Synthesis of 1,4-benzoxazepine derivatives via a novel domino aziridine ring-opening and isocyanide-insertion reaction

Article abstract of DOI:10.1002/adsc.201300650

A novel and efficient domino process has been developed for the synthesis of 1,4-benzoxazepine derivatives from a range of readily accessible N-tosylaziridines, 2-iodophenols and isocyanides. This process involves the aziridine ring-opening reaction with 2-iodophenol, followed by a palladium-catalyzed isocyanide-insertion reaction. This regioselective and high-yielding transformation could be extended to its application for the synthesis of natural products and biologically interesting heterocycles. Copyright

Full text of DOI:10.1002/adsc.201300650

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DOI: 10.1002/adsc.201300650
Synthesis of 1,4-Benzoxazepine Derivatives via a Novel Domino
Aziridine Ring-Opening and Isocyanide-Insertion Reaction
a
a
a
a,
Fei Ji, Mei-fang Lv, Wen-bin Yi, and Chun Cai *
a
Chemical Engineering College, Nanjing University of Science and Technology, Nanjing 210094, Peopleꢀs Republic of
China
Fax : (+86)-25-8431-5030; e-mail: c.cai@mail.njust.edu.cn
Received: July 24, 2013; Published online: November 14, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300650.
Abstract: A novel and efficient domino process has
been developed for the synthesis of 1,4-benzoxaze-
pine derivatives from a range of readily accessible
N-tosylaziridines, 2-iodophenols and isocyanides.
This process involves the aziridine ring-opening re-
action with 2-iodophenol, followed by a palladium-
catalyzed isocyanide-insertion reaction. This regio-
selective and high-yielding transformation could be
extended to its application for the synthesis of natu-
ral products and biologically interesting heterocy-
cles.
separation of the intermediates and minimizes the
amount of chemical waste, energy, and time. Thus, the
development of a novel domino reaction for the syn-
thesis of 1,4-benzoxazepine derivatives is highly de-
sired.
The aziridines as strained three-membered ring het-
erocycles are considered to be important building
blocks for the synthesis of various N-heterocyclic
[
5]
compounds. They could undergo ring-opening reac-
tions with various nucleophilic reagents to provide
[6]
a direct access to a range of substituted amines,
which are important precursors and have been ex-
[7]
Keywords: aziridine ring-opening; 1,4-benzoxaze-
pines; domino process; isocyanide-insertion; regio-
selectivity
plored in the synthesis of biological heterocycles.
Recently, the Alper group reported a new domino
aziridine ring-opening/carboxamidation reaction of N-
tosylaziridine and 2-halophenols for the synthesis of
[
7a]
1
,4-benzoxazepine derivatives. However, the use of
toxic carbon monoxide and the harsh reaction condi-
tions limited the further use and exploration of these
[
8]
The importance of N-heterocyclic scaffolds in nature reactions. Isocyanides, a kind of unsaturated mole-
products, pharmaceuticals and synthetic materials is cules similar to carbon monoxide, have emerged as
[1]
well known, thus significant efforts have been devot- versatile building blocks in the construction of medici-
[9]
ed to the discovery of methodologies for the prepara- nally important molecules and natural products.
tion of N-heterocyclic compounds in organic chemis- Thus, transition metal-catalyzed reactions involving
[
10]
try. Among them, 1,4-benzoxazepine derivatives are isocyanides have been studied intensively.
This
a very important class of seven-membered rings and methodology has several advantages such as simple
such structural units could widely exist in numerous handling, mild conditions and wide substrate scope
[
8]
medicinal heterocyclic molecules with promising bio- (variable group at nitrogen) over carbon monoxide.
[
2]
logical and pharmaceutical activities. These biologi- As a result, the transition metal-catalyzed isocyanide-
cal characteristics have stimulated organic researchers insertion reactions could offer more opportunities for
to explore synthetic methods for 1,4-benzoxazepines the synthesis of N-heterocyclic compounds. In this
[
3]
and their structural analogues. However, the major context, we designed a palladium-catalyzed domino
drawback of the reported methods is their multistep reaction involving 2-iodophenols 1, N-tosylaziridines
[3a]
nature,
which prevents them from generating 2, and isocyanides 3, which would allow an efficient
a large number of structural derivatives efficiently. access to 1,4-benzoxazepines 4 by the aziridine ring-
Over the past decades, domino reactions as one of the opening and isocyanide-insertion events (Scheme 1).
most powerful tools for generating complex molecules To the best of our knowledge, this novel domino strat-
[
4]
have attracted more and more attention. This strat- egy based on isocyanides and aziridines has not been
egy leads to the rapid formation of two or more new reported up to now.
bonds under the same conditions, which avoids the
Adv. Synth. Catal. 2013, 355, 3401 – 3406
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3401

Fei Ji et al.
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choice (Table 1, entries 2, and 6–9). Moreover, no ob-
vious improvement in the yield was observed when
the solvent was switched to DMF, MeCN, THF and
1,4-dioxane (Table 1, entries 2, and 10–13).
The structure of the novel 1,4-benzoxazepine 4a
was fully characterized by mass spectrometry,
1
13
H NMR, C NMR spectroscopy and elemental anal-
ysis. The trans stereochemistry of the product 4a was
confirmed by single-crystal X-ray analysis (Figure 1).
According to this single-crystal structure analysis of
4
a, the effect of the steric hindrance may lead to the
Scheme 1. Approaches towards the 1,4-benzoxazepine struc-
ture.
Z configuration of the imino bond (see the Support-
ing Information).
To test the hypothesis, we commenced the investi-
gation with 2-iodophenol 1a (0.55 mmol), N-tosyl-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
aziridine of cyclohexene 2a (0.50 mmol) and tert-butyl
isocyanide 3a (0.75 mmol) as model substrates cata-
lyzed by Pd(PPh ) Cl (5 mol%) and Cs CO
2 equiv.) in toluene under reflux for 24 h. The de-
sired 1,4-benzoxazepine 4a was obtained in 65% yield
Table 1, entry 1). To our delight, the isolated yield of
a was enhanced further to 78% when the amount of
ACHTUNGTRENNUNG
3
2
2
2
3
(
(
4
base was increased (Table 1, entry 2). However, fur-
ther raising the amount of base to 4 equiv. could not
improve the yield significantly (Table 1, entry 3).
When K CO and DIPEA were employed as bases in-
2
3
stead of Cs CO , the desired product 4a was isolated
2
3
in 62% and 23% yields, respectively (Table 1, en-
tries 4 and 5). Next, several Pd catalysts were
screened: it was found that Pd ACHTUNGTRENUNG( PPh ) Cl was the best Figure 1. Single-crystal X-ray analysis of 4a.
3 2 2
[a]
Table 1. Optimization of the domino aziridine ring-opening/isocyanide-insertion reaction.
o
[b]
Entry
Catalyst
Pd(PPh ) Cl
Base (equiv.)
Cs CO (2.0)
Solvent
T [ C]
Yield [%]
1
2
3
4
5
6
7
8
9
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMF
110
110
110
110
110
110
110
110
110
110
66
65
78
75
23
62
45
66
75
32
72
67
75
65
3
2
2
2
2
2
2
2
3
Pd(PPh ) Cl
Pd(PPh ) Cl
Pd(PPh ) Cl
Pd
Pd(OAc)
Pd
Pd(amphos) Cl
PdCl /Xantphos (1:2)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
3
2
2 3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (4.0)
2 3
3
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
DIPEA (3.0)
K CO (3.0)
3
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh ) Cl
3
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
2
2 3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(dppf)Cl
Cs CO (3.0)
2 3
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
2
2
2 3
Cs CO (3.0)
2
2
3
10
11
12
13
Pd
Pd(PPh ) Cl
Pd(PPh ) Cl
Pd(PPh ) Cl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh ) Cl
Cs CO (3.0)
3
2
2
2
2
2
2 3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
THF
MeCN
1,4-dioxane
3
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
81
3
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Cs CO (3.0)
100
3
2
2
3
[
a]
Reaction conditions: 2-iodophenol 1a (0.55 mmol), N-tosylaziridine of cyclohexene 2a (0.50 mmol), tert-butyl isocyanide
a (0.75 mmol), 5 mol% Pd catalyst, base, solvent (4 mL), 24 h.
Isolated yield based on 2a.
3
[
b]
3402
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Adv. Synth. Catal. 2013, 355, 3401 – 3406

Synthesis of 1,4-Benzoxazepine Derivatives via a Novel Domino Aziridine Ring-Opening
[a,b]
Table 2. Synthesis of 1,4-benzoxazepine derivatives.
[
a]
Reaction conditions: 2-iodophenols 1 (0.55 mmol), N-tosylaziridines 2 (0.50 mmol), isocyanides 3 (0.75 mmol), Pd-
(PPh ) Cl (5 mol%), Cs CO (3 equiv.), toluene (4 mL), reflux, 24 h.
ACHTUNGTRENNUNG
3
2
2
2
3
[
[
b]
c]
Isolated yield based on 2.
Reation conditions: 2-bromophenol (0.55 mmol), N-tosylaziridine of cyclohexene (0.50 mmol), tert-butyl isocyanide
0.75 mmol), Pd(PPh ) Cl (5 mol%), Cs CO (3 equiv.), toluene (4 mL), reflux, 24 h.
(
ACHTUNGTRENNUNG
3
2
2
2
3
With these optimized reaction conditions in hand, 4c and 4d). The reactions employing various substitut-
we turned our attention to the scope of the domino ed 2-iodophenols bearing a 4-chloro, 4-methyl or 4-
aziridine ring-opening/isocyanide-insertion reaction. phenyl group also worked well to furnish the corre-
Different N-tosylaziridines, 2-iodophenols and isocya- sponding 1,4-benzoxazepines (Table 2, 4e, 4i and 4l).
nides were examined and the results are summarized Unfortunately, no reaction occurred when 2-iodo-4-ni-
in Table 2. Various N-tosylaziridines, including cyclic trophenol was employed in this transformation. We
and acyclic ones, reacted with tert-butyl isocyanide reasoned that the introduction of the electron-with-
and 2-iodophenol, leading to the desired 1,4-benzox- drawing nitro group decreased the nucleophilic activi-
ACHTUNGTRENNUNGa zepines in high yields (Table 2, 4a–4d). As for acyclic ty of 2-iodo-4-nitrophenol. Next, different isocyanides
N-tosylaziridines, the reaction was completely regiose- were also applied to probe the scope of the reaction.
lective and provided only one regioisomer (Table 2, Generally, when tert-butyl isocyanide and 1,1,3,3-tet-
Adv. Synth. Catal. 2013, 355, 3401 – 3406
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asc.wiley-vch.de
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Fei Ji et al.
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bromophenol, N-tosylaziridine of cyclohexene and
tert-butyl isocyanide afforded the corresponding 1,4-
benzoxazepine 4a in only 52% yield. The lower rate
of oxidative addition of the Ar-Br moiety to the in
situ formed palladium(0) species may lead to the dif-
ference in behaviour compared to that of 2-iodo-
AHCTUNGTRENNUNG
control experiments were conducted as shown in
Scheme 2. When 2-iodophenol 1a and N-tosylaziridine
2
a were treated in toluene using Cs CO (3 equiv.) as
2 3
a base, the ring-opening product 5a was obtained in
up to 94% yield. Next, amide 5a was reacted with
tert-butyl isocyanide 3a under the optimized condi-
tions, leading to the desired product 4a in 81% yield.
The experimental results revealed that this domino
reaction may proceed through the base-catalyzed
ring-opening of N-tosylaziridines with 2-iodophenols,
and then the palladium-catalyzed isocyanide-insertion
reaction.
Scheme 2. Investigation of the reaction mechanism.
ramethylbutyl isocyanide were used, the reaction gave
On the basis of these preliminary results and the
the corresponding products in high yields (Table 2, 4a control experiments conducted, a possible catalytic
and 4n). However, when cyclohexyl isocyanide was cycle involved in the present process was outlined as
[
7a,8]
applied in this process under the optimized condi- shown in Scheme 3.
Initially, the base-catalyzed
tions, the desired product was only obtained in mod- ring-opening reaction of N-tosylaziridine 2 with 2-io-
erate yield (Table 2, 4p). Finally, the treatment of 2- dophenol 1 generates the amide 5. The oxidative ad-
Scheme 3. Possible mechanism of the domino aziridine ring-opening/isocyanide-insertion reaction.
3404
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Adv. Synth. Catal. 2013, 355, 3401 – 3406

Synthesis of 1,4-Benzoxazepine Derivatives via a Novel Domino Aziridine Ring-Opening
dition of 5 to the in situ generated palladium(0) spe- Acknowledgements
cies leads to the palladium complex 6. This could then
We are thankful for financial support by NUST research
undergo the isocyanide-insertion process to give the
intermediate 7. Next, with the aid of the base, hydro-
ACHTUNGTRENNUNGi odic acid is extruded out of 7 to generate the eight-
funding (2011ZDJH07). We also acknowledge the Analysis
and Testing Center of Nanjing University of Science and
Technology for the NMR analyses reported in this work.
membered product 8. Finally, reductive elimination
affords 4, regenerating the palladium (0) catalyst.
In conclusion, we have demonstrated a novel and
efficient procedure for the synthesis of a new class of
substituted 1,4-benzoxazepine derivatives via a palladi-
um-catalyzed three-component domino reaction of N-
tosylaziridines, 2-iodophenols and isocyanides. This
process involved the aziridine ring-opening reaction
with 2-iodophenol, followed by palladium-catalyzed
isocyanide-insertion reaction. This domino reaction
was successful with a range of N-tosylaziridines, 2-io-
dophenols and isocyanides, and could afford the de-
sired products in moderate to high yields. More im-
portantly, this methodology may be instrumental to
the rapid construction of important 1,4-benzoxazepine
derivatives with promising biological and pharmaceut-
ical activities.
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1
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