Synthesis of 2,3-allenylamides utilizing [1,2]-phospha-Brook rearrangement and their application to gold-catalyzed cycloisomerization providing 2-aminofuran derivatives

Article abstract of DOI:10.1039/c6cc06591k

An efficient synthetic method for 2,3-allenylamides having an oxygen functionality at the 2-position, which are difficult to access by conventional methods, was newly developed by utilizing the [1,2]-phospha-Brook rearrangement under Br?nsted base catalysis. Further manipulation of the 2,3-allenylamides via gold-catalyzed cycloisomerization enables the formation of 2-aminofuran derivatives.

Full text of DOI:10.1039/c6cc06591k

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DOI: 10.1039/C6CC06591K
Journal Name
COMMUNICATION
Synthesis of 2,3-allenylamides utilizing [1,2]-phospha-Brook
rearrangement and their application to gold-catalyzed
cycloisomerization providing 2-aminofuran derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Azusa Kondoh,a Sho Ishikawa,b Takuma Aoki,b and Masahiro Terada*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient synthetic method for 2,3-allenylamides having an
oxygen functionality at the 2-position, which are difficult to access
by conventional methods, was newly developed by utilizing the
[1,2]-phospha-Brook rearrangement under Brønsted base catalysis.
Further manipulation of the 2,3-allenylamides via gold-catalyzed
cycloisomerization enables the formation of 2-aminofuran
derivatives.
2,3-Allenylcarbonyl compounds are one of the privileged
classes of compounds used as building blocks in organic
synthesis.1 A variety of transformations have been developed
on the basis of their versatile reactivities, including
nucleophilic cyclization catalyzed by transition metals,2
cycloaddition under nucleophilic catalysis,3 and others.4 A new
synthesis of 2,3-allenylcarbonyl compounds also has received
considerable attention in recent years.5,6 Particularly, the
synthesis of 2,3-allenylcarbonyl compounds having
a
functional group on a specific sp2-carbon is attractive because
it paves the way for the synthesis of highly functionalized
complex molecules through the transformation of 2,3-
allenylcarbonyl compounds. In this context, we have
developed an efficient synthesis of 2,3-allenylcarbonyl
compounds having an oxygen-functionality at the 2-position,7
which are difficult to access by conventional methods, by
utilizing the [1,2]-phospha-Brook rearrangement under
Brønsted base catalysis.8,9 Our reaction design is shown in
Scheme
1
Synthesis of 2,3-allenylamides utilizing [1,2]-phospha-Brook
rearrangement and their application to metal-catalyzed cycloisomerization
to oxygen, i.e., the [1,2]-phospha-Brook rearrangement,
proceeds to form enolate
protonation at the γ-position by the conjugated acid of the
Brønsted base catalyst or provides desired 2,3-allenylamide 3.
B. Finally, the regioselective
2
Our methodology is characterized by the catalytic generation
of enolate (propargyl anion) having an oxygen-functionality
under mild reaction conditions. Generally, the synthesis of
related funtionalized allenes involves the stoichiometric
generation of allenyl or propargyl anions under highly basic
Scheme 1. α-Alkynylketoamide
1
is employed as the substrate
and is treated with phosphite
2 in the presence of a catalytic
amount of Brønsted base (Scheme 1a). The deprotonation of
phosphite by the Brønsted base and the following 1,2-
selective addition of the resulting anion of provide alkoxide A.
2
conditions.7 As an application of newly synthesized
3, we also
2
The migration of the dialkoxyphosphoryl moiety from carbon
have developed a cycloisomerization reaction catalyzed by π-
acidic transition metals,2 which provides 2-aminofuran
derivative
hopes that
4
having an oxygen-functionality at the 3-position, in
would be a useful building block for the
a.Research and Analytical Center for Giant Molecules, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan. Email:
mterada@m.tohoku.ac.jp.
4
construction of six-membered cyclic frameworks through the
Diels-Alder reaction (Scheme 1b).10 Herein we report an
efficient synthesis of 2,3-allenylamides utilizing the [1,2]-
phospha-Brook rearrangement catalyzed by a phosphazene
b.Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-
ku, Sendai 980-8578, Japan.
† Electronic Supplementary Information (ESI) available: Experimental procedures
screening of reaction conditions, and characterization data. See
DOI: 10.1039/x0xx00000x
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base, and the cycloisomerization of the 2,3-allenylamides 92% by employing 1.5 equivalents of 2a (entry 14). In order to
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catalyzed by a cationic Au(I) complex.
At the outset of our study, N-benzyl-2-oxo-N-phenyloct-3- 5aa
clarify whether or not 3aa was formed bDyOtIh: 1e0i.1s0o3m9/eCr6izCaCt0io65n9o1Kf
5aa was treated with a catalytic amount of P1-tBu in the
,
ynamide (1a) was employed as the initial substrate. 1a was presence or absence of 2a. This resulted in the recovery of 5aa
treated with 1.0 equivalent of diethyl phosphite (2a) in the and the formation of only a trace amount of 3aa, thus clearly
presence of 10 mol% Brønsted base in toluene at room suggesting that 3aa was formed through the regioselective
temperature. The use of a tertiary amine, such as N,N- protonation of the enolate intermediate (B in Scheme 1) at the
diisopropylethylamine, provided no adduct, and 72% of γ-position.11 Considering the acidity of diethyl phosphite
starting 1a was recovered (Table 1, entry 1). The use of DBU (dimethyl phosphite, pKa = 18.4 in DMSO, estimated)12 and the
and TBD accelerated the consumption of 1a, and desired 2,3- conjugated acid of P1-tBu (pKBH+ = 15.7 in DMSO),13 the latter
allenylamide 3aa was obtained along with propargylic amide would preferentially serve as the proton source, and its
5aa, albeit in low yield (entries 2 and 3). A dramatic bulkiness would contribute to the regioselective protonation
improvement of the yield of 3aa was achieved by employing at the less hindered γ-position to provide 2,3-allenylamide 3aa
phosphazene bases, such as P1-tBu and P2-tBu (entries 4 and With the optimum reaction conditions in hand, the scope of α-
5). In particular, P1-tBu facilitated the regioselective alkynylketoamide and phosphite was investigated (Table 2).
.
1
2
protonation and desired 3aa was obtained in 63% yield along At first, different phosphites were tested (entries 1-3).
with 2% of 5aa (entry 4). The use of tBuOK resulted in an Dimethyl phosphite (2b) gave a similar result to 2a (entry 1).
almost 1:1 mixture of 3aa and 5aa in moderate combined yield Sterically hindered diisopropyl phosphite (2c) provided only a
(entry 6). Whereas the solvents screened did not increase the trace amount of the desired product and a substantial amount
yield of 3aa so much (entries 7-11), the decrease of the of recovered 1a (entry 2). The reaction of diphenyl phosphite
reaction temperature was beneficial as it improved the mass
balance of the reaction and increased the yield of 3aa. The propargylic amide 5ad in 16% NMR yield (entry 3). Then the
optimum reaction temperature was 40 °C; at that effect of substituents on the nitrogen of α-alkynylketoamide
temperature, 3aa was obtained in 82% yield, and 5aa was not was examined. Both diphenylamide 1b and dibenzylamide 1c
(2d) proceeded to provide 3ad in 76% yield along with
—
1
detected (entry 13). Finally, the yield of 3aa was improved to
were applicable to the reaction, affording corresponding
products 3ba and 3ca in good yields, respectively (entries 4
Table 1 Optimization of reaction conditionsa
Table 2 Scope of ketoamides 1 and phosphites 2a
entry
1
R1
R2
R3
2
R4
3
yield
(%)b
94
1
2
3
4
5
6
7
8
1a
1a
1a
1b Ph
1c Bn
1d Me
1e (CH2)2O(CH2)2 nBu
1f
1g
1h Ph
1i
1j
1k
1l
1m Ph
1n Ph
1o Ph
1p Ph
1q Ph
Ph
Ph
Ph
Bn
Bn
Bn
Ph
Bn
Me
nBu
nBu
nBu
nBu
nBu
nBu
2b
2c
2d
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
Me
iPr
Ph
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
3ab
3ac
3ad
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
3ja
3ka
3la
<5c
76d
79
temp.
(°C)
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
—20
—40
—40
yieldb (%)
5aa
0
entry
base
solvent
3aa
0
15
28
63
63
32
55
59
66
26
70
79
82
92 (91)
1a
72
6
0
0
0
0
0
0
0
20
3
0
0
83
1
2
3
4
5
6
7
8
9
10
11
12
13
14c
iPr2NEt
DBU
TBD
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
THF
DMF
EtOH
toluene
toluene
toluene
80e
69f
92
12
18
2
28
34
2
Ph
Ph
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
cHex
tBu
Ph
Ph
(CH2)4Cl
(CH2)4Br
(CH2)4OAc
P1-tBu
P2-tBu
tBuOK
P1-tBu
P1-tBu
P1-tBu
P1-tBu
P1-tBu
P1-tBu
P1-tBu
P1-tBu
9g
10
11
12
13
14
15
16
17
18
19
56h
96i
90i
80
Bn
Ph
Ph
Ph
3
71
86
87
89
<1
<1
7
1
<1
<1
(CH2)4OTHP 2a
3ma
3na
3oa
3pa
3qa
(CH2)4OTBS
(CH2)4OH
TIPS
2a
2a
2a
2a
80
<1j
80
0
H
aReaction conditions: 1a (0.25 mmol), 2a (0.25 mmol), base (0.025 mmol), solvent
(1.0 mL). bYields were determined by 1H and 31P NMR analyses of the crude
mixture. Trimethyl phosphate was used as the internal standard. Isolated yield is
shown in parentheses. c0.38 mmol 2a (1.5 eq.) was used.
aReaction conditions: 1 (0.25 mmol), 2 (0.38 mmol), P1-tBu (0.025 mmol), toluene
(1.0 mL), —40°C, 3 h. bIsolated yields unless otherwise noted. c60% of 1a was
recovered. d5ad (16% NMR yield) was formed. e5da (13% NMR yield) was formed.
f5ea (20% NMR yield) was formed. gThe reaction was conducted for 3.5 h. h5ga
(29% NMR yield) was formed. iNMR yields. j5pa (95% NMR yield) was formed.
2 | J. Name., 2012, 00, 1-3
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and 5). Less sterically hindered dialkyl amides, such as
dimethyl amide 1d and morpholine amide 1e, provided
corresponding allenylamides 3da and 3ea in moderate yields
along with propargylic amides, indicating that the size of the
amide moiety influenced the regioselectivity of the enolate
protonation (entries 6 and 7). Next, the substituents on the
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DOI: 10.1039/C6CC06591K
alkyne terminus of
1 were screened. Cyclohexyl-substituted 1f
underwent the reaction smoothly and 3fa was obtained in
good yield (entry 8). The reaction of tert-butyl- substituted 1g
resulted in the formation of a mixture of 3ga and 5ga (entry 9).
The reactions of 1h and 1i having a phenyl group proceeded to
provide allenylamides 3ha and 3ia, which were incompatible
with silica-gel column purification (entries 10 and 11). A variety
of functional groups including chloro, bromo, and even
unprotected hydroxy groups, were tolerated under the
reaction conditions, and the corresponding products were
obtained in good yields (entries 12-17). The reaction of
triisopropylsilyl-substituted 1p provided only alkynylamide 5pa
(entry 18). The results of the reactions of 1g and 1p (entries 9
and 18) suggested that the bulky substituents on the alkyne
terminus prevented regioselective protonation at the γ-
position. 1q having a terminal alkyne moiety underwent the
reaction without any problems to afford 3qa in good yield
(entry 19).
Scheme 2 Intramolecular Diels-Alder reaction of 3aa
an allene moiety to provide 6aa 14
,
which indicated the
potential of the newly synthesized allenylamide for diverse
applications (Scheme 2). Then, investigation of the substrate
scope for the cycloisomerization was carried out (Table 3). The
reaction of diphenylamide 3ba proceeded smoothly to provide
4ba in good yield; in contrast, the reaction of dibenzylamide
3ca did not take place (entries 2 and 3). Then, substrates
having various substituents on the allene moiety were
examined. The reaction of cyclohexyl- and tert-butyl-
substituted 3fa and 3ga proceeded without any problems to
afford 4fa and 4ga in good yields, respectively (entries 4 and 5).
When the reaction was performed with 3ha, which had a
phenyl group on the allene moiety, the intramolecular Diels-
Alder reaction proceeded in preference to the
cycloisomerization even in the presence of Au(I) catalyst (entry
6). In contrast, the reaction of dibenzylamide 3ia having a
phenyl group on the allene moiety proceeded to afford 4ia in
Next, we investigated the cycloisomerization of 2,3-
allenylamides
3 into 2-aminofuran derivatives 4 as an
application of newly synthesized 2,3-allenylamides
3
.
good yield (entry 7). 2,3-Allenylamides
of functional groups were applicable to the cycloisomerization,
and corresponding 2-aminofuran derivatives were obtained
in moderate to good yields (entries 8-13). The reaction of 3qa
provided Diels-Alder product 6qa as the major product (entry
14).
The investigation was further extended to the one-pot
synthesis of 2-aminofuran derivative 4aa from α-
alkynylketoamide 1a (Scheme 3). 1a was treated with 2a in the
3 possessing a variety
Screening for π-acidic transition metals as catalyst revealed
that the cationic Au(I) complex efficiently catalyzed the
cycloisomerization of 3aa in toluene at 70 °C, providing 4aa in
85% yield (Table 3, entry 1).11 It is noteworthy that heating 3aa
in the absence of Au(I) catalyst accelerated the intramolecular
Diels-Alder reaction of the phenyl group on the nitrogen with
4
Table 3 Cycloisomerization of 2,3-allenylamides 3 into 2-aminofuran derivatives 4a
presence of 10 mol% P1-tBu in toluene at
adding 10 mol% AcOH to deactivate P1-tBu,
—
40 °C for 3 h. After
mol%
4
Au(PPh3)NTf2 was added, and the resulting mixture was stirred
at 70 °C for 12 h to furnish desired product 4aa in 57% yield.
Finally, the transformation of 2-aminofuran derivatives was
attempted.15 4aa was found to participate in the Diels-Alder
reaction with maleic anhydride and N-methylmaleimide and
subsequent dehydrative aromatization to furnish 2-
aminophenol derivatives 7a and 7b in high yields (Scheme
4).10c
entry
1
2
3
4
5
6
7
8
3
R1
R2
R3
4
yield (%)b
85
80
<1
3aa
3ba
3ca
3fa
3ga
3ha
3ia
3ja
3ka
3la
Ph
Ph
Bn
Ph
Ph
Ph
Bn
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Bn
Ph
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
nBu
nBu
nBu
cHex
tBu
Ph
Ph
(CH2)4Cl
(CH2)4Br
(CH2)4OAc
(CH2)4OTHP
(CH2)4OTBS
(CH2)4OH
H
4aa
4ba
4ca
4fa
4ga
4ha
4ia
4ja
4ka
4la
78
83
15c
64
83
69
50
60
9
10
11
12
13
14
3ma
3na
3oa
3qa
4ma
4na
4oa
4qa
79
42
12d
aReaction conditions: 3, Au(PPh3)NTf2 (4 mol%), toluene (0.125 M), 70 °C, 12 h.
bIsolated yields. c6ha (68% NMR yield) was formed. d6qa (52% NMR yield) was
formed.
Scheme 3 One-pot synthesis of 2-aminofuran derivative 4aa from 1a
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Asymmetry, 2009
¸20, 259; (c) S. Yu and S. Ma, Chem.
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Commun., 2011, 47, 5384.
DOI: 10.1039/C6CC06591K
6
For selected very recent examples, see: (a) A. Roy, B. A. Bhat
and S. D. Lepore, Org. Lett. 2016, 18, 1230; (b) X. Wang, X.
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Scheme 4 Transformation of 4aa
In conclusion, we have newly developed an efficient synthesis
of 2,3-allenylamides by utilizing the [1,2]-phospha-Brook
rearrangement under Brønsted base catalysis. The reaction
involves the generation of an amide enolate by treatment of α-
alkynylketoamide with phosphite through the [1,2]-phospha-
Brook rearrangement, followed by the regioselective
protonation at the γ-position to provide 2,3-allenylamides
having an oxygen functionality at the 2-position, which are
difficult to access by conventional methods. Thus synthesized
2,3-allenylamides were subsequently applied to the gold-
catalyzed cycloisomerization to afford 2-aminofuran
derivatives. The one-pot synthesis of 2-aminofuran derivatives
as well as their further transformation into 2-aminophenol
derivatives was also achieved.
This work was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalysts” from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan, and a Grant-in-Aid for Scientific Research from the
Japan Society for the Promotion of Science (JSPS).
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15 The diethoxyphosphoryl moiety of
4 could be removed
under basic conditions. See the ESI for details.
4 | J. Name., 2012, 00, 1-3
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