Reactions of Nitrosobenzenes with Cyclobutanones by Activation with a Lewis Acid

Article abstract of DOI:10.1055/s-0036-1588469

Formal [4+2] cycloaddition between 3-ethoxy-2-alkylcyclobutanones and nitrosobenzene proceeded by activation with Me ₃ SiOTf to afford 6-alkyl-2-phenyl-2 H -1,2-oxazin-5(6 H)-one by regioselective cleavage of the more substituted C2-C3 bond of the cyclobutanone ring. On the other hand, reactions of 3-phenylcyclobutanones and 2-benzyloxycyclobutanone with nitrosobenzene gave γ,δ-unsaturated and cyclic hydroxamic acid derivatives, respectively, by cleavage of a cyclobutanone C1-C2 bond.

Full text of DOI:10.1055/s-0036-1588469

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–D
cluster
A
en
Y. Shima et al.
Cluster
Synlett
Reactions of Nitrosobenzenes with Cyclobutanones by Activation
with a Lewis Acid
O
Yusuke Shima
O
Ph
Emiko Igarashi
N
O
R
Tomoyuki Yoshimura
Jun-ichi Matsuo*
OH
Ph
Ph
O
Me3SiOTf
R
O
N
OEt
O
Division of Pharmaceutical Sciences, Graduate School of Medical
Sciences, Kanazawa University, Kakuma-machi, Kanazawa
920-1192, Japan
O
Ph
Me3SiOTf
Ph
N
Me3SiOTf
N
O
Ph
jimatsuo@p.kanazawa-u.ac.jp
O
OBn
5 examples
up to 67% yield
regioselective
OBn
Published as part of the Cluster C–C Activation
Dedicated to Professor Teruaki Mukaiyama in celebration of his
90th birthday
selective cleavage of
the C1–C2 bond
selective cleavage of
the C2–C3 bond
Received: 26.04.2017
Accepted after revision: 23.05.2017
Published online: 05.07.2017
ponent, and unsaturated bonds such as C=O,16 C=C,17 C=N,18
N=N19 bonds reacted to give the corresponding six-mem-
bered cyclic compounds. In this paper, we report not only
formal [4+2] cycloaddition reactions of nitrosobenzenes
with cyclobutanones but also formation of γ,δ-unsaturated
hydroxamic acid derivatives by activation with a Lewis acid.
First, screening of Lewis acids for activation of cyclobu-
tanones was carried out in a reaction of cyclobutanone 1a
and nitrosobenzene (Table 1). It was found that AlCl3 and
TiCl4 showed moderate activity (Table 1, entries 1 and 2),
and Me3SiOTf was the most effective Lewis acid (Table 1,
entry 6). Formal [4+2] cycloadduct 2a was formed selec-
tively by cleavage of the more substituted C2–C3 bond of
cyclobutanones16 without formation of another regioiso-
DOI: 10.1055/s-0036-1588469; Art ID: st-2017-u0293-c
Abstract Formal [4+2] cycloaddition between 3-ethoxy-2-alkylcy-
clobutanones and nitrosobenzene proceeded by activation with Me3-
SiOTf to afford 6-alkyl-2-phenyl-2H-1,2-oxazin-5(6H)-one by regiose-
lective cleavage of the more substituted C2–C3 bond of the
cyclobutanone ring. On the other hand, reactions of 3-phenylcyclobu-
tanones and 2-benzyloxycyclobutanone with nitrosobenzene gave γ,δ-
unsaturated and cyclic hydroxamic acid derivatives, respectively, by
cleavage of a cyclobutanone C1–C2 bond.
Key words cyclobutanone, nitrosobenzene, cycloaddition, Lewis acid,
carbon–carbon bond cleavage, hydroxamic acid
Among reactions of nitrosobenzenes with carbonyl
compounds, the nitroso aldol reaction,1 which is a valuable
synthetic method for introduction of nitrogen or oxygen at
the α position of a carbonyl group, has been extensively
studied, and various regioselective2 and enantioselective3
methods have been established.4 In addition to the nitroso
aldol reaction, reactions of nitrosobenzene with carboxylic
acid chloride and formaldehyde were reported to give N-(p-
chlorophenyl)hydroxamic acid5 and N-phenylhydroxamic
acid,6 respectively. Frongia and Piras reported that organo-
catalyzed reaction of cyclobutanones with nitrosobenzene
gave 5-hydroxy-γ-lactams.7 Seayad and Zhang reported N-
heterocyclic carbene-catalyzed amidation of aldehydes
with nitrosobenzene to N-arylhydroxamic acids.8
Cycloaddition of nitrosobenzenes is a powerful method
for the synthesis of heterocyclic compounds bearing both
oxygen and nitrogen atoms, and [4+2],9,10 [3+3],11 [3+2]12
cycloaddition reactions have been reported (other cycload-
dition reactions).13,14 We have studied formal [4+2] cycload-
dition of 3-ethoxycyclobutanones with molecules having
unsaturated bonds.15 Cyclobutanones worked as a C4 com-
Table 1 Screening of Lewis Acidsa
O
O
t-Bu
t-Bu
Lewis acid
O
N
+
O
CH2Cl2
Ph
N
OEt
r.t., 40 min
Ph
1a
2a
Entry
Lewis acid
Yield (%)b
1
2
3
4
5
6
AlCl3
45
TiCl4
43
SnCl4
18
EtAlCl2
BF3-Et2O
Me3SiOTf
26
17
54 (67)c (41)d
a Reaction conditions: Compound 1a (3.0 equiv), nitrosobenzene (1.0
equiv), and Lewis acid (3.3 equiv) were used.
b Isolated yield.
c Reaction time was 10 min.
d Reaction temperature was 0 °C.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

B
Y. Shima et al.
Cluster
Synlett
mer. A shorter reaction time improved the yield of 2a to
67%, but lowering the reaction temperature from room
temperature to 0 °C did not improve the yield of 2a.
position reacted with nitrosobenzene. Therefore, dialkyl
substitution of cyclobutanone 3 was required for efficient
synthesis of γ,δ-unsaturated hydroxamic acid 4.
Reactions of nitrosobenzene with some 3-ethoxycy-
clobutanones were performed under the optimized reac-
tion conditions using Me3SiOTf at room temperature for ten
minutes. 3-Ethoxycyclobutanones were prepared by [2+2]
cycloaddition of ethyl vinyl ether and in situ formed ketene
from carboxylic acid chloride and trialkylamine.20 3-Ethox-
ycyclobutanone (1b) reacted with nitrosobenzene to give 2-
phenyl-2H-1,2-oxazin-5(6H)-one (2b) in 48% yield (Table 2,
entry 1). The reversed regioselectivity2a was observed in
hetero Diels–Alder reaction between Danishefsky’s diene
and nitrosobenzene.9a Substituted cyclobutanones 1a,c–e
also reacted with nitrosobenzene to afford the correspond-
ing cycloadducts 2a,c–e in moderate yields (Table 2, entries
2–5). It seemed that cis/trans ratios of cyclobutanones 1a,c–
e did not affect the efficiency of the present cycloaddition.
The structure was determined by NOE experiments of 2e.
Table 3 Reactions of 3-Phenylcyclobutanones 3 with Nitrosobenzene
to γ,δ-Unsaturated Hydroxamic Acid 4a
O
R
Ph
R
O
N
Me3SiOTf
O
N
+
OH
Ph
CH2Cl2
–20 °C, 20 min
Ph
Ph
3
R
R
4
Entry
3
4
Yield (%)b
1
2
3
4
3a (R = H)
4a
4b
4c
4d
86
83
40
66
3b (R = –(CH2)2–)
3c (R = –(CH2)3–)
3d (R = –(CH2)4–)
O
Table 2 Formal [4+2] Cycloaddition between 3-Ethoxycyclobutanones
Ph
O
with Nitrosobenzenea
N
5
12 (22)c
OH
Ph
O
Ph
3e
O
R
R
Me3SiOTf
4e
O
N
+
O
CH2Cl2
r.t., 10 min
a Reaction conditions: Cyclobutanone 3 (1.0 equiv), nitrosobenzene (1.0
equiv), Me3SiOTf (1.1 equiv), CH2Cl2, –20 °C, 30 min.
b Isolated yield.
Ph
N
OEt
1a–e
Ph
2a–e
c Reaction temperature: r.t. to 40 °C.
Entry
R
1 (cis/trans)
Product
Yield (%)b
1
2
3
4
5
H
1b
2b
2c
2d
2e
2a
48
60
58
57
67
The present synthesis of γ,δ-unsaturated hydroxamic
acid was performed by using some nitrosobenzene deriva-
tives 5 that were prepared by Molander’s method23 (Table
4). It was found that substitution with electron-releasing
and electron-withdrawing groups did not significantly af-
fect the yields of unsaturated hydroxamic acids 6a–c, and
they were obtained in high yields.
Me
Bn
1c (36:64)
1d (69:31)
1e (34:66)
1a (47:53)
i-Pr
t-Bu
a Reaction conditions: Cyclobutanone (3.0 equiv), nitrosobenzene (1.0
equiv), and Me3SiOTf (3.3 equiv) were used.
b Isolated yield.
Table 4 Reactions of 3a with some Nitrosobenzenes 5a
Next, reactions of nitrosobenzene with cyclobutanone 3
that did not have the 3-ethoxy group21 were studied (Table
3). Cyclobutanones 3 that had the phenyl group at the 3-po-
sition were prepared by modified Ghosez’s method22 for
[2+2] cycloaddition using keteniminium salts and styrene
derivatives. 2,2-Dialkylcyclobutanones 3a–d reacted with
nitrosobenzene in the presence of Me3SiOTf at –20 °C to
provide γ,δ-unsaturated hydroxamic acids 4a–d in 40–86%
yields (Table 3, entries 1–4). Products that were formed by
cleavage of the C2–C3 bond of 3 or by isomerization of the
double bond of 4 were not obtained. 2-Methylcyclobuta-
none 3e gave γ,δ-unsaturated hydroxamic acid 4e in a low
yield (Table 3, 12%, entry 5), and raising the reaction tem-
perature to 40 °C slightly improved the yield of 4e to 22%
yield. Neither 2,2,3-triphenylcyclobutanone nor 3-phenyl-
cyclobutanone that did not have alkyl substituents at the 2-
X
O
O
O
N
OH
Me3SiOTf
N
+
Ph
CH2Cl2
–20 °C, 20 min
Ph
X
3a
5a–c
6a–c
Entry
X
5
6
Yield (%)b
1
2
3
OMe
Me
5a
6a
6b
6c
79
93
91
5b
5c
CO2Me
a Reaction conditions: cyclobutanone 3a (1.0 equiv), nitrosobenzene 5 (1.5
equiv), Me3SiOTf (1.1 equiv), CH2Cl2, –20 °C, 30 min.
b Isolated yield.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

C
Y. Shima et al.
Cluster
Synlett
Reaction of 2-benzyloxycyclobutanone (7) with nitoro-
benze gave cyclized hydroxamic acid derivative 8 in 69%
yield (Scheme 1).24 A higher reaction temperature was re-
quired for the reaction of 7 compared with the reaction of
2,2-dialkylcyclobutanones 3.
cause other products that had an isomerized carbon–car-
bon double bond were not detected. The low yield of hy-
droxamic acid 4e from 2-methyl-3-phenylcyclobutanone
(3e) was attributed to less opportunity for cis configuration
between the α-methyl group and the N=O bond in 12 com-
pared with the case of 2,2-dimethyl-3-phenylcyclobuta-
none (3a). When 2-benzyloxycyclobutanone 7 was subject-
ed in the present reaction, the C1–C2 bond of the cyclobu-
tanone ring was cleaved by assistance of the benzyloxy
group as shown in 16. This type of ring cleavage has often
been seen in the case of donor–acceptor cyclobutanes.10a,25
Cyclization of thus-formed intermediate 17 gave 8.
The obtained compounds 2 were new compounds, and
the synthetic utility of cycloadduct 2 was clarified by che-
moselective transformation of 2a (Scheme 3). Thus, chemo-
selective reduction of the C=C double bond of 2a to 18 took
place by using DIBAL. The use of L-selectride in THF at –78
°C for one hour also gave 18 in 60% yield. Chemoselective
cleavage of the N–O bond was performed by using Pd(OH)2
and H2 to give 19.
O
O
OBn
Ph
Me3SiOTf
O
N
N
O
+
CH2Cl2
reflux, 10 min
Ph
7
OBn
69%
8
Scheme 1 Reaction of 2-benzyloxycyclobutanone (7) with nitrosoben-
zene
A proposed reaction mechanism giving 2, 15, and 8 is
shown in Scheme 2. 3-Ethoxycyclbutanones (R1 = OEt) 9
were activated with Me3SiTOf, and the more substituted
C2–C3 bond of the cyclobutanone ring was cleaved regiose-
lectively to afford an intermediate 10. The nitrogen atom of
nitorosobenzene attacked the carbon atom of the oxonium
cation of the intermediate 10 followed by cyclization to give
11, and elimination of ethanol provided 2. On the other
hand, an intermediate 12 was formed by the reaction of ni-
trosobenzene with the carbonyl carbon of 9 that did not
have an alkoxy group as the R1 group. Proton transfer took
place as 13 to give unsaturated compound 14, from which
γ,δ-unsaturated hydroxamic acid 15 was formed. A concert-
ed mechanism for the ring cleavage of 12 was proposed be-
O
O
O
Pd(OH)2/C
t-Bu
t-Bu
t-Bu
H
2 (balloon)
DIBALH
O
O
OH
THF
–78 °C, 1 h
68%
MeOH
reflux, 25 min
52%
N
N
NH
Ph
19
Ph
18
Ph
2a
Scheme 3 Transformation of 2a
In conclusion, we have developed three types of
Me3SiOTf-mediated reactions between cyclobutanones and
nitorosobenzene. The effects of substituents of cyclobuta-
nones were important for the determination of reaction
pathways. Cyclobutanones bearing a 3-ethoxy group gave a
formal [4+2] cycloadduct, 2-phenyl-2H-1,2-oxazin 5(6H)-
one 2, by cleavage of their more substituted C2–C3 bonds
regioselectively.26 Reactions of cyclobutanones that did not
have a 3-ethoxy group afforded γ,δ-unsaturated hyroxamic
acid derivatives 4 and 6 or cyclized hydroxamic acid 8 by
cleavage of their more substituted C1–C2 bonds. It is noted
that γ,δ-unsaturated hydroxamic acids were formed direct-
ly from 2-alkylcyclobutanones. Therefore, the present reac-
tions strengthen the synthetic utility of cyclobutanones in
organic synthesis.
SiMe3
O
SiMe3
O
O
N
O
O
R2
R1
R2
R2
R2
R1 = OEt
– EtOH
Ph
O
O
EtO
N
N
OEt
9
10
Ph
11
Ph
2
R1 = Ph, H
H
O
OH
H
Ph
N
O
R2
Ph
N
Ph
N
H
Me3SiO
O
O
R2 = Me
R1
R1
SiMe3
13
R1
SiMe3
14
12
O
Ph
N
R2 = OBn
OH
R1
Funding Information
H
H
15
SiMe3
This work was supported by JSPS KAKENHI Grant Number 15K07855.apaJn
O
O
Ph
O
S
o
ecity
o
f
r
hte
Pro
m
o
itn
of
S
ecin
c
e
1(
5
K
0
7
8
5
5)
N
Ph
Ph
OBn
N
O
N
O
Me3SiO
Supporting Information
16
OBn
OBn
17
8
Supporting information for this article is available online at
Scheme 2 Proposed mechanism
https://doi.org/10.1055/s-0036-1588469.
S
u
p
p
o
nrtogI
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

D
Y. Shima et al.
Cluster
Synlett
References and Notes
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(26) 6-tert-Butyl-2-phenyl-2H-1,2-oxazin 5(6H)-one (2a)
–
Typical Procedure
To a stirred mixture of cyclobutanone 1a (143 mg, 0.84 mmol)
and nitrosobenzene (30 mg, 0.28 mmol) in CH2Cl2 (2 mL) was
added Me3SiOTf (0.17 mL, 0.92 mmol) at r.t. The reaction
mixture was stirred for 10 min. The reaction was quenched by
addition of potassium sodium tartrate. The mixture was
extracted with EtOAc, and the organic extracts were washed
with brine, dried over anhydrous Na2SO4, filtered, and concen-
trated. The crude product was purified by preparative thin-layer
chromatography on silica gel (hexane/Et2O = 1:2) to afford 2a
(43.6 mg, 0.19 mmol, 67%) as a brown solid; mp 69.0–70.0 °C.
1H NMR (600 MHz, CDCl3): δ = 1.16 (s, 9 H), 4.20 (s, 1 H), 5.34 (d,
J = 7.2 Hz, 1 H), 7.14 (t, J = 7.5 Hz, 1 H), 7.22 (d, J = 8.4 Hz, 2 H),
7.40 (t, J = 8.1 Hz, 2 H), 7.71 (d, J = 7.8 Hz, 1 H). 13C NMR (100
MHz, CDCl3): δ = 26.7, 34.7, 91.2, 99.3, 114.5, 124.4, 129.5,
(11) (a) Pagar, V. V.; Jadhav, A. M.; Liu, R.-S. J. Am. Chem. Soci. 2011,
133, 20728. (b) Das, S.; Chakrabarty, S.; Daniliuc, C. G.; Studer,
A. Org. Lett. 2016, 18, 2784.
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Studer, A. Angew. Chem. Int. Ed. 2014, 53, 5964. (b) Varshnaya, R.
K.; Banerjee, P. Eur. J. Org. Chem. 2016, 4059.
139.8, 140.6, 188.1. IR (CHCl3): 3018, 1576, 1223, 1209 cm–1
.
HRMS: m/z calcd for C14H18NO2: 232.13375; found: 232.13476.
(13) Other cycloaddition reactions: (a) Murphy, W. S.; Raman, K. P.
Tetrahedron Lett. 1980, 21, 319. (b) Adam, W.; Bottle, S.; Peters,
K. Tetrahedron Lett. 1991, 32, 4283.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D