, 134,
9721.
(13) Lactam carboxylic acid 1a was readily prepared in five steps
starting from commercially available N-methylphthalimide.
Me
N
Me
N
O
O
O
1) Mg, 4-bromo-1-butene
2) Et3SiH, BF3•OEt2
3a (85%)
(20) The addition of 2,2,6,6-tetramethylpiperide 1-oxyl (TEMPO) or
2,6-di-tert-butyl-4-methylphenol (BHT) to the reaction mixture
of 1a completely shut down the formation of 2a, indicating that
these reactions should be a radical process. Hydrogen abstrac-
tion from the substrate would occur at the potentially reactive
C–H bond adjacent to the nitrogen atom. In the case of 2d,
hydrogen abstraction on the endocyclic carbon atom would be
favored because the generated radical species could be much
more stabilized by the conjugated system of isoindolinone the
benzylic radical on the exocyclic position.
Me
N
O
1) OsO4, NMO
2) NaIO4 aq
CO2H
3) NaClO2, NaH2PO4
2-methyl-2-butene
1a (29%)
Scheme 3
(14) (a) Dohi, T.; Ueda, S.; Iwasaki, K.; Tsunoda, Y.; Morimoto, K.;
Kita, Y. Beilstein J. Org. Chem. 2018, 14, 1087. (b) Dohi, T.;
Takenaga, N.; Goto, A.; Maruyama, A.; Kita, Y. Org. Lett. 2007, 9,
3129.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–D