Gold(I)/(III)-catalyzed synthesis of 2-substituted piperidines; valency-controlled cyclization modes

Article abstract of DOI:10.1016/j.tetlet.2015.09.115

Strategic use of hard gold(III) and soft gold(I) catalysts provides facile access to two types of 2-substituted piperidines from propargylic alcohols. Thus, heating propargylic alcohols in the presence of AuBr₃ results in cyclization to furnish

Full text of DOI:10.1016/j.tetlet.2015.09.115

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Gold(I)/(III)-catalyzed synthesis of 2-substituted piperidines; valency-
controlled cyclization modes
⇑
Nobuyoshi Morita , Tomonori Tsunokake, Yuji Narikiyo, Mayuka Harada, Tatsuyuki Tachibana,
⇑
Yuta Saito, Shintaro Ban, Yoshimitsu Hashimoto, Iwao Okamoto, Osamu Tamura
Showa Pharmaceutical University, Machida, Tokyo 194-8543, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Strategic use of hard gold(III) and soft gold(I) catalysts provides facile access to two types of 2-substituted
Received 7 August 2015
Revised 24 September 2015
Accepted 25 September 2015
Available online xxxx
piperidines from propargylic alcohols. Thus, heating propargylic alcohols in the presence of AuBr
in cyclization to furnish piperidines having an acetylenic moiety, due to activation of the propargylic
hydroxyl group by coordination of gold(III). On the other hand, the catalyst [Ph PAuNTf PhMe induces
Meyer–Schuster rearrangement of propargylic alcohols to give ,b-unsaturated ketones, which undergo
3
results
3
2 2
]
a
intramolecular aza-Michael addition to afford piperidines bearing a carbonyl group, due to activation of
the triple bond by coordination of gold(I).
Keywords:
Oxophilic gold(III) catalyst
Ó 2015 Published by Elsevier Ltd.
p-Philic gold(I) catalyst
Cyclization
Meyer–Schuster rearrangement
Aza-Michael addition
Piperidine1 is a structural component of many alkaloids, such
,2
OH
O
3
3
4
as (À)-coniine, (À)-sedamine and (+)-pelletierine (Fig. 1), as well
as a wide range of compounds with various pharmacological prop-
erties. Therefore, efficient synthesis of functionalized piperidines
remains an important topic in organic synthesis.5
Me
N
H
Ph
N
Me
N
H
Me
(-)-Coniine
(-)-Sedamine
(+)-Pelletierine
We recently reported that strategic use of hard gold(III) catalyst
and soft gold(I) catalyst enables divergent synthesis of cyclic ethers
from propargylic alcohols (Scheme 1).6 Thus, use of gold(III)
catalyst results in cyclization to furnish cyclic ethers 2 having an
acetylenic moiety, due to coordination of gold(III) to the oxygen
atom at the propargylic position (route A, Scheme 1). On the other
hand, treatment of propargylic alcohols 1 with soft gold(I) catalyst
Figure 1. Natural products containing 2-substituted piperidine structure.
followed by aza-Michael addition8 to furnish piperidines bearing
a carbonyl group (route B, Scheme 2).
Based on our previous results on cyclic ether formation, we
started our study by examining the reaction of model substrate
1a in the presence of various oxophilic gold(III) catalysts in
induces Meyer–Schuster rearrangement to afford
ketones 3, which undergo intramolecular oxa-Michael addition to
furnish cyclic ethers bearing carbonyl group (route B,
a,b-unsaturated
4
a
ClCH
2
CH
2
Cl (DCE) (Table 1). Treatment of propargylic alcohol 1a
) afforded the desired product
Scheme 1). In order to extend our strategy, we turned our attention
to the use of nitrogen nucleophile in place of oxygen nucleophile
and explored the synthesis of piperidines from propargylic alco-
hols bearing a nitrogen functionality at the terminal position
with gold(III) catalyst (5 mol% AuBr
3
2a in 57% yield (entry 1). During optimization of the reaction
conditions, we found that heating the reaction solution dramatically
accelerated cyclization. Thus, the reaction of propargylic alcohol 1a
(Scheme 2). Here, we report a hard gold(III)-catalyzed cyclization
with 5 mol% AuBr
yield within 10 min (entry 2). Other gold(III) catalysts (AuCl
HAuCl O) also enabled the cyclization, affording 2a in 35%
Á3H
and 77% yields, respectively (entries 3 and 4). Cationic gold species
generated from 5 mol% AuBr and 15 mol% AgNTf gave a complex
3
at 50 °C gave the desired product 2a in high
3
or
to give piperidines having an acetylenic moiety (route A, Scheme 2)
7
and
a
soft gold(I)-catalyzed Meyer–Schuster rearrangement
4
2
3
2
mixture (entry 5). Furthermore, although the scope and limitation
of gold(III)-catalyzed cyclization about substituents at the position
of alkyne were researched, the reaction with propargylic alcohol
⇑
Corresponding authors. Tel./fax: +81 42 721 1579.
E-mail addresses: morita@ac.shoyaku.ac.jp (N. Morita), tamura@ac.shoyaku.ac.jp
(
O. Tamura).
http://dx.doi.org/10.1016/j.tetlet.2015.09.115
040-4039/Ó 2015 Published by Elsevier Ltd.
0
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2
N. Morita et al. / Tetrahedron Letters xxx (2015) xxx–xxx
a
H
a: Au(III) catalyst activates
Au
b
O
hydroxy group (Cyclization)
OH
n
route A
route B
n
O
Hard Activation
R
Nu
R
1
2
b: Au(I) catalyst activates unsaturated bond
Meyer-Schuster Rearrangement)
Soft Activation
(
O
intramolecular
Michael addition
O
R
n
OH
R
O
n
3
4
Scheme 1. Gold(I)/(III)-catalyzed synthesis of two types of cyclic ethers (our previous work).
Cyclization
route A
N
a: Hard Au(III) catalyst
activates hydroxy group
PG
R
a
H
Au
O
NHPG
b
b: Soft Au(I) catalyst
activates unsaturated bond
R
O
route B
Meyer-Schuster Rearrangement R
N
PG
Aza-Michael Addition
Scheme 2. Synthesis of piperidines from propargylic alcohols via hard gold(III)-catalyzed cyclization and soft gold(I)-catalyzed Meyer–Schuster rearrangement followed by
aza-Michael addition.
Table 1
Table 2
Optimization of the reaction conditions in gold(III)-catalyzed cyclization
Gold(III)-catalyzed cyclization of propargylic alcohols with various protecting groups
AuBr3
5 mol%)
OH
NHTs
OH
NHPG
Catalyst
ClCH CH Cl Ph
(
N
N
Ts
PG
ClCH CH Cl Ph
Ph
2
2
Ph
2
2
1
a
Temp. Time
2a
1
Temp. Time
2
Entry
Catalyst (mol%)
Temp
rt
50 °C
50 °C
50 °C
50 °C
Time
Yield (%)
Entry
1 PG
Temp
Time
Yield (%)
1
2
3
4
5
AuBr
AuBr
3
(5)
(5)
(5)
2 days
10 min
2 days
10 min
10 min
57
83
35
77
1
2
3
4
1b Ms
1c Ns
1d Boc
1e H
50 °C
50 °C
Reflux
50 °C
15 min
10 min
1 day
2b 64
2c 82
No reaction
3
AuCl
HAuCl
AuBr
AgNTf
3
4
Á3H
2
O (5)
2 days
Complex mixture
3
(5)
Complex mixture
2
(15)
of starting material 1a (entries 1 and 2). Cationic gold species gen-
erated from Ph PAuCl (5 mol%) and AgSbF (5 mol%) furnished the
desired product 3a in 50% yield (entry 3). Finally, treatment of
propargylic alcohol 1a with 1 mol% [Ph PAuNTf PhMe and 1
in toluene afforded piperidine 3a in 81%
3
6
bearing other substituents instead of phenyl group afforded no
desired product.
To examine the effect of protecting group of nitrogen atoms in
propargylic alcohols in the gold(III)-catalyzed cyclization, we con-
3
2 2
]
6
,7c
equivalent of MeOH
yield (entry 4).
ducted reactions of substrates 1b–e with 5 mol% AuBr
3
(Table 2).
Next, we examined the effect of the protecting group of propar-
gylic alcohols 1b–e in the Meyer–Schuster rearrangement followed
by aza-Michael addition (Table 4). Treatment of propargylic
Propargylic alcohols protected by a sulfonyl group, 1b and 1c, were
smoothly transformed into the corresponding piperidines 2b and 2c
in 64% and 82% yields, respectively (entries 1 and 2), whereas the
reaction with propargylic alcohol protected by carbamate 1d gave
no cyclization product 2d (entry 3). In the case of propargylic alcohol
alcohols 1b and 1c bearing
a sulfonyl group with 1 mol%
[Ph PAuNTf PhMe and 1 equivalent of MeOH in toluene afforded
3
2 2
]
piperidines 3b and 3c in 86% and 78% yields, respectively (entries
1 and 2). In contrast to the gold(III)-catalyzed cyclization (Table 2,
entry 3), propargylic alcohol 1d having a carbamate moiety was
also converted into the corresponding piperidine 3d in 74% yield
(entry 3). It should be noted that the cyclization product 3d is an
1
e with free amine, the reaction gave a complex mixture (entry 4).
According to our HSAB strategy using gold catalysts,6 we next
,9
7
examined gold(I)-catalyzed Meyer–Schuster rearrangement fol-
lowed by aza-Michael addition8 from propargylic alcohol 1a
3
f
(Table 3). The reaction with AuCl (15 mol%) or Ph
3
PAuCl (15 mol
important synthetic intermediate for optically active piperidine
3
10
%
) afforded a trace amount of the desired product 3a with recovery
alkaloids (À)-sedamine and (À)-lobeline (Scheme 3). In contrast
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N. Morita et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 3
Table 5
Optimization of the reaction conditions in gold(I)-catalyzed Meyer–Schuster rear-
rangement followed by aza-Michael addition
Gold(I)-catalyzed Meyer–Schuster rearrangement followed by aza-Michael addition
with various propargylic alcohols
OH
NHTs
[Ph PAuNTf ] PhMe
NHTs
3 2 2
Catalyst
OH
O
O
(
1 mol%)
Ph
N
R
N
rt, Time
Solvent
Ts
Ph
1 eq. MeOH
toluene
rt, Time
Ts
R
1
3
1a
3a
Entry
Catalyst (mol%)
AuCl (15)
Solvent
Time
Yield (%)
Entry
R
Time
Yield (%)
1
2
3
CH
CH
2
Cl
Cl
2
2
3 days
3 days
1 day
Trace
Trace
50
Ph
Ph
3
PAuCl (15)
PAuCl (5)
2
1
2
3
4
1f p-MeO-C
1g n-Hex
1h c-Hex
1i t-Bu
6
H
4
1 day
1 day
4 days
5 days
3f 76
3g 77
3h 75
3i 32
3
Toluene
AgSbF
[Ph PAuNTf
equiv MeOH
6
(5)
4
3
2
2
] PhMe (1)
Toluene
1 day
81
1
Table 4
a
H
Gold(I)-catalyzed Meyer–Schuster rearrangement followed by aza-Michael addition
with propargylic alcohols having various protecting groups
Au
O
NHPG
Au(III)
Au
b
1
2
3
[
Ph PAuNTf ] PhMe
a: Hard Activation
3
2 2
A
OH
NHPG
R
O
(1 mol%)
MeOH
Ph
N
Au(I) b: Soft Activation
1
eq. MeOH
PG
Ph
1
3
toluene
rt, Time
OH
NHPG
OH2
NHPG
Au
Au
R
Entry
1 PG
Time
Yield (%)
Me
Me
R
O
H
1
2
3
4
1b Ms
1c Ns
1d Boc
1e H
1 day
1 day
4 days
5 days
3b 86
3c 78
3d 74
O
H
NHPG
O
No reaction
3
H O
2
Au
R
NHPG
3
Me
R
O
3
OH
Scheme 4. Mechanistic proposal for gold(III)-catalyzed cyclization and gold(I)-
catalyzed Meyer–Schuster rearrangement followed by aza-Michael addition.
Ph
N
Me
O
(-)-Sedamine
the hydroxyl group to induce cyclization by intramolecular nucle-
ophilic substitution, affording piperidine 2 having an acetylenic
moiety (activation a). On the other hand, soft gold(I) in complex
A promotes addition of methanol to generate an allenyl ether,6
Ph
N
OH
O
Boc
3d
Ph
N
Ph
,7c
Me
(
-)-Lobeline
which undergoes hydrolysis to give
activation b). This ketone cyclizes smoothly to afford 3.
In summary, we present gold(I)/(III)-catalyzed syntheses of two
a,b-unsaturated ketone
(
Scheme 3. Formal synthesis of natural products (À)-sedamine and (À)-lobeline
from 3d produced by gold(I)-catalyzed Meyer–Schuster rearrangement followed by
aza-Michael addition of 1d.
types of piperidines from propargylic alcohols, by making use of
the hard-soft principle. We are currently applying the method to
the synthesis of biologically active piperidine derivatives.
Experimental and theoretical investigations of the reaction mech-
anism are also in progress.
to 1b–d, free amine 1e did not undergo Meyer–Schuster rearrange-
ment at all (entry 4).
Next, we examined the effect of terminal substituents of
propargylic alcohols 1 on gold(I)-catalyzed Meyer–Schuster rear-
rangement followed by aza-Michael addition. As shown in Table 5,
the present cyclization can be applied not only for 1f having aro-
matic terminal groups (entry 1), but also for 1g and 1h bearing
alkyl groups (entries 2 and 3). Even propargylic alcohol 1i having
a bulky tert-butyl group underwent sequential Meyer–Schuster
rearrangement–aza-Michael reaction to afford 3i, although the
yield was rather low (entry 4).
Acknowledgments
This research is supported by the Platform Project for Support-
ing Drug Discovery and Life Science Research (Platform for Drug
Discovery, Informatics, and Structural Life Science) from the Min-
istry of Education, Culture, Sports, Science (MEXT), and Japan
Agency for Medical Research and development (AMED).
A plausible mechanistic model for gold-catalyzed synthesis of
the two types of piperidines is shown in Scheme 4. In both cases,
Supplementary data
6
,11
complex A
would be formed as a common reaction intermedi-
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.09.
115.
ate, whose character would play a pivotal role in determining the
reaction pathway. Hard gold(III) in complex A strongly activates
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4
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Please cite this article in press as: Morita, N.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.115