Ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with amines: Rapid access to 2-substituted 4-hydroxyindole

Article abstract of DOI:10.1021/ol501837b

An efficient ring-opening cyclization of cyclohexane-1,3-dione-2- spirocyclopropanes with primary amines has been developed. The reaction proceeded at room temperature without any additives to provide 2-substituted tetrahydroindol-4-ones in good to excellent yields without the formation of the 3-substituted isomers. The obtained product was readily converted into a 2-substituted 4-hydroxyindole derivative via a synthetically useful indoline intermediate.

Full text of DOI:10.1021/ol501837b

Letter
pubs.acs.org/OrgLett
Ring-Opening Cyclization of Cyclohexane-1,3-dione-2-
spirocyclopropanes with Amines: Rapid Access to 2‑Substituted
4‑Hydroxyindole
Hisanori Nambu,* Masahiro Fukumoto, Wataru Hirota, and Takayuki Yakura*
Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Sugitani, Toyama 930-0194, Japan
S
* Supporting Information
ABSTRACT: An efficient ring-opening cyclization of cyclo-
hexane-1,3-dione-2-spirocyclopropanes with primary amines
has been developed. The reaction proceeded at room
temperature without any additives to provide 2-substituted
tetrahydroindol-4-ones in good to excellent yields without the
formation of the 3-substituted isomers. The obtained product
was readily converted into a 2-substituted 4-hydroxyindole derivative via a synthetically useful indoline intermediate.
Scheme 1. Ring-Opening Cyclization of Cyclopropanes
yclopropanes serve as valuable building blocks in organic
synthesis.1 They have strong ring strain, and the relief of
C
the strain provides a high chemical reactivity. Among the large
variety of interesting and efficient reactions of cyclopropanes,1
one of the most extensively investigated reactions is the formal
cycloaddition of doubly activated cyclopropanes for the
construction of highly functionalized carbo- and heterocyclic
compounds,2,3 in which ring opening of their cyclopropanes
results in the formation of a 1,3-dipole equivalent as a C3 unit.
Another unique application of activated 1-acylcyclopropanes is
as C4 units (from three cyclopropane carbons and one carbonyl
carbon); this has been used for the synthesis of heterocycles.4−7
Thus, 1-acyl-1-(alkoxycarbonyl)cyclopropanes have been re-
acted with primary amines at >140 °C to produce 4-
alkoxycarbonyl-2,3-dihydropyrroles (Scheme 1, eq 1).4 Similar
reactions using various activating groups such as nitro and
cyano groups5 and Lewis acid activation6 have also been
reported. In these reactions, a nucleophilic attack of amine at
the doubly activated β-carbon of the acyl group results in
formation of a γ-amino ketone, which easily undergoes
cyclization to construct the pyrrolidine ring. As an alternative
approach, Jabin and co-workers have reported a pyrrole
synthesis involving a ring-opening cyclization of a 1,1-
diacylcyclopropane.7 The reaction of a 1,1-diacylcyclopropane
with amine in the presence of a catalytic amount of
trifluoroacetic acid (TFA) in toluene at reflux provides an α-
cyclopropyl imine, which when treated with silica gel, followed
by air oxidation, gives the corresponding pyrrole through a
Cloke-type rearrangement8 (Scheme 1, eq 2). These examples
offer useful synthetic tools for obtaining nitrogen-containing 5-
membered rings.
substituted indoles.11 As a part of our efforts on the synthesis
of functionalized indoles,12 we investigated the ring-opening
cyclization of cyclohexane-1,3-dione-2-spirocyclopropane with
primary amines as a new potential route to indole frameworks
(Scheme 1, eq 3). Herein, we report that ring-opening
cyclizations of cyclohexane-1,3-dione-2-spirocyclopropane
with amines proceed smoothly at room temperature without
any additives to give 2,3,6,7-tetrahydro-1H-indol-4(5H)-ones,
which are readily converted to highly substituted indoles
through the indoline intermediate.
Indoles that possess a cyclohexane-fused pyrrolidine frame-
work are important structural components of a wide range of
biologically active alkaloids and pharmaceutical agents.9
Although many synthetic methodologies for indoles have
been investigated,10 much effort continues to be devoted to
developing efficient synthetic routes, especially to highly
Received: June 25, 2014
Published: July 17, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/ol501837b | Org. Lett. 2014, 16, 4012−4015

Organic Letters
Letter
At the outset of this work, the ring-opening cyclization of
spirocyclopropane was explored using the known 6,6-dimethyl-
1-phenylspiro[2.5]octane-4,8-dione (1a), readily prepared by
Table 2. Ring-Opening Cyclization of Spirocyclopropane 1a
with a Variety of Amines 2
a
Muller’s method,13 and 1.5 equiv of benzylamine (2a). Among
̈
the reported conditions,4−7 we first chose the use of Ni(ClO4)2·
6H2O14 as a catalyst in CH2Cl2 at reflux according to France’s
procedure,6 due to the mild conditions at relatively low reaction
temperature (Table 1, entry 1). The reaction proceeded with
amine
b
entry
R
time (h)
yield (%)
Table 1. Ring-Opening Cyclization of Spirocyclopropane 1a
with Benzylamine (2a)
a
1
2
3
4
5
2b
2c
2d
2e
2f
PMB
allyl
nBu
tBu
Ph
3
5
3b
92
98
95
85
64
76
39
86
31
3c
3d
3e
3f
2.5
60
4
c
6
2f
Ph
2
3f
7
2g
2g
2h
2i
4-MeOC6H4
4-MeOC6H4
4
3g
3g
3h
b
c
entry
additive
solvent (M)
temp (°C) time (h) yield (%)
c
8
1
d
d
1
2
Ni(ClO4)2
CH2Cl2 (0.1)
toluene (0.1)
CH3CN (0.1)
toluene (0.1)
toluene (0.1)
toluene (0.5)
CH2Cl2 (0.5)
CH3CN (0.5)
THF (0.5)
reflux
reflux
80
9
97
83
95
97
93
97
93
97
97
97
9
H
2
e
TFA
0.5
1
10
Cbz
24
f
3
MgSO4
a
b
All reactions were performed on a 0.2 mmol scale. Isolated yield.
These reactions were conducted in toluene at 70 °C. 10 equiv of
ammonia solution (25% in water) was used. PMB: p-methoxybenzyl.
4
reflux
rt
1
c
d
5
48
6
rt
3
7
rt
3.5
5
in 92%, 98%, and 95% yields, respectively (entries 1−3). The
reaction with tert-butylamine (2e) as a sterically hindered
amine resulted in good yield of 3e, but a significantly longer
time was required to reach completion (60 h, 85% yield, entry
4). In the case of aromatic amines such as aniline (2f) and p-
anisidine (2g), moderate product yields (39−64% yields) were
obtained owing to the formation of several side products
(entries 5 and 7). Gratifyingly, switching the solvent to toluene
and heating at 70 °C greatly improved the product yields,
providing the dihydropyrroles 3f and 3g in 76% and 86% yields,
respectively (entries 6 and 8). The use of ammonia solution
(25% in water) afforded the N-unprotected product 3h in
moderate yield (31% yield, entry 9). The reaction with
CbzNH2 and the corresponding amide prepared with NaH
did not give the desired product (entry 10).
Using benzylamine (2a) as an amine nucleophile, we
explored the scope of the reaction with respect to the
substituents on the cyclopropane (Table 3). The reaction of
p-tolyl-substituted cyclopropane 1b proceeded smoothly to
completion within 2.5 h, giving the 2-substituted tetrahy-
droindol-4-one 3i as the sole product in 86% yield (entry 1).
The use of 1-aryl-substituted spirocyclopropanes 1c and 1d
with a chlorine atom present at the para- and ortho-positions on
the benzene ring provided the corresponding products 3j and
3k in 97% yield (entries 2 and 3). The reaction of cyclopropane
1e, bearing a 2,3,4,5,6-pentafluorophenyl group, resulted in a
good yield of 3l, although the reaction time was prolonged (48
h, 90% yield, entry 4). The use of cyclopropane 1f, possessing
an alkyl substituent on the cyclopropane ring, led to a high
yield of 3m (97% yield), but the reaction time was much longer
(168 h) than those with aromatic substituents (entry 5). As
might be expected, the reaction of 1f in THF at reflux reduced
the reaction time (8 h) and provided 3m in 96% yield.16 The
ring-opening cyclization of simple cyclopropane substrate 1g17
proceeded uneventfully and afforded the desired product 3n in
92% yield (entry 7).
8
rt
9
rt
3
10
THF (0.5)
reflux
0.5
a
b
All reactions were performed on a 0.2 mmol scale. Concentration of
c
d
starting material 1a. Isolated yield. A catalytic amount of Ni(ClO4)2·
6H2O (15 mol %) was used. A catalytic amount of TFA (0.2 equiv)
was used. 0.5 equiv of MgSO4 was used.
e
f
ring-opening and cyclization, and 2-phenyl-substituted tetrahy-
droindol-4-one 3a was obtained after 9 h in 97% yield with no
evidence of the formation of the 3-phenyl-substituted product.
Next, we tested the formation of the imine according to Jabin’s
method (entry 2).7 Unexpectedly, the reaction of 1a in the
presence of a catalytic amount of TFA in toluene at reflux
afforded, in 83% yield, the cyclization product 3a, not the
corresponding imine product. The use of MgSO4 as a desiccant
in CH3CN at 80 °C15 resulted in a high yield of 3a (95% yield,
entry 3). Since ring-opening reactions of doubly activated
cyclopropane under heating conditions were reported by
Lhommet4 and Charette5 (Scheme 1, eq 1), the reaction in
toluene at reflux was examined (entry 4). As a result, a high
product yield of 3a was obtained without any additives (97%
yield). To our surprise, the reaction proceeded at room
temperature to afford 3a in 93% yield, although a much longer
reaction time (48 h) was needed to reach completion (entry 5).
Remarkably, the reaction at a higher molar concentration of 1a
(0.5 M) shortened the reaction time to 3 h and led to high yield
of 3a (97% yield, entry 6). In contrast to the ring-opening of
nonspirocyclopropanes which required further activation or
heating conditions,4−7 1a does not need any activation due to
the spiro activation.1b A survey of solvents revealed that toluene
and THF were suitable for this reaction in terms of product
yield and reaction rate (entries 6−10).
Having optimized the reaction conditions, we investigated
the reaction of 1a with a range of amines 2 in THF (Table 2).
The use of p-methoxybenzylamine (PMBNH2, 2b), allylamine
(2c) and n-butylamine (2d) regioselectively afforded the
corresponding 2-phenyl-substituted products 3b, 3c, and 3d
A plausible mechanism for the ring-opening cyclization of
spirocyclopropane 1 with amine 2 is shown in Scheme 2. On
the basis of previous reports,4−7 two routes to the 2-substituted
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Organic Letters
Letter
corresponding imine product 6 could not be detected even with
a catalytic amount of TFA (Table 1, entry 2).7
Using the present protocol, we prepared the 4-hydroxyindole
8 (Scheme 3). Ring-opening cyclization of spirocyclopropane
Table 3. Ring-Opening Cyclization of a Variety of
Spirocyclopropanes 1b−g with Benzylamine (2a)
a
Scheme 3. Synthesis of 4-[(tert-
Butyldimethylsilyl)oxy]indole 8 from Spirocyclopropane 1h
1h, prepared from 2-diazocyclohexane-1,3-dione and styrene
according to the procedure of Muller,13 with 2a proceeded
̈
smoothly at room temperature to give tetrahydroindol-4-one
3o in 95% yield. According to the literature procedure,19
bromination of 3o with CuBr2 in EtOAc at reflux was followed
by aromatization using a combination of LiBr and Li2CO3 to
afford 4-hydroxyindoline 7 in 82% yield. Since indoline is a
ubiquitous scaffold found in a large range of biologically active
alkaloids and pharmaceutically active compounds,20 this
synthetic route to indoles via indoline intermediates offers a
distinctive advantage. While dehydrogenation of indoline 7
possessing a free phenolic hydroxyl group gave a complex
mixture of products, oxidation of tert-butyldimethylsilyl (TBS)-
protected 4-hydroxyindoline with chloranil furnished 4-
hydroxyindole derivative 8 in 87% yield from 7.
In conclusion, we have developed a regioselective ring-
opening cyclization of cyclohexane-1,3-dione-2-spirocyclopro-
panes with primary amines. The reaction proceeds at room
temperature without any additives to provide tetrahydroindol-
4-ones in good-to-excellent yields. To the best of our
knowledge, this is the first example of the synthesis of indole
frameworks using the ring-opening reaction of cyclopropanes.
The present protocol is applicable to the synthesis of 2-
substituted 4-hydroxyindoles via a synthetically useful indoline
intermediate. Further application of this method to the
synthesis of biologically active indole alkaloids as well as
mechanistic studies are currently in progress.
a
All reactions were performed on a 0.2 mmol scale with 1.5 equiv of
b
c
benzylamine (2a) in THF at room temperature. Isolated yield. The
reaction was conducted in THF at reflux.
Scheme 2. Plausible Mechanism
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, and 1H and 13
C
NMR spectra for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
tetrahydroindol-4-one 3 are proposed. In one route (route a),
ring-opening of the spirocyclopropane 1 proceeds through an
SN2-like displacement by the nucleophilic amine 2 at the more
electrophilic substituted carbon on the cyclopropane A,5
leading to γ-amino ketone 4 regioselectively. Nucleophilic
attack of amine to the carbonyl carbon 4 followed by
dehydration of hemiaminal 5 provides tetrahydroindol-4-one
3. An alternative route employing imine rearrangement8,18 is
also proposed (route b). However, the present reaction would
proceed through γ-amino ketone 4 (route a) because the
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: nambu@pha.u-toyama.ac.jp.
*E-mail: yakura@pha.u-toyama.ac.jp.
Notes
The authors declare no competing financial interest.
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Letter
Hibino, S. Nat. Prod. Rep. 2013, 30, 694. (f) Marcos, I. S.; Moro, R. F.;
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ACKNOWLEDGMENTS
■
This research was supported, in part, by a Grant-in-Aid for
Scientific Research (C) (Grant No. 24590001) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
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